Memory T cells: a hurdle to immunologic tolerance

Polyfunctional donor-reactive T cells are associated with acute T-cell-mediated rejection of the kidney transplant

Acute T-cell-mediated rejection (aTCMR) still remains a clinical problem after kidney transplantation despite significant improvements in immunosuppressive regimens. Polyfunctional T cells, i.e. T cells producing multiple pro-inflammatory cytokines, are believed to be the most relevant T cells in an immune response. The aim of this study was to determine whether polyfunctional donor-reactive T cells are associated with aTCMR. In a case-control study, 49 kidney transplant recipients with a biopsy-proven aTCMR in the first year after transplantation were included, as well as 51 controls without aTCMR. Circulating donor-reactive T cells were identified by the expression of CD137 after short-term co-culture with donor antigen-presenting cells. Polyfunctional donor-reactive T cells were further characterized by dissection into different T-cell subsets encompassing the spectrum of naïve to terminally differentiated effector T cells. Prior to kidney transplantation, proportions of donor-reactive CD4+ (0.03% versus 0.02%; P < 0.01) and CD8+ (0.18% versus 0.10%; P < 0.01) CD137++ T cells were significantly higher in recipients with a biopsy-proven aTCMR versus non-rejectors. Polyfunctionality was higher (P = 0.03) in this subset of CD137-expressing T cells. These cells were predominantly of the EM/EMRA-phenotype, with polyfunctional donor-reactive CD137++CD4+ T cells predominantly co-expressing CD28 whereas approximately half of the polyfunctional CD137++CD8+ T cells co-expressed CD28. In addition, at the time of aTCMR, polyfunctional donor-reactive CD137++ CD4+, but not CD8+, T cells, were specifically decreased by 75% compared to before transplantation in recipients with as well as those without an aTCMR. Prior to transplantation, the proportion of polyfunctional donor-reactive CD137++ T cells is associated with the occurrence of a biopsy-proven aTCMR within the first year after transplantation.

Natural killer cells- from innate cells to the discovery of adaptability

Natural Killer (NK) cells have come a long way since their first description in the 1970's. The most recent reports of their adaptive-like behavior changed the way the immune system dichotomy is described. Adaptive NK cells present characteristics of both the innate and adaptive immune system. This NK cell subpopulation undergoes a clonal-like expansion in response to an antigen and secondary encounters with the same antigen result in an increased cytotoxic response. These characteristics can be of extreme importance in the clinical setting, especially as adoptive immunotherapies, since NK cells present several advantages compared other cell types. This review will focus on the discovery and the path to the current knowledge of the adaptive NK cell population.

Severe delayed graft function in a living-related kidney transplant recipient due to combination of alloimmunity, autoimmunity, and heterologous immunity: A case report

BACKGROUND

Delayed graft function is a manifestation of acute kidney injury unique to transplantation usually related to donor ischemia or recipient immunological causes. Ischemia also considered the most important trigger for innate immunity activation and production of non-HLA antibodies. While ischemia is inevitable after deceased donor transplantation, this complication is rare after living transplantation. Heterologous Immunity commonly used to describe the activation of T cells recognizing specific pathogen-related antigens as well unrelated antigens is common post-viral infection. In transplant-setting induction of heterologous immunity that cross-react with HLA-antigens and subsequent reactivation of memory T cells can lead to allograft rejection.

METHODS

Here we describe a non-sensitized child with ESRD secondary to lupus nephritis and recent history of COVID-19 infection who experienced 17 days of anuria after first kidney living transplantation from her young HLA-haploidentical uncle donor. Graft histology showed acute cellular rejection, evidence of mild antibody-mediated rejection and vascular wall necrosis in some arterioles suggesting possibility of intraoperative graft ischemia. Both pre- and post-transplant sera showed very high level of several non-HLA antibodies.

RESULTS

The patient was treated for cellular and antibody-mediated rejection while maintained on hemodialysis before her graft function started to improve on day seventeen post transplantation.

CONCLUSION

The cellular rejection likely trigged by ischemia that activated T-cells-mediated immunity. The high level of non- HLA-antibodies further aggravated the damage and the rapid onset of rejection may be partly related to memory T-cell activation induced by heterologous immunity.

Non-cytotoxic functions of CD8 T cells: “repentance of a serial killer”

Cytotoxic CD8 T cells (CTLs) are classically described as the “serial killers” of the immune system, where they play a pivotal role in protective immunity against a wide spectrum of pathogens and tumors. Ironically, they are critical drivers of transplant rejection and autoimmune diseases, a scenario very similar to the famous novel “The strange case of Dr. Jekyll and Mr. Hyde”. Until recently, it has not been well-appreciated whether CTLs can also acquire non-cytotoxic functions in health and disease. Several investigations into this question revealed their non-cytotoxic functions through interactions with various immune and non-immune cells. In this review, we will establish a new classification for CD8 T cell functions including cytotoxic and non-cytotoxic. Further, we will discuss this novel concept and speculate on how these functions could contribute to homeostasis of the immune system as well as immunological responses in transplantation, cancer, and autoimmune diseases.

Targeting inflammation and immune activation to improve CTLA4-Ig-based modulation of transplant rejection

For the last few decades, Calcineurin inhibitors (CNI)-based therapy has been the pillar of immunosuppression for prevention of organ transplant rejection. However, despite exerting effective control of acute rejection in the first year post-transplant, prolonged CNI use is associated with significant side effects and is not well suited for long term allograft survival. The implementation of Costimulation Blockade (CoB) therapies, based on the interruption of T cell costimulatory signals as strategy to control allo-responses, has proven potential for better management of transplant recipients compared to CNI-based therapies. The use of the biologic cytotoxic T-lymphocyte associated protein 4 (CTLA4)-Ig is the most successful approach to date in this arena. Following evaluation of the BENEFIT trials, Belatacept, a high-affinity version of CTLA4-Ig, has been FDA approved for use in kidney transplant recipients. Despite its benefits, the use of CTLA4-Ig as a monotherapy has proved to be insufficient to induce long-term allograft acceptance in several settings. Multiple studies have demonstrated that events that induce an acute inflammatory response with the consequent release of proinflammatory cytokines, and an abundance of allograft-reactive memory cells in the recipient, can prevent the induction of or break established immunomodulation induced with CoB regimens. This review highlights advances in our understanding of the factors and mechanisms that limit CoB regimens efficacy. We also discuss recent successes in experimentally designing complementary therapies that favor CTLA4-Ig effect, affording a better control of transplant rejection and supporting their clinical applicability.

Tacrolimus before CTLA4Ig and rapamycin promotes vascularized composite allograft survival in MGH miniature swine

BACKGROUND

We evaluated the outcome of vertical rectus abdominus myocutaneous flap (VRAM) allotransplantation in a mini-pig model, using a combined co-stimulation blockade (Co-SB) and mechanistic target of rapamycin inhibition (mTORi)-based regimen, with or without preceding calcineurin inhibition (CNI).

MATERIALS AND METHODS

VRAM allotransplants were performed between SLA-mismatched MGH miniature swine. Group A (n = 2) was treated continuously with the mTOR inhibitor rapamycin from day -1 in combination with the Co-SB agent cytotoxic T lymphocyte antigen 4-Ig (CTLA4-Ig) from post-operative day (POD) 0. In group B (n = 3), animals received tacrolimus daily from POD 0 to POD 13, followed by rapamycin daily from POD 7 and CTLA4-Ig weekly from POD 7-28. Graft rejection was determined by Banff criteria and host cellular and humoral immunity monitored.

RESULTS

In group A, allografts developed grade-I acute rejection by POD 2 and POD 7, and reached grade-IV by POD 17 and POD 20, respectively. By contrast, in group B, two allografts demonstrated grade-I rejection on POD 30 and grade-IV on POD 74, while the third exhibited grade-I rejection starting on POD 50, though this animal had to be euthanized on POD 58 due to Pneumocystis jirovecii infection. Time-to-event incidence of grade-I rejection was significantly lower in group A compared to group B. During the first 3 weeks post-transplant, no significant differences in anti-donor immunity were observed between the groups.

CONCLUSION

A short course of CNI, followed by combined Co-SB and mTORi significantly delays acute rejection of VRAM allografts in SLA-mismatched miniature swine.

Activated-memory T cells influence naïve T cell fate: a noncytotoxic function of human CD8 T cells

T cells are endowed with the capacity to sense their environment including other T cells around them. They do so to set their numbers and activation thresholds. This form of regulation has been well-studied within a given T cell population - i.e., within the naïve or memory pool; however, less is known about the cross-talk between T cell subsets. Here, we tested whether memory T cells interact with and influence surrounding naïve T cells. We report that human naïve CD8 T cells (T_N) undergo phenotypic and transcriptional changes in the presence of autologous activated-memory CD8 T cells (T_Mem). Following in vitro co-culture with activated central memory cells (T_CM), ~3% of the T_N acquired activation/memory canonical markers (CD45RO and CD95) in an MHC-I dependent-fashion. Using scRNA-seq, we also observed that ~3% of the T_N acquired an activated/memory signature, while ~84% developed a unique activated transcriptional profile hybrid between naïve and activated memory. Pseudotime trajectory analysis provided further evidence that T_N with an activated/memory or hybrid phenotype were derived from T_N. Our data reveal a non-cytotoxic function of T_Mem with potential to activate autologous T_N into the activated/memory pool. These findings may have implications for host-protection and autoimmunity that arises after vaccination, infection or transplantation.

Recombinant HBsAg of the Wild-Type and the G145R Escape Mutant, included in the New Multivalent Vaccine against Hepatitis B Virus, Dramatically Differ in their Effects on Leukocytes from Healthy Donors In Vitro

Immune-escape hepatitis B virus (HBV) mutants play an important role in HBV spread. Recently, the multivalent vaccine Bubo®-Unigep has been developed to protect against both wild-type HBV and the most significant G145R mutant. Here, we compared the effects of recombinant HBsAg antigens, wild-type and mutated at G145R, both included in the new vaccine, on activation of a human high-density culture of peripheral blood mononuclear cells (PBMC) in vitro. The antigens were used either alone or in combination with phytohemagglutinin (PHA). None of the antigens alone affected the expression of CD40, HLA-DR or CD279. Wild-type HBsAg enhanced CD86 and CD69 expression, and induced TNF-α, IL-10, and IFN-γ, regardless of the anti-HBsAg status of donor. In the presence of PHA, wild-type HBsAg had no effect on either of the tested surface markers, but increased IFN-γ and IL-10 and inhibited IL-2. In contrast, the G145R mutant alone did not affect CD86 expression, it induced less CD69, and stimulated IL-2 along with lowering levels of TNF-α, IL-10, and IFN-γ. The G145R mutant also suppressed PHA-induced activation of CD69. The dramatic differences in the immune responses elicited by wild-type HBsAg and the G145R mutant HBsAg suggest distinct adaptive capabilities of the G145R mutant HBV.

Heterologous Immunity of Virus-Specific T Cells Leading to Alloreactivity: Possible Implications for Solid Organ Transplantation

Exposure of the adaptive immune system to a pathogen can result in the activation and expansion of T cells capable of recognizing not only the specific antigen but also different unrelated antigens, a process which is commonly referred to as heterologous immunity. While such cross-reactivity is favourable in amplifying protective immune responses to pathogens, induction of T cell-mediated heterologous immune responses to allo-antigens in the setting of solid organ transplantation can potentially lead to allograft rejection. In this review, we provide an overview of murine and human studies investigating the incidence and functional properties of virus-specific memory T cells cross-reacting with allo-antigens and discuss their potential relevance in the context of solid organ transplantation.

Cross-species higher sensitivities of FcγRIIIA/FcγRIV to afucosylated IgG for enhanced ADCC

Background

Expressing afucosylated human IgG1 antibodies with Chinese hamster ovary (CHO) cells deficient of α-(1,6)-fucosyltransferase (FUT8) is being more and more accepted as a routine method to enhance antibody-dependent cellular cytotoxicity (ADCC) of therapeutic antibodies, especially for anti-cancer regimens. However, in pre-clinical studies relying on disease models other than mice and primates, e.g., those underrepresented species for infectious diseases, it is less clear whether such afucosylated antibodies can demonstrate enhanced therapeutic index. This is because the orthologues of human FcγRIIIA or mouse FcγRIV from those species have not been well characterized.

Methods

We set up a luciferase-based ADCC assay with Jurkat reporter cells expressing FcγRIIIA/FcγRIV from human, mouse, rat, hamster, guinea pig, ferret, rabbit, cat, dog, pig and monkey, and also produced human, mouse, hamster, rabbit and pig IgG from wild type and Fut8^−/− CHO cells or hybridomas.

Results

We confirmed that enhanced stimulation through FcγRIIIA/FcγRIV by afucosylated IgG, as compared with wild type IgG, is a cross-species phenomenon.

Conclusions

Thus, efficacy and toxicology studies of the next generation afucosylated therapeutic IgG and Fc fusion proteins in these underrepresented animal models should be expected to generate translatable data for treating human diseases, leading to the expanded applications of this new class of glycoengineered biologics.

Transplantation and immunosuppression: a review of novel transplant-related immunosuppressant drugs

Immunosuppressive drugs used in the transplantation period are generally defined as induction and maintenance therapy. The use of immunosuppressants, which are particularly useful and have fewer side effects, decreased both mortality and morbidity. Many drugs such as steroids, calcineurin inhibitors (cyclosporine-A, tacrolimus), antimetabolites (mycophenolate mofetil, azathioprine), and mTOR inhibitors (sirolimus, everolimus) are used as immunosuppressive agents. Although immunosuppressant drugs cause many side effects such as hypertension, infection, and hyperlipidemia, they are the agents that should be used to prevent organ rejection. This shows the importance of individualized drug use. The optimal immunosuppressive therapy post-transplant is not established. Therefore, discovering less toxic but more potent new agents is of great importance, and new experimental and clinical studies are needed in this regard.Our review discussed the mechanism of immunosuppressants, new agents' discovery, and current therapeutic protocols in the transplantation.

Influenza virus infection selectively triggers the accumulation and persistence of more potent Helios-expressing Foxp3+ regulatory T cells in the lungs

Foxp3⁺ regulatory T cells (Tregs) represent a special lineage of CD4⁺ T cells. Analysis of Treg response during primary and secondary influenza virus infection clearly demonstrates a robust accumulation of Tregs into the infected lungs and the existence of a population of long-lived antigen-specific memory Tregs in the same tissues after resolution of the infection. However, it remains unknown whether these Tregs co-express Helios, a member of the Ikaros transcription factor family. In this study, Foxp3⁺ Helios⁺ and Foxp3⁺ Helios^- Tregs in the lungs, mLNs and spleens of influenza virus-infected and uninfected control mice were tracked. The data show that while there is a co-existence of Foxp3⁺ Helios⁺ and Foxp3⁺ Helios^- Tregs in the tissues, the accumulated Tregs in the lungs and lung-draining mediastinal lymph nodes (mLNs) of the infected mice are highly enriched for Foxp3⁺ Helios⁺ cells. It was further demonstrated that, after the clearance of primary infection, Foxp3⁺ Helios⁺ cells have the ability to persist in the tissues over their Helios^- counterparts. More importantly, Foxp3⁺ Helios⁺ Tregs accumulated in an accelerated kinetics during recall response to reinfection. In vitro analysis of Treg suppressive function revealed that Foxp3⁺ Helios⁺ Tregs are more capable of suppressing influenza virus-specific CD8⁺ T cell activation, cytokine production and proliferation. Together, the data provide new insights into Treg responses during primary and secondary influenza virus infection and suggest that Foxp3⁺ Helios⁺ Tregs predominantly drive the Treg responses.

Immunotherapeutic strategy based on anti-OX40L and low dose of IL-2 to prolong graft survival in sensitized mice by inducing the generation of CD4+ and CD8+ Tregs

Alloreactive memory cells play a critical role after a second transplant and are difficult to suppress. This study investigated the effect of an immunotherapeutic strategy that combines anti-OX40L, rapamycin (Rapa), and a low dose of IL-2 in a memory cell-based adoptive model. In this model, the median survival time (MST) of the grafts of the combined treatment group was significantly extended compared to that of the control group and other treatment groups. A similar effect was observed regarding a reduction in memory T cells (Tm) and inflammatory cytokines production. Also, the percentages of Foxp3⁺ regulatory T cells (Tregs) increased in our model. In addition, mounting evidence has shown CD8⁺CD122⁺ T cells are also Tregs. We found that the group of CD8⁺CD122⁺PD1⁺ T cells was markedly increased in the combined treatment group, especially in the graft. We further demonstrated that CD8⁺CD122⁺PD1⁺ T cells could suppress activated T cells. Our data suggest that anti-OX40L combined with Rapa and a low dose of IL-2 can suppress Tm, modulate CD4 and CD8 Tregs, and induce long-term heart allograft survival in sensitized mice.

Vitamin D3 combined with antibody agents suppresses alloreactive memory T-cell responses to induce heart allograft long-term survival

BACKGROUND

The pre-stored memory T cells in organ transplant patient carry a high risk of allograft rejection. The current study aimed to determine whether the allogenic response of adoptively transferred memory T cells in mice was suppressed by vitamin D3 monotherapy alone or in combination with monoclonal antibody treatment.

METHODS

Prior to vascularized heterotopic heart transplantation, naïve C57BL/6 mice were primed with memory T cells. Recipient mice were administered vitamin D3 alone or in combination with monoclonal antibodies (anti-CD40L/ anti-LFA-1). Memory T cells and CD4⁺ forkhead box P3⁺ T cells in recipient spleens were measured using flow cytometry. Additionally, the expression of cytokines was measured by ELISA and quantitative PCR. Inflammatory factors in the grafts were identified by hematoxylin and eosin staining.

RESULTS

Vitamin D3 in conjunction with anti-CD40L/ anti-LFA-1 antibodies were administered according to the median survival time from 6.5 to 80 days. The results revealed that grafts were protected through the prevention of inflammatory cell infiltration. Combined treatment decreased the mRNA levels of IL-2, IFN-γ and IL-10 and increased the mRNA levels of IL-4, Foxp3 and TGF-β in the allograft. Rejection was suppressed by a reduction of CD4⁺CD44^high CD62L⁺ and CD8⁺ CD44^high CD62L⁺ memory T cells, the induction of regulatory T cells in the recipient spleen and a reduction of serum IL-2, IFN-γ and IL-10 levels.

CONCLUSION

Vitamin D3 efficiently protected allografts from memory T-cell allo-responses when combined with anti-CD40L/anti-LFA-1 antibodies therapy.

Diverse Routes of Allograft Tolerance Disruption by Memory T Cells

Memory T lymphocytes constitute a significant problem in tissue and organ transplantation due their contribution to early rejection and their relative resistance to tolerance-promoting therapies. Memory cells generated by environmental antigen exposure, as with T cells in general, harbor a high frequency of T cell receptors (TCR) spontaneously cross-reacting with allogeneic major histocompatibility complex (MHC) molecules. This phenomenon, known as ‘heterologous’ immunity, is thought to be a key barrier to transplant tolerance induction since such memory cells can potentially react directly with essentially any prospective allograft. In this review, we describe two additional concepts that expand this commonly held view of how memory cells contribute to transplant immunity and tolerance disruption. Firstly, autoimmunity is an additional response that can comprise an endogenously generated form of heterologous alloimmunity. However, unlike heterologous immunity generated as a byproduct of indiscriminate antigen sensitization, autoimmunity can generate T cells that have the unusual potential to interact with the graft either through the recognition of graft-bearing autoantigens or by their cross-reactive (heterologous) alloimmune specificity to MHC molecules. Moreover, we describe an additional pathway, independent of significant heterologous immunity, whereby immune memory to vaccine- or pathogen-induced antigens also may impair tolerance induction. This latter form of immune recognition indirectly disrupts tolerance by the licensing of naïve alloreactive T cells by vaccine/pathogen directed memory cells recognizing the same antigen-presenting cell in vivo. Thus, there appear to be recognition pathways beyond typical heterologous immunity through which memory T cells can directly or indirectly impact allograft immunity and tolerance.

Changes of T-cell Immunity Over a Lifetime

T-cell immunity undergoes a complex and continuous remodeling with aging. Understanding those dynamics is essential in refining immunosuppression. Aging is linked to phenotypic and metabolic changes in T-cell immunity, many resulting into impaired function and compromised effectiveness. Those changes may impact clinical immunosuppression with evidences suggesting age-specific efficacies of some (CNI and mammalian target of rapamycin inhibitors) but not necessarily all immunosuppressants. Metabolic changes of T cells with aging have only recently been appreciated and may provide novel ways of immunosuppression. Here, we provide an update on changes of T-cell immunity in aging.

A kinetic model of T cell autoreactivity in autoimmune diseases

We construct a mathematical model of kinetic type in order to describe the immune system interactions in the context of autoimmune disease. The interacting populations are self-antigen presenting cells, self-reactive T cells and the set of immunosuppressive cells consisting of regulatory T cells and Natural Killer cells. The main aim of our work is to develop a qualitative analysis of the model equations and investigate the existence of biologically realistic solutions. Having this goal in mind we describe the interactions between cells during an autoimmune reaction based on biological considerations that are given in the literature and we show that the corresponding system of integro-differential equations has finite positive solutions. The asymptotic behaviour of the solution of the system is also studied. We complement our mathematical analysis with numerical simulations that study the sensitivity of the model to parameters related to proliferation of immunosuppressive cells, destruction of self-antigen presenting cells and self-reactive T cells and tolerance of SRTCs to self-antigens.

Longitudinal Changes in Cd4+, Cd8+ T Cell Phenotype and Activation Marker Expression Following Antiretroviral Therapy Initiation among Patients with Cryptococcal Meningitis

Despite improvement in the prognosis of HIV/AIDS (human immunodeficiency virus/acquired immune deficiency syndrome) patients on antiretroviral therapy (ART), cryptococcal meningitis (CM) still causes 10–15% mortality among HIV-infected patients. The immunological impact of ART on the CD4⁺ and CD8⁺ T cell repertoire during cryptococcal co-infection is unclear. We determined longitudinal phenotypic changes in T cell subsets among patients with CM after they initiated ART. We hypothesized that ART alters the clonotypic phenotype and structural composition of CD4⁺ and CD8⁺ T cells during CM co-infection. For this substudy, peripheral blood mononuclear cells (PBMC) were isolated at four time points from CM patients following ART initiation during the parent study (ClinicalTrials.gov number, NCT01075152). Phenotypic characterization of CD4⁺ and CD8⁺ T cells was done using T cell surface marker monoclonal antibodies by flow cytometry. There was variation in the expression of immunophenotypic markers defining central memory (CD27⁺CD45R0⁺), effector memory (CD45R0⁺CD27^–), immune activation (CD38⁺ and Human Leucocyte Antigen DR (HLA-DR⁺), and exhaustion (Programmed cell death protein one (PD-1) in the CD4⁺ T cell subset. In comparison to the CD4⁺ T cell population, the CD8⁺ central memory subset declined gradually with minimal increase in the effector memory subset. Both CD4⁺ and CD8⁺ T cell immune exhaustion and activation markers remained elevated over 12 weeks. The relative surge and decline in the expression of T cell surface markers outlines a variation in the differentiation of CD4⁺ T cells during ART treatment during CM co-infection.

Active targeted delivery of immune therapeutics to lymph nodes

PURPOSE OF REVIEW

Organ transplantation is a life-saving procedure and the only option for patients with end-organ failure. Immune therapeutics have been key to the success of organ transplantation. However, immune therapeutics are still unable to eliminate graft rejection and their toxicity has been implicated in poorer long-term transplant outcomes. Targeted nanodelivery has the potential to enhance not only the therapeutic index but also the bioavailability of the immune therapeutics. One of the key sites of immune therapeutics delivery is lymph node where the priming of immune cells occur. The focus of this review is on nanomedicine research to develop the targeted delivery of immune therapeutics to lymph nodes for controlling immune activation.

RECENT FINDINGS

As nanomedicine creates its niche in clinical care, it provides novel immunotherapy platforms for transplant recipients. Draining lymph nodes are the primary loci of immune activation and represent a formidable site for delivery of wide variety of immune therapeutics. There have been relentless efforts to improve the properties of nanomedicines, to have in-depth knowledge of antigen and drug loading, and, finally, to explore various routes of passive and active targeted delivery to lymph nodes.

SUMMARY

The application of nanotechnology principles in the delivery of immune therapeutics to the lymph node has created enormous excitement as a paradigm shifting approach that enables targeted delivery of a gamut of molecules to achieve a desired immune response. Therefore, innovative strategies that improve their efficacy while reducing their toxicity are among the highest unmet needs in transplantation.

Immunological memory: What's in a name?

Immunological memory is one of the core topics of contemporary immunology. Yet there are many discussions about what this concept precisely means, which components of the immune system display it, and in which phyla it exists. Recent years have seen the multiplication of claims that immunological memory can be found in "innate" immune cells and in many phyla beyond vertebrates (including invertebrates, plants, but also bacteria and archaea), as well as the multiplication of concepts to account for these phenomena, such as "innate immune memory" or "trained immunity". The aim of this critical review is to analyze these recent claims and concepts, and to distinguish ideas that have often been misleadingly associated, such as memory, adaptive immunity, and specificity. We argue that immunological memory is a gradual and multidimensional phenomenon, irreducible to any simple dichotomy, and we show why adopting this new view matters from an experimental and therapeutic point of view.

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.

Donor-Derived Regulatory Dendritic Cell Infusion Maintains Donor-Reactive CD4+CTLA4hi T Cells in Non-Human Primate Renal Allograft Recipients Treated with CD28 Co-Stimulation Blockade

Donor-derived regulatory dendritic cell (DCreg) infusion before transplantation, significantly prolongs renal allograft survival in non-human primates. This is associated with enhanced expression of the immunoregulatory molecules cytotoxic T-lymphocyte-associated antigen (Ag) 4 (CTLA4) and programmed cell death protein 1 (PD1) by host donor-reactive T cells. In rodents and humans, CD28 co-stimulatory pathway blockade with the fusion protein CTLA4:Ig (CTLA4Ig) is associated with reduced differentiation and development of regulatory T cells (Treg). We hypothesized that upregulation of CTLA4 by donor-reactive CD4+ T cells in DCreg-infused recipients treated with CTLA4Ig, might be associated with higher incidences of donor-reactive CD4+ T cells with a Treg phenotype. In normal rhesus monkeys, allo-stimulated CD4+CTLA4hi, but not CD4+CTLA4med/lo T cells exhibited a regulatory phenotype, irrespective of PD1 expression. CTLA4Ig significantly reduced the incidence of CD4+CTLA4hi, but not CD4+CTLA4med/lo T cells following allo-stimulation, associated with a significant reduction in the CD4+CTLA4hi/CD4+CTLA4med/lo T cell ratio. In CTLA4Ig-treated renal allograft recipient monkeys, there was a marked reduction in circulating donor-reactive CD4+CTLA4hi T cells. In contrast, in CTLA4Ig-treated monkeys with DCreg infusion, no such reduction was observed. In parallel, the donor-reactive CD4+CTLA4hi/CD4+CTLA4med/lo T cell ratio was reduced significantly in graft recipients without DCreg infusion, but increased in those given DCreg. These observations suggest that pre-transplant DCreg infusion promotes and maintains donor-reactive CD4+CTLA4hi T cells with a regulatory phenotype after transplantation, even in the presence of CD28 co-stimulation blockade.

Current status of costimulatory blockade in renal transplantation

PURPOSE OF REVIEW

The review will focus on the impact and current status of costimulatory blockade in renal transplantation.

RECENT FINDINGS

The mainstay of immunosuppression in kidney transplantation is calcineurin inhibitors (CNIs) which have reduced acute rejection rates but failed to improve long-term allograft survival. Their cardiometabolic side-effects and nephrotoxicity have shifted the focus of investigation to CNI-free regimens. Costimulation blockade with belatacept, a second generation, higher avidity variant of cytotoxic T-lymphocyte associated protein 4 has emerged as part of a CNI-free regimen. Belatacept has demonstrated superior glomerular filtration rate compared with CNIs, albeit with an increased risk of early and histologically severe rejection. Focus on optimizing the belatacept regimen is underway. ASKP1240, which blocks the cluster of differentiation 40 (CD40)/CD154 costimulatory pathway, has just completed a phase 2 trial with a CNI-free regimen. CFZ533, an anti-CD40, is also poised to be tested in a phase 2 trial in renal transplantation. Nonagonistic CD28 antibodies have re-emerged with two anti-CD28 candidates in preclinical development.

SUMMARY

A reliable, CNI-free regimen that maintains low acute rejection rates and improves long-term renal allograft survival has become an achievable goal with costimulation blockade. The task of clinicians and researchers is to find the optimal combinations to maintain safety and improve efficacy.

Caerulomycin A suppresses the differentiation of naïve T cells and alleviates the symptoms of experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a highly detrimental autoimmune disease of the central nervous system. There is no cure for it but the treatment typically focuses on subsiding severity and recurrence of the disease. Experimental autoimmune encephalomyelitis (EAE) is an animal model of MS. It is characterized by frequent relapses due to the generation of memory T cells. Caerulomycin A (CaeA) is known to suppress the Th1 cells, Th2 cells, and Th17 cells. Interestingly, it enhances the generation of regulatory T cells (Tregs). Th1 cells and Th17 cells are known to aggravate EAE, whereas Tregs suppress the disease symptoms. Consequently, in the current study we evaluated the influence of CaeA on EAE. Intriguingly, we observed by whole body imaging that CaeA regressed the clinical symptoms of EAE. Further, there was reduction in the pool of Th1 cells, Th17 cells, and CD8 T cells. The mechanism involved in suppressing the EAE symptoms was due to the inhibition in the generation of effector and central memory T cells and induction of the expansion of Tregs. In essence, these findings implicate that CaeA may be considered as a potent future immunosuppressive drug.

A composite score associated with spontaneous operational tolerance in kidney transplant recipients

New challenges in renal transplantation include using biological information to devise a useful clinical test for discerning high- and low-risk patients for individual therapy and ascertaining the best combination and appropriate dosages of drugs. Based on a 20-gene signature from a microarray meta-analysis performed on 46 operationally tolerant patients and 266 renal transplant recipients with stable function, we applied the sparse Bolasso methodology to identify a minimal and robust combination of six genes and two demographic parameters associated with operational tolerance. This composite score of operational tolerance discriminated operationally tolerant patients with an area under the curve of 0.97 (95% confidence interval 0.94-1.00). The score was not influenced by immunosuppressive treatment, center of origin, donor type, or post-transplant lymphoproliferative disorder history of the patients. This composite score of operational tolerance was significantly associated with both de novo anti-HLA antibodies and tolerance loss. It was validated by quantitative polymerase chain reaction using independent samples and demonstrated specificity toward a model of tolerance induction. Thus, our score would allow clinicians to improve follow-up of patients, paving the way for individual therapy.

Mechanisms of Tolerance Induction by Hematopoietic Chimerism: The Immune Perspective

Hematopoietic chimerism is one of the effective approaches to induce tolerance to donor‐derived tissue and organ grafts without administration of life‐long immunosuppressive therapy. Although experimental efforts to develop such regimens have been ongoing for decades, substantial cumulative toxicity of combined hematopoietic and tissue transplants precludes wide clinical implementation. Tolerance is an active immunological process that includes both peripheral and central mechanisms of mutual education of coresident donor and host immune systems. The major stages include sequential suppression of early alloreactivity, establishment of hematopoietic chimerism and suppressor cells that sustain the state of tolerance, with significant mechanistic and temporal overlap along the tolerization process. Efforts to devise less toxic transplant strategies by reduction of preparatory conditioning focus on modulation rather than deletion of residual host immunity and early reinstitution of regulatory subsets at the central and peripheral levels. Stem Cells Translational Medicine2017;6:700–712

Patterns of Early Rejection in Renal Retransplantation: A Single-Center Experience

It has been reported that kidney retransplant patients had high rates of early acute rejection due to previous sensitization. In addition to the acute antibody-mediated rejection (ABMR) that has received widespread attention, the early acute T-cell-mediated rejection (TCMR) may be another important issue in renal retransplantation. In the current single-center retrospective study, we included 33 retransplant patients and 90 first transplant patients with similar protocols of induction and maintenance therapy. Analysis focused particularly on the incidence and patterns of early acute rejection episodes, as well as one-year graft and patient survival. Excellent short-term clinical outcomes were obtained in both groups, with one-year graft and patient survival rates of 93.9%/100% in the retransplant group and 92.2%/95.6% in the first transplant group. Impressively, with our strict immunological selection and desensitization criteria, the retransplant patients had a very low incidence of early acute ABMR (6.1%), which was similar to that in the first transplant patients (4.4%). However, a much higher rate of early acute TCMR was observed in the retransplant group than in the first transplant group (30.3% versus 5.6%, P < 0.001). Acute TCMR that develops early after retransplantation should be monitored in order to obtain better transplant outcomes.

Advances in diagnostics for transplant rejection

INTRODUCTION

Identification of allograft injury, including acute clinical and subclinical injury, is vital in increasing the longevity of the transplanted organ. Acute rejection, which occurs as a result of a variety of immune and non-immune factors including the infiltration of immune cells and antibodies to the donor specific epitopes, poses a significant risk to the organ. Recent years have marked an increase in the discovery of new genomic, transcriptomic, and proteomic biomarkers in molecular diagnostics, which offer better potential for personalized management of the transplanted organ by providing earlier detection of rejection episodes. Areas covered: This review was compiled from key word searches of full-text publications relevant to the field. Expert commentary: Many of the recent advancements in the molecular diagnostics of allograft injury show much promise, but before they can be fully realized further validation in larger sample sets must be conducted. Additionally, for better informed therapeutic decisions, more work must be completed to differentiate between different causes of injury. Moreover, the diagnostics field is looking at methodologies that allow for multiplexing, the ability to identify multiple targets simultaneously, in order to provide more robust biomarkers and better understanding.

Costimulatory Blockade and Use of mTOR Inhibitors: Avoiding Injury Part 2

Kidney transplantation immunosuppression relies on a calcineurin inhibitor backbone. Calcineurin inhibitors have reduced early-acute rejection rates but failed to improve long-term allograft survival. Their nephrotoxicity has shifted the focus of investigation to calcineurin inhibitor-free regimens. Costimulation blockade with belatacept, a second generation, higher avidity variant of CTLA4-Ig, has emerged as part of a calcineurin inhibitor-free regimen. Belatacept has demonstrated superior glomerular filtration rate compared with calcineurin inhibitors albeit with an increased risk of early and histologically severe rejection. Focus on optimizing the belatacept regimen to reduce the acute rejection rate while maintaining superior renal function is underway. Belatacept has also been utilized as part of a calcineurin inhibitor-free conversion strategy in stable renal transplant recipients and has demonstrated superior improvement in glomerular filtration rate with conversion vs calcineurin inhibitor continuation. Additional work is underway to better define the role of belatacept in patients on calcineurin inhibitors with allograft dysfunction not due to rejection.

Targeted Delivery of Immunomodulators to Lymph Nodes

Active-targeted delivery to lymph nodes represents a major advance toward more effective treatment of immune-mediated disease. The MECA79 antibody recognizes peripheral node addressin molecules expressed by high endothelial venules of lymph nodes. By mimicking lymphocyte trafficking to the lymph nodes, we have engineered MECA79-coated microparticles containing an immunosuppressive medication, tacrolimus. Following intravenous administration, MECA79-bearing particles showed marked accumulation in the draining lymph nodes of transplanted animals. Using an allograft heart transplant model, we show that targeted lymph node delivery of microparticles containing tacrolimus can prolong heart allograft survival with negligible changes in tacrolimus serum level. Using MECA79 conjugation, we have demonstrated targeted delivery of tacrolimus to the lymph nodes following systemic administration, with the capacity for immune modulation in vivo.

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.

Prospective Clinical Testing of Regulatory Dendritic Cells in Organ Transplantation

Dendritic cells (DC) are rare, professional antigen-presenting cells with ability to induce or regulate alloimmune responses. Regulatory DC (DCreg) with potential to down-modulate acute and chronic inflammatory conditions that occur in organ transplantation can be generated in vitro under a variety of conditions. Here, we provide a rationale for evaluation of DCreg therapy in clinical organ transplantation with the goal of promoting sustained, donor-specific hyporesponsiveness, while lowering the incidence and severity of rejection and reducing patients’ dependence on anti-rejection drugs. Generation of donor- or recipient-derived DCreg that suppress T cell responses and prolong transplant survival in rodents or non-human primates has been well-described. Recently, good manufacturing practice (GMP)-grade DCreg have been produced at our Institution for prospective use in human organ transplantation. We briefly review experience of regulatory immune therapy in organ transplantation and describe our experience generating and characterizing human monocyte-derived DCreg. We propose a phase I/II safety study in which the influence of donor-derived DCreg combined with conventional immunosuppression on subclinical and clinical rejection and host alloimmune responses will be examined in detail.

The Evolving Roles of Memory Immune Cells in Transplantation

Memory cells are the products of immune responses but also exert significant impact on subsequent immunity and immune tolerance, thus placing them in a unique position in transplant research. Memory cells are heterogeneous, including not only memory T cells but also memory B cells and innate memory cells. Memory cells are a critical component of protective immunity against invading pathogens, especially in immunosuppressed patients, but they also mediate graft loss and tolerance resistance. Recent studies suggest that some memory cells unexpectedly act as regulatory cells, promoting rather than hindering transplant survival. This functional diversity makes therapeutic targeting of memory cells a challenging task in transplantation. In this article, we highlight recent advances in our understanding of memory cells, focusing on diversity of memory cells and mechanisms involved in their induction and functions. We also provide a broad overview on the challenges and opportunities in targeting memory cells in the induction of transplant tolerance.

T-cell alloreactivity and transplantation outcome: a budding role for heterologous immunity?

PURPOSE OF REVIEW

Despite the association between alloreactive T cells and poor graft survival, the mechanisms behind T-cell-mediated rejection are still under investigation. In this review, we will discuss the latest insights into the impact of T-cell alloreactivity on solid organ transplantation and hematopoietic stem cell transplantation (HSCT), with special emphasis on the potential impact of heterologous immunity.

RECENT FINDINGS

A large part of the memory T-cell repertoire is induced upon virus infections, and evidence for a role of T-cell receptor cross-reactivity of virus-induced memory T cells against allogeneic human leukocyte antigen (HLA) is accumulating in experimental and clinical solid organ transplantation studies. In HSCT, strong alloreactive potential of naïve T cells causes concerns for graft-versus-host disease while additional HLA-DP matching is suggested to prevent CD4 alloreactivity. Furthermore, virus-induced memory T cells hamper mixed chimerism induction, pointing once more towards a role for heterologous immunity.

SUMMARY

Both memory and naïve T cells contribute to the alloimmune response after transplantation. Monitoring for T-cell phenotypes could help predict rejection episodes and/or graft-versus-host disease, allowing timely intervention. Tailoring donor lymphocyte infusions and additional HLA matching could prevent strong alloreactivity in HSCT. Furthermore, the potential role of heterologous immunity in T-cell alloreactivity and transplantation is gaining interest.

Memory T Cell Migration

Immunological memory is a key feature of adaptive immunity. It provides the organism with long-lived and robust protection against infection. In organ transplantation, memory T cells pose a significant threat by causing allograft rejection that is generally resistant to immunosuppressive therapy. Therefore, a more thorough understanding of memory T cell biology is needed to improve the survival of transplanted organs without compromising the host’s ability to fight infections. This review will focus on the mechanisms by which memory T cells migrate to the site where their target antigen is present, with particular emphasis on their migration to transplanted organs. First, we will define the known subsets of memory T cells (central, effector, and tissue resident) and their circulation patterns. Second, we will review the cellular and molecular mechanisms by which memory T cells migrate to inflamed and non-inflamed tissues and highlight the emerging paradigm of antigen-driven, trans-endothelial migration. Third, we will discuss the relevance of this knowledge to organ transplantation and the prevention or treatment of allograft rejection.

Polyclonal and monoclonal antibodies in renal transplant: an update

PURPOSE OF REVIEW

There are several monoclonal and polyclonal antibodies used in renal transplantation today, this article will discuss several agents, their updates and newer agents.

RECENT FINDINGS

Antithymocyte globulin and interleukin-2 (IL-2) receptor blocker continue to be used as induction agents. The risk of acute rejection was higher in IL-2 receptor blockers mainly in the first year, but graft survivals were similar in both groups long term. Belatacept is the only approved intravenous maintenance immunosuppressive therapy which provides the benefit of glomerular filtration rate preservation, but it was associated with a higher risk of acute rejection and post-transplant lymphoproliferative disorder. Bortezomib may help decrease donor-specific antibody levels, but there are limited data to support its use for desensitization or rejection. Eculizumab may help in antibody-mediated rejection in some cases but has not shown promising long-term effects in high-risk individuals. Newer agents have been continuously tested for improved efficacy and safety.

SUMMARY

Transplantation is the standard of care for end-stage renal disease patients, but we still have a long way to go, as we need to improve long-term outcomes. The manipulation of the immune system is a delicate undertaking, with risks of adverse events; therefore, risk versus benefit needs to be carefully evaluated and treatment needs to be individualized.

Role of Memory T Cells and Perspectives for Intervention in Organ Transplantation

Memory T cells are necessary for protective immunity against invading pathogens, especially under conditions of immunosuppression. However, their presence also threatens transplant survival, making transplantation a great challenge. Significant progress has been achieved in recent years in advancing our understanding of the role that memory T cells play in transplantation. This review focuses on the latest advances in our understanding of the involvement of memory T cells in graft rejection and transplant tolerance and discusses potential strategies for targeting memory T cells in order to minimize allograft rejection and optimize clinical outcomes.

Co-Stimulatory Blockade of the CD28/CD80-86/CTLA-4 Balance in Transplantation: Impact on Memory T Cells?

CD28 and CTLA-4 are prototypal co-stimulatory and co-inhibitory cell surface signaling molecules interacting with CD80/86, known to be critical for immune response initiation and regulation, respectively. Initial “bench-to-beside” translation, two decades ago, resulted in the development of CTLA4-Ig, a biologic that targets CD80/86 and prevents T-cell costimulation. In spite of its proven effectiveness in inhibiting allo-immune responses, particularly in murine models, clinical experience in kidney transplantation with belatacept (high-affinity CTLA4-Ig molecule) reveals a high incidence of acute, cell-mediated rejection. Originally, the etiology of belatacept-resistant graft rejection was thought to be heterologous immunity, i.e., the cross-reactivity of the pool of memory T cells from pathogen-specific immune responses with alloantigens. Recently, the standard view that memory T cells arise from effector cells after clonal contraction has been challenged by a “developmental” model, in which less differentiated memory T cells generate effector cells. This review delineates how this shift in paradigm, given the differences in co-stimulatory and co-inhibitory signal depending on the maturation stage, could profoundly affect our understanding of the CD28/CD80-86/CTLA-4 blockade and highlights the potential advantages of selectively targeting CD28, instead of CD80/86, to control post-transplant immune responses.

Tolerance of Lung Allografts Achieved in Nonhuman Primates via Mixed Hematopoietic Chimerism

While the induction of transient mixed chimerism has tolerized MHC-mismatched renal grafts in nonhuman primates and patients, this approach has not been successful for more immunogenic organs. Here, we describe a modified delayed-tolerance-induction protocol resulting in three out of four monkeys achieving long-term lung allograft survival without ongoing immunosuppression. Two of the tolerant monkeys displayed stable mixed lymphoid chimerism, and the other showed transient chimerism. Serial biopsies and post-mortem specimens from the tolerant monkeys revealed no signs of chronic rejection. The tolerant recipients also exhibited T cell unresponsiveness and a lack of alloantibody. This is the first report of durable mixed chimerism and successful tolerance induction of MHC-mismatched lungs in primates.

Hematopoietic stem cell infusion/transplantation for induction of allograft tolerance

PURPOSE OF REVIEW

The present review updates the current status of basic, preclinical, and clinical research on donor hematopoietic stem cell infusion for allograft tolerance induction.

RECENT FINDINGS

Recent basic studies in mice provide evidence of significant involvement of both central deletional and peripheral regulatory mechanisms in induction and maintenance of allograft tolerance effected through a mixed chimerism approach with donor hematopoietic stem cell infusion. The presence of heterologous memory T cells in primates hampers the induction of persistent chimerism. Durable mixed chimerism, however, now has been recently induced in inbred major histocompatibility complex-mismatched swine, resulting in tolerance of vascularized composite tissue allografts. In clinical transplantation, allograft tolerance has been achieved in human leukocyte antigen-mismatched kidney transplantation after the induction of transient mixed chimerism or persistent full donor chimerism.

SUMMARY

Tolerance induction in clinical kidney transplantation has been achieved by donor hematopoietic stem cell infusion. Improving the consistency and safety of tolerance induction and extending successful protocols to other organs, and to organs from deceased donors, are critical next steps to bringing tolerance to a wider range of clinical applications.

Recollective homeostasis and the immune consequences of peritransplant depletional induction therapy

One's cellular immune repertoire is composed of lymphocytes in multiple stages of maturation - the dynamic product of their responses to antigenic challenges and the homeostatic contractions necessary to accommodate immune expansions within physiologic norms. Given that alloreactivity is predominantly a cross-reactive phenomenon that is stochastically distributed throughout the overall T-cell repertoire, one's allospecific repertoire is similarly made up of cells in a variety of differentiation states. As such, the continuous expansion and elimination of activated memory populations, producing a 'recollective homeostasis' of sorts, has the potential over time to alter the maturation state and effector composition of both ones protective and alloreactive T-cell repertoire. Importantly, a T cell's maturation state significantly influences its response to numerous immunomodulatory therapies used in organ transplantation, including depletional antibody induction. In this review, we discuss clinically utilized depletional induction strategies, how their use alters a transplant recipient's cellular immune repertoire, and how a recipient's repertoire influences the clinical effects of induction therapy.

Serine protease inhibitor-6 differentially affects the survival of effector and memory alloreactive CD8-T cells

The clonal expansion of effector T cells and subsequent generation of memory T cells are critical in determining the outcome of transplantation. While cytotoxic T lymphocytes induce direct cytolysis of target cells through secretion of Granzyme-B (GrB), they also express cytoplasmic serine protease inhibitor-6 (Spi6) to protect themselves from GrB that has leaked from granules. Here, we studied the role of GrB/Spi6 axis in determining clonal expansion of alloreactive CD8-T cells and subsequent generation of memory CD8-T cells in transplantation. CD8-T cells from Spi6(-/-) mice underwent more GrB mediated apoptosis upon alloantigen stimulation in vitro and in vivo following adoptive transfer into an allogeneic host. Interestingly, while OT1.Spi6(-/-) CD8 T cells showed significantly lower clonal expansion following skin transplants from OVA mice, there was no difference in the size of the effector memory CD8-T cells long after transplantation. Furthermore, lack of Spi6 resulted in a decrease of short-lived-effector-CD8-cells but did not impact the pool of memory-precursor-effector-CD8-cells. Similar results were found in heart transplant models. Our findings suggest that the final alloreactive CD8-memory-pool-size is independent from the initial clonal-proliferation as memory precursors express low levels of GrB and therefore are independent of Spi6 for survival. These data advance our understanding of memory T cells generation in transplantation and provide basis for Spi6 based strategies to target effector T cells.