Asialo GM1(+) CD8(+) T cells play a critical role in costimulation blockade-resistant allograft rejection

Steady-state, therapeutic, and helminth-induced IL-4 compromise protective CD8 T cell bystander activation

Memory CD8 T cells (Tmem) can be activated into innate-like killers by cytokines like IL-12, IL-15, and/or IL-18; but mechanisms regulating this phenomenon (termed bystander activation) are not fully resolved. We found strain-intrinsic deficiencies in bystander activation using specific pathogen-free mice, whereby basal IL-4 signals antagonize IL-18 sensing. We show that therapeutic and helminth-induced IL-4 impairs protective bystander-mediated responses against pathogens. However, this IL-4/IL-18 axis does not completely abolish bystander activation but rather tunes the expression of direct versus indirect mediators of cytotoxicity (granzymes and interferon-γ, respectively). We show that antigen-experience overrides strain-specific deficiencies in bystander activation, leading to uniform IL-18 receptor expression and enhanced capacity for bystander activation/cytotoxicity. Our data highlight that bystander activation is not a binary process but tuned/deregulated by other cytokines that are elevated by contemporaneous infections. Further, our findings underscore the importance of antigen-experienced Tmem to dissect the contributions of bystander Tmem in health and disease.

Marginal Zone B Cells Are Necessary for the Formation of Anti-donor IgG After Allogeneic Sensitization

BACKGROUND

The formation of anti-major histocompatibility complex (MHC) antibodies is a significant barrier for many patients awaiting organ transplantation. Patients with preformed anti-MHC antibodies have limited options for suitable donors, and the formation of donor-specific anti-MHC antibodies after transplantation is a harbinger of graft rejection. Despite the recognized importance of anti-MHC antibodies, the mechanisms responsible for the differentiation of B cells after exposure to allogeneic antigens are poorly understood.

METHODS

To evaluate the differentiation of B cells in response to allogeneic antigen, we used a model of H-2 b C57Bl/6 sensitization with H-2 d antigen. We used a class I MHC tetramer-based approach to identify allogeneic B cells and flow cytometric crossmatch to identify allogeneic IgM and IgG.

RESULTS

We found that although the formation of anti-H-2 d IgG was robust, few class-switched B cells and germinal center B cells were formed. Antigen-specific B cells did not express classical memory B-cell markers after sensitization but had an IgM + CD21 + marginal zone B-cell phenotype. The frequency of marginal zone B cells increased after sensitization. Depletion of marginal zone B cells before sensitization or skin grafting resulted in a significant diminution of anti-H-2 d IgG and fewer germinal center B cells. Adoptive transfer experiments revealed that marginal zone B cells more efficiently differentiated into germinal center B cells and anti-donor IgG-producing cells than follicular B cells.

CONCLUSIONS

These results demonstrate an important role for marginal zone B cells as a reservoir of alloreactive B cells that are activated by allogeneic antigens.

A Membrane Lipid Signature Unravels the Dynamic Landscape of Group 1 ILCs

In an era where the established lines between cell identities are blurred by intra-lineage plasticity, distinguishing between stable and transitional states becomes imperative. This challenge is particularly pronounced within the Group 1 ILC lineage, where the similarity and plasticity between NK cells and ILC1s obscure their classification and the assignment of their unique contributions to immune regulation. This study exploits the unique property of Asialo-GM1 (AsGM1)-a membrane lipid associated with cytotoxic attributes absent in ILC1s-as a definitive criterion to distinguish between these cells. By prioritizing cytotoxic potential as the cardinal differentiator, our strategic use of the AsGM1 signature achieved precise delineation of NK cells and ILC1s across tissues, validated by RNA-seq analysis. This capability extends beyond steady-state classifications, adeptly capturing the binary classification of NK cells and ILC1s during acute liver injury. By leveraging two established models of NK-to-ILC1 plasticity driven by TGFβ and Toxoplasma gondii , we demonstrate the stability of the AsGM1 signature, which sharply contrasts with the loss of Eomes. This signature identified a spectrum of known and novel NK cell derivatives-ILC1-like entities that bridge traditional binary classifications in aging and infection. The early detection of the AsGM1 signature at the immature NK (iNK) stage, preceding Eomes, and its stability, unaffected by transcriptional reprogramming that typically alters Eomes, position AsGM1 as a unique, site-agnostic marker for fate mapping NK-to-ILC1 plasticity. This provides a powerful tool to explore the expanding heterogeneity within the Group 1 ILC landscape, effectively transcending the ambiguity inherent to the NK-to-ILC1 continuum.

Differential induction of donor-reactive Foxp3+ regulatory T cell via blockade of CD154 vs CD40

Recently published studies in both murine models and a meta-analysis of non-human primate renal transplant studies showed that anti-CD154 reagents conferred a significant survival advantage over CD40 blockers in both animal models and across multiple organs. Here we sought to compare the induction of donor-reactive forkhead box P3+-induced regulatory T cells (Foxp3+ iTreg) in mice treated with anti-CD154 versus anti-CD40 monoclonal antibodies (mAbs). Results indicated that while treatment with anti-CD154 mAb resulted in a significant increase in the frequency of donor-reactive CD4+ Foxp3+ iTreg following transplantation, treatment with anti-CD40 or Cd40 deficiency failed to recapitulate this result. Because we recently identified CD11b as an alternate receptor for CD154 during alloimmunity, we interrogated the role of CD154:CD11b interactions in the generation of Foxp3+ iTreg and found that blockade of CD11b in Cd40-/- recipients resulted in increased donor-reactive Foxp3+ iTreg as compared with CD40 deficiency alone. Mechanistically, CD154:CD11b inhibition decreased interleukin (IL)-1β from CD11b+ and CD11c+ dendritic cells, and blockade of IL-1β synergized with CD40 deficiency to promote Foxp3+ iTreg induction and prolong allograft survival. Taken together, these data provide a mechanistic basis for the observed inferiority of anti-CD40 blockers as compared with anti-CD154 mAb and illuminate an IL-1β-dependent mechanism by which CD154:CD11b interactions prevent the generation of donor-reactive Foxp3+ iTreg during transplantation.

Memory T follicular helper cells drive donor-specific antibodies independent of memory B cells and primary germinal center and alloantibody formation

Human leukocyte antigen antibodies are important immunologic mediators of renal allograft loss and are difficult to control. The inability to permanently eliminate donor-specific antibodies (DSA) is partly due to an incomplete understanding of the cellular mechanisms driving alloantibody formation, recurrence, and maintenance. Memory T follicular helper (mTfh) cells rapidly interact with memory B cells upon antigen re-exposure for anamnestic humoral responses, but little is known about Tfh memory in transplantation. We hypothesized that alloreactive mTfh cells form after transplantation and play a critical role in DSA formation following alloantigen re-encounter. To test this hypothesis, we utilized murine skin allograft models to identify and characterize Tfh memory and interrogate its ability to mediate alloantibody responses. We identified alloreactive Tfh memory as a mediator of accelerated humoral alloresponses independent of memory B cells and primary germinal center, or DSA, formation. Furthermore, we demonstrate that mTfh-driven alloantibody formation is susceptible to CD28 costimulation blockade. These findings provide novel insight into a pathologic role for memory Tfh in alloantibody responses and strongly support shifting therapeutic focus from the singular targeting of B cell lineage cells and alloantibodies themselves to multimodal strategies that include inhibition of mTfh cells to treat DSA.

TIGIT agonism alleviates costimulation blockade-resistant rejection in a regulatory T cell-dependent manner

Belatacept-based immunosuppression in kidney transplantation confers fewer off-target toxicities than calcineurin inhibitors but comes at a cost of increased incidence and severity of acute rejection, potentially due to its deleterious effect on both the number and function of Foxp3+ regulatory T cells (Tregs). TIGIT is a CD28 family coinhibitory receptor expressed on several subsets of immune cells including Tregs. We hypothesized that coinhibition through TIGIT signaling could function to ameliorate costimulation blockade-resistant rejection. The results demonstrate that treatment with an agonistic anti-TIGIT antibody, when combined with costimulation blockade by CTLA-4Ig, can prolong allograft survival in a murine skin graft model compared with CTLA-4Ig alone. Further, this prolongation of graft survival is accompanied by an increase in the frequency and number of graft-infiltrating Tregs and a concomitant reduction in the number of CD8+ T cells in the graft. Through the use of Treg-specific TIGIT conditional knockout animals, we demonstrated that the TIGIT-mediated reduction in the graft-infiltrating CD8+ T cell response is dependent on signaling of TIGIT on Foxp3+ Tregs. Our results highlight both the key functional role of TIGIT on Foxp3+ Tregs under conditions in which CTLA-4 is blocked and the therapeutic potential of TIGIT agonism to optimize costimulation blockade-based immunosuppression.

