Recognizing Complexity of CD8 T Cells in Transplantation

The major role of CD8+ T cells in clinical and experimental transplantation is well documented and acknowledged. Nevertheless, the precise impact of CD8+ T cells on graft tissue injury is not completely understood, thus impeding the development of specific treatment strategies. The goal of this overview is to consider the biology and functions of CD8+ T cells in the context of experimental and clinical allotransplantation, with special emphasis on how this cell subset is affected by currently available and emerging therapies.

p40 homodimers bridge ischemic tissue inflammation and heterologous alloimmunity in mice via IL-15 transpresentation

Virus-induced memory T cells often express functional cross-reactivity, or heterologous immunity, to other viruses and to allogeneic MHC molecules that is an important component of pathogenic responses to allogeneic transplants. During immune responses, antigen-reactive naive and central memory T cells proliferate in secondary lymphoid organs to achieve sufficient cell numbers to effectively respond, whereas effector memory T cell proliferation occurs directly within the peripheral inflammatory microenvironment. Mechanisms driving heterologous memory T cell proliferation and effector function expression within peripheral tissues remain poorly understood. Here, we dissected proliferation of heterologous donor-reactive memory CD8+ T cells and their effector functions following infiltration into heart allografts with low or high intensities of ischemic inflammation. Proliferation within both ischemic conditions required p40 homodimer-induced IL-15 transpresentation by graft DCs, but expression of effector functions mediating acute allograft injury occurred only in high-ischemic allografts. Transcriptional responses of heterologous donor-reactive memory CD8+ T cells were distinct from donor antigen-primed memory CD8+ T cells during early activation in allografts and at graft rejection. Overall, the results provide insights into mechanisms driving heterologous effector memory CD8+ T cell proliferation and the separation between proliferation and effector function that is dependent on the intensity of inflammation within the tissue microenvironment.

Advancing mouse models for transplantation research

Mouse models have been instrumental in understanding mechanisms of transplant rejection and tolerance, but cross-study reproducibility and translation of experimental findings into effective clinical therapies are issues of concern. The Mouse Models in Transplantation symposium gathered scientists and physician-scientists involved in basic and clinical research in transplantation to discuss the strengths and limitations of mouse transplant models and strategies to enhance their utility. Participants recognized that increased procedure standardization, including the use of prespecified, defined endpoints, and statistical power analyses, would benefit the field. They also discussed the generation of new models that incorporate environmental and genetic variables affecting clinical outcomes as potentially important. If implemented, these strategies are expected to improve the reproducibility of mouse studies and increase their translation to clinical trials and, ideally, new Food and Drug Administration-approved drugs.

Select symbionts drive high IgA levels in the mouse intestine

Immunoglobulin A (IgA) is an important factor in maintaining homeostasis at mucosal surfaces, yet luminal IgA levels vary widely. Total IgA levels are thought to be driven by individual immune responses to specific microbes. Here, we found that the prebiotic, pectin oligosaccharide (pec-oligo), induced high IgA levels in the small intestine in a T cell-dependent manner. Surprisingly, this IgA-high phenotype was retained after cessation of pec-oligo treatment, and microbiome transmission either horizontally or vertically was sufficient to retain high IgA levels in the absence of pec-oligo. Interestingly, the bacterial taxa enriched in the overall pec-oligo bacterial community differed from IgA-coated microbes in this same community. Rather, a group of ethanol-resistant microbes, highly enriched for Lachnospiraceae bacterium A2, drove the IgA-high phenotype. These findings support a model of intestinal adaptive immunity in which a limited number of microbes can promote durable changes in IgA directed to many symbionts.

Sex-Biased T-cell Exhaustion Drives Differential Immune Responses in Glioblastoma

Sex differences in glioblastoma (GBM) incidence and outcome are well recognized, and emerging evidence suggests that these extend to genetic/epigenetic and cellular differences, including immune responses. However, the mechanisms driving immunologic sex differences are not fully understood. Here, we demonstrate that T cells play a critical role in driving GBM sex differences. Male mice exhibited accelerated tumor growth, with decreased frequency and increased exhaustion of CD8+ T cells in the tumor. Furthermore, a higher frequency of progenitor exhausted T cells was found in males, with improved responsiveness to anti-PD-1 treatment. Moreover, increased T-cell exhaustion was observed in male GBM patients. Bone marrow chimera and adoptive transfer models indicated that T cell-mediated tumor control was predominantly regulated in a cell-intrinsic manner, partially mediated by the X chromosome inactivation escape gene Kdm6a. These findings demonstrate that sex-biased predetermined behavior of T cells is critical for inducing sex differences in GBM progression and immunotherapy response.

SIGNIFICANCE

Immunotherapies in patients with GBM have been unsuccessful due to a variety of factors, including the highly immunosuppressive tumor microenvironment in GBM. This study demonstrates that sex-biased T-cell behaviors are predominantly intrinsically regulated, further suggesting sex-specific approaches can be leveraged to potentially improve the therapeutic efficacy of immunotherapy in GBM. See related commentary by Alspach, p. 1966. This article is featured in Selected Articles from This Issue, p. 1949.

Autoantibodies against DNA topoisomerase I promote renal allograft rejection by increasing alloreactive T cell responses

Antibodies reactive to self-antigens are an important component of posttransplant immune responses. The generation requirements and functions of autoantibodies, as well as the mechanisms of their influence on alloimmune responses, still remain to be determined. Our study investigated the contribution of autoimmunity during rejection of renal allografts. We have previously characterized a mouse model in which the acute rejection of a life-supporting kidney allograft is mediated by antibodies. At rejection, recipient sera screening against >4000 potential autoantigens revealed DNA topoisomerase I peptide 205-219 (TI-I205-219) as the most prominent epitope. Subsequent analysis showed TI-I205-219-reactive autoantibodies are induced in nonsensitized recipients of major histocompatibility complex-mismatched kidney allografts in a T cell-dependent manner. Immunization with TI-I205-219 broke self-tolerance, elicited TI-I205-219 immunoglobin G autoantibodies, and resulted in acute rejection of allogeneic but not syngeneic renal transplants. The graft loss was associated with increased priming of donor-reactive T cells but not with donor-specific alloantibodies elevation. Similarly, passive transfer of anti-TI-I205-219 sera following transplantation increased donor-reactive T cell activation with minimal effects on donor-specific alloantibody levels. The results identify DNA topoisomerase I as a novel self-antigen in transplant settings and demonstrate that autoantibodies enhance activation of donor-reactive T cells following renal transplantation.

Failure of Costimulatory Blockade-induced Regulatory T Cells to Sustain Long-term Survival of High Ischemic Allografts

BACKGROUND

Costimulatory blockade-induced allograft tolerance has been achieved in rodent models, but these strategies do not translate well to nonhuman primate and clinical transplants. One confounder that may underlie this discrepancy is the greater ischemic inflammation imposed on the transplants. In mice, cardiac allografts subjected to prolonged cold ischemic storage (CIS) before transplant have increased ischemia-reperfusion injury, which amplifies infiltrating endogenous memory CD8 T-cell activation within hours after transplantation to mediate acute graft inflammation and cytotoxic lymphocyte-associated molecule-4 immunoglobulin-resistant rejection. This study tested strategies inhibiting memory CD8 T-cell activation within such high ischemic allografts to achieve long-term survival.

METHODS

A/J (H-2 a ) hearts subjected to 0.5 or 8 h of CIS were transplanted to C57BL/6 (H-2 b ) recipients and treatment with peritransplant costimulatory blockade. At 60 d posttransplant, regulatory T cells (Treg) were depleted in recipients of high ischemic allografts with anti-CD25 monoclonal antibody (mAb) or diphtheria toxin.

RESULTS

Whereas peritransplant (days 0 and +1) anti-lymphocyte function-associated antigen-1 mAb and anti-CD154 mAb prolonged survival of >60% allografts subjected to minimal CIS for >100 d, only 20% of allografts subjected to prolonged CIS survived beyond day 80 posttransplant and rejection was accompanied by high titers of donor-specific antibody. Peritransplant anti-lymphocyte function-associated antigen-1, anti-tumor necrosis factor-α, and anti-CD154 mAb plus additional anti-CD154 mAb on days 14 and 16 obviated this donor-specific antibody and promoted Treg-mediated tolerance and survival of 60% of high ischemic allografts beyond day 100 posttransplant, but all allografts failed by day 120.

CONCLUSIONS

These studies indicate a strategy inducing prolonged high ischemic allograft survival through Treg-mediated tolerance that is not sustained indefinitely.

Water channel aquaporin 4 is required for T cell receptor mediated lymphocyte activation

Aquaporins are a family of ubiquitously expressed transmembrane water channels implicated in a broad range of physiological functions. We have previously reported that aquaporin 4 (AQP4) is expressed on T cells and that treatment with a small molecule AQP4 inhibitor significantly delays T cell mediated heart allograft rejection. Using either genetic deletion or small molecule inhibitor, we show that AQP4 supports T cell receptor mediated activation of both mouse and human T cells. Intact AQP4 is required for optimal T cell receptor (TCR)-related signaling events, including nuclear translocation of transcription factors and phosphorylation of proximal TCR signaling molecules. AQP4 deficiency or inhibition impairs actin cytoskeleton rearrangements following TCR crosslinking, causing inferior TCR polarization and a loss of TCR signaling. Our findings reveal a novel function of AQP4 in T lymphocytes and identify AQP4 as a potential therapeutic target for preventing TCR-mediated T cell activation.

Macrophage-inducible C-type lectin activates B cells to promote T cell reconstitution in heart allograft recipients

Diminishing homeostatic proliferation of memory T cells is essential for improving the efficacy of lymphoablation in transplant recipients. Our previous studies in a mouse heart transplantation model established that B lymphocytes secreting proinflammatory cytokines are critical for T cell recovery after lymphoablation. The goal of the current study was to identify mediators of B cell activation following lymphoablation in allograft recipients. Transcriptome analysis revealed that macrophage-inducible C-type lectin (Mincle, Clec4e) expression is up-regulated in B cells from heart allograft recipients treated with murine anti-thymocyte globulin (mATG). Recipient Mincle deficiency diminishes B cell production of pro-inflammatory cytokines and impairs T lymphocyte reconstitution. Mixed bone marrow chimeras lacking Mincle only in B lymphocytes have similar defects in T cell recovery. Conversely, treatment with a synthetic Mincle ligand enhances T cell reconstitution after lymphoablation in non-transplanted mice. Treatment with agonistic CD40 mAb facilitates T cell reconstitution in CD4 T cell-depleted, but not in Mincle-deficient, recipients indicating that CD40 signaling induces T cell proliferation via a Mincle-dependent pathway. These findings are the first to identify an important function of B cell Mincle as a sensor of damage-associated molecular patterns released by the graft and demonstrate its role in clinically relevant settings of organ transplantation.

The Role of LAG3 in Antibody Responses To Kidney Transplantation

The role of lymphocyte activation gene-3 (LAG3) in T cell functions is well studied, however its role in humoral immune responses remains poorly characterized. The goal of this study was to test the role of recipient LAG3 in a mouse model of renal allograft rejectionNaive B6.LAG3−/− mice have elevated numbers of follicular and memory T cells, and plasma cells compared to WT mice, as well as increased frequencies of memory T cells reactive to H-2Dd, H-2Ds, H-2Dq and H-2Dk alloantigens. When kidneys were transplanted from C3H donors to B6 WT and LAG3−/− mice, all C3H kidney allografts survived for > 60d in WT recipients, whereas LAG3−/− recipients rapidly reject allografts and have elevated serum creatinine levels at d14 posttransplant. Graft histology at rejection revealed minimal T cell infiltration, diffuse C4d staining, atrophic peritubular capillaries, endothelial swelling and edema characteristic of antibody mediated rejection (AMR). Compared to WT, LAG3−/− recipients had elevated frequencies of anti-donor IFNγ producing T cells and increased levels of donor specific antibody (DSA) against MHC-I and MHC-II. Depletion of CD8 T cells in LAG3−/− recipients did not alter rejection kinetics, while B cell depletion significantly extended C3H kidney allograft survival, suggesting the predominant role of alloantibody rather than T cell mediated rejection in these mice. However, neither T nor B cell conditional knockout recipients rejected the allograft demonstrating LAG3 expression on both cell types is necessary to mediate rejection.

These findings demonstrate that LAG3 regulates both T and B cell functions in response to kidney allografts, and is an attractive therapeutic target for the prevention of AMR.

Supported by grants from NIH (P01AI087586-10)

Aquaporin 4 Is A Mediator Of Essential Dendritic Cell Function

Aquaporins are transmembrane water channels implicated in a range of physiologic functions. We previously reported that Aquaporin 4 (AQP4) is expressed by T cells and treatment with a small molecule AQP4 inhibitor (AER-270) significantly delays T cell mediated mouse heart allograft rejection. Our findings did not exclude a role for antigen presenting cells (APCs), and the purpose of this study is to investigate the requirement for AQP4 in dendritic cell (DC) functions.

Stimulation with AQP4−/− spleen APCs with the addition of antigenic peptide, resulted in reduced frequencies of IFNγ producing MAR T cells compared to WT APCs. We confirmed that splenic DCs express AQP4. Bone marrow derived dendritic cells (BMDCs) from WT and AQP4−/− mice were stimulated with LPS or protein antigen, the absence or inhibition of AQP4 resulted in no defects in up-regulation of maturation markers following stimulation. WT and AQP4−/− BMDCs were pulsed with Lucifer Yellow, FITC-dextran and OVA-AF647 to assess antigen uptake with no observable defect after AQP4 inhibition or in AQP4−/− BMDCs. However, AQP4−/− BMDCs pulsed with OVA protein had reduced expression of SIINFEKL peptide-MHC-I complexes. In contrast, when pulsed with OVA-derived SIINFEKL peptide, AER-270 treated WT BMDCs or AQP4−/− BMDCs showed no change in peptide-MHC complex expression suggesting a role for AQP4 in antigen processing. Consistent with this, the absence or inhibition of AQP4 resulted in decreased DQ-OVA signal, indicating that AQP4 is required for the antigen loading into the lysosome.

Our data indicate that AQP4 plays a critical role in the processing of antigens by dendritic cells that impacts their ability to prime T cells, and can be therapeutically targeted in a transplant setting.

Supported by grants from NIH (R56AI152368-01)

C1q as a potential tolerogenic therapeutic in transplantation

In 1963, Lepow and colleagues resolved C1, the first component of the classical pathway, into three components, which they named C1q, C1r, and C1s. All three of these components were demonstrated to be involved in causing hemolysis in vitro. For over 30 years after that seminal discovery, the primary function attributed to C1q was as part of the C1 complex that initiated the classical pathway of the complement cascade. Then, a series of papers reported that isolated C1q could bind to apoptotic cells and facilitate their clearance by macrophages. Since then, rheumatologists have recognized that C1q is an important pattern recognition receptor (PRR) that diverts autoantigen containing extracellular vesicles from immune recognition. This critical function of C1q as a regulator of immune recognition has been largely overlooked in transplantation. Now that extracellular vesicles released from transplants have been identified as a major agent of immune recognition, it is logical to consider the potential impact of C1q on modulating the delivery of allogeneic extracellular vesicles to antigen presenting cells. This concept has clinical implications in the possible use of C1q or a derivative as a biological therapeutic to down-modulate immune responses to transplants.

Recipient myeloperoxidase-producing cells regulate antibody-mediated acute versus chronic kidney allograft rejection

Antibody-mediated rejection (ABMR) continues to be a major problem undermining the success of kidney transplantation. Acute ABMR of kidney grafts is characterized by neutrophil and monocyte margination in the tubular capillaries and by graft transcripts indicating NK cell activation, but the myeloid cell mechanisms required for acute ABMR have remained unclear. Dysregulated donor-specific antibody (DSA) responses with high antibody titers are induced in B6.CCR5-/- mice transplanted with complete MHC-mismatched A/J kidneys and are required for rejection of the grafts. This study tested the role of recipient myeloid cell production of myeloperoxidase (MPO) in the cellular and molecular components of acute ABMR. Despite induction of equivalent DSA titers, B6.CCR5-/- recipients rejected A/J kidneys between days 18 and 25, with acute ABMR, whereas B6.CCR5-/-MPO-/- recipients rejected the grafts between days 46 and 54, with histopathological features of chronic graft injury. On day 15, myeloid cells infiltrating grafts from B6.CCR5-/- and B6.CCR5-/-MPO-/- recipients expressed marked phenotypic and functional transcript differences that correlated with the development of acute versus chronic allograft injury, respectively. Near the time of peak DSA titers, activation of NK cells to proliferate and express CD107a was decreased within allografts in B6.CCR5-/-MPO-/- recipients. Despite high titers of DSA, depletion of neutrophils reproduced the inhibition of NK cell activation and decreased macrophage infiltration but increased monocytes producing MPO. Overall, recipient myeloid cells producing MPO regulate graft-infiltrating monocyte/macrophage function and NK cell activation that are required for DSA-mediated acute kidney allograft injury, and their absence switches DSA-mediated acute pathology and graft outcomes to chronic ABMR.

Natural Killer Cells: Critical Effectors During Antibody-mediated Rejection of Solid Organ Allografts

Antibody-mediated rejection (AMR) is an important cause of graft loss and continues to present a formidable obstacle to successful transplantation. Unresolved problems continue to be the absence of effective strategies to ablate the donor-specific antibody (DSA) response as well as to attenuate the antibody-mediated graft tissue injury. While the properties of DSA that cause greater graft tissue injury and the characteristic microvascular pathology of the graft injury are well documented, the mechanisms underlying the injury mediated by the antibodies remains unclear. Recent transcriptome interrogation of kidney and heart biopsies procured during ongoing AMR has indicated the expression of genes associated with natural killer (NK) cell activation that is absent during T cell-mediated rejection. The expression of NK cell transcripts during AMR correlates with the presence of CD56+ cells in the microcirculation inflammation observed during AMR. Several mouse models have recently demonstrated the role of NK cells in antibody-mediated chronic vasculopathy in heart allografts and the requirement for NK cell activation during acute AMR of kidney allografts. In the latter model, NK cell activation within kidney allografts is regulated by the activation of myeloid cells producing myeloperoxidase. Overall, the studies to date indicate that AMR constitutes a complex series of DSA-induced interactions with components of the innate immune response. The innate immune participants and their expressed effector functions resulting in the rejection are beginning to be identified. The identification of these components should uncover novel targets that can be used to attenuate acute graft tissue injury in the presence of DSA.

B cell-derived IL-1β and IL-6 drive T cell reconstitution following lymphoablation

Understanding the mechanisms of T cell homeostatic expansion is crucial for clinical applications of lymphoablative therapies. We previously established that T cell recovery in mouse heart allograft recipients treated with anti-thymocyte globulin (mATG) critically depends on B cells and is mediated by B cell-derived soluble factors. B cell production of interleukin (IL)-1β and IL-6 is markedly upregulated after heart allotransplantation and lymphoablation. Neutralizing IL-1β or IL-6 with mAb or the use of recipients lacking mature IL-1β, IL-6, IL-1R, MyD88, or IL-6R impair CD4⁺ and CD8⁺ T cell recovery and significantly enhance the graft-prolonging efficacy of lymphoablation. Adoptive co-transfer experiments demonstrate a direct effect of IL-6 but not IL-1β on T lymphocytes. Furthermore, B cells incapable of IL-1β or IL-6 production have diminished capacity to mediate T cell reconstitution and initiate heart allograft rejection upon adoptive transfer into mATG treated B cell deficient recipients. These findings reveal the essential role of B cell-derived IL-1β and IL-6 during homeostatic T cell expansion in a clinically relevant model of lymphoablation.

Marginal zone B cells induce T cell homeostatic proliferation following antibody-mediated lymphoablation

Antibody-mediated lymphoablation is commonly used in clinical transplantation. Using mouse model of heart transplantation and murine anti-thymocyte globulin (mATG) lymphoablation, we previously showed that B cell-derived IL-1β and IL-6 are critical mediators of homeostatic T cell proliferation. While marginal zone (MZ) B cells are best known as important sensors and amplifiers of inflammation, their role in depletion-induced lymphopenia has not been previously addressed. The goal of this study was to test the contribution of distinct B cell subsets to proinflammatory cytokine production and T cell homeostatic reconstitution. B6 (H-2b) recipients of BALB/c (H-2d) heart allografts were treated with mATG (d. 0, 4). At d. 8 posttransplant, the proportion of IL-1β, IL-6 and TNF-α secreting cells was significantly higher within MZ vs follicular (FO) B cell subset. Depletion of both FO and MZ B cells with anti-CD20 mAb in addition to mATG (d. -3, 11) significantly diminished T cell reconstitution and prolonged heart allograft survival compared to mATG treatment alone. Notably, specific MZ B cell depletion with anti-VLA4 and anti-LFA1α mAbs (d. -4, -2) resulted in similarly impaired T cells recovery along with improved heart allograft survival. Consistent with these findings, mATG treated Notch2flox/flox CD19Cre/+ mice, which lack MZ B cells but contain normal numbers of FO B cells, had delayed T cell recovery and prolonged heart allograft survival compared to WT recipients. These results show the critical role of MZ B cells in stimulating homeostatic T cell proliferation in lymphopenic heart allograft recipients and suggest that transient removal of MZ B cells may improve the efficacy of lymphoablative induction therapies.

Recipient LAG3 deficiency results in antibody-mediated rejection of mouse renal allografts

The functions of coinhibitory receptor lymphocyte activation gene-3 (LAG3) in T cells are well studied, however its role in humoral immune responses remains poorly characterized. The goal of this study was to test the role of recipient LAG3 in a mouse model of renal allograft rejection

Compared to WT animals, naïve B6.LAG3−/− mice have increased splenic cellularity and higher frequencies of CD44hi memory T cells, CXCR5hi follicular T cells, and B220+CD138+plasma cells yet do not develop spontaneous autoimmunity. Furthermore, naïve B6.LAG3−/−(H-2Db) mice have increased frequencies of memory T cells against H-2Dd, H-2Ds, H-2Dq and H-2Dk alloantigens. C3H (H-2Dk) kidney allografts were transplanted into B6.WT or B6.LAG3−/− recipients after bilateral nephrectomy. Whereas 4/4 WT recipients accepted C3H allografts for longer than 60 d, recipient LAG3 deficiency led to rapid allograft rejection (MST of 14 d, n=5) with serum creatinine levels of 0.1 and 1.35 mg/dl respectively at d14 posttransplant. Graft histology at rejection revealed minimal T cell infiltration, diffuse C4d staining, atrophic peritubular capillaries, endothelial swelling and edema characteristic of antibody mediated rejection (AMR). Compared to WT, LAG3−/− recipients had elevated frequencies of anti-donor IFNg producing T cells and increased levels of anti-donor MHC-II antibodies. Recipient CD8 T cell depletion did not alter the rejection kinetics in LAG3−/− recipients (MST of 16 d), while B cell depletion significantly extended C3H kidney allograft survival (MST of >30 d).

These results suggest the predominant role of alloantibody rather than T cells in renal allograft injury and identify LAG3 is a potential therapeutic target for AMR prevention and treatment.

B cell-derived IL-1β and IL-6 drive homeostatic T cell recovery following lymphoablation

T cell reconstitution after lymphoablation in allograft recipients may lead to acute rejection and poor graft outcome. Using a mouse model of heart transplantation and murine anti-thymocyte globulin (mATG) we showed that while T cell recovery depends on B cells, B cell MHC expression is dispensable. The goal of this study was to investigate the role of B cell-derived IL-1β and IL-6 in T cell homeostatic reconstitution after mATG lymphoablation. The NanoString analysis showed that mATG upregulated IL-1β and IL-6 gene expression in recipient B cells. To validate NanoString findings, IL-1β and IL-6 expression was measured by intracellular flow cytometry. Following heart allograft placement and mATG treatment, B cells are the main source of IL-1β and IL-6, and these cytokines mutually regulate each other’s production. Either treatment with anti-IL-1β mAb or the use of IL-1R1 KO, caspase-1 KO and MyD88 KO recipients diminished both CD4+ and CD8+ T cell recovery along with improved heart allograft survival and decreased donor-reactive IFN-γ producing cells after mATG treatment. Similarly, in vivo IL-6 neutralization with mAb or using IL-6 KO recipients delayed CD4+ and CD8+ T cell recovery, markedly prolonged heart allograft survival and decreased donor-specific T cell responses suggesting that IL-1β and IL-6 are essential for optimal T cell recovery. Adoptive B cell transfers into μMT mice showed that B cell-derived IL-1β and IL-6 supported CD8+ T cells recovery. The absence of either IL-1R or IL-6R on CD8+ T cells markedly impaired their homeostatic recovery following mATG depletion. These findings reveal the essential role of B cell-derived IL-1β and IL-6 during homeostatic T cell expansion in a clinically relevant model of lymphoablation.

Recipient expression of C-type lectin receptor Mincle promotes rapid T cell homeostatic recovery following antibody mediated lymphoablation

Antibody-mediated lymphocyte depletion is widely used in transplantation. We recently showed that T cell recovery in mouse heart allograft recipients treated with anti-thymocyte globulin (mATG) depends on B cells activated via CD40 and is mediated by B cell-derived IL-1β and IL-6. Mincle (Clec-4e) is a FcRγ-coupled C-type lectin receptor that binds ligands released by pathogens or damaged cells and signals through the Card-9 pathway. While Mincle functions are well characterized in myeloid cells, its significance in B cells remains to be determined. Our goal was to investigate the role of Mincle in initiating B cell cytokine production in lymphopenic heart allograft recipients. The NanoString and RT-PCR analyses of B cells showed that mATG depletion resulted in B cell up-regulation of Mincle and Card-9 gene expression. Flow cytometry confirmed cell surface Mincle expression in B cells from mATG-treated recipients but not from naïve mice. Treatment with anti-CD154 mAb in addition to mATG decreased the B cell expression of Mincle, IL-1β, IL-6 and TNF-α. Moreover, B cell IL-1β and TNF-α expression were reduced in mATG treated Mincle KO allograft recipients compared to WT controls. Most importantly, Mincle KO recipients treated with mATG had severely impaired T cell recovery and significantly prolonged allograft survival compared to WT recipients. We propose that B cells activation via CD40 results in up-regulation of cell surface Mincle. Endogenous ligands released from necrotic graft cells bind to Mincle thus inducing B cells to secrete proinflammatory cytokines and facilitate CD8+ T cell proliferation. These data identify Mincle as a novel therapeutic target to improve the efficacy of lymphoablation in allograft recipients.

Early T cell infiltration is modulated by programed cell death-1 protein and its ligand (PD-1/PD-L1) interactions in murine kidney transplants

Allogeneic transplants elicit dynamic T cell responses that are modulated by positive and negative co-stimulatory receptors. Understanding mechanisms that intrinsically modulate the immune responses to transplants is vital to develop rational treatment for rejection. Here, we have investigated the impact of programed cell death-1 (PD-1) protein, a negative co-stimulatory receptor, on the rejection of MHC incompatible kidney transplants in mice. T cells were found to rapidly infiltrate the kidneys of A/J mice transplanted to C57BL/6 mice, which peaked at six days and decline by day 14. The T cells primarily encircled tubules with limited infiltration of the tubular epithelium. Lipocalin 2 (LCN2), a marker of tubular injury, also peaked in the urine at day six and then declined. Notably, flow cytometry demonstrated that most of the T cells expressed PD-1 (over 90% of CD8 and about 75% of CD4 cells) at day six. Administration of blocking antibody to PD-L1, the ligand for PD-1, before day six increased T cell infiltrates and urinary LCN2, causing terminal acute rejection. In contrast, blocking PD-1/PD-L1 interactions after day six caused only a transient increase in urinary LCN2. Depleting CD4 and CD8 T cells virtually eliminated LCN2 in the urine in support of T cells injuring tubules. Thus, our data indicate that PD-1/PD-L1 interactions are not just related to chronic antigenic stimulation of T cells but are critical for the regulation of acute T cell responses to renal transplants.

Anti-donor MHC Class II Alloantibody Induces Glomerular Injury in Mouse Renal Allografts Subjected to Prolonged Cold Ischemia

BACKGROUND

The mechanisms underlying the effects of prolonged cold-ischemia storage on kidney allografts are poorly understood.

METHODS

To investigate effects of cold ischemia on donor-reactive immune responses and graft pathology, we used a mouse kidney transplantation model that subjected MHC-mismatched BALB/c kidney allografts to cold-ischemia storage for 0.5 or 6 hours before transplant into C57BL/6 mice.

RESULTS

At day 14 post-transplant, recipients of allografts subjected to 6 versus 0.5 hours of cold-ischemia storage had increased levels of anti-MHC class II (but not class I) donor-specific antibodies, increased donor-reactive T cells, and a significantly higher proportion of transplant glomeruli infiltrated with macrophages. By day 60 post-transplant, allografts with a 6 hour cold-ischemia time developed extensive glomerular injury compared with moderate pathology in allografts with 0.5 hour of cold-ischemia time. Pathology was associated with increased serum levels of anti-class 2 but not anti-class 1 donor-specific antibodies. Recipient B cell depletion abrogated early macrophage recruitment, suggesting augmented donor-specific antibodies, rather than T cells, increase glomerular pathology after prolonged cold ischemia. Lymphocyte sequestration with sphingosine-1-phosphate receptor 1 antagonist FTY720 specifically inhibited anti-MHC class II antibody production and abrogated macrophage infiltration into glomeruli. Adoptive transfer of sera containing anti-donor MHC class II antibodies or mAbs against donor MHC class II restored early glomerular macrophage infiltration in FTY720-treated recipients.

CONCLUSIONS

Post-transplant inflammation augments generation of donor-specific antibodies against MHC class II antigens. Resulting MHC class II-reactive donor-specific antibodies are essential mediators of kidney allograft glomerular injury caused by prolonged cold ischemia.

The neonatal Fc receptor: Key to homeostasic control of IgG and IgG-related biopharmaceuticals

IgG and albumin are the most abundant proteins in the circulation and have the longest half-lives. These properties are due to a unique receptor, the neonatal Fc receptor (FcRn). Although FcRn is named for its function of transferring IgG across the placenta from maternal to fetal circulation, FcRn functions throughout life to maintain IgG and albumin concentrations. FcRn protects IgG and albumin from intracellular degradation and recycles them back into the circulation. Clinical trials have confirmed that pathogenic antibodies can be depleted by blocking this homeostatic function of FcRn. Moreover, understanding the molecular interactions between IgG and FcRn has resulted in the design of therapeutic monoclonal antibodies with more efficacious pharmacokinetics. As a result of genetic engineering these monoclonals can be delivered at lower doses and at longer intervals. More recent findings have demonstrated that FcRn enhances phagocytosis by neutrophils, immune complex clearance by podocytes and antigen presentation by dendritic cells, macrophages, and B cells. This minireview highlights the relevance of FcRn to transplantation.

Aquaporin 4 inhibition alters chemokine receptor expression and T cell trafficking

Aquaporins (AQPs) are water channels that mediate a variety of biological processes. However, their role in the immune system is poorly understood. We recently reported that AQP4 is expressed by naïve and memory T cells and that AQP4 blockade with a small molecule inhibitor prolongs murine heart allograft survival at least partially through diminishing T cell activation, proliferation and trafficking. The goal of this study was to determine how AQP4 function impacts T cells in the absence of antigen stimulation. AQP4 inhibition transiently reduced the number of circulating CD4+ and CD8+ T cells in naïve non-transplanted mice in the absence of systemic T cell depletion. Adoptive transfer studies demonstrated T cell intrinsic effect of AQP4 inhibition. AQP4 blockade altered T cell gene and protein expression of chemokine receptors S1PR1 and CCR7, and their master regulator KLF-2, and reduced chemotaxis toward S1P and CCL21. Consistent with the in vitro data, in vivo AQP4 inhibition reduced T lymphocyte numbers in the lymph nodes with simultaneous accumulation in the liver. Our findings indicate that blocking AQP4 reversibly alters T lymphocyte trafficking pattern. This information can be explored for the treatment of undesirable immune responses in transplant recipients or in patients with autoimmune diseases.

IL-1β And IL-6 Facilitate T Cell Reconstitution In Heart Allograft Recipients Treated With Anti-Thymocyte Globulin

Antibody-mediated lymphocyte depletion is used in solid organ transplantation. T cell reconstitution after lymphoablation may lead to acute rejection and poor graft outcome. We investigated the mechanisms of T cell reconstitution in heart allograft recipients treated with murine anti-thymocyte globulin (mATG).

B cell depletion with anti-mouse CD20 mAb significantly diminished T cell reconstitution in B6 (H-2b) recipients of BALB/c (H-2d) heart allografts treated with mATG. However, cognate T/B cell interactions were dispensable for T cell recovery suggesting the potential role of B cell derived cytokines. The NanoString gene expression analysis of isolated B cells showed that mATG upregulated IL-1β and IL-6 expression in CD4 T cell-dependent manner. Intracellular flow cytometry confirmed that B cells are the major source of IL-1β and IL-6 following mATG treatment. The proportion of IL-1β secreting B cells was increased in mATG treated compared to untreated recipients, whereas IL-6 producing B cells were not significantly affected by lymphoablation. Neutralization of either IL-1β or IL-6 with mAb significantly decreased T cell recovery and extended heart allograft survival in mATG treated recipients. To further test the role of B cell derived IL-1β, we injected either WT or Caspase-1−/− B cells into B cell deficient μMT mice followed by BALB/C heart transplantation and mATG treatment. Caspase-1 deficiency in injected B cells resulted in significantly lower CD8 T cell recovery.

Our results show the novel functions of IL-1β and IL-6 in regulating homeostatic T cell recovery following mATG treatment. IL-1β and IL-6 neutralization may be a promising strategy for enhancing the efficacy of mATG lymphoablation in transplant recipients.

The role of Aquaporin-4 in T cell activation

Aquaporins (AQPs) are membrane channels that mediate a variety of biological processes via water and other small molecule movement. Several aquaglyceroporins have been shown to regulate key immune functions. However, the role of AQP4 in immune cells is poorly characterized. We recently reported that AQP4 is expressed by both naïve and memory CD4 and CD8 T cells. Furthermore, AQP4 blockade with a small molecule inhibitor, AER-270, reduced T cell activation and proliferation leading to a significant prolongation of fully MHC-mismatched murine heart allograft survival. The goal of this study was to determine the mechanisms by which AQP4 facilitates T cell activation and TCR signaling. Wild-type (WT) but not AQP4−/− T cells treated with AER-270 were protected from lysis in a hypoosmotic shock assay demonstrating AQP4 requirement for water transport in T cells. Following αCD3/αCD28 stimulation, AQP4−/− T cells demonstrated inferior activation compared to WT T cells. Similar decreased activation was observed after treatment of WT T cells with AER-270. Consistent with reduced T cell activation, AQP4 inhibition diminished Ca2+flux in WT but not AQP4−/− T cells in response to TCR crosslinking. AQP4 inhibition also resulted in altered pattern of protein tyrosine phosphorylation at 2 min following TCR crosslinking. These results indicate that despite T cell expression of several water channels, AQP4 function is critical for optimal T cell activation and functions. Our study identifies a novel role of water channels in T cell biology and suggests AQP4 inhibition as a promising strategy to modulate T cell alloresponses following transplantation.

Kidney Allograft Recipient Absence of Myeloperoxidase Decreases Donor-Specific Antibody Titers and Attenuates Antibody-Mediated Allograft Rejection

The incidence of antibody-mediated rejection (AMR) in clinical kidney transplants is increasingly observed. We have reported that strong donor-specific antibody (DSA) responses are induced in B6.CCR5−/−mice transplanted with complete MHC mismatched A/J kidney allografts and that NK cells play a critical role in acute injury and graft failure. Since this acute AMR is accompanied by intense macrophage infiltration into the allograft, we tested the role of recipient-derived myeloperoxidase (MPO) production on kidney allograft survival. B6.CCR5−/−and B6.CCR5−/−MPO−/−mice were transplanted with complete MHC mismatched A/J kidney grafts. Allografts were rejected between days 18 and 25 post-transplant in B6.CCR5−/−recipients but not until days 46–54 in B6.CCR5−/−MPO−/− recipients. DSA titers in B6.CCR5−/−MPO−/−recipients were 4–5 fold lower than those induced in the B6.CCR5−/− recipients. There was also a 60% decrease in the number of donor-reactive T cells producing IFN-g in the spleens of the B6.CCR5−/−MPO−/− vs. B6.CCR5−/−recipients on day 7 post-transplant. Despite the extended survival, qPCR analyses indicated slight increases in mRNA encoding TNFa, IL-6 and FasL in kidney allografts on day 14 post-transplant in B6.CCR5−/−MPO−/− vs. B6.CCR5−/−recipients as well as the pro-fibrogenic factors P-selectin and connective tissue growth factor on day 50 post-transplant. These results suggest MPO regulates the magnitude of the DSA and T cell response in kidney transplant recipients and that the decreased DSA titers attenuate acute AMR but induce the indolent development of interstitial fibrosis and glomerular injury that will eventually lead to graft dysfunction and failure at later times.

p40 Homodimers Indirectly Induce Endogenous Donor-Reactive Memory CD8 T Cell Proliferation within High-Risk Allografts via IL-15 signaling

Increasing cold ischemic storage (CIS) time from 0.5 to 8 hrs prior to transplant promotes the increased proliferation and effector functions of endogenous donor-reactive memory CD4 and CD8 T cells infiltrating mouse cardiac allografts 24–48 hrs after transplant. This increased mCD8 T cell proliferation within high-risk allografts requires mCD4 T cell help via CD40-CD154 interactions with graft dendritic cells to induce IL-12p40 homodimer (p40 HD) production required for this mCD8 T cell proliferation. This study tested if p40HD directly or indirectly stimulate the mCD8 T cell proliferation within high risk allografts. While endogenous mCD8 T cells within highly ischemic allografts expressed both IL12Rβ1 and β2, most proliferating CD8 T cells did not express the IL12Rβ1 needed for p40 HD mediated signaling. Moreover, endogenous mCD8 T cells isolated from allografts did not proliferate ex vivo when cultured with p40 HD. qPCR analysis indicated increased mRNA expression of IL2Rα (CD25), β(CD122) and IL15 Rain infiltrating mCD8 T cells purified from allografts subjected to prolonged vs. minimal CIS on day 2 post-transplant. ELISA indicated longer CIS time markedly increased IL-12p40 and IL-15 protein in the allografts and these increases were dependent on recipient CD4 T cells. Blocking of p40 or IL-15 signaling with anti-p40 or anti-CD122 mAb inhibited endogenous mCD8 T cell proliferation within high-risk cardiac allografts and extended the survival of the high-risk allografts from day 18 to day 54 in CTLA-4Ig conditioned recipients. These results suggest that the p40 HD stimulate proliferative cytokines such as IL-15, which directly stimulate endogenous donor-reactive mCD8 T cell proliferation within the higher-risk allografts.

Interleukin-27 promotes CD8+ T cell reconstitution following antibody-mediated lymphoablation

Antibody-mediated lymphoablation is used in solid organ and stem cell transplantation and autoimmunity. Using murine anti-thymocyte globulin (mATG) in a mouse model of heart transplantation, we previously reported that the homeostatic recovery of CD8+ T cells requires help from depletion-resistant memory CD4+ T cells delivered through CD40-expressing B cells. This study investigated the mechanisms by which B cells mediate CD8+ T cell proliferation in lymphopenic hosts. While CD8+ T cell recovery required MHC class I expression in the host, the reconstitution occurred independently of MHC class I, MHC class II, or CD80/CD86 expression on B cells. mATG lymphoablation upregulated the B cell expression of several cytokine genes, including IL-15 and IL-27, in a CD4-dependent manner. Neither treatment with anti-CD122 mAb nor the use of IL-15Rα-/- recipients altered CD8+ T cell recovery after mATG treatment, indicating that IL-15 may be dispensable for T cell proliferation in our model. Instead, IL-27 neutralization or the use of IL-27Rα-/- CD8+ T cells inhibited CD8+ T cell proliferation and altered the phenotype and cytokine profile of reconstituted CD8+ T cells. Our findings uncover what we believe is a novel role of IL-27 in lymphopenia-induced CD8+ T cell proliferation and suggest that targeting B cell-derived cytokines may increase the efficacy of lymphoablation and improve transplant outcomes.

Determinant roles of dendritic cell-expressed Notch Delta-like and Jagged ligands on anti-tumor T cell immunity

Background

Notch intercellular communication instructs tissue-specific T-cell development and function. In this study, we explored the roles of dendritic cell (DC)-expressed Notch ligands in the regulation of T-cell effector function.

Methods

We generated mice with CD11c lineage-specific deletion of Notch Delta-like ligand (Dll)1 and Jagged (Jag)2. Using these genetically-ablated mice and engineered pharmacological Notch ligand constructs, the roles of various Delta-like and Jagged ligands in the regulation of T-cell-mediated immunity were investigated. We assessed tumor growth, mouse survival, cytokine production, immunophenotyping of myeloid and lymphoid populations infiltrating the tumors, expression of checkpoint molecules and T-cell function in the experimental settings of murine lung and pancreatic tumors and cardiac allograft rejection. Correlative studies were also performed for the expression of NOTCH ligands, NOTCH receptors and PD-1 on various subsets of myeloid and lymphoid cells in tumor-infiltrating immune cells analyzed from primary human lung cancers.

Results

Mice with CD11c lineage-specific deletion of Notch ligand gene Dll1, but not Jag2, exhibited accelerated growth of lung and pancreatic tumors concomitant with decreased antigen-specific CD8⁺T-cell functions and effector-memory (Tem) differentiation. Increased IL-4 but decreased IFN-γ production and elevated populations of T-regulatory and myeloid-derived suppressor cells were observed in Dll1-ablated mice. Multivalent clustered DLL1-triggered Notch signaling overcame DC Dll1 deficiency and improved anti-tumor T-cell responses, whereas the pharmacological interference by monomeric soluble DLL1 construct suppressed the rejection of mouse tumors and cardiac allograft. Moreover, monomeric soluble JAG1 treatment reduced T-regulatory cells and improved anti-tumor immune responses by decreasing the expression of PD-1 on CD8⁺Tem cells. A significant correlation was observed between DC-expressed Jagged and Delta-like ligands with Tem-expressed PD-1 and Notch receptors, respectively, in human lung tumor-infiltrates.

Conclusion

Our data show the importance of specific expression of Notch ligands on DCs in the regulation of T-cell effector function. Thus, strategies incorporating selectively engineered Notch ligands could provide a novel approach of therapeutics for modulating immunity in various immunosuppressive conditions including cancer.

Electronic supplementary material

The online version of this article (10.1186/s40425-019-0566-4) contains supplementary material, which is available to authorized users.

Measuring alloreactive B cell responses in transplant recipients

PURPOSE

Despite advanced immunosuppression, donor-specific antibodies (DSA) remain the leading cause of acute and chronic transplant tissue injury. Comprehensive evaluation of anti-donor humoral immune responses is critical for successful prevention, diagnosis and treatment of antibody-mediated rejection. This review summarizes the evolution of techniques used for this purpose in experimental and clinical transplantation.

RECENT FINDINGS

For decades, measuring DSA serum levels was the only way to assess recipient humoral immunity. Recently, the interest shifted from quantifying circulating DSA to the analyses of various B cell subsets and most importantly, of donor antigen-specific B cells. State-of-the-art approaches have been developed by studies of model antigens, infectious agents and autoimmunity. These methods are now being adopted by the transplantation field.

SUMMARY

The complexity of humoral immunity caused by organ transplantation necessitates complementary approaches assessing both DSA and various B cell subsets to successfully target antibody-mediated rejection.

Graft dendritic cell p40 homodimers activate donor-reactive endogenous memory CD8 T cells within higher risk allografts

Memory T cells with donor-reactivity pose a major barrier to successful transplantation and tolerance induction. Longer cold ischemic storage (CIS) prior to transplantation promotes endogenous donor-reactive memory CD8 T cell infiltration and activation within cardiac allografts to directly mediate CTLA-4Ig-resistant rejection between days 15–22 post-transplant vs. > 60 days for allografts subjected to minimal CIS. Using BrdU labeling, graftinfiltrating memory CD4 and CD8 T cells proliferated much more strongly within allografts subjected to 8 vs. 0.5 hr CIS and proliferation of endogenous memory CD8 T cells within the higher risk allografts required recipient CD4 T cells, graft dendritic cells (DC) and graft expression of class II MHC, CD40 and IL-12p40, but not p35. IL-12p40 but not IL-12 p35 or IL-23p19 mRNA was elevated at 48 hr post-transplant in allografts subjected to 8 vs. 0.5 hr CIS and the increase in p40 mRNA was reduced to the levels seen in 0.5 CIS allografts by recipient CD4 T cell depletion, but was restored by treating with agonist anti-CD40 mAb. p40 homodimers, but not p70 heterodimers, were increased in allografts subjected to 8 hr CIS and were dependent on recipient CD4 T cells. Peri-transplant anti-p40 mAb reversed CTLA-4Ig-resistant rejection of higher risk allografts and peri-transplant p40 homodimers induced endogenous memory CD8 T cell proliferation within allografts subjected to 0.5 hr CIS to the levels observed in allografts subjected to 8 hr CIS. These data indicate that endogenous memory CD8 T cell activation within higher risk cardiac allografts requires CD4 T cell help via CD40-CD154 interactions with graft DC to induce their production of p40 homodimers.

Aquaporin 4 blockade improves survival of murine heart allografts subjected to prolonged cold ischemia

Prolonged cold ischemia storage (CIS) is a leading risk factor for poor transplant outcome. Existing strategies strive to minimize ischemia-reperfusion injury in transplanted organs, yet there is a need for novel approaches to improve outcomes of marginal allografts and expand the pool of donor organs suitable for transplantation. Aquaporins (AQPs) are a family of water channels that facilitate homeostasis, tissue injury, and inflammation. We tested whether inhibition of AQP4 improves the survival of fully MHC-mismatched murine cardiac allografts subjected to 8 hours of CIS. Administration of a small molecule AQP4 inhibitor during donor heart collection and storage and for a short-time posttransplantation improves the viability of donor graft cells, diminishes donor-reactive T cell responses, and extends allograft survival in the absence of other immunosuppression. Furthermore, AQP4 inhibition is synergistic with cytotoxic T lymphocyte-associated antigen 4-Ig in prolonging survival of 8-hour CIS heart allografts. AQP4 blockade markedly reduced T cell proliferation and cytokine production in vitro, suggesting that the improved graft survival is at least in part mediated through direct effects on donor-reactive T cells. These results identify AQPs as a promising target for diminishing donor-specific alloreactivity and improving the survival of high-risk organ transplants.

Prolonged cold ischemia storage of renal allografts shifts DSA specificity and enhances glomerular injury

Prolonged cold ischemia storage (CIS) of donor organs is a major risk factor for allograft injury. We tested how increased CIS time influences humoral alloimmunity in a mouse model of renal transplantation.

B6 (H-2b) mice received BALB/c (H-2d) renal allografts subjected to 0.5 h or 6 h CIS. At 14 d posttransplant, 6 h CIS recipients had increased titers of donor-MHC class II but not class I reactive IgG DSA, elevated frequencies of both class I and class II-reactive antibody secreting cells, higher frequencies of donor-reactive IFNγ-secreting T cells and higher proportion of graft glomeruli infiltrated with macrophages compared to 0.5 h CIS controls. B cell depletion inhibited DSA generation, intragraft C4d deposition and macrophage glomerular infiltration on d14.

Regardless of CIS duration, serum creatinine levels at d 60 posttransplant were comparable to non-transplanted mice. By d 60, 0.5 h CIS grafts had moderate segmental sclerosis in a limited number of glomeruli. In contrast, 6 h CIS grafts had extensive glomerular injury including thickened capillary loops, sclerosis, thrombotic microangiopathy, intracapillary fibrin thrombi, and mesangiolysis, and elevated intragraft levels of acute kidney injury markers NGAL and osteopontin. In addition, 6 h CIS recipients had increased serum levels of anti-class II but not anti-class I IgG DSA whereas the frequencies of IFNγ-producing donor-reactive T cells were low and comparable in both groups.

Our findings suggest that the augmented humoral rather than cellular immune responses account for the transplant glomerulopathy after prolonged CIS. Posttransplant inflammation spreads the specificity of DSA responses from class I to class II and may thus influence renal allograft pathology.

Aquaporin 4 blockade alters T cell trafficking through a novel mechanism of S1PR1 regulation

The water channels aquaporins mediate a variety of immune functions. We previously demonstrated that aquaporin 4 (AQP4) is expressed by CD4 and CD8 T cells and blockade of AQP4 with small molecule inhibitor AER-270/271 significantly prolongs survival of heart allografts in two robust models of rejection. The goal of this study was to determine the effects of AQP4 blockade on T cells.

Administration of AER-271 into naïve non-transplanted mice (250 μg i.p. every 6 h on d. 0–4) decreased numbers of circulating CD4 and CD8 T cells, but not B cells, by >90%, compared to untreated controls. The T cell frequencies and numbers in the secondary lymphoid organs (SLOs) were not significantly affected by AER-271, suggesting the lack of circulating T cells is not due to systemic depletion but rather to altered T cell trafficking.

The effect of AER-271 treatment was transient, as circulating T cell numbers returned to pretreatment levels by d 21. AER-271 treated animals promptly rejected heart allografts transplanted 24 d after treatment cessation (MST 6 d, n=4).

We next tested the effects of AQP4 blockade on Sphingosine-1 Phosphate Receptor 1 (S1PR1), a key mediator of T cell trafficking. S1PR1 mRNA expression was reduced in isolated mouse spleen T cells within 3 h of in vitro incubation with AER-270. Decreased S1PR1 mRNA expression translated into altered chemotaxis, as AER-270 reduced T cell migration toward S1PR1 ligand, Sphingosine-1 Phosphate (S1P), in a transwell system.

Our data suggest that AER-270/271 treatment results in altered S1PR1 transcription thus changing T cell trafficking and prolonging allograft survival. Therefore, AQP4 blockade may be an attractive therapeutic strategy in transplantation and other immune-mediated diseases.

Donor-specific antibody mediates chronic, but not acute, kidney allograft rejection in the absence of natural killer cells

Dysregulated donor-specific antibody (DSA) responses are induced in B6.CCR5−/− mice transplanted with complete MHC mismatched kidney allografts and NK cells play a critical role in acute graft injury and failure. The current study investigated the consequence of high DSA titers on graft outcomes in the presence vs. absence of NK cell activation within the kidney graft. B6.CCR5−/− recipients of complete MHC mismatched A/J allografts or semi allogeneic (A/J x B6)F1 kidney grafts responded with equivalent serum DSA titers that reached peak by day 14 post-transplant. B6.CCR5−/− recipients rejected A/J allografts between days 15–30, whereas B6 isografts and (A/J x B6)F1 semi allogeneic grafts survived past day 65. On day 7 post-transplant NK cell infiltration into A/J allografts was composed of distinct Ly49b+ cell populations expressing low and high levels of NK1.1 and both NK1.1high and NK1.1low cells increased in A/J allografts with time to rejection. In contrast, NK cell infiltration into (A/J x B6)F1 grafts on day 7 post-transplant was composed entirely of NK1.1high cells that decreased to background/isograft numbers thereafter. (A/J x B6)F1 grafts at day 65 post-transplant had severe chronic injury with prominent interstitial fibrosis, glomerulopathy and arteriopathy that was accompanied by gene expression of profibrogenic factors. These results indicate that NK cells synergize with DSA to cause acute kidney allograft failure whereas high titers of DSA in the absence of NK cell activation cannot cause acute antibody-mediated rejection but induce the indolent development of interstitial fibrosis and glomerular injury that leads to graft dysfunction and failure at later times after transplantation.

Allograft dendritic cell p40 homodimers activate donor-reactive memory CD8+ T cells

Recipient endogenous memory T cells with donor reactivity pose an important barrier to successful transplantation and costimulatory blockade-induced graft tolerance. Longer ischemic storage times prior to organ transplantation increase early posttransplant inflammation and negatively impact early graft function and long-term graft outcome. Little is known about the mechanisms enhancing endogenous memory T cell activation to mediate tissue injury within the increased inflammatory environment of allografts subjected to prolonged cold ischemic storage (CIS). Endogenous memory CD4+ and CD8+ T cell activation is markedly increased within complete MHC-mismatched cardiac allografts subjected to prolonged versus minimal CIS, and the memory CD8+ T cells directly mediate CTLA-4Ig-resistant allograft rejection. Memory CD8+ T cell activation within allografts subjected to prolonged CIS requires memory CD4+ T cell stimulation of graft DCs to produce p40 homodimers, but not IL-12 p40/p35 heterodimers. Targeting p40 abrogates memory CD8+ T cell proliferation within the allografts and their ability to mediate CTLA-4Ig-resistant allograft rejection. These findings indicate a critical role for memory CD4+ T cell-graft DC interactions to increase the intensity of endogenous memory CD8+ T cell activation needed to mediate rejection of higher-risk allografts subjected to increased CIS.

Depletion-Resistant CD4 T Cells Enhance Thymopoiesis During Lymphopenia

Lymphoablation is routinely used in transplantation, and its success is defined by the balance of pathogenic versus protective T cells within reconstituted repertoire. While homeostatic proliferation and thymopoiesis may both cause T cell recovery during lymphopenia, the relative contributions of these mechanisms remain unclear. The goal of this study was to investigate the role of the thymus during T cell reconstitution in adult allograft recipients subjected to lymphoablative induction therapy. Compared with euthymic mice, thymectomized heart allograft recipients demonstrated severely impaired CD4 and CD8 T cell recovery and prolonged heart allograft survival after lymphoablation with murine anti-thymocyte globulin (mATG). The injection with agonistic anti-CD40 mAb or thymus transplantation only partially restored T cell reconstitution in mATG-treated thymectomized mice. After mATG depletion, residual CD4 T cells migrated into the thymus and enhanced thymopoiesis. Conversely, depletion of CD4 T cells before lymphoablation inhibited thymopoiesis at the stage of CD4^- CD8^- CD44^hi CD25⁺ immature thymocytes. This is the first demonstration that the thymus and peripheral CD4 T cells cooperate to ensure optimal T cell reconstitution after lymphoablation. Targeting thymopoiesis through manipulating functions of depletion-resistant helper T cells may thus improve therapeutic benefits and minimize the risks of lymphoablation in clinical settings.

Total Recall: Can We Reshape T Cell Memory by Lymphoablation?

Despite recent advances in immunosuppression, donor-reactive memory T cells remain a serious threat to successful organ transplantation. To alleviate damaging effects of preexisting immunologic memory, lymphoablative induction therapies are used as part of standard care in sensitized recipients. However, accumulating evidence suggests that memory T cells have advantages over their naive counterparts in surviving depletion and expanding under lymphopenic conditions. This may at least partially explain the inability of existing lymphoablative strategies to improve long-term allograft outcome in sensitized recipients, despite the well-documented decrease in the frequency of early acute rejection episodes. This minireview summarizes the insights gained from both experimental and clinical transplantation as to the effects of existing lymphoablative strategies on memory T cells and discusses the latest research developments aimed at improving the efficacy and safety of lymphoablation.

CD4+ T lymphocytes produce adiponectin in response to transplants

Adiponectin is a pleiotropic cytokine with diverse immunomodulatory effects on macrophages and lymphocytes. In the current paradigm, lymphocytes and macrophages respond to adiponectin that is produced by adipocytes and other parenchymal cells. Using a model of chronic arterial inflammation in cardiac transplants, we found that T cells derived from the recipient migrate to the heart and produce adiponectin locally. The evidence that T cells produce significant amounts of adiponectin is based on 3 experimental approaches. First, CD4+ T cells isolated from the blood and spleen after cardiac transplantation express mRNA for adiponectin. Second, reconstitution of T cell-deficient recipients with transgenic CD4+ T cells that express receptors for donor antigens results in arterial infiltrates containing T cells and increased mRNA expression for adiponectin in cardiac transplants. Third, CD4+ T cells isolated from the allograft secrete adiponectin in vitro. Taken together, these data indicate that adiponectin-competent cells originating in the recipient migrate into the transplant. Establishing T cells as a source of adiponectin provides a new dimension, to our knowledge, to the modulatory effects of adiponectin on immune responses.

Role of Memory T Cells in Allograft Rejection and Tolerance

Memory T cells are characterized by their low activation threshold, robust effector functions, and resistance to conventional immunosuppression and costimulation blockade. Unlike their naïve counterparts, memory T cells reside in and recirculate through peripheral non-lymphoid tissues. Alloreactive memory T cells are subdivided into different categories based on their origins, phenotypes, and functions. Recipients whose immune systems have been directly exposed to allogeneic major histocompatibility complex (MHC) molecules display high affinity alloreactive memory T cells. In the absence of any prior exposure to allogeneic MHC molecules, endogenous alloreactive memory T cells are regularly generated through microbial infections (heterologous immunity). Regardless of their origin, alloreactive memory T cells represent an essential element of the allograft rejection process and a major barrier to tolerance induction in clinical transplantation. This article describes the different subsets of alloreactive memory T cells involved in transplant rejection and examine their generation, functional properties, and mechanisms of action. In addition, we discuss strategies developed to target deleterious allospecific memory T cells in experimental animal models and clinical settings.

Thymus is required for T cell reconstitution following lymphoablation

Lymphoablation is used as induction therapy in transplantation. Following lymphoablation, peripheral lymphopenia triggers homeostatic T cell proliferation and thymopoiesis. However, the relative contribution of each pathway to T cell recovery is unknown. The goal of this study was to test the role of thymus during T cell reconstitution following lymphoablation. We use a mouse model of heterotopic heart transplantation (BALB/c to B6) and a murine analog of Thymoglobulin (mATG). We previously reported that the recovery of CD8 T cells after mATG depletion requires residual memory CD4 T cells and CD40 expressing B cells. CD4 T cell depletion or CD154 blockade significantly reduced the numbers of single positive CD8 thymocytes generated in mATG treated mice. To test if thymus is required for T cell reconstitution, thymectomized and euthymic heart allograft recipients were treated with mATG on days 0 and 4. Thymectomy impaired recovery of CD4 and CD8 T cells compared to euthymic mice (10% vs. 1.7% CD4 and 3% vs 0.06% CD8 T cells on d 12). The transplantation of thymi from B6 WT or RAG1−/− mice under the kidney capsule of thymectomized mice partially restored T cell reconstitution following depletion. Adoptive transfer of CD4 T cells (10×106 i.v. d −1) or treatment with agonistic anti-CD40 mAb (0.1mg i.v. d 0, 2) also restored T cell recovery in thymectomized mice. These results indicate that depletion-resistant memory CD4 T cells and thymus are both required for effective CD8 T cell recovery. Our findings may lead to therapies that specifically target reconstitution of naïve vs memory T cells following lymphoablation.

Inhibition of Endogenous Memory CD8 T Cell Infiltration into Cardiac Allografts Subjected to Prolonged Cold Ischemic Storage is Required to Promote Long-Term Graft Survival Naoki Kohei, Hidetoshi

Endogenous memory CD8 T cells infiltrate MHC-mismatched cardiac allografts and produce IFN-γ in response to donor class I MHC within 24 hrs after graft reperfusion in mice. We have reported that prolonged cold ischemic graft storage (CIS) provokes intense inflammation within hours after allograft reperfusion and promotes CTLA-4Ig-resistant endogenous memory CD8 T cell rejection of the allograft by day 28 post-transplant (whereas CTLA-4Ig prolongs survival of allografts subjected to minimal CIS > day 60). We tested strategies inhibiting endogenous memory CD8 T cell infiltration into cardiac allografts subjected to prolonged CIS to promote long-term allograft survival. Peri-transplant anti-LFA-1 mAb and anti-CD154 mAb treatment of recipients of allografts subjected to minimal CIS prolonged survival of 60% of allograft > 100 days. In contrast, this treatment prolonged only 20% of allografts subjected to prolonged CIS beyond day 80 post-transplant, with rejection accompanied by the appearance of high titers of donor-specific antibody (DSA >10,000 vs. <100 in long-term survivors). Use of a new regimen: peri-transplant recipient treatment with anti-LFA-1 mAb, anti-TNFa mAb, and anti-CD154 mAb plus additional doses of anti-CD154 mAb on days 14 and 16 post-transplant promoted long-term survival of 60% allografts subjected to prolonged CIS past day 100 post-transplant. Recipients with surviving allografts accepted donor skin allografts, but rejected third-party skin allografts. These studies indicate long-term survival of allografts subjected to prolonged CIS by using strategies that include reagents inhibiting endogenous memory CD8 T cell infiltration into the allografts and reagents that inhibit the production of DSA.

NK Cells Play a Critical Role in Mediating Inflammation and Graft Failure During Antibody-Mediated Rejection of Kidney Allografts

The incidence of antibody-mediated kidney graft rejection has increased, but the key cellular and molecular participants underlying this graft injury remain unclear. We previously reported that kidney allograft rejection in CCR5−/− mice is dependent on donor-specific antibody (DSA). The current study determined if cells expressing cytotoxic function contributed to antibody-mediated kidney allograft rejection in CCR5−/− recipients. Wild type C57BL/6, B6.CCR5−/− and CD8−/−/CCR5−/− mice were transplanted with complete MHC mismatched A/J kidney grafts and intra-graft inflammatory components were followed to rejection. B6.CCR5−/− and CD8−/−/CCR5−/− recipients rejected the allografts by day 35 whereas 65% of allografts in wild type recipients survived past day 80 post-transplant. Allograft rejection in wild type recipients was associated with high titers of DSA, equivalent to those induced in CCR5-deficient recipients. Rejected allografts in C57BL/6, B6.CCR5−/− and CD8−/−/CCR5−/− recipients’ expressed high levels of VCAM-1 and MMP7 mRNA that was associated with high serum titers of DSA. At rejection in wild type and CD8−/−/CCR5−/− recipients, kidney allografts also expressed genes associated with NK cell (Sh2d1B1 and MYBL1) but not with T cell (CXCR6) activity during inflammation. High levels of perforin and granzyme B mRNA expression in kidney allografts peaked on day 6 post-transplant in all recipients. NK cell depletion in CD8−/−/CCR5−/− recipients reduced this expression to background levels and promoted long-term survival of 40% of kidney allografts. These results support a role for NK cells in increasing inflammation during antibody-mediated kidney allograft injury and rejection.

Memory CD4 T Cells Induce Antibody-Mediated Rejection of Renal Allografts

Despite advances in immunosuppression, antibody-mediated rejection is a serious threat to allograft survival. Alloreactive memory helper T cells can induce potent alloantibody responses and often associate with poor graft outcome. Nevertheless, the ability of memory T cells to elicit well characterized manifestations of antibody-mediated rejection has not been tested. We investigated helper functions of memory CD4 T cells in a mouse model of renal transplantation. Whereas the majority of unsensitized C57Bl/6 recipients spontaneously accepted fully MHC-mismatched A/J renal allografts, recipients containing donor-reactive memory CD4 T cells rapidly lost allograft function. Increased serum creatinine levels, high serum titers of donor-specific alloantibody, minimal T cell infiltration, and intense C4d deposition in the grafts of sensitized recipients fulfilled all diagnostic criteria for acute renal antibody-mediated rejection in humans. IFNγ neutralization did not prevent the renal allograft rejection induced by memory helper T cells, and CD8 T cell depletion at the time of transplantation or depletion of both CD4 and CD8 T cells also did not prevent the renal allograft rejection induced by memory helper T cells starting at day 4 after transplantation. However, B cell depletion inhibited alloantibody generation and significantly extended allograft survival, indicating that donor-specific alloantibodies (not T cells) were the critical effector mechanism of renal allograft rejection induced by memory CD4 T cells. Our studies provide direct evidence that recipient T cell sensitization may result in antibody-mediated rejection of renal allografts and introduce a physiologically relevant animