Emerging translational strategies and challenges for enhancing regulatory T cell therapy for graft-versus-host disease

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a curative therapy for many types of cancer. Genetic disparities between donor and host can result in immune-mediated attack of host tissues, known as graft versus host disease (GVHD), a major cause of morbidity and mortality following HSCT. Regulatory CD4+ T cells (Tregs) are a rare cell type crucial for immune system homeostasis, limiting the activation and differentiation of effector T cells (Teff) that are self-reactive or stimulated by foreign antigen exposure. Adoptive cell therapy (ACT) with Treg has demonstrated, first in murine models and now in patients, that prophylactic Treg infusion can also suppress GVHD. While clinical trials have demonstrated Treg reduce severe GVHD occurrence, several impediments remain, including Treg variability and practical need for individualized Treg production for each patient. Additionally, there are challenges in the use of in vitro expansion techniques and in achieving in vivo Treg persistence in context of both immune suppressive drugs and in lymphoreplete patients being treated for GVHD. This review will focus on 3 main translational approaches taken to improve the efficacy of tTreg ACT in GVHD prophylaxis and development of treatment options, following HSCT: genetic modification, manipulating TCR and cytokine signaling, and Treg production protocols. In vitro expansion for Treg ACT presents a multitude of approaches for gene modification to improve efficacy, including: antigen specificity, tissue targeting, deletion of negative regulators/exhaustion markers, resistance to immunosuppressive drugs common in GVHD treatment. Such expansion is particularly important in patients without significant lymphopenia that can drive Treg expansion, enabling a favorable Treg:Teff ratio in vivo. Several potential therapeutics have also been identified that enhance tTreg stability or persistence/expansion following ACT that target specific pathways, including: DNA/histone methylation status, TCR/co-stimulation signaling, and IL-2/STAT5 signaling. Finally, this review will discuss improvements in Treg production related to tissue source, Treg subsets, therapeutic approaches to increase Treg suppression and stability during tTreg expansion, and potential for storing large numbers of Treg from a single production run to be used as an off-the-shelf infusion product capable of treating multiple recipients.

Retinoic acid signaling acts as a rheostat to balance Treg function

Regulatory T cells (Tregs) promote immune homeostasis by maintaining self-tolerance and regulating inflammatory responses. Under certain inflammatory conditions, Tregs can lose their lineage stability and function. Previous studies have reported that ex vivo exposure to retinoic acid (RA) enhances Treg function and stability. However, it is unknown how RA receptor signaling in Tregs influences these processes in vivo. Herein, we employed mouse models in which RA signaling is silenced by the expression of the dominant negative receptor (DN) RARα in all T cells. Despite the fact that DNRARα conventional T cells are hypofunctional, Tregs had increased CD25 expression, STAT5 pathway activation, mTORC1 signaling and supersuppressor function. Furthermore, DNRARα Tregs had increased inhibitory molecule expression, amino acid transporter expression, and metabolic fitness and decreased antiapoptotic proteins. Supersuppressor function was observed when wild-type mice were treated with a pharmacologic pan-RAR antagonist. Unexpectedly, Treg-specific expression of DNRARα resulted in distinct phenotypes, such that a single allele of DNRARα in Tregs heightened their suppressive function, and biallelic expression led to loss of suppression and autoimmunity. The loss of Treg function was not cell intrinsic, as Tregs that developed in a noninflammatory milieu in chimeric mice reconstituted with DNRARα and wild-type bone marrow maintained the enhanced suppressive capacity. Fate mapping suggested that maintaining Treg stability in an inflammatory milieu requires RA signaling. Our findings indicate that RA signaling acts as a rheostat to balance Treg function in inflammatory and noninflammatory conditions in a dose-dependent manner.

PD-L1 signaling selectively regulates T cell lymphatic transendothelial migration

Programmed death-1 (PD-1) and its ligand PD-L1 are checkpoint molecules which regulate immune responses. Little is known about their functions in T cell migration and there are contradictory data about their roles in regulatory T cell (Treg) function. Here we show activated Tregs and CD4 effector T cells (Teffs) use PD-1/PD-L1 and CD80/PD-L1, respectively, to regulate transendothelial migration across lymphatic endothelial cells (LECs). Antibody blockade of Treg PD-1, Teff CD80 (the alternative ligand for PD-L1), or LEC PD-L1 impairs Treg or Teff migration in vitro and in vivo. PD-1/PD-L1 signals through PI3K/Akt and ERK to regulate zipper junctional VE-cadherin, and through NFκB-p65 to up-regulate VCAM-1 expression on LECs. CD80/PD-L1 signaling up-regulates VCAM-1 through ERK and NFκB-p65. PD-1 and CD80 blockade reduces tumor egress of PD-1high fragile Tregs and Teffs into draining lymph nodes, respectively, and promotes tumor regression. These data provide roles for PD-L1 in cell migration and immune regulation.

Treg tissue stability depends on lymphotoxin beta-receptor- and adenosine-receptor-driven lymphatic endothelial cell responses

Regulatory T cell (Treg) lymphatic migration is required for resolving inflammation and prolonging allograft survival. Focusing on Treg interactions with lymphatic endothelial cells (LECs), we dissect mechanisms and functional consequences of Treg transendothelial migration (TEM). Using three genetic mouse models of pancreatic islet transplantation, we show that Treg lymphotoxin (LT) αβ and LEC LTβ receptor (LTβR) signaling are required for efficient Treg migration and suppressive function to prolong allograft survival. Inhibition of LT signaling increases Treg conversion to Foxp3loCD25lo exTregs. In a transwell-based model of TEM across polarized LECs, non-migrated Tregs become exTregs. Such conversion is regulated by LTβR nuclear factor κB (NF-κB) signaling in LECs, which increases interleukin-6 (IL-6) production and drives exTreg conversion. Migrating Tregs are ectonucleotidase CD39hi and resist exTreg conversion in an adenosine-receptor-2A-dependent fashion. Human Tregs migrating across human LECs behave similarly. These molecular interactions can be targeted for therapeutic manipulation of immunity and suppression.

ICOSL+ plasmacytoid dendritic cells as inducer of graft-versus-host disease, responsive to a dual ICOS/CD28 antagonist

Acute graft-versus-host disease (aGVHD) remains a major complication of allogeneic hematopoietic cell transplantation (HCT). CD146 and CCR5 are proteins that mark activated T helper 17 (Th17) cells. The Th17 cell phenotype is promoted by the interaction of the receptor ICOS on T cells with ICOS ligand (ICOSL) on dendritic cells (DCs). We performed multiparametric flow cytometry in a cohort of 156 HCT recipients and conducted experiments with aGVHD murine models to understand the role of ICOSL⁺ DCs. We observed an increased frequency of ICOSL⁺ plasmacytoid DCs, correlating with CD146⁺CCR5⁺ T cell frequencies, in the 64 HCT recipients with gastrointestinal aGVHD. In murine models, donor bone marrow cells from ICOSL-deficient mice compared to those from wild-type mice reduced aGVHD-related mortality. Reduced aGVHD resulted from lower intestinal infiltration of pDCs and pathogenic Th17 cells. We transplanted activated human ICOSL⁺ pDCs along with human peripheral blood mononuclear cells into immunocompromised mice and observed infiltration of intestinal CD146⁺CCR5⁺ T cells. We found that prophylactic administration of a dual human ICOS/CD28 antagonist (ALPN-101) prevented aGVHD in this model better than did the clinically approved belatacept (CTLA-4-Fc), which binds CD80 (B7-1) and CD86 (B7-2) and interferes with the CD28 T cell costimulatory pathway. When started at onset of aGVHD signs, ALPN-101 treatment alleviated symptoms of ongoing aGVHD and improved survival while preserving antitumoral cytotoxicity. Our data identified ICOSL⁺-pDCs as an aGVHD biomarker and suggest that coinhibition of the ICOSL/ICOS and B7/CD28 axes with one biologic drug may represent a therapeutic opportunity to prevent or treat aGVHD.

In vitro expanded human regulatory T cells (tTreg) and CD4+ effector T cells (Teff) display distinct metabolic demands and response to lactate

Thymus-derived regulatory T cells (tTreg) have been used as a cellular therapeutic to prevent or treat autoimmunity, graft rejection, and graft versus host disease (GVHD) in humans. Metabolic pathways affect tTreg differentiation, expansion and suppressive function. Recent experiments challenge the initial paradigm that tTregs depend on fatty acid oxidation (FAO) and glycolysis. Murine studies also show that central memory and effector tTreg have distinct metabolic profiles, but human studies to date use pooled populations of tTreg. The goal of the current study was to identify and compare the metabolic pathways critical for naïve and memory human tTreg, and to determine whether these pathways differ from naïve human CD4 T cells. We found that memory tTreg have higher rates of glycolysis and oxidative phosphorylation (OXPHOS) than naïve tTreg, both at rest and after stimulation. Activated naïve and memory tTreg had high basal OXPHOS, but neither glucose nor FAO were major substrates. Consistent with this, culture in low glucose did not affect expansion of either naïve or memory tTreg, but greatly decreased that of CD4 Teff. Extracellular lactate increased tTreg basal OCR and, in contrast to the lactate-dependent inhibition of CD4 T cells, enhanced tTreg expansion. This is consistent with differential expression of a class of lactate transporters (monocarboxylate transporters, or MCT) where Treg have increased expression of MCT1/2, which favor import whereas Teff have increased expression of MCT4, which favors export. These data demonstrate that naïve and memory tTreg should be analyzed separately, and provide a molecular insight into why tTreg, but not Teff, function in a low glucose, high lactate environment.

Multiply restimulated human thymic regulatory T cells express distinct signature regulatory T-cell transcription factors without evidence of exhaustion

BACKGROUND AIMS

Adoptive transfer of suppressive CD4+CD25+ thymic regulatory T cells (tTregs) can control auto- and alloimmune responses but typically requires in vitro expansion to reach the target cell number for efficacy. Although the adoptive transfer of expanded tTregs purified from umbilical cord blood ameliorates graft-versus-host disease in patients receiving hematopoietic stem cell transplantation for lymphohematopoietic malignancy, individual Treg products of 100 × 10⁶ cells/kg are manufactured over an extended 19-day time period using a process that yields variable products and is both laborious and costly. These limitations could be overcome with the availability of 'off the shelf' Treg.

RESULTS

Previously, the authors reported a repetitive restimulation expansion protocol that maintains Treg phenotype (CD4+25++127-Foxp3+), potentially providing hundreds to thousands of patient infusions. However, repetitive stimulation of effector T cells induces a well-defined program of exhaustion that leads to reduced T-cell survival and function. Unexpectedly, the authors found that multiply stimulated human tTregs do not develop an exhaustion signature and instead maintain their Treg gene expression pattern. The authors also found that tTregs expanded with one or two rounds of stimulation and tTregs expanded with three or five rounds of stimulation preferentially express distinct subsets of a group of five transcription factors that lock in Treg Foxp3expression, Treg stability and suppressor function. Multiply restimulated Tregs also had increased transcripts characteristic of T follicular regulatory cells, a Treg subset.

DISCUSSION

These data demonstrate that repetitively expanded human tTregs have a Treg-locking transcription factor with stable FoxP3 and without the classical T-cell exhaustion gene expression profile-desirable properties that support the possibility of off-the-shelf Treg therapeutics.

Regulatory T Cells Condition Lymphatic Endothelia for Enhanced Transendothelial Migration

Regulatory T cells (Tregs) express high levels of cell surface lymphotoxin alpha beta (LTα1β2) to activate the LT beta receptor (LTβR) on the lymphatic endothelial cells (LECs), modulating LEC adhesion molecules, intercellular junctions, and chemokines. We demonstrate a role for Tregs through this pathway to condition the permissiveness of lymphatic endothelia for transendothelial migration (TEM), thus gating leukocyte traffic. Human Tregs share the same property with murine Tregs. Activation of TLR2 on Tregs during inflammation specifically augments LTα1β2-LTβR signaling, which further enhances the permissiveness of LECs to facilitate TEM. The conditioning of endothelia may promote the resolution of inflammation by directing leukocytes out of tissues to lymphatic vessels and draining lymph nodes (dLNs). Thus, Tregs interact with lymphatic endothelia under homeostasis and inflammation and dictate endothelial permissiveness and gating mechanisms for subsequent leukocyte migration through endothelial barriers.

The lymph node stromal laminin α5 shapes alloimmunity

Lymph node stromal cells (LNSCs) regulate immunity through constructing lymphocyte niches. LNSC-produced laminin α5 (Lama5) regulates CD4+ T cells but the underlying mechanisms of its functions are poorly understood. Here we show that depleting Lama5 in LNSCs resulted in decreased Lama5 protein in the LN cortical ridge (CR) and around high endothelial venules (HEVs). Lama5 depletion affected LN structure with increased HEVs, upregulated chemokines, and cell adhesion molecules, and led to greater numbers of Tregs in the T cell zone. Mouse and human T cell transendothelial migration and T cell entry into LNs were suppressed by Lama5 through the receptors α6 integrin and α-dystroglycan. During immune responses and allograft transplantation, depleting Lama5 promoted antigen-specific CD4+ T cell entry into the CR through HEVs, suppressed T cell activation, and altered T cell differentiation to suppressive regulatory phenotypes. Enhanced allograft acceptance resulted from depleting Lama5 or blockade of T cell Lama5 receptors. Lama5 and Lama4/Lama5 ratios in allografts were associated with the rejection severity. Overall, our results demonstrated that stromal Lama5 regulated immune responses through altering LN structures and T cell behaviors. This study delineated a stromal Lama5-T cell receptor axis that can be targeted for immune tolerance modulation.

CD4 T cell sphingosine 1-phosphate receptor (S1PR)1 and S1PR4 and endothelial S1PR2 regulate afferent lymphatic migration

Sphingosine 1-phosphate (S1P) and S1P receptors (S1PRs) regulate migration of lymphocytes out of thymus to blood and lymph nodes (LNs) to efferent lymph, whereas their role in other tissue sites is not known. Here, we investigated the question of how these molecules regulate leukocyte migration from tissues through afferent lymphatics to draining LNs (dLNs). S1P, but not other chemokines, selectively enhanced human and murine CD4 T cell migration across lymphatic endothelial cells (LECs). T cell S1PR1 and S1PR4, and LEC S1PR2, were required for migration across LECs and into lymphatic vessels and dLNs. S1PR1 and S1PR4 differentially regulated T cell motility and vascular cell adhesion molecule-1 (VCAM-1) binding. S1PR2 regulated LEC layer structure, permeability, and expression of the junction molecules VE-cadherin, occludin, and zonulin-1 through the ERK pathway. S1PR2 facilitated T cell transcellular migration through VCAM-1 expression and recruitment of T cells to LEC migration sites. These results demonstrated distinct roles for S1PRs in comodulating T cell and LEC functions in migration and suggest previously unknown levels of regulation of leukocytes and endothelial cells during homeostasis and immunity.

Mitochondrial Integrity Regulated by Lipid Metabolism Is a Cell-Intrinsic Checkpoint for Treg Suppressive Function

Regulatory T cells (Tregs) subdue immune responses. Central to Treg activation are changes in lipid metabolism that support their survival and function. Fatty acid binding proteins (FABPs) are a family of lipid chaperones required to facilitate uptake and intracellular lipid trafficking. One family member, FABP5, is expressed in T cells, but its function remains unclear. We show that in Tregs, genetic or pharmacologic inhibition of FABP5 function causes mitochondrial changes underscored by decreased OXPHOS, impaired lipid metabolism, and loss of cristae structure. FABP5 inhibition in Tregs triggers mtDNA release and consequent cGAS-STING-dependent type I IFN signaling, which induces heightened production of the regulatory cytokine IL-10 and promotes Treg suppressive activity. We find evidence of this pathway, along with correlative mitochondrial changes in tumor infiltrating Tregs, which may underlie enhanced immunosuppression in the tumor microenvironment. Together, our data reveal that FABP5 is a gatekeeper of mitochondrial integrity that modulates Treg function.

Distinct Regulatory and Effector T Cell Metabolic Demands during Graft-Versus-Host Disease

Despite graft-versus-host disease (GVHD) prophylactic agents, the success and wider utilization of allogeneic hematopoietic stem cell transplantation (allo-HSCT) is limited by GVHD. Increasing donor graft regulatory T cell (Treg):effector T cell (Teff) ratios can substantially reduce GVHD in cancer patients, but pre-HSCT conditioning regimens and GVHD create a challenging inflammatory environment for Treg stability, persistence, and function. Metabolism plays a crucial role in T cell and Treg differentiation, and development of effector function. Although glycolysis is a main driver of allogeneic T cell-driven GVHD, oxidative phosphorylation is a main driver of Treg suppressor function. This review focuses on recent advances in our understanding of Treg metabolism in the context of GVHD, and discusses potential therapeutic applications of Tregs in the prevention or treatment of GVHD in cancer patients.

Rorc restrains the potency of ST2+ regulatory T cells in ameliorating intestinal graft-versus-host disease

Soluble stimulation-2 (ST2) is increased during graft-versus-host disease (GVHD), while Tregs that express ST2 prevent GVHD through unknown mechanisms. Transplantation of Foxp3- T cells and Tregs that were collected and sorted from different Foxp3 reporter mice indicated that in mice that developed GVHD, ST2+ Tregs were thymus derived and predominantly localized to the intestine. ST2-/- Treg transplantation was associated with reduced total intestinal Treg frequency and activation. ST2-/- versus WT intestinal Treg transcriptomes showed decreased Treg functional markers and, reciprocally, increased Rorc expression. Rorc-/- T cells transplantation enhanced the frequency and function of intestinal ST2+ Tregs and reduced GVHD through decreased gut-infiltrating soluble ST2-producing type 1 and increased IL-4/IL-10-producing type 2 T cells. Cotransfer of ST2+ Tregs sorted from Rorc-/- mice with WT CD25-depleted T cells decreased GVHD severity and mortality, increased intestinal ST2+KLRG1+ Tregs, and decreased type 1 T cells after transplantation, indicating an intrinsic mechanism. Ex vivo IL-33-stimulated Tregs (TregIL-33) expressed higher amphiregulin and displayed better immunosuppression, and adoptive transfer prevented GVHD better than control Tregs or TregIL-33 cultured with IL-23/IL-17. Amphiregulin blockade by neutralizing antibody in vivo abolished the protective effect of TregIL-33. Our data show that inverse expression of ST2 and RORγt in intestinal Tregs determines GVHD and that TregIL-33 has potential as a cellular therapy avenue for preventing GVHD.

Induction of immunosuppressive functions and NF-κB by FLIP in monocytes

Immunosuppression is a hallmark of tumor progression, and treatments that inhibit or deplete monocytic myeloid-derived suppressive cells could promote anti-tumor immunity. c-FLIP is a central regulator of caspase-8-mediated apoptosis and necroptosis. Here we show that low-dose cytotoxic chemotherapy agents cause apoptosis linked to c-FLIP down-regulation selectively in monocytes. Enforced expression of c-FLIP or viral FLIP rescues monocytes from cytotoxicity and concurrently induces potent immunosuppressive activity, in T cell cultures and in vivo models of tumor progression and immunotherapy. FLIP-transduced human blood monocytes can suppress graft versus host disease. Neither expression of FLIP in granulocytes nor expression of other anti-apoptotic genes in monocytes conferred immunosuppression, suggesting that FLIP effects on immunosuppression are specific to monocytic lineage and distinct from death inhibition. Mechanistically, FLIP controls a broad transcriptional program, partially by NF-κB activation. Therefore, modulation of FLIP in monocytes offers a means to elicit or block immunosuppressive myeloid cells.

Signaling and transcriptional regulation of MDSC activity remains largely undefined. Here the authors show that monocytic MDSC immunosuppression is triggered by c-FLIP and requires NFκB, implicate this axis in cancer prognosis and response to therapy, and employ ectopic FLIP to treat immunopathology.

The vimentin intermediate filament network restrains regulatory T cell suppression of graft-versus-host disease

Regulatory T cells (Tregs) are critical for maintaining immune homeostasis. However, current Treg immunotherapies do not optimally treat inflammatory diseases in patients. Understanding the cellular processes that control Treg function may allow for the augmentation of therapeutic efficacy. In contrast to activated conventional T cells, in which protein kinase C-θ (PKC-θ) localizes to the contact point between T cells and antigen-presenting cells, in human and mouse Tregs, PKC-θ localizes to the opposite end of the cell in the distal pole complex (DPC). Here, using a phosphoproteomic screen, we identified the intermediate filament vimentin as a PKC-θ phospho target and show that vimentin forms a DPC superstructure on which PKC-θ accumulates. Treatment of mouse Tregs with either a clinically relevant PKC-θ inhibitor or vimentin siRNA disrupted vimentin and enhanced Treg metabolic and suppressive activity. Moreover, vimentin-disrupted mouse Tregs were significantly better than controls at suppressing alloreactive T cell priming in graft-versus-host disease (GVHD) and GVHD lethality, using a complete MHC-mismatch mouse model of acute GVHD (C57BL/6 donor into BALB/c host). Interestingly, vimentin disruption augmented the suppressor function of PKC-θ-deficient mouse Tregs. This suggests that enhanced Treg activity after PKC-θ inhibition is secondary to effects on vimentin, not just PKC-θ kinase activity inhibition. Our data demonstrate that vimentin is a key metabolic and functional controller of Treg activity and provide proof of principle that disruption of vimentin is a feasible, translationally relevant method to enhance Treg potency.

The purinergic receptor P2RX7 directs metabolic fitness of long-lived memory CD8+ T cells

Extracellular ATP (eATP) is an ancient 'danger signal' used by eukaryotes to detect cellular damage¹. In mice and humans, the release of eATP during inflammation or injury stimulates both innate immune activation and chronic pain through the purinergic receptor P2RX7^2-4. It is unclear, however, whether this pathway influences the generation of immunological memory, a hallmark of the adaptive immune system that constitutes the basis of vaccines and protective immunity against re-infection^5,6. Here we show that P2RX7 is required for the establishment, maintenance and functionality of long-lived central and tissue-resident memory CD8⁺ T cell populations in mice. By contrast, P2RX7 is not required for the generation of short-lived effector CD8⁺ T cells. Mechanistically, P2RX7 promotes mitochondrial homeostasis and metabolic function in differentiating memory CD8⁺ T cells, at least in part by inducing AMP-activated protein kinase. Pharmacological inhibitors of P2RX7 provoked dysregulated metabolism and differentiation of activated mouse and human CD8⁺ T cells in vitro, and transient P2RX7 blockade in vivo ameliorated neuropathic pain but also compromised production of CD8⁺ memory T cells. These findings show that activation of P2RX7 by eATP provides a common currency that both alerts the nervous and immune system to tissue damage, and promotes the metabolic fitness and survival of the most durable and functionally relevant memory CD8⁺ T cell populations.

Adaptive NK Cells Resist Regulatory T-cell Suppression Driven by IL37

Natural killer (NK) cells are capable of fighting viral infections and cancer. However, these responses are inhibited by immune suppressor cells in the tumor microenvironment. Tumor progression promotes the recruitment and generation of intratumoral regulatory T cells (Treg), associated with a poor prognosis in cancer patients. Here, we show that canonical NK cells are highly susceptible to Treg-mediated suppression, in contrast to highly resistant CD57⁺ FcεRγ^-NKG2C⁺ adaptive (CD56⁺CD3^-) NK cells that expand in cytomegalovirus exposed individuals. Specifically, Tregs suppressed canonical but not adaptive NK-cell proliferation, IFNγ production, degranulation, and cytotoxicity. Treg-mediated suppression was associated with canonical NK-cell downregulation of TIM3, a receptor that activates NK-cell IFNγ production upon ligand engagement, and upregulation of the NK-cell inhibitory receptors PD-1 and the IL1 receptor family member, IL1R8 (SIGIRR or TIR8). Treg production of the IL1R8 ligand, IL37, contributed to the phenotypic changes and diminished function in Treg-suppressed canonical NK cells. Blocking PD-1, IL1R8, or IL37 abrogated Treg suppression of canonical NK cells while maintaining NK-cell TIM3 expression. Our data uncover new mechanisms of Treg-mediated suppression of canonical NK cells and identify that adaptive NK cells are inherently resistant to Treg suppression. Strategies to enhance the frequency of adaptive NK cells in the tumor microenvironment or to blunt Treg suppression of canonical NK cells will enhance the efficacy of NK-cell cancer immunotherapy. Cancer Immunol Res; 6(7); 766-75. ©2018 AACR.

Effects of MicroRNA on Regulatory T Cells and Implications for Adoptive Cellular Therapy to Ameliorate Graft-versus-Host Disease

Regulatory T cells (Tregs) are key mediators of the immune system. MicroRNAs (miRNAs) are a family of ~22 nucleotide non-coding RNAs that are processed from longer precursors by the RNases Drosha and Dicer. miRNA regulates protein expression posttranscriptionally through mRNA destabilization or translational silencing. A critical role for miRNA in Treg function was initially discovered when both Dicer and Drosha knockout (KO) mice were found to develop a fatal autoimmune disease phenotypically similar to Foxp3 KO mice.

Human in vitro-induced regulatory T cells display Dlgh1dependent and PKC-θ restrained suppressive activity

In vitro induced human regulatory T cells (iTregs) have demonstrated in vivo therapeutic utility, but pathways regulating their function have not been elucidated. Here, we report that human iTregs generated in vitro from naïve cord blood cells preferentially recruit Disc large homolog 1 (Dlgh1) and exclude protein kinase C (PKC)-θ from immunological synapses formed on supported lipid bilayers with laterally mobile ICAM-1 and anti-CD3 mAb. Also, iTregs display elevated Dlgh1 overall and Dlgh1-dependent p38 phosphorylation, higher levels of phosphatase and tensin homolog (PTEN), and diminished Akt phosphorylation. Pharmacological interruption of PKC-θ increases and Dlgh1 silencing decreases the ability of iTregs to suppress interferon-γ production by CD4⁺CD25^- effector T cells (Teff). Comparison with expanded cord blood-derived CD4⁺CD25^hi tTreg and expanded Teffs from the same donors indicate that iTreg are intermediate between expanded CD4⁺CD25^hi tTregs and Teffs, whereas modulation of suppressive activities by PKC-θ and Dlgh1 signaling pathways are shared.

A robust in vitro model for trans-lymphatic endothelial migration

Trans-endothelial migration (TEM) is essential for leukocyte circulation. While much is known about trans-blood endothelial migration, far less is known about trans-lymphatic endothelial migration. We established an in vitro system to evaluate lymphatic TEM for various cell types across primary mouse and human lymphatic endothelial cells (LEC), and validated the model for the murine LEC cell line SVEC4-10. T cells exhibited enhanced unidirectional migration from the basal (abluminal) to the apical (luminal) surface across LEC, whereas for blood endothelial cells (BEC) they migrated similarly in both directions. This preferential, vectorial migration was chemotactic toward many different chemoattractants and dose-dependent. Stromal protein fibers, interstitial type fluid flow, distribution of chemokines in the stromal layer, and inflammatory cytokines influenced LEC phenotype and leukocyte TEM. Activated and memory CD4 T cells, macrophages, and dendritic cell (DC) showed chemoattractantΔdriven vectorial migration, while CD8 T cell migration across LEC was not. The system was further validated for studying cancer cell transmigration across lymphatic endothelium. This model for lymphatic TEM for various migrating and endothelial cell types possesses the capacity to be high-throughput, highly reproducible and integrate the complexities of lymphatic biology, stromal variability, chemoattractant distribution, and fluid flow.

Toward revision of antimicrobial therapies in hematopoietic stem cell transplantation: target the pathogens, but protect the indigenous microbiota

Host microbiota plays important roles in providing colonization resistance to pathogens and instructing development and function of the immune system. Antibiotic treatments intended to target pathogens further weaken the host defenses and may paradoxically increase the risk of systemic infections. This consequence is especially problematic in patients undergoing hematopoietic stem cell transplantation, where the mucosal defenses are already weakened by the conditioning regimens. This review discusses the roles that indigenous microbiota plays in protecting the host and maintaining immune homeostasis. In addition, we highlight possible strategies that are being developed to allow targeted antimicrobial therapy against pathogens, while minimizing the harm to indigenous microbiota.

Protein Kinase C-θ and vimentin modulate multiple facets of Regulatory T-cell function

Regulatory T-cells (Tregs) prevent autoimmune and alloimmune reactions. Augmenting Treg function may enhance Treg therapies for these diseases. Treg-specific inhibition of protein kinase C-theta (PKC-θ) enhances Treg function. However, it is unclear whether PKC-θ inhibition can boost Treg function in systemic inflammatory conditions.

In a mouse model of acute GVHD, we found that Tregs treated with the PKC-θ inhibitor AEB071 reduced GVHD mortality and severity significantly better than DMSO treated Tregs. Compared to DMSO, AEB071 treated Tregs significantly reduced conventional T-cells (Tcon) proliferation on D4 after transplant. Multi-photon microscopy showed that AEB071 treated Tregs significantly increased Tcon velocity and displacement compared to DMSO.

Mechanistically, AEB071 augments expression of Neuropilin-1 (Nrp1) and Lymphocyte activation gene 3 (Lag3). Antibody blockade of Nrp1 and Lag3 in transwell suppression assays reduced the effect of AEB071 on Treg function. PKC-θ inhibition also reduces phosphorylation of mTORC2 targets FoxO3a and Akt phospho-site S473, but not mTORC1 targets S6, 4E-BP1 or Akt phospho-site T308. Compared to DMSO, AEB071 treatment significantly increased fatty acid uptake and oxygen consumption rate (OCR).

Phosphoproteomic analysis identified a significant alteration in the interaction between PKC-θ and the intermediate filament vimentin after AEB071 treatment, which was confirmed by confocal. Vimentin siRNA treatment also significantly reduces PKC-θ/vimentin interaction, increases Treg function, Nrp1 expression and OCR.

In summary, PKC-θ and vimentin modulate multiple aspects of Treg function, and altering these molecules may enhance the efficacy of Treg therapeutics.

Umbilical cord blood-derived T regulatory cells to prevent GVHD: kinetics, toxicity profile, and clinical effect

We studied the safety and clinical outcomes of patients treated with umbilical cord blood (UCB)-derived regulatory T cells (Tregs) that expanded in cultures stimulated with K562 cells modified to express the high-affinity Fc receptor (CD64) and CD86, the natural ligand of CD28 (KT64/86). Eleven patients were treated with Treg doses from 3-100 × 10(6) Treg/kg. The median proportion of CD4(+)FoxP3(+)CD127(-) in the infused product was 87% (range, 78%-95%), and we observed no dose-limiting infusional adverse events. Clinical outcomes were compared with contemporary controls (n = 22) who received the same conditioning regimen with sirolimus and mycophenolate mofetil immune suppression. The incidence of grade II-IV acute graft-versus-host disease (GVHD) at 100 days was 9% (95% confidence interval [CI], 0-25) vs 45% (95% CI, 24-67) in controls (P = .05). Chronic GVHD at 1 year was zero in Tregs and 14% in controls. Hematopoietic recovery and chimerism, cumulative density of infections, nonrelapse mortality, relapse, and disease-free survival were similar in the Treg recipients and controls. KT64/86-expanded UCB Tregs were safe and resulted in low risk of acute GVHD.

Evaluation of TCR Gene Editing Achieved by TALENs, CRISPR/Cas9, and megaTAL Nucleases

Present adoptive immunotherapy strategies are based on the re-targeting of autologous T-cells to recognize tumor antigens. As T-cell properties may vary significantly between patients, this approach can result in significant variability in cell potency that may affect therapeutic outcome. More consistent results could be achieved by generating allogeneic cells from healthy donors. An impediment to such an approach is the endogenous T-cell receptors present on T-cells, which have the potential to direct dangerous off-tumor antihost reactivity. To address these limitations, we assessed the ability of three different TCR-α-targeted nucleases to disrupt T-cell receptor expression in primary human T-cells. We optimized the conditions for the delivery of each reagent and assessed off-target cleavage. The megaTAL and CRISPR/Cas9 reagents exhibited the highest disruption efficiency combined with low levels of toxicity and off-target cleavage, and we used them for a translatable manufacturing process to produce safe cellular substrates for next-generation immunotherapies.

Scaffold protein Disc large homolog 1 is required for T-cell receptor-induced activation of regulatory T-cell function

Foxp3(+)CD4(+)CD25(high) regulatory T cell (Treg) suppression of inflammation depends on T-cell receptor-mediated Nuclear Factor of Activated T cells c1 (NFATc1) activation with reduced Akt activity. We investigated the role of the scaffold protein Disc large homolog 1 (Dlgh1) in linking the T-cell receptor to this unique signaling outcome. The Treg immunological synapse (IS) recruited fourfold more Dlgh1 than conventional CD4(+) T-cell IS. Tregs isolated from patients with active rheumatoid arthritis, or treated with tumor necrosis factor-α, displayed reduced function and diminished Dlgh1 recruitment to the IS. Furthermore, Dlgh1 silencing abrogated Treg function, impaired NFATc1 activation, reduced phosphatase and tensin homolog levels, and increased Akt activation. Dlgh1 operates independently of the negative feedback pathway mediated by the related adapter protein Carma1 and thus presents an array of unique targets to selectively manipulate Treg function.

Clinical perspectives for regulatory T cells in transplantation tolerance

Three main types of CD4+ regulatory T cells can be distinguished based upon whether they express Foxp3 and differentiate naturally in the thymus (natural Tregs) or are induced in the periphery (inducible Tregs); or whether they are FoxP3 negative but secrete IL-10 in response to antigen (Tregulatory type 1, Tr1 cells). Adoptive transfer of each cell type has proven highly effective in mouse models at preventing graft vs. host disease (GVHD) and autoimmunity. Although clinical application was initially hampered by low Treg frequency and unfavorable ex vivo expansion properties, several phase I trials are now being conducted to assess their effect on GVHD following hematopoietic stem cell transplantation (HSCT) and in type I diabetes. Human Treg trials for HSCT recipients have preceded other indications because GVHD onset is precisely known, the time period needed for prevention relatively short, initial efficacy is likely to provide life-long protection, and complications of GVHD can be lethal. This review will summarize the clinical trials conducted to date that have employed Tregs to prevent GVHD following HSCT and discuss recent advances in Treg cellular therapy.

Massive ex vivo expansion of human natural regulatory T cells (T(regs)) with minimal loss of in vivo functional activity

Graft-versus-host disease (GVHD) is a frequent and severe complication after hematopoietic cell transplantation. Natural CD4(+)CD25(+) regulatory T cells (nT(regs)) have proven highly effective in preventing GVHD and autoimmunity in murine models. Yet, clinical application of nT(regs) has been severely hampered by their low frequency and unfavorable ex vivo expansion properties. Previously, we demonstrated that umbilical cord blood (UCB) nT(regs) could be purified and expanded in vitro using good manufacturing practice (GMP) reagents; however, the initial number of nT(regs) in UCB units is limited, and average yield after expansion was only 1 × 10(9) nT(regs). Therefore, we asked whether yield could be increased by using peripheral blood (PB), which contains far larger quantities of nT(regs). PB nT(regs) were purified under GMP conditions and expanded 80-fold to yield 19 × 10(9) cells using anti-CD3 antibody-loaded, cell-based artificial antigen-presenting cells (aAPCs) that expressed the high-affinity Fc receptor and CD86. A single restimulation increased expansion to ~3000-fold and yield to >600 × 10(9) cells while maintaining Foxp3 expression and suppressor function. nT(reg) expansion was ~50 million-fold when flow sort-purified nT(regs) were restimulated four times with aAPCs. Indeed, cryopreserved donor nT(regs) restimulated four times significantly reduced GVHD lethality induced by the infusion of human T cells into immune-deficient mice. The capability to efficiently produce donor cell banks of functional nT(regs) could transform the treatment of GVHD and autoimmunity by providing an off-the-shelf, cost-effective, and proven cellular therapy.

Repression of the genome organizer SATB1 in regulatory T cells is required for suppressive function and inhibition of effector differentiation

Regulatory T cells (T(reg) cells) are essential for self-tolerance and immune homeostasis. Lack of effector T cell (T(eff) cell) function and gain of suppressive activity by T(reg) cells are dependent on the transcriptional program induced by Foxp3. Here we report that repression of SATB1, a genome organizer that regulates chromatin structure and gene expression, was crucial for the phenotype and function of T(reg) cells. Foxp3, acting as a transcriptional repressor, directly suppressed the SATB1 locus and indirectly suppressed it through the induction of microRNAs that bound the SATB1 3' untranslated region. Release of SATB1 from the control of Foxp3 in T(reg) cells caused loss of suppressive function, establishment of transcriptional T(eff) cell programs and induction of T(eff) cell cytokines. Our data support the proposal that inhibition of SATB1-mediated modulation of global chromatin remodeling is pivotal for maintaining T(reg) cell functionality.

Infusion of ex vivo expanded T regulatory cells in adults transplanted with umbilical cord blood: safety profile and detection kinetics

Acute graft-versus-host disease (aGVHD) is associated with high risk of morbidity and mortality and is a common complication after double umbilical cord blood (UCB) transplantation. To reduce these risks, we established a method of CD4(+)CD25(+)FoxP3(+) T regulatory cell (Treg) enrichment from cryopreserved UCB followed by a 18 (+) 1-day expansion culture including anti-CD3/anti-CD28 antibody-coated beads and recombinant human interleukin-2. In a "first-in-human" clinical trial, we evaluated the safety profile of UCB Treg in 23 patients. Patients received a dose of 0.1-30 × 10(5)UCB Treg/kg after double UCB transplantation. The targeted Treg dose was achieved in 74% of cultures, with all products being suppressive in vitro (median 86% suppression at a 1:4 ratio). No infusional toxicities were observed. After infusion, UCB Treg could be detected for 14 days, with the greatest proportion of circulating CD4(+)CD127(-)FoxP3(+) cells observed on day (+)2. Compared with identically treated 108 historical controls without Treg, there was a reduced incidence of grade II-IV aGVHD (43% vs 61%, P = .05) with no deleterious effect on risks of infection, relapse, or early mortality. These results set the stage for a definitive study of UCB Treg to determine its potency in preventing allogeneic aGVHD. This study is registered at http://www.clinicaltrials.gov as NCT00602693.

Protein kinase C-theta mediates negative feedback on regulatory T cell function

T cell receptor (TCR)-dependent regulatory T cell (Treg) activity controls effector T cell (Teff) function and is inhibited by the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha). Protein kinase C-theta (PKC-theta) recruitment to the immunological synapse is required for full Teff activation. In contrast, PKC-theta was sequestered away from the Treg immunological synapse. Furthermore, PKC-theta blockade enhanced Treg function, demonstrating PKC-theta inhibits Treg-mediated suppression. Inhibition of PKC-theta protected Treg from inactivation by TNF-alpha, restored activity of defective Treg from rheumatoid arthritis patients, and enhanced protection of mice from inflammatory colitis. Treg freed of PKC-theta-mediated inhibition can function in the presence of inflammatory cytokines and thus have therapeutic potential in control of inflammatory diseases.

Immune regulatory cells in umbilical cord blood: T regulatory cells and mesenchymal stromal cells

A major goal in haematopoietic cell transplantation (HCT) is to retain the lymphohaematopoietic potential of the cell transfer without its side effects. In addition to the physical injury caused by the conditioning regimen, donor T cells can react to alloantigens of the recipient and cause graft-versus-host disease (GVHD), which accounts for the largest share of morbidity and mortality after HCT. Immune modulator cells, such as regulatory T cells (Tregs) and mesenchymal stromal cells (MSCs) have shown promise in their ability to control GVHD and yet, in preclinical models, preserve the graft-versus-malignancy effect. Initially, MSCs and Tregs have been isolated from adult sources, such as bone marrow or peripheral blood, respectively. More recent studies have indicated that umbilical cord blood (UCB) is a rich source of both cell types. We will review the current data on UCB-derived Tregs and MSCs and their therapeutic implications.

Progress towards Understanding the Genetic Pathogenesis of Systemic Lupus Erythematosus

In order to better understand the genetic factors that initiate systemic lupus erythematosus (SLE), we are using both linkage and association approaches to identify susceptibility genes for the disease. Association studies have recently identified three HLA Class II haplotypes as well as a functional missense polymorphism in protein tyrosine phosphatase (PTP) PTPN22 as important risk alleles for SLE. Here, we will review these data, and explain how these findings contribute to an understanding of the genetic architecture of human SLE.

B cell receptor basal signaling regulates antigen-induced Ig light chain rearrangements

BCR editing in the bone marrow contributes to B cell tolerance by orchestrating secondary Ig rearrangements in self-reactive B cells. We have recently shown that loss of the BCR or a pharmacologic blockade of BCR proximal signaling pathways results in a global "back-differentiation" response in which immature B cells down-regulate genes important for the mature B cell program and up-regulate genes characteristic of earlier stages of B cell development. These observations led us to test the hypothesis that self-Ag-induced down-regulation of the BCR, and not self-Ag-induced positive signals, lead to Rag induction and hence receptor editing. Supporting this hypothesis, we found that immature B cells from xid (x-linked immunodeficiency) mice induce re-expression of a Rag2-GFP bacterial artificial chromosome reporter as well as wild-type immature B cells following Ag incubation. Incubation of immature B cells with self-Ag leads to a striking reversal in differentiation to the pro-/pre-B stage of development, consistent with the idea that back-differentiation results in the reinduction of genes required for L chain rearrangement and receptor editing. Importantly, Rag induction, the back-differentiation response to Ag, and editing in immature and pre-B cells are inhibited by a combination of phorbol ester and calcium ionophore, agents that bypass proximal signaling pathways and mimic BCR signaling. Thus, mimicking positive BCR signals actually inhibits receptor editing. These findings support a model whereby Ag-induced receptor editing is inhibited by BCR basal signaling on developing B cells; BCR down-regulation removes this basal signal, thereby initiating receptor editing.

Attenuation of IL-7 Receptor Signaling Is Not Required for Allelic Exclusion

Allelic exclusion prevents pre-B cells from generating more than one functional H chain, thereby ensuring the formation of a unique pre-BCR. The signaling processes underlying allelic exclusion are not clearly understood. IL-7R-dependent signals have been clearly shown to regulate the accessibility of the Ig H chain locus. More recent work has suggested that pre-BCR-dependent attenuation of IL-7R signaling returns the H chain loci to an inaccessible state; this process has been proposed to underlie allelic exclusion. Importantly, this model predicts that preventing pre-BCR-dependent down-regulation of IL-7R signaling should interfere with allelic exclusion. To test this hypothesis, we made use of transgenic mice that express a constitutively active form of STAT5b (STAT5b-CA). STAT5b-CA expression restores V(D)J recombination in IL-7R(-/-) B cells, demonstrating that IL-7 regulates H chain locus accessibility and V(D)J recombination via STAT5 activation. To examine the effects of constitutively active STAT5b on allelic exclusion, we crossed STAT5b-CA mice (which express the IgM(b) allotype) to IgM(a) allotype congenic mice. We found no difference in the percentage of IgM(a)/IgM(b)-coexpressing B cells in STAT5b-CA vs littermate control mice; identical results were observed when crossing STAT5b-CA mice with hen egg lysozyme (HEL) H chain transgenic mice. The HEL transgene enforces allelic exclusion, preventing rearrangement of endogenous H chain genes; importantly, rearrangement of endogenous H chain genes was suppressed to a similar degree in STAT5b-CA vs HEL mice. Thus, attenuation of IL-7R/STAT5 signaling is not required for allelic exclusion.

Genomic view of systemic autoimmunity in MRLlpr mice

MRLlpr mice develop spontaneous systemic autoimmunity with many hallmarks of the human disease systemic lupus erythematosus. Although a variety of genes have been implicated in this model, disease pathogenesis is still poorly understood. In an effort to identify novel genes and pathways, we performed genome-wide mRNA expression analysis in the spleens and kidneys of MRLlpr mice throughout the disease course. Samples were collected from cohorts of C57BL/6, MRL+/+ and MRLlpr mice, and profiled by flow cytometry and gene expression microarrays. Serum autoantibodies and renal pathology were studied in parallel. We identified 236 genes in MRLlpr spleen that showed significant threefold or greater changes in expression between 6 and 20 weeks. Of interest, a number of interferon-responsive genes were expressed early, and remained dysregulated throughout the disease course. Many chemokines, cell surface proteins, transcription factors and cytokines, including IFN-gamma, also showed altered expression as disease progressed. Analysis of kidneys indicated the presence of severe inflammation that coincided with evidence for changes in kidney function and elevated expression of IFN-inducible genes, complement components and antigen presentation genes. These data provide a unique genomic view of the progression to fatal autoimmunity in MRLlpr mice, and provide new candidate genes and pathways to explore.

In vivo assessment of the relative contributions of deletion, anergy, and editing to B cell self-tolerance

In normal B cell development, a large percentage of newly formed cells bear receptors with high levels of self-reactivity that must be tolerized before entry into the mature B cell pool. We followed the fate of self-reactive B cells expressing high affinity anti-hen egg lysozyme (HEL) Ag receptors exposed in vivo to membrane HEL in a setting in which the anti-HEL L chain was "knocked-in" at the endogenous L chain locus. These mice demonstrated extensive and efficient L chain receptor editing responses and had B cell numbers comparable to those found in animals lacking membrane Ag. BrdU labeling indicated that the time required for editing in response to membrane HEL was approximately 6 h. In mice transgenic for soluble HEL, anti-HEL B cells capable of editing showed evidence for both editing and anergy. These data identify receptor editing as a major physiologic mechanism by which highly self-reactive B cells are tolerized to membrane and soluble self-Ags.

Basal immunoglobulin signaling actively maintains developmental stage in immature B cells

In developing B lymphocytes, a successful V(D)J heavy chain (HC) immunoglobulin (Ig) rearrangement establishes HC allelic exclusion and signals pro-B cells to advance in development to the pre-B stage. A subsequent functional light chain (LC) rearrangement then results in the surface expression of IgM at the immature B cell stage. Here we show that interruption of basal IgM signaling in immature B cells, either by the inducible deletion of surface Ig via Cre-mediated excision or by incubating cells with the tyrosine kinase inhibitor herbimycin A or the phosphatidylinositol 3-kinase inhibitor wortmannin, led to a striking “back-differentiation” of cells to an earlier stage in B cell development, characterized by the expression of pro-B cell genes. Cells undergoing this reversal in development also showed evidence of new LC gene rearrangements, suggesting an important role for basal Ig signaling in the maintenance of LC allelic exclusion. These studies identify a previously unappreciated level of plasticity in the B cell developmental program, and have important implications for our understanding of central tolerance mechanisms.

Gene rearrangement is a hallmark of B cell maturation. By interrupting basal cell signaling through the rearranged IgM receptor, immature B cells "back-differentiate" to an earlier stage in their development

Fatal Acute Lymphoblastic Leukemia in Mice Transgenic for B Cell-Restricted bcl-xLand c-myc

Expression of the c-myc gene is frequently dysregulated in malignant tumors and translocations of c-myc into the Ig H chain locus are associated with Burkitt's-type lymphoma. There is indirect evidence that bcl-x, an anti-apoptotic member of the bcl-2 gene family, may also contribute to a variety of B lymphoid tumors. In this study, we show that mice transgenic for both B cell-restricted c-myc and bcl-x(L) developed aggressive, acute leukemias expressing early B lineage and stem cell surface markers. Of interest, the tumor cells proliferated and differentiated down the B cell developmental pathway following in vitro treatment with IL-7. Analysis of sorted leukemic cells from spleen indicated constitutive expression of sterile micro and kappa transcripts in combination with evidence for D-J(H) DNA rearrangements. Several B cell-specific genes were either not expressed or were expressed at low levels in primary tumor cells and were induced following culture with IL-7. IL-7 also increased V-Jkappa and V-DJ(H) rearrangements. These data demonstrate oncogenic synergy between c-myc and bcl-x(L) in a new mouse model for acute lymphoblastic leukemia. Tumors in these animals target an early stage in B cell development characterized by the expression of both B lineage and stem cell genes.

Late immature B cells (IgMhighIgDneg) undergo a light chain receptor editing response to soluble self-antigen

Receptor editing is an important mechanism to modify the Ag specificity of newly developing B cells that are reactive with self-Ags. Previous studies have suggested that late immature B cells, bearing high levels of IgM on their cell surface, are unable to initiate receptor editing and instead are deleted by apoptosis. Using the hen egg lysozyme transgenic system, we show that IgM(high) late-immature B cells are fully capable of receptor editing to soluble self-Ag. This was demonstrated by the induction of new endogenous light chain locus rearrangements and by a single-cell flow cytometric assay using a recombination-activating gene 2-green fluorescence protein reporter transgene. These studies suggest that the developmental window available for immature B cells to edit their Ig receptors, at least in response to certain soluble Ags, extends through the IgM(high) late immature B cell stage.

Visualization of the genesis and fate of isotype-switched B cells during a primary immune response

The life history of isotype-switched B cells is unclear, in part, because of an inability to detect rare antigen-specific B cells at early times during the immune response. To address this issue, a small population of B cells carrying targeted antibody transgenes capable of class switching was monitored in immunized mice. After contacting helper T cells, the first switched B cells appeared in follicles rather than in the red pulp, as was expected. Later, some of the switched B cells transiently occupied the red pulp and marginal zone, whereas others persisted in germinal centers (GCs). Antigen-experienced IgM B cells were rarely found in GCs, indicating that these cells switched rapidly after entering GCs or did not persist in this environment.

Ig light chain receptor editing in anergic B cells

Receptor editing in the bone marrow (BM) serves to modify the Ag receptor specificity of immature self-reactive B cells, while anergy functionally silences self-reactive clones. Here, we demonstrate that anergic B cells in hen egg lysozyme Ig (HEL-Ig)/soluble HEL double transgenic mice show evidence of having undergone receptor editing in vivo, as demonstrated by the presence of elevated levels of endogenous kappa light chain rearrangements in the BM and spleen. In an in vitro IL-7-driven BM culture system, HEL-Ig BM B cells grown in the presence of soluble HEL down-regulated surface IgM expression and also showed induction of new endogenous kappa light chain rearrangements. Using a panel of soluble protein ligands with reduced affinity for the HEL-Ig receptor, the editing response was shown to correlate in a dose-dependent fashion with the strength of signaling through the B cell receptor. The finding that the level of B cell receptor cross-linking sufficient to induce anergy in B cells is also capable of engaging the machinery required for receptor editing suggests an intimate relationship between these two mechanisms in maintaining B cell tolerance.

CD5 maintains tolerance in anergic B cells

Clonal anergy of autoreactive B cells is a key mechanism regulating tolerance. Here, we show that anergic B cells express significant surface levels of CD5, a molecule normally found on T cells and a subset of B-1 cells. Breeding of the hen egg lysozyme (HEL) transgenic model for B cell anergy onto the CD5 null background resulted in a spontaneous loss of B cell tolerance in vivo. Evidence for this included elevated levels of anti-HEL immunoglobulin M (IgM) antibodies in the serum of CD5(-/-) mice transgenic for both an HEL-specific B cell receptor (BCR) and soluble lysozyme. "Anergic" B cells lacking CD5 also showed enhanced proliferative responses in vitro and elevated intracellular Ca(2+) levels at rest and after IgM cross-linking. These data support the hypothesis that CD5 negatively regulates Ig receptor signaling in anergic B cells and functions to inhibit autoimmune B cell responses.

Fc gammaRIIB1 inhibition of BCR-mediated phosphoinositide hydrolysis and Ca2+ mobilization is integrated by CD19 dephosphorylation

The B cell receptor for immunoglobulin G, Fc gammaRIIB1, is a potent transducer of signals that block antigen-induced B cell activation. Coligation of Fc gammaRIIB1 with B lymphocyte antigen receptors (BCR) causes premature termination of phosphoinositide hydrolysis and Ca2+ mobilization and inhibits proliferation. This inhibitory signal is mediated in part by phosphorylation of Fc gammaRIIB1 and recruitment of phosphatases; however, the molecular target(s) of effectors is unknown. Here we report that Fc gammaRIIB1 inhibition of BCR signaling is mediated in part by selective dephosphorylation of CD19, a BCR accessory molecule and coreceptor. CD19 dephosphorylation leads to failed CD19 association with phosphatidylinositol 3-kinase, and this in turn leads to termination of inositol-1,4,5-trisphosphate production, intracellular Ca2+ release, and Ca2+ influx. The results define a molecular circuit by which Fc gammaRIIB signals block phosphoinositide hydrolysis.

Distinct mechanisms mediate SHC association with the activated and resting B cell antigen receptor

Ligation of the B cell antigen receptor (BCR) complex initiates tyrosine phosphorylation of the receptor's transducer components, Ig-alpha and Ig-beta and tyrosine kinase-dependent accumulation of GTP-bound, activated p21ras. The mechanism of receptor coupling to p21ras activation and the roles of Ig-alpha and Ig-beta are unknown. The results reported here indicate that the resting, nonphosphorylated BCR associates with the Grb-2/Sos-linker SHC via the Ig-alpha immunoreceptor-based tyrosine activation motif (ITAM). Ig-alpha specificity of this interaction is determined by the sequence DCSM found in Ig-alpha, but not Ig-beta. Tyrosine phosphorylation of Ig-alpha and Ig-beta ITAM allows recruitment of SHC, which now binds directly to both Ig-alpha and Ig-beta via a phosphotyrosine/SH2 interaction. In confirmation of recent studies by Saxton et al. (J. Immunol. 1994. 153: 623) receptor ligation leads to tyrosine phosphorylation of SHC and to the formation of a phospho-SHC/Grb2/Sos complex. In view of previous studies which demonstrated p21ras co-capping with ligated BCR, the data presented here suggest that Ig-alpha/beta- and SHC tyrosine phosphorylation-dependent recruitment of the Grb2/Sos complex to the receptor can occur and may provide a mechanism by which the nucleotide exchange activity of Sos could mediate activation of BCR-localized p21ras.

Human and mouse killer-cell inhibitory receptors recruit PTP1C and PTP1D protein tyrosine phosphatases

NK cells express cell surface receptors for MHC class I proteins (KIR). Engagement of these receptors inhibits NK cell cytotoxic programs. KIR can be expressed on T cells, and their engagement also results in inhibition of effector functions initiated by the CD3/TCR complex. While human KIR genes belong to the Ig gene superfamily, mouse KIR belong to a family of dimeric lectins. Despite these distinct evolutionary origins, we show here that both HLA-Cw3-specific human p58.183 receptors and H-2D d/k-specific mouse Ly49A receptors recruit the same protein tyrosine phosphatases, PTP1C and PTP1D, upon phosphorylation of critical intracytoplasmic tyrosine residues. These results document a common pathway by which diverse KIR can down-regulate NK and T cell activation programs, and further define the sequence of the immunoreceptor tyrosine-based inhibitory motif (ITIM), initially described in FcgammaRIIB1, and expressed in both human and mouse KIR.

Human and mouse killer-cell inhibitory receptors recruit PTP1C and PTP1D protein tyrosine phosphatases

NK cells express cell surface receptors for MHC class I proteins (KIR). Engagement of these receptors inhibits NK cell cytotoxic programs. KIR can be expressed on T cells, and their engagement also results in inhibition of effector functions initiated by the CD3/TCR complex. While human KIR genes belong to the Ig gene superfamily, mouse KIR belong to a family of dimeric lectins. Despite these distinct evolutionary origins, we show here that both HLA-Cw3-specific human p58.183 receptors and H-2D d/k-specific mouse Ly49A receptors recruit the same protein tyrosine phosphatases, PTP1C and PTP1D, upon phosphorylation of critical intracytoplasmic tyrosine residues. These results document a common pathway by which diverse KIR can down-regulate NK and T cell activation programs, and further define the sequence of the immunoreceptor tyrosine-based inhibitory motif (ITIM), initially described in FcgammaRIIB1, and expressed in both human and mouse KIR.

Recruitment and activation of PTP1C in negative regulation of antigen receptor signaling by Fc gamma RIIB1

Coligation of the Fc receptor on B cells, Fc gamma RIIB1, with the B cell antigen receptor (BCR) leads to abortive BCR signaling. Here it was shown that the Fc gamma RIIB1 recruits the phosphotyrosine phosphatase PTP1C after BCR coligation. This association is mediated by the binding of a 13-amino acid tyrosine-phosphorylated sequence to the carboxyl-terminal Src homology 2 domain of PTP1C and activates PTP1C. Inhibitory signaling and PTP1C recruitment are dependent on the presence of the tyrosine within the 13-amino acid sequence. Inhibitory signaling mediated by Fc gamma RIIB1 is deficient in motheaten mice which do not express functional PTP1C. Thus, PTP1C is an effector of BCR-Fc gamma RIIB1 negative signal cooperativity.

Acidic residues are involved in substrate recognition by two soluble protein tyrosine phosphatases, PTP-5 and rrbPTP-1

The mechanisms for substrate recognition by two cytoplasmic protein tyrosine phosphatases, PTP-5 and rrbPTP-1, were investigated. Phosphorylation sites on tyrosine-phosphorylated casein, a model PTP substrate, were characterized. Two peptides based on casein phosphorylation sites and one peptide based on the tyrosine phosphorylation site of reduced, carboxamidomethylated and maleylated (RCM) lysozyme were tested as PTP substrates. The three peptides were dephosphorylated by PTP-5 and rrbPTP-1 at rates comparable to those of the corresponding sites on the intact proteins. This indicates that peptides based on the two model PTP substrates, casein and RCM-lysozyme, contained all or most of the structural information necessary for PTP-5 and rrbPTP-1 substrate recognition. Structural elements required for substrate recognition by PTP-5 and rrbPTP-1 were also investigated. Km values for dephosphorylation of three simple aromatic phosphate esters (phosphotyrosine, p-nitrophenyl phosphate, and phenyl phosphate) by rrbPTP-1 were about 5000-fold higher than those obtained for the peptide and protein substrates. This indicates that recognition of protein and peptide substrates involves structural elements in addition to the phosphate group and the aromatic tyrosine ring of phosphotyrosine. Analysis of the effects of truncations and Ala for polar substitutions on the reactivity with PTP-5 and rrbPTP-1 of peptides based on casein, RCM-lysozyme, and angiotensin II indicated that Asp or Glu within the first five residues on the N-terminal side of phosphotyrosine increased peptide reactivity with both PTP's. Asn residues were unable or only weakly able to substitute for Asp residues.(ABSTRACT TRUNCATED AT 250 WORDS)