Selective CD28 blockade impacts T cell differentiation during homeostatic reconstitution following lymphodepletion

Introduction

Costimulation blockade targeting the CD28 pathway provides improved long-term renal allograft survival compared to calcineurin inhibitors but may be limited as CTLA-4-Ig (abatacept, belatacept) blocks both CD28 costimulation and CTLA-4 coinhibition. Directly targeting CD28 while leaving CTLA-4 intact may provide a mechanistic advantage. Fc-silent non-crosslinking CD28 antagonizing domain antibodies (dAb) are currently in clinical trials for renal transplantation. Given the current standard of care in renal transplantation at most US centers, it is likely that lymphodepletion via thymoglobulin induction therapy could be used in patients treated with CD28 antagonists. Thus, we investigated the impact of T cell depletion (TCD) on T cell phenotype following homeostatic reconstitution in a murine model of skin transplantation treated with anti-CD28dAb.

Methods

Skin from BALB/cJ donors was grafted onto C56BL/6 recipients which were treated with or without 0.2mg anti-CD4 and 10μg anti-CD8 one day prior to transplant and with or without 100μg anti-CD28dAb on days 0, 2, 4, 6, and weekly thereafter. Mice were euthanized six weeks post-transplant and lymphoid cells were analyzed by flow cytometry.

Results

Anti-CD28dAb reversed lymphopenia-induced differentiation of memory CD4+ T cells in the spleen and lymph node compared to TCD alone. Mice treated with TCD+anti-CD28dAb exhibited significantly improved skin graft survival compared to anti-CD28dAb alone, which was also improved compared to no treatment. In addition, the expression of CD69 was reduced on CD4+ and CD8+ T cells in the spleen and lymph node from mice that received TCD+anti-CD28dAb compared to TCD alone. While a reduced frequency of CD4+FoxP3+ T cells was observed in anti-CD28dAb treated mice relative to untreated controls, this was balanced by an increased frequency of CD8+Foxp3+ T cells that was observed in the blood and kidney of mice given TCD+anti-CD28dAb compared to TCD alone.

Discussion

These data demonstrate that CD28 signaling impacts the differentiation of both CD4+ and CD8+ T cells during homeostatic reconstitution following lymphodepletion, resulting in a shift towards fewer activated memory T cells and more CD8+FoxP3+ T cells, a profile that may underpin the observed prolongation in allograft survival.

Role of the ITAM-Bearing Receptors Expressed by Natural Killer Cells in Cancer

Natural Killer (NK) cells are innate lymphoid cells (ILCs) capable of recognizing and directly killing tumor cells. They also secrete cytokines and chemokines, which participate in the shaping of the adaptive response. NK cells identify tumor cells and are activated through a net positive signal from inhibitory and activating receptors. Several activating NK cell receptors are coupled to adaptor molecules containing an immunoreceptor tyrosine-based activation motif (ITAM). These receptors include CD16 and the natural cytotoxic receptors NKp46, NKp44, NKp30 in humans. The powerful antitumor NK cell response triggered by these activating receptors has made them attractive targets for exploitation in immunotherapy. In this review, we will discuss the different activating receptors associated with ITAM-bearing cell surface receptors expressed on NK cells, their modulations in the tumor context and the various therapeutic tools developed to boost NK cell responses in cancer patients.

Functional visualization of NK Cell-mediated killing of metastatic single tumor cells

Natural killer (NK) cells lyse invading tumor cells to limit metastatic growth in the lung, but how some cancers evade this host protective mechanism to establish a growing lesion is unknown. Here, we have combined ultra-sensitive bioluminescence imaging with intravital two-photon microscopy involving genetically encoded biosensors to examine this question. NK cells eliminated disseminated tumor cells from the lung within 24 hr of arrival, but not thereafter. Intravital dynamic imaging revealed that 50% of NK-tumor cell encounters lead to tumor cell death in the first 4 hr after tumor cell arrival, but after 24 hr of arrival, nearly 100% of the interactions result in the survival of the tumor cell. During this 24-hr period, the probability of ERK activation in NK cells upon encountering the tumor cells was decreased from 68% to 8%, which correlated with the loss of the activating ligand CD155/PVR/Necl5 from the tumor cell surface. Thus, by quantitatively visualizing, the NK-tumor cell interaction at the early stage of metastasis, we have revealed the crucial parameters of NK cell immune surveillance in the lung.

When a Friend Becomes Your Enemy: Natural Killer Cells in Atherosclerosis and Atherosclerosis-Associated Risk Factors

Atherosclerosis (ATS), the change in structure and function of arteries with associated lesion formation and altered blood flow, is the leading cause of cardiovascular disease, the number one killer worldwide. Beyond dyslipidemia, chronic inflammation, together with aberrant phenotype and function of cells of both the innate and adaptive immune system, are now recognized as relevant contributors to atherosclerosis onset and progression. While the role of macrophages and T cells in atherosclerosis has been addressed in several studies, Natural Killer cells (NKs) represent a poorly explored immune cell type, that deserves attention, due to NKs’ emerging contribution to vascular homeostasis. Furthermore, the possibility to re-polarize the immune system has emerged as a relevant tool to design new therapies, with some succesfull exmples in the field of cancer immunotherapy. Thus, a deeper knowledge of NK cell pathophysiology in the context of atherosclerosis and atherosclerosis-associated risk factors could help developing new preventive and treatment strategies, and decipher the complex scenario/history from “the risk factors for atherosclerosis” Here, we review the current knowledge about NK cell phenotype and activities in atherosclerosis and selected atherosclerosis risk factors, namely type-2 diabetes and obesity, and discuss the related NK-cell oriented environmental signals.

Invariant NKT Cells Promote the Development of Highly Cytotoxic Multipotent CXCR3+CCR4+CD8+ T Cells That Mediate Rapid Hepatocyte Allograft Rejection

Hepatocyte transplant represents a treatment for metabolic disorders but is limited by immunogenicity. Our prior work identified the critical role of CD8+ T cells, with or without CD4+ T cell help, in mediating hepatocyte rejection. In this study, we evaluated the influence of invariant NKT (iNKT) cells, uniquely abundant in the liver, upon CD8-mediated immune responses in the presence and absence of CD4+ T cells. To investigate this, C57BL/6 (wild-type) and iNKT-deficient Jα18 knockout mice (cohorts CD4 depleted) were transplanted with allogeneic hepatocytes. Recipients were evaluated for alloprimed CD8+ T cell subset composition, allocytotoxicity, and hepatocyte rejection. We found that CD8-mediated allocytotoxicity was significantly decreased in iNKT-deficient recipients and was restored by adoptive transfer of iNKT cells. In the absence of both iNKT cells and CD4+ T cells, CD8-mediated allocytotoxicity and hepatocyte rejection was abrogated. iNKT cells enhance the proportion of a novel subset of multipotent, alloprimed CXCR3+CCR4+CD8+ cytolytic T cells that develop after hepatocyte transplant and are abundant in the liver. Alloprimed CXCR3+CCR4+CD8+ T cells express cytotoxic effector molecules (perforin/granzyme and Fas ligand) and are distinguished from alloprimed CXCR3+CCR4-CD8+ T cells by a higher proportion of cells expressing TNF-α and IFN-γ. Furthermore, alloprimed CXCR3+CCR4+CD8+ T cells mediate higher allocytotoxicity and more rapid allograft rejection. Our data demonstrate the important role of iNKT cells in promoting the development of highly cytotoxic, multipotent CXCR3+CCR4+CD8+ T cells that mediate rapid rejection of allogeneic hepatocytes engrafted in the liver. Targeting iNKT cells may be an efficacious therapy to prevent rejection of intrahepatic cellular transplants.

Transient regulatory T-cell targeting triggers immune control of multiple myeloma and prevents disease progression

Multiple myeloma remains a largely incurable disease of clonally expanding malignant plasma cells. The bone marrow microenvironment harbors treatment-resistant myeloma cells, which eventually lead to disease relapse in patients. In the bone marrow, CD4⁺FoxP3⁺ regulatory T cells (Tregs) are highly abundant amongst CD4⁺ T cells providing an immune protective niche for different long-living cell populations, e.g., hematopoietic stem cells. Here, we addressed the functional role of Tregs in multiple myeloma dissemination to bone marrow compartments and disease progression. To investigate the immune regulation of multiple myeloma, we utilized syngeneic immunocompetent murine multiple myeloma models in two different genetic backgrounds. Analyzing the spatial immune architecture of multiple myeloma revealed that the bone marrow Tregs accumulated in the vicinity of malignant plasma cells and displayed an activated phenotype. In vivo Treg depletion prevented multiple myeloma dissemination in both models. Importantly, short-term in vivo depletion of Tregs in mice with established multiple myeloma evoked a potent CD8 T cell- and NK cell-mediated immune response resulting in complete and stable remission. Conclusively, this preclinical in-vivo study suggests that Tregs are an attractive target for the treatment of multiple myeloma.

Natural killer cells and dendritic epidermal γδ T cells orchestrate type 1 conventional DC spatiotemporal repositioning toward CD8+ T cells

Successful immune responses rely on a regulated delivery of the right signals to the right cells at the right time. Here we show that natural killer (NK) and dendritic epidermal γδ T cells (DETCs) use similar mechanisms to spatiotemporally orchestrate conventional type 1 dendritic cell (cDC1) functions in the spleen, skin, and its draining lymph nodes (dLNs). Upon MCMV infection in the spleen, cDC1 clusterize with activated NK cells in marginal zones. This XCR1-dependent repositioning of cDC1 toward NK cells allows contact delivery of IL-12 and IL-15/IL-15Rα by cDC1, which is critical for NK cell responses. NK cells deliver granulocyte-macrophage colony-stimulating factor (GM-CSF) to cDC1, guiding their CCR7-dependent relocalization into the T cell zone. In MCMV-infected skin, XCL1-secreting DETCs promote cDC1 migration from the skin to the dLNs. This XCR1-dependent licensing of cDC1 both in the spleen and skin accelerates antiviral CD8⁺ T cell responses, revealing an additional mechanism through which cDC1 bridge innate and adaptive immunity.

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Upon viral infection in the spleen, NK cells clusterize with cDC1 in the marginal zone

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This XCL1/XCR1-dependent interaction allows mutual delivery of activating signals

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NK cell GM-CSF directs cDC1 migration to T cell zone boosting CD8+ T cell priming

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In the skin, DETCs contact cDC1 via XCL1/XCR1 to promote antiviral CD8+ T cell priming

NK/ILC1 cells mediate neuroinflammation and brain pathology following congenital CMV infection

Congenital human cytomegalovirus (cHCMV) infection of the brain is associated with a wide range of neurocognitive sequelae. Using infection of newborn mice with mouse cytomegalovirus (MCMV) as a reliable model that recapitulates many aspects of cHCMV infection, including disseminated infection, CNS infection, altered neurodevelopment, and sensorineural hearing loss, we have previously shown that mitigation of inflammation prevented alterations in cerebellar development, suggesting that host inflammatory factors are key drivers of neurodevelopmental defects. Here, we show that MCMV infection causes a dramatic increase in the expression of the microglia-derived chemokines CXCL9/CXCL10, which recruit NK and ILC1 cells into the brain in a CXCR3-dependent manner. Surprisingly, brain-infiltrating innate immune cells not only were unable to control virus infection in the brain but also orchestrated pathological inflammatory responses, which lead to delays in cerebellar morphogenesis. Our results identify NK and ILC1 cells as the major mediators of immunopathology in response to virus infection in the developing CNS, which can be prevented by anti-IFN-γ antibodies.

Co-inhibitory Receptor Signaling in T-Cell-Mediated Autoimmune Glomerulonephritis

Autoimmune glomerulonephritis occurs as a consequence of autoantibodies and T-cell effector functions that target autoantigens. Co-signaling through cell surface receptors profoundly influences the optimal activation of T cells. The scope of this review is signaling mechanisms and the functional roles of representative T-cell co-inhibitory receptors in the regulation of autoimmune glomerulonephritis, along with current therapeutic challenges mainly on preclinical trials. Co-inhibitory receptors utilize both shared and unique signaling pathway, suggesting specialized functions that provide the rationale behind therapies for autoimmune glomerulonephritis by targeting these inhibitory receptors. These receptors largely suppress Th1 immunity, modify Th17 and Th2 immune response, and enhance Treg function. Anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA4) immunoglobulin (Ig), which is able to block both activating CD28 and inhibitory CTLA4 signaling, has been shown in preclinical and clinical investigations to have effects on glomerular disease. Other inhibitory receptors for treating glomerulonephritis have not been clinically tested, and efficacy of manipulating these pathways requires further preclinical investigation. While immune checkpoint inhibition using anti-CTLA4 antibodies and anti-programmed cell death 1 (PD-1)/PD-L1 antibodies has been approved for the treatment of several cancers, blockade of CTLA4 and PD-1/PD-L1 is associated with adverse effects that resemble autoimmune disorders, including systemic vasculitis. A renal autoimmune vasculitis model features an initial Th17 dominancy followed later by a Th1-dominant outcome and Treg cells that attenuate autoreactive T-cell function. Toward the development of effective therapies for T-cell-mediated autoimmune glomerulonephritis, it would be preferable to pay attention to the impact of the inhibitory pathways in immunological renal disease settings.

Costimulation Blockade in Vascularized Composite Allotransplantation

Vascular composite allotransplantation (VCA) is a field under research and has emerged as an alternative option for the repair of severe disfiguring defects that result from infections or traumatic amputation in a selected group of patients. VCA is performed in centers with appropriate expertise, experience and adequate resources to effectively manage the complexity and complications of this treatment. Lifelong immunosuppressive therapy, immunosuppression associated complications, and the effects of the host immune response in the graft are major concerns in VCA. VCA is considered a quality of life transplant and the risk-benefit ratio is dissimilar to life saving transplants. Belatacept seems a promising drug that prolongs patient and graft survival in kidney transplantation and it could also be an alternative approach to VCA immunosuppression. In this review, we are summarizing current literature about the role of costimulation blockade, with a focus on belatacept in VCA.

CD11b is a novel alternate receptor for CD154 during alloimmunity

Antagonism of the CD154/CD40 pathway is a highly effective means of inducing long-term graft survival in preclinical models. Using a fully allogeneic murine transplant model, we found that CD154 blockade was more effective in prolonging graft survival than was CD40 blockade, raising the possibility that CD154 binds a second receptor. To test this, we queried the impact of CD154 antagonism in the absence of CD40. Data indicated that anti-CD154 functioned to reduce graft-infiltrating CD8⁺ T cells in both WT and CD40^-/- hosts. Because it has recently been reported that CD154 can ligate CD11b, we addressed the impact of blocking CD154-CD11b interactions during transplantation. We utilized a specific peptide antagonist that prevents CD154 binding of CD11b but has no effect on CD154-CD40 interactions. CD154:CD11b antagonism significantly increased the efficacy of anti-CD40 in prolonging allograft survival as compared to anti-CD40 plus control peptide. Mechanistically, CD154:CD11b antagonism functioned to reduce the frequency of graft-infiltrating CD8⁺ T cells and innate immune cells. These data therefore demonstrate that blocking CD154 interactions with both CD40 and CD11b is required for optimal inhibition of alloimmunity and provide an explanation for why CD40 blockers may be less efficacious than anti-CD154 reagents for the inhibition of allograft rejection.

Memory T cell-mediated rejection is mitigated by FcγRIIB expression on CD8+ T cells

Donor-reactive memory T cells generated via heterologous immunity represent a potent barrier to long-term graft survival following transplantation because of their increased precursor frequency, rapid effector function, altered trafficking patterns, and reduced reliance on costimulation signals for activation. Thus, the identification of pathways that control memory T cell survival and secondary recall potential may provide new opportunities for therapeutic intervention. Here, we discovered that donor-specific effector/memory CD8⁺ T cell populations generated via exposure to acute vs latent vs chronic infections contain differential frequencies of CD8⁺ T cells expressing the inhibitory Fc receptor FcγRIIB. Results indicated that frequencies of FcγRIIB-expressing CD8⁺ donor-reactive memory T cells inversely correlated with allograft rejection. Furthermore, adoptive T cell transfer of Fcgr2b^-/- CD8⁺ T cells resulted in an accumulation of donor-specific CD8⁺ memory T cells and enhanced recall responses, indicating that FcγRIIB functions intrinsically to limit T cell CD8⁺ survival in vivo. Lastly, we show that deletion of FcγRIIB on donor-specific CD8⁺ memory T cells precipitated costimulation blockade-resistant rejection. These data therefore identify a novel cell-intrinsic inhibitory pathway that functions to limit the risk of memory T cell-mediated rejection following transplantation and suggest that therapeutic manipulation of this pathway could improve outcomes in sensitized patients.

Vascularized composite allotransplantation combined with costimulation blockade induces mixed chimerism and reveals intrinsic tolerogenic potential

Vascularized composite allotransplantation (VCA) has become a valid therapeutic option to restore form and function after devastating tissue loss. However, the need for high-dose multidrug immunosuppression to maintain allograft survival is still hampering more widespread application of VCA. In this study, we investigated the immunoregulatory potential of costimulation blockade (CoB; CTLA4-Ig and anti-CD154 mAb) combined with nonmyeoablative total body irradiation (TBI) to promote allograft survival of VCA in a fully MHC-mismatched mouse model of orthotopic hind limb transplantation. Compared with untreated controls (median survival time [MST] 8 days) and CTLA4-Ig treatment alone (MST 17 days), CoB treatment increased graft survival (MST 82 days), and the addition of nonmyeloablative TBI led to indefinite graft survival (MST > 210 days). Our analysis suggests that VCA-derived BM induced mixed chimerism in animals treated with CoB and TBI + CoB, promoting gradual deletion of alloreactive T cells as the underlying mechanism of long-term allograft survival. Acceptance of donor-matched secondary skin grafts, decreased ex vivo T cell responsiveness, and increased graft-infiltrating Tregs further indicated donor-specific tolerance induced by TBI + CoB. In summary, our data suggest that vascularized BM-containing VCAs are immunologically favorable grafts promoting chimerism induction and long-term allograft survival in the context of CoB.

Asialo GM1-positive liver-resident CD8 T cells that express CD44 and LFA-1 are essential for immune clearance of hepatitis B virus

Persistent hepatitis B virus (HBV) infection results in chronic liver diseases that may progress to chronic hepatitis, liver cirrhosis, and subsequent hepatocellular carcinoma. Previous studies demonstrated that adaptive immunity, in particular CD8 T cells, is critical in HBV elimination. Recent studies have revealed a distinct tissue-localized T cell lineage, tissue-resident memory (TRM) cells, that is crucial for protective immunity in peripheral tissues. In this study, we showed that treatment with an anti-asialo GM1 (ASGM1) antibody (Ab), which depletes NK cells, led to impairment of HBV clearance in a mouse animal model. Unexpectedly, the ability to clear HBV was not significantly impaired in NFIL3 KO mice, which are deficient in NK cells, implying that other non-NK ASGM1-positive immune cells mediate HBV clearance. We isolated intrahepatic ASGM1-positive cells from NFIL3 KO mice and analyzed the immune phenotype of these cells. Our results demonstrated a distinct population of CD44⁺ LFA-1^hi CD8 T cells that were the major intrahepatic ASGM1-positive immune cells in NFIL3 KO mice. Importantly, transcriptome analysis revealed that these ASGM1-positive CD8 T cells had distinct gene profiles and shared a similar core gene signature with TRM cells. In addition to both transcriptional and phenotypic liver residency characteristics, ASGM1-positive CD8 T cells were able to home to and be retained in the liver after adoptive transfer. Taken together, our study results indicate that these ASGM1-positive liver-resident CD8 T cells are the major effector immune cells mediating anti-HBV immunity.

Signaling through the Inhibitory Fc Receptor FcγRIIB Induces CD8+ T Cell Apoptosis to Limit T Cell Immunity

Effector CD8+ T cells are important mediators of adaptive immunity, and receptor-ligand interactions that regulate their survival may have therapeutic potential. Here, we identified a subset of effector CD8+ T cells that expressed the inhibitory fragment crystallizable (Fc) receptor FcγRIIB following activation and multiple rounds of division. CD8+ T cell-intrinsic genetic deletion of Fcgr2b increased CD8+ effector T cell accumulation, resulting in accelerated graft rejection and decreased tumor volume in mouse models. Immunoglobulin G (IgG) antibody was not required for FcγRIIB-mediated control of CD8+ T cell immunity, and instead, the immunosuppressive cytokine fibrinogen-like 2 (Fgl2) was a functional ligand for FcγRIIB on CD8+ T cells. Fgl2 induced caspase-3/7-mediated apoptosis in Fcgr2b+, but not Fcgr2b-/-, CD8+ T cells. Increased expression of FcγRIIB correlated with freedom from rejection following withdrawal from immunosuppression in a clinical trial of kidney transplant recipients. Together, these findings demonstrate a cell-intrinsic coinhibitory function of FcγRIIB in regulating CD8+ T cell immunity.

Circulating T follicular helper cells are a biomarker of humoral alloreactivity and predict donor-specific antibody formation after transplantation

Donor-specific antibodies (DSAs) contribute to renal allograft loss. However, biomarkers to guide clinical management of DSA posttransplant or detect humoral alloimmune responses before alloantibodies develop are not available. Circulating T follicular helper (cTfh) cells are CD4⁺ CXCR5⁺ Tfh-like cells in the blood that have been associated with alloantibodies in transplant recipients, but whether they precede antibody formation for their evaluation as a predictive biomarker in transplant is unknown. To evaluate the ability of cTfh cells to predict DSA, we used murine transplant models to determine the temporal relationship between cTfh cells, germinal center formation, and DSA development. We observed that donor-reactive CD4⁺ CXCR5⁺ cTfh cells expand after allotransplant. These cTfh cells were equivalent to graft-draining lymph node-derived Tfh cells in their ability to provide B cell help for antibody production. cTfh cell expansion and differentiation into ICOS⁺ PD-1⁺ cells temporally correlated with germinal center alloreactivity and preceded the generation of DSAs in instances of modified and unmodified alloantibody formation. Importantly, delayed costimulation blockade initiated after the detection of ICOS⁺ PD-1⁺ cTfh cells prevented DSAs. These findings suggest that cTfh cells could serve as a biomarker for humoral alloreactivity before the detection of alloantibodies and inform therapeutic approaches to prevent DSAs.

NK cells induce hepatic ER stress to promote insulin resistance in obesity through osteopontin production

High-fat diet (HFD) induced hepatic endoplasmic reticulum (ER) stress drives insulin resistance (IR) and steatosis. NK cells in adipose tissue play an important role in the pathogenesis of IR in obesity. Whether NK cells in the liver can induce hepatic ER stress and thus promote IR in obesity is still unknown. We demonstrate that HFD-fed mice display elevated production of proinflammatory cytokine osteopontin (OPN) in hepatic NK cells, especially in CD49a⁺ DX5^- tissue-resident NK (trNK) cells. Obesity-induced ER stress, IR, and steatosis in the liver are ameliorated by ablating NK cells with neutralizing antibody in HFD-fed mice. OPN treatment enhances the expression of ER stress markers, including p-PERK, p-eIF2, ATF4, and CHOP in both murine liver tissues and HL-7702, a human liver cell line. Pretreatment of HL-7702 cells with OPN promotes hyperactivation of JNK and subsequent decrease of tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1), resulting in impaired insulin signaling, which can be reversed by inhibiting ER stress. Collectively, we demonstrate that hepatic NK cells induce obesity-induced hepatic ER stress, and IR through OPN production.

Impact of selective CD28 blockade on virus-specific immunity to a murine Epstein-Barr virus homolog

CTLA-4Ig (belatacept) blocks the CD80/CD86 ligands for both CD28 and CTLA-4; thus, in addition to the intended effect of blocking CD28-mediated costimulation, belatacept also has the unintended effect of blocking CTLA-4-mediated coinhibition. Recently, anti-CD28 domain antibodies (dAb) that selectively target CD28 while leaving CTLA-4 intact were shown to more effectively inhibit alloimmune responses and prolong graft survival. However, the impact of selective CD28 blockade on protective immunity has not been extensively investigated. Here, we sought to compare the impact of CTLA-4Ig vs anti-CD28dAb on CD8⁺ T cell immunity to a transplant-relevant pathogen, a murine homolog of Epstein-Barr virus. Mice were infected with murine gammaherpesvirus-68 (MHV) and treated with vehicle, CTLA-4Ig, or anti-CD28dAb. Although anti-CD28dAb resulted in a decrease in virus-specific CD8⁺ T cell numbers as compared to CTLA-4Ig, cytolytic function and the expression of markers of high-quality effectors were not different from CTLA-4Ig treated animals. Importantly, MHV-68 viral load was not different between the treatment groups. These results suggest that preserved CTLA-4 coinhibition limits MHV-specific CD8⁺ T cell accumulation, but the population that remains retains cytolytic function and migratory capacity and is not inferior in its ability to control viral burden relative to T cell responses in CTLA-4Ig-treated animals.

Chemerin Suppresses Breast Cancer Growth by Recruiting Immune Effector Cells Into the Tumor Microenvironment

Infiltration of immune cells into the tumor microenvironment (TME) can regulate growth and survival of neoplastic cells, impacting tumorigenesis and tumor progression. Correlations between the number of effector immune cells present in a tumor and clinical outcomes in many human tumors, including breast, have been widely described. Current immunotherapies utilizing checkpoint inhibitors or co-stimulatory molecule agonists aim to activate effector immune cells. However, tumors often lack adequate effector cell numbers within the TME, resulting in suboptimal responses to these agents. Chemerin (RARRES2) is a leukocyte chemoattractant widely expressed in many tissues and is known to recruit innate leukocytes. CMKLR1 is a chemotactic cellular receptor for chemerin and is expressed on subsets of dendritic cells, NK cells, and macrophages. We have previously shown that chemerin acts as a tumor suppressive cytokine in mouse melanoma models by recruiting innate immune defenses into the TME. Chemerin/RARRES2 is down-regulated in many tumors, including breast, compared to normal tissue counterparts. Here, using a syngeneic orthotopic EMT6 breast carcinoma model, we show that forced overexpression of chemerin by tumor cells results in significant recruitment of NK cells and T cells within the TME. While chemerin secretion by EMT6 cells did not alter their phenotypic behavior in vitro, it did significantly suppress tumor growth in vivo. To define the cellular effectors required for this anti-tumor phenotype, we depleted NK cells or CD8+ T cells and found that either cell type is required for chemerin-dependent suppression of EMT6 tumor growth. Finally, we show significantly reduced levels of RARRES2 mRNA in human breast cancer samples compared to matched normal tissues. Thus, for the first time we have shown that increasing chemerin expression within the breast carcinoma TME can suppress growth by recruitment of NK and T cells, thereby supporting this approach as a promising immunotherapeutic strategy.

Targeting natural killer cells in solid tumors

Natural killer (NK) cells are innate lymphoid cells endowed with cytolytic activity and a capacity to secrete cytokines and chemokines. Several lines of evidence suggest that NK cells play an important role in anti-tumor immunity. Some therapies against hematological malignacies make use of the immune properties of NK cells, such as their ability to kill residual leukemic blasts efficiently after conditioning during haploidentical hematopoietic stem cell transplantation. However, knowledge on NK cell infiltration and the status of NK cell responsiveness in solid tumors is limited so far. The pro-angiogenic role of the recently described NK cell-like type 1 innate lymphoid cells (ILC1s) and their phenotypic resemblance to NK cells are confounding factors that add a level of complexity, at least in mice. Here, we review the current knowledge on the presence and function of NK cells in solid tumors as well as the immunotherapeutic approaches designed to harness NK cell functions in these conditions, including those that aim to reinforce conventional anti-tumor therapies to increase the chances of successful treatment.

Role of NKp46+ natural killer cells in house dust mite-driven asthma

House dust mite (HDM)‐allergic asthma is driven by T helper 2 (Th2) lymphocytes, but also innate immune cells control key aspects of the disease. The precise function of innate natural killer (NK) cells during the initiation and propagation of asthma has been very confusing, in part because different, not entirely specific, strategies were used to target these cells. We show that HDM inhalation rapidly led to the accumulation of NK cells in the lung‐draining lymph nodes and of activated CD69⁺ NK cells in the bronchoalveolar lumen. However, genetically engineered Ncr1‐DTA or Ncr1‐DTR mice that constitutively or temporarily lack NK cells, still developed all key features of acute or chronic HDM‐driven asthma, such as bronchial hyperreactivity, Th2 cytokine production, eosinophilia, mucus overproduction, and Th2‐dependent immunoglobulin serum titers. The same results were obtained by administration of conventional NK1.1 or asialo‐GM1 NK cell‐depleting antibodies, antibody‐mediated blocking of the NKG2D receptor, or genetic NKG2D deficiency. Thus, although NK cells accumulate in allergen‐challenged lungs, our findings comprehensively demonstrate that these cells are not required for HDM‐driven asthma in the mouse.

Characterization of eomesodermin and T-bet expression by allostimulated CD8+ T cells of healthy volunteers and kidney transplant patients in relation to graft outcome

Memory T cell (Tmem) responses play a critical role in the outcome of allo-transplantation. While the role of the T-box transcription factor Eomesodermin (Eomes) in the maintenance of antigen-specific Tmem is well studied, little is known about Eomes⁺ CD8⁺ T cell responses after transplantation. We evaluated the phenotype and function of allo-reactive Eomes⁺ CD8⁺ T cells in healthy volunteers and kidney transplant patients and their relation to transplant outcome. High Eomes expression by steady-state CD8⁺ T cells correlated with effector and memory phenotype. Following allo-stimulation, the expression of both the T-box proteins Eomes and T-bet by proliferating cells increased significantly, where high expression of Eomes and T-bet correlated with higher incidence of allo-stimulated IFNγ⁺ TNFα⁺ CD8⁺ T cells. In patients with no subsequent rejection, Eomes but not T-bet expression by donor-stimulated CD8⁺ T cells, increased significantly after transplantation. This was characterized by increased Eomes^hi T-bet^-/lo and decreased Eomes^-/lo T-bet^hi CD8⁺ T cell subsets, with no significant changes in the Eomes^hi T-bet^hi CD8⁺ T cell subset. No upregulation of exhaustion markers programmed-death-1 (PD-1) and cytotoxic-T-lymphocyte-associated-antigen-4 (CTLA4) by donor-stimulated Eomes⁺ CD8⁺ T cells was observed. Before transplantation, in patients without rejection, there were higher incidences of Eomes^hi T-bet^-/lo , and lower incidences of Eomes^hi T-bet^hi and Eomes^-/lo T-bet^hi donor-stimulated CD8⁺ T cell subsets, compared to those with subsequent rejection. Overall, our findings indicate that high Eomes expression by allo-stimulated T-bet⁺ CD8⁺ T cells is associated with enhanced effector function, and that an elevated incidence of donor-stimulated CD8⁺ T cells co-expressing high levels of Eomes and T-bet before transplantation, may correlate with an increased incidence of acute cellular rejection.

CD122 signaling in CD8+ memory T cells drives costimulation-independent rejection

Interrupting T cell costimulatory signals as a strategy to control undesired immune responses, such as occur in autoimmunity or transplantation, has the potential to alleviate many of the unwanted side effects associated with current immunosuppressive therapies. Belatacept, a high-affinity version of CTLA4-Ig that blocks ligand ligation to CD28, has been approved for use in kidney transplant recipients. Despite the long-term benefits associated with its use, such as improved renal function and lower cardiovascular risk, a subset of patients treated with belatacept experience elevated rates of acute T cell-mediated rejection, tempering enthusiasm for its use. Here we demonstrate that costimulation-independent T cell alloreactivity relies on signaling through CD122, the shared IL-2 and IL-15 receptor β-chain. Combined costimulatory and CD122 blockade improved survival of transplanted tissue in mice and nonhuman primates by controlling proliferation and effector function of CD8+ T cells. The high-affinity IL-2 receptor was dispensable for memory CD8+ T cell responses, whereas signaling through CD122 as a component of the high-affinity IL-15 receptor was critical for costimulation-independent memory CD8+ T cell recall, distinguishing specific roles for IL-2 and IL-15 in T cell activation. These studies outline a novel approach for clinical optimization of costimulatory blockade strategies in transplantation by targeting CD122.

2B4 Mediates Inhibition of CD8+ T Cell Responses via Attenuation of Glycolysis and Cell Division

We recently showed that 2B4 expression on memory T cells in human renal transplant recipients was associated with reduced rates of rejection. To investigate whether 2B4 functionally underlies graft acceptance during transplantation, we established an experimental model in which 2B4 was retrogenically expressed on donor-reactive murine CD8+ T cells (2B4rg), which were then transferred into naive recipients prior to skin transplantation. We found that constitutive 2B4 expression resulted in significantly reduced accumulation of donor-reactive CD8+ T cells following transplantation and significantly prolonged graft survival following transplantation. This marked reduction in alloreactivity was due to reduced proliferation of CD8+ Thy1.1+ 2B4rg cells as compared with control cells, underpinned by extracellular flux analyses demonstrating that 2B4-deficient (2B4KO) CD8+ cells activated in vitro exhibited increased glycolytic capacity and upregulation of gene expression profiles consistent with enhanced glycolytic machinery as compared with wild type controls. Furthermore, 2B4KO CD8+ T cells primed in vivo exhibited significantly enhanced ex vivo uptake of a fluorescent glucose analogue. Finally, the proliferative advantage associated with 2B4 deficiency was only observed in the setting of glucose sufficiency; in glucose-poor conditions, 2B4KO CD8+ T cells lost their proliferative advantage. Together, these data indicate that 2B4 signals function to alter T cell glucose metabolism, thereby limiting the proliferation and accumulation of CD8+ T cells. Targeting 2B4 may therefore represent a novel therapeutic strategy to attenuate unwanted CD8+ T cell responses.

Salvia Miltiorrhiza Ameliorates Liver Fibrosis by Activating Hepatic Natural Killer Cells in Vivo and in Vitro

Natural killer (NK) cells are known for their ability to kill activated hepatic stellate cells (HSCs), which has been confirmed both in patients and animal models. But the killing function is depressed in period of advanced liver injury. Salvia Miltiorrhiza (SM), a Chinese herbal medicine for invigorating blood circulation and eliminating stasis, is widely used to treat liver fibrosis in clinic. Nevertheless, the immunological mechanism remains unclearly. Here, we put forward the hypothesis that the anti-fibrotic effect of SM is concerned with boosting the activation of hepatic NK cells. Liver fibrosis was induced with carbon tetrachloride (CCl₄) and effects of SM on NK cells and HSC (JS-1 cell line, HSC) were investigated in vivo and in vitro. Hepatic NK cells were isolated from C57BL/6 mice, and pre-incubated with SM before they were co-cultured with HSCs. We found that SM increased frequency of NK cells, enhanced activities of NKG2D and Nkp46 on NK cells and inhibited activation of HSCs in vivo and in vitro. SM could promote the activities of NK cells by increasing the expressions of NKG2D and IFN-γ before or after co-cultured with HSCs in vitro. Besides, SM could partially antagonize ASGM-1-induced NK cell depletion and enhance the cell activities to inhibit HSCs activation in vitro. Therefore, our work provided a new insight into the anti-fibrotic mechanism that SM could enhance the activities of NK cell to reduce liver fibrosis in vivo and in vitro.

Transplantation preferentially induces a KLRG-1lo CD127hi differentiation program in antigen-specific CD8+ T cells

Models of infection have shaped our understanding of programmed memory T cell differentiation, yet whether these models apply to memory programming in the context of transplantation has yet to be defined. Previous work has identified differences in the response of antigen-specific CD8⁺ T cells to cognate antigen based on the environment in which the antigen is presented. Thus, we hypothesized that programming of antigen specific CD8⁺ T cells responding to graft and pathogen may be dissimilar. Here we find that antigen-specific CD8⁺ T cells primed by a skin graft contract faster than those primed by gammaherpesvirus (gHV), yet are able to expand more rapidly upon rechallenge. Moreover, graft-primed antigen-specific CD8⁺ T cells exhibited higher frequencies of cells secreting IL-2 and demonstrate lower expression of KLRG-1, which are qualities suggestive of increased recall potential. Additionally, the expression of CD127 at a memory time point suggests graft-elicited CD8⁺ antigen specific T cells are maintained in a less terminally-differentiated state compared to gHV-elicited CD8⁺ antigen specific T cells, despite fewer cells being present at that time point. Taken together, our findings suggest that the surface marker expression and functional profiles of T cells depends on the priming conditions and may be used to predict immunologic risk following transplantation after traditional allosensitization or heterologous immune priming.

Pan-Influenza A Protection by Prime-Boost Vaccination with Cold-Adapted Live-Attenuated Influenza Vaccine in a Mouse Model

Influenza virus infections continually pose a major public health threat with seasonal epidemics and sporadic pandemics worldwide. While currently licensed influenza vaccines provide only strain-specific protection, antigenic drift and shift occasionally render the viruses resistant to the host immune responses, which highlight the need for a vaccine that provides broad protection against multiple subtypes. In this study, we suggest a vaccination strategy using cold-adapted, live attenuated influenza vaccines (CAIVs) to provide a broad, potent, and safe cross-protection covering antigenically distinct hemagglutinin (HA) groups 1 and 2 influenza viruses. Using a mouse model, we tested different prime-boost combinations of CAIVs for their ability to induce humoral and T-cell responses, and protective efficacy against H1 and H5 (HA group 1) as well as H3 and H7 (HA group 2) influenza viruses. Notably, even in the absence of antibody-mediated neutralizing activity or HA inhibitory activity in vitro, CAIVs provided a potent protection against heterologous and heterosubtypic lethal challenges in vivo. Heterologous combination of prime (H1)-boost (H5) vaccine strains showed the most potent cross-protection efficacy. In vivo depletion experiments demonstrated not only that T cells and natural killer cells contributed to the cross-protection, but also the involvement of antibody-dependent mechanisms for the cross-protection. Vaccination-induced antibodies did not enhance the infectivity of heterologous viruses, and prime vaccination did not interfere with neutralizing antibody generation by the boost vaccination, allaying vaccine safety concerns associated with heterogeneity between the vaccines and challenge strains. Our data show that CAIV-based strategy can serve as a simple but powerful option for developing a "truly" universal influenza vaccine providing pan-influenza A protection, which has not been achieved yet by other vaccine strategies. The promising results of potency, breadth, and safety demonstrated in the mouse model support further studies in higher animal models for clinical relevance.

Selective CD28 blockade attenuates CTLA-4-dependent CD8+ memory T cell effector function and prolongs graft survival

Memory T cells pose a significant problem to successful therapeutic control of unwanted immune responses during autoimmunity and transplantation, as they are differentially controlled by cosignaling receptors such as CD28 and CTLA-4. Treatment with abatacept and belatacept impede CD28 signaling by binding to CD80 and CD86, but they also have the unintended consequence of blocking the ligands for CTLA-4, a process that may inadvertently boost effector responses. Here, we show that a potentially novel anti-CD28 domain antibody (dAb) that selectively blocks CD28 but preserves CTLA-4 coinhibition confers improved allograft survival in sensitized recipients as compared with CTLA-4 Ig. However, both CTLA-4 Ig and anti-CD28 dAb similarly and significantly reduced the accumulation of donor-reactive CD8+ memory T cells, demonstrating that regulation of the expansion of CD8+ memory T cell populations is controlled in part by CD28 signals and is not significantly impacted by CTLA-4. In contrast, selective CD28 blockade was superior to CTLA-4 Ig in inhibiting IFN-γ, TNF, and IL-2 production by CD8+ memory T cells, which in turn resulted in reduced recruitment of innate CD11b+ monocytes into allografts. Importantly, this superiority was CTLA-4 dependent, demonstrating that effector function of CD8+ memory T cells is regulated by the balance of CD28 and CTLA-4 signaling.

Genetic Depletion or Hyperresponsiveness of Natural Killer Cells Do Not Affect Atherosclerosis Development

Rationale:

Chronic inflammation is central in the development of atherosclerosis. Both innate and adaptive immunities are involved. Although several studies have evaluated the functions of natural killer (NK) cells in experimental animal models of atherosclerosis, it is not yet clear whether NK cells behave as protective or proatherogenic effectors. One of the main caveats of previous studies was the lack of specificity in targeting loss or gain of function of NK cells.

Objectives:

We used 2 selective genetic approaches to investigate the role of NK cells in atherosclerosis: (1) Ncr1 iCre/+ R26 lsl− DTA/+ mice in which NK cells were depleted and (2) Noé mice in which NK cells are hyperresponsive.

Methods and Results:

No difference in atherosclerotic lesion size was found in Ldlr −/− (low-density lipoprotein receptor null) mice transplanted with bone marrow (BM) cells from Ncr1 iCre R26R lsl− DTA , Noé , or wild-type mice. Also, no difference was observed in plaque composition in terms of collagen content, macrophage infiltration, or the immune profile, although Noé chimera had more IFN (interferon)-γ–producing NK cells, compared with wild-type mice. Then, we investigated the NK-cell selectivity of anti–asialoganglioside M1 antiserum, which was previously used to conclude the proatherogenicity of NK cells. Anti–asialoganglioside M1 treatment decreased atherosclerosis in both Ldlr −/− mice transplanted with Ncr1 iCre R26R lsl− DTA or wild-type bone marrow, indicating that its antiatherogenic effects are unrelated to NK-cell depletion, but to CD8 + T and NKT cells. Finally, to determine whether NK cells could contribute to the disease in conditions of pathological NK-cell overactivation, we treated irradiated Ldlr −/− mice reconstituted with either wild-type or Ncr1 iCre R26R lsl− DTA bone marrow with the viral mimic polyinosinic:polycytidylic acid and found a significant reduction of plaque size in NK-cell–deficient chimeric mice.

Conclusions:

Our findings, using state-of-the-art mouse models, demonstrate that NK cells have no direct effect on the natural development of hypercholesterolemia-induced atherosclerosis, but may play a role when an additional systemic NK-cell overactivation occurs.

Selective CD28 Blockade Results in Superior Inhibition of Donor-Specific T Follicular Helper Cell and Antibody Responses Relative to CTLA4-Ig

Donor-specific antibodies (DSAs) are a barrier to improved long-term outcomes after kidney transplantation. Costimulation blockade with CTLA4-Ig has shown promise as a potential therapeutic strategy to control DSAs. T follicular helper (Tfh) cells, a subset of CD4⁺ T cells required for optimal antibody production, are reliant on the CD28 costimulatory pathway. We have previously shown that selective CD28 blockade leads to superior allograft survival through improved control of CD8⁺ T cells relative to CTLA4-Ig, but the impact of CD28-specific blockade on CD4⁺ Tfh cells is unknown. Thus, we identified and characterized donor-reactive Tfh cells in a murine skin transplant model and then used this model to evaluate the impact of selective CD28 blockade with an anti-CD28 domain antibody (dAb) on the donor-specific Tfh cell-mediated immune response. We observed that the anti-CD28 dAb led to superior inhibition of donor-reactive CXCR5⁺ PD-1^high Tfh cells, CD95⁺ GL7⁺ germinal center B cells and DSA formation compared with CTLA4-Ig. Interestingly, donor-reactive Tfh cells differentially upregulated CTLA4 expression, suggesting an important role for CTLA4 in mediating the superior inhibition observed with the anti-CD28 dAb. Therefore, selective CD28 blockade as a novel approach to control Tfh cell responses and prevent DSA after kidney transplantation warrants further study.

Toll-like receptor pathways and innate responses to allografts

PURPOSE OF REVIEW

There is an increasing appreciation of the role of the innate immune system in modulating alloimmunity following organ transplantation. This review will outline the studies that have investigated the role of the Toll-like receptors in allograft rejection.

RECENT FINDINGS

Evidence indicates that Toll like receptors are involved in acute allograft rejection by inducing dendritic cells to mature and migrate to the draining lymph nodes and initiate Th1 alloimmune responses. Within the last year the impact of Toll-like receptor activation on transplantation tolerance has been investigated. Two studies have shown that Toll-like receptor activation can prevent the induction of transplantation tolerance whereas our own study has shown that deficiency in Toll-like receptor signaling facilitates transplantation tolerance. Finally, a recent study indicated that fragmented hyaluronan, part of the extracellular matrix, might be one of the endogenous Toll-like receptor activators in the setting of alloimmunty.

SUMMARY

Toll-like receptors play an important role in modulating the alloimmune response. Their manipulation may have profound effects on the induction of transplantation tolerance. Future investigation into other aspects of innate immunity and their influence on solid organ transplantation will likely yield important insights to the field.

Graft-Versus-Host Disease Following Liver Transplantation: Development of a High-Incidence Rat Model and a Selective Prevention Method

Graft-versus-host disease (GvHD) following liver transplantation (LT) is a rare but serious complication with no presently available animal model and no preventive measures. To develop a rat model of GvHD after LT (LT-GvHD), we preconditioned hosts with sublethal irradiation plus reduction of natural killer (NK) cells with anti-CD8α mAb treatment, which invariably resulted in acute LT-GvHD. Compared with those in the peripheral counterpart, graft CD4⁺ CD25^- passenger T cells showed lower alloreactivities in mixed leukocyte culture. Immunohistology revealed that donor CD4⁺ T cells migrated and formed clusters with host dendritic cells in secondary lymphoid organs, with early expansion and subsequent accumulation in target organs. For selectively preventing GvHD, donor livers were perfused ex vivo with organ preservation media containing anti-TCRαβ mAb. T cell-depleted livers almost completely suppressed clinical GvHD such that host rats survived for >100 days. Our results showed that passenger T cells could develop typical LT-GvHD if resistant cells such as host radiosensitive cells and host radioresistant NK cells were suppressed. Selective ex vivo T cell depletion prevented LT-GvHD without affecting host immunity or graft function. This method might be applicable to clinical LT in prediagnosed high-risk donor-recipient combinations and for analyzing immunoregulatory mechanisms of the liver.

Mechanisms of graft rejection after lung transplantation

PURPOSE OF REVIEW

To date, outcomes after lung transplantation are far worse than after transplantation of other solid organs. New insights into mechanisms that contribute to graft rejection and tolerance after lung transplantation remain of great interest. This review examines the recent literature on the role of innate and adaptive immunity in shaping the fate of lung grafts.

RECENT FINDINGS

Innate and adaptive immune cells orchestrate allograft rejection after transplantation. Innate immune cells such as neutrophils are recruited to the lung graft early after reperfusion and subsequently promote allograft rejection. Although it is widely recognized that CD4 T lymphocytes in concert with CD8 T cells promote graft rejection, regulatory Foxp3 CD4 T, central memory CD8 T cells, and natural killer cells can facilitate tolerance.

SUMMARY

This review highlights interactions between innate and adaptive immune pathways and how they contribute to lung allograft rejection. These findings lay a foundation for the design of new therapeutic strategies that target both innate and adaptive immune responses.

What's Hot, What's New From the 2016 American Transplant Congress

From June 11-15, 2016 the American Transplant Congress, the joint meeting of the American Society of Transplantation and the American Society of Transplant Surgeons, held its annual meeting in Boston, MA. The meeting, attended by 5200 registrants, included pre-meeting conferences, focused topic sessions, and hundreds of high-quality presentations from the transplant field. This meeting report highlights key findings from specific basic science, translational, and clinical research presentations deemed to have notable impact in thematic areas. In particular, there were a number of transformative studies indicating important advances in the understanding of alloimmunity, chronic rejection, tolerance, and organ-specific outcomes. Many of these results are discussed in the context of the published literature to showcase rapid advances in the transplant field.

Enhanced Requirement for TNFR2 in Graft Rejection Mediated by Low-Affinity Memory CD8+ T Cells during Heterologous Immunity

The affinity of a TCR binding to peptide:MHC profoundly impacts the phenotype and function of effector and memory cell differentiation. Little is known about the effect of low-affinity priming on memory cell generation and function, which is particularly important in heterologous immunity, when microbe-specific T cells cross-react with allogeneic Ag and mediate graft rejection. We found that low-affinity-primed memory CD8(+) T cells produced high levels of TNF ex vivo in response to heterologous rechallenge compared with high-affinity-primed memory T cells. Low-affinity secondary effectors significantly upregulated TNFR2 on the cell surface and contained a higher frequency of TNFR2(hi) proliferating cells. Low-affinity-primed secondary effectors concurrently downregulated TNF production. Importantly, blockade of TNFR2 attenuated graft rejection in low- but not high-affinity-primed animals. These data establish a functional connection between TNF signaling and TCR-priming affinity and have implications for the immunomodulation of pathogenic T cell responses during transplantation.

T Cell Cosignaling Molecules in Transplantation

The ultimate outcome of alloreactivity versus tolerance following transplantation is potently influenced by the constellation of cosignaling molecules expressed by immune cells during priming with alloantigen, and the net sum of costimulatory and coinhibitory signals transmitted via ligation of these molecules. Intense investigation over the last two decades has yielded a detailed understanding of the kinetics, cellular distribution, and intracellular signaling networks of cosignaling molecules such as the CD28, TNF, and TIM families of receptors in alloimmunity. More recent work has better defined the cellular and molecular mechanisms by which engagement of cosignaling networks serve to either dampen or augment alloimmunity. These findings will likely aid in the rational development of novel immunomodulatory strategies to prolong graft survival and improve outcomes following transplantation.

Anti-Leukocyte Function-Associated Antigen 1 Therapy in a Nonhuman Primate Renal Transplant Model of Costimulation Blockade-Resistant Rejection

Costimulation blockade with the fusion protein belatacept provides a desirable side effect profile and improvement in renal function compared with calcineurin inhibition in renal transplantation. This comes at the cost of increased rates of early acute rejection. Blockade of the integrin molecule leukocyte function-associated antigen 1 (LFA-1) has been shown to be an effective adjuvant to costimulation blockade in a rigorous nonhuman primate (NHP) model of islet transplantation; therefore, we sought to test this combination in an NHP renal transplant model. Rhesus macaques received belatacept maintenance therapy with or without the addition of LFA-1 blockade, which was achieved using a murine-derived LFA-1-specific antibody TS1/22. Additional experiments were performed using chimeric rhesus IgG1 (TS1/22R1) or IgG4 (TS1/22R4) variants, each engineered to limit antibody clearance. Despite evidence of proper binding to the target molecule and impaired cellular egress from the intravascular space indicative of a therapeutic effect similar to prior islet studies, LFA-1 blockade failed to significantly prolong graft survival. Furthermore, evidence of impaired protective immunity against cytomegalovirus was observed. These data highlight the difficulties in translating treatment regimens between organ models and suggest that the primarily vascularized renal model is more robust with regard to belatacept-resistant rejection than the islet model.

NK Cells Help Induce Anti-Hepatitis B Virus CD8+ T Cell Immunity in Mice

Although recent clinical studies demonstrate that NK cell function is impaired in hepatitis B virus (HBV)-persistent patients, whether or how NK cells play a role in anti-HBV adaptive immunity remains to be explored. Using a mouse model mimicking acute HBV infection by hydrodynamic injection of an HBV plasmid, we observed that although serum hepatitis B surface Ag and hepatitis B envelope Ag were eliminated within 3 to 4 wk, HBV might persist for >8 wk in CD8(-/-) mice and that adoptive transfer of anti-HBV CD8(+) T cells restored the ability to clear HBV in HBV-carrier Rag1(-/-) mice. These results indicate that CD8(+) T cells are critical in HBV elimination. Furthermore, NK cells increased IFN-γ production after HBV plasmid injection, and NK cell depletion led to significantly increased HBV persistence along with reduced frequency of hepatitis B core Ag-specific CD8(+) T cells. Adoptive transfer of IFN-γ-sufficient NK cells restored donor CD8(+) T cell function, indicating that NK cells positively regulated CD8(+) T cells via secreting IFN-γ. We also observed that NK cell depletion correlated with decreased effector memory CD8(+) T cell frequencies. Importantly, adoptive transfer experiments showed that NK cells were involved in anti-HBV CD8(+) T cell recall responses. Moreover, DX5(+)CD49a(-) conventional, but not DX5(-)CD49a(+) liver-resident, NK cells were involved in improving CD8(+) T cell responses against HBV. Overall, the current study reveals that NK cells, especially DX5(+)CD49a(-) conventional NK cells, promote the antiviral activity of CD8(+) T cell responses via secreting IFN-γ in a mouse model mimicking acute HBV infection.

Defective CD8 Signaling Pathways Delay Rejection in Older Recipients

CD8+ T cells play a cardinal feature in response to alloantigens and are able to generate effector/memory T cells independently from CD4+ T cells. To investigate the impact of aging on CD8 T cells, we used a fully mismatched mouse skin transplant model. Our findings showed a prolonged allograft survival in older recipients associated with a significant increase of CD4+ and CD8+ CD44high CD62Llow effector/memory T cells and a reduced systemic IFNγ production. When reconstituting young CBA Rag-1 mice that lack mature T and B cells with old CD8+ T cells expressing clonal anti-H2K T cell receptor (TCR) alloreactive for MHC I, graft survival was significantly prolonged and comparable to those receiving young CD8+ T cells. Moreover, our data showed that reduced systemic IFNγ levels observed in old recipients had been linked to a compromised expression of the IL-2R β subunit (CD122) by old CD8+ T cells. In addition, we observed an impaired IFNγ production on IL-2 receptor activation. At the same time, gene profiling analysis of old CD8 T cells demonstrated reduced chemokine ligand-3 and CD40L expression that resulted in compromised CD8+ T cell/dendritic cell communication, leading to impaired migratory and phagocytic activity of CD11c cells.Collectively, our study demonstrated that aging delays allograft rejection. CD8 T cells play a critical role in this process linked to a compromised production of IFNγ, in addition to a defective IL-2 receptor signaling machinery and a defective communication between CD8 T cells and dendritic cells.

Eomesodermin(lo) CTLA4(hi) Alloreactive CD8+ Memory T Cells Are Associated With Prolonged Renal Transplant Survival Induced by Regulatory Dendritic Cell Infusion in CTLA4 Immunoglobulin-Treated Nonhuman Primates

BACKGROUND

Memory T cells (Tmem), particularly those resistant to costimulation blockade (CB), are a major barrier to transplant tolerance. The transcription factor Eomesodermin (Eomes) is critical for Tmem development and maintenance, but its expression by alloactivated T cells has not been examined in nonhuman primates.

METHODS

We evaluated Eomes and coinhibitory cytotoxic T lymphocyte antigen-4 (CTLA4) expression by alloactivated rhesus monkey T cells in the presence of CTLA4 immunoglobulin, both in vitro and in renal allograft recipients treated with CTLA4Ig, with or without regulatory dendritic cell (DCreg) infusion.

RESULTS

In normal monkeys, CD8+ T cells expressed significantly more Eomes than CD4+ T cells. By contrast, CD8+ T cells displayed minimal CTLA4. Among T cell subsets, central Tmem (Tcm) expressed the highest levels of Eomes. Notably, Eomes(lo)CTLA4(hi) cells displayed higher levels of CD25 and Foxp3 than Eomes(hi)CTLA4(lo) CD8+ T cells. After allostimulation, distinct proliferating Eomes(lo)CTLA4(hi) and Eomes(hi)CTLA4(lo) CD8+ T cell populations were identified, with a high proportion of Tcm being Eomes(lo)CTLA4(hi). CB with CTLA4Ig during allostimulation of CD8+ T cells reduced CTLA4 but not Eomes expression, significantly reducing Eomes(lo)CTLA4(hi) cells. After transplantation with CB and rapamycin, donor-reactive Eomes(lo)CTLA4(hi) CD8+ T cells were reduced. However, in monkeys also given DCreg, absolute numbers of these cells were elevated significantly.

CONCLUSIONS

Low Eomes and high CTLA4 expression by donor-reactive CD8+ Tmem is associated with prolonged renal allograft survival induced by DCreg infusion in CTLA4Ig-treated monkeys. Prolonged allograft survival associated with DCreg infusion may be related to maintenance of donor-reactive Eomes(lo)CTLA4(hi) Tcm.

Adipose Natural Killer Cells Regulate Adipose Tissue Macrophages to Promote Insulin Resistance in Obesity

Obesity-induced inflammation mediated by immune cells in adipose tissue appears to participate in the pathogenesis of insulin resistance. We show that natural killer (NK) cells in adipose tissue play an important role. High-fat diet (HFD) increases NK cell numbers and the production of proinflammatory cytokines, notably TNFα, in epididymal, but not subcutaneous, fat depots. When NK cells were depleted either with neutralizing antibodies or genetic ablation in E4bp4(+/-) mice, obesity-induced insulin resistance improved in parallel with decreases in both adipose tissue macrophage (ATM) numbers, and ATM and adipose tissue inflammation. Conversely, expansion of NK cells following IL-15 administration or reconstitution of NK cells into E4bp4(-/-) mice increased both ATM numbers and adipose tissue inflammation and exacerbated HFD-induced insulin resistance. These results indicate that adipose NK cells control ATMs as an upstream regulator potentially by producing proinflammatory mediators, including TNFα, and thereby contribute to the development of obesity-induced insulin resistance.

Influenza Virus Targets Class I MHC-Educated NK Cells for Immunoevasion

The immune response to influenza virus infection comprises both innate and adaptive defenses. NK cells play an early role in the destruction of tumors and virally-infected cells. NK cells express a variety of inhibitory receptors, including those of the Ly49 family, which are functional homologs of human killer-cell immunoglobulin-like receptors (KIR). Like human KIR, Ly49 receptors inhibit NK cell-mediated lysis by binding to major histocompatibility complex class I (MHC-I) molecules that are expressed on normal cells. During NK cell maturation, the interaction of NK cell inhibitory Ly49 receptors with their MHC-I ligands results in two types of NK cells: licensed (“functional”), or unlicensed (“hypofunctional”). Despite being completely dysfunctional with regard to rejecting MHC-I-deficient cells, unlicensed NK cells represent up to half of the mature NK cell pool in rodents and humans, suggesting an alternative role for these cells in host defense. Here, we demonstrate that after influenza infection, MHC-I expression on lung epithelial cells is upregulated, and mice bearing unlicensed NK cells (Ly49-deficient NKC^KD and MHC-I-deficient B2m^-/- mice) survive the infection better than WT mice. Importantly, transgenic expression of an inhibitory self-MHC-I-specific Ly49 receptor in NKC^KD mice restores WT influenza susceptibility, confirming a direct role for Ly49. Conversely, F(ab’)₂-mediated blockade of self-MHC-I-specific Ly49 inhibitory receptors protects WT mice from influenza virus infection. Mechanistically, perforin-deficient NKC^KD mice succumb to influenza infection rapidly, indicating that direct cytotoxicity is necessary for unlicensed NK cell-mediated protection. Our findings demonstrate that Ly49:MHC-I interactions play a critical role in influenza virus pathogenesis. We suggest a similar role may be conserved in human KIR, and their blockade may be protective in humans.

Influenza virus has developed a number of immune-evasion mechanisms to prolong its survival within the host. Development of functional NK cells is dependent on multiple factors such as the interaction between MHC-I and Ly49 receptors. NK cells that develop in the absence of these interactions are referred to as ‘unlicensed’ and represent up to half of the total number of NK cells. We show that significant MHC-I upregulation on lung epithelial cells following influenza virus infection most likely allows influenza virus to evade detection by licensed NK cells. Importantly, we demonstrate that unlicensed NK cells play a major role in protecting mice from influenza infection. Both Ly49- and MHC-I-deficient mice, which possess unlicensed NK cells, exhibit better survival than WT mice when infected with a lethal dose of influenza virus. Survival of Ly49-deficient mice is associated with reduced viral titers and lung pathology, compared to the infected WT mice. Moreover, disrupting the interaction between MHC-I and inhibitory Ly49 receptors protects WT mice from a lethal influenza virus infection. These results suggest that the so-called unlicensed NK cells, previously characterized as being hyporesponsive, actually possess potent antiviral activity, and are crucial for protection from influenza virus and possibly other viral infections.

Low-Affinity Memory CD8+ T Cells Mediate Robust Heterologous Immunity

Heterologous immunity is recognized as a significant barrier to transplant tolerance. Whereas it has been established that pathogen-elicited memory T cells can have high or low affinity for cross-reactive allogeneic peptide-MHC, the role of TCR affinity during heterologous immunity has not been explored. We established a model with which to investigate the impact of TCR-priming affinity on memory T cell populations following a graft rechallenge. In contrast to high-affinity priming, low-affinity priming elicited fully differentiated memory T cells with a CD45RB(hi) status. High CD45RB status enabled robust secondary responses in vivo, as demonstrated by faster graft rejection kinetics and greater proliferative responses. CD45RB blockade prolonged graft survival in low affinity-primed mice, but not in high affinity-primed mice. Mechanistically, low affinity-primed memory CD8(+) T cells produced more IL-2 and significantly upregulated IL-2Rα expression during rechallenge. We found that CD45RB(hi) status was also a stable marker of priming affinity within polyclonal CD8(+) T cell populations. Following high-affinity rechallenge, low affinity-primed CD45RB(hi) cells became CD45RB(lo), demonstrating that CD45RB status acts as an affinity-based differentiation switch on CD8(+) T cells. Thus, these data establish a novel mechanism by which CD45 isoforms tune low affinity-primed memory CD8(+) T cells to become potent secondary effectors following heterologous rechallenge. These findings have direct implications for allogeneic heterologous immunity by demonstrating that despite a lower precursor frequency, low-affinity priming is sufficient to generate memory cells that mediate potent secondary responses against a cross-reactive graft challenge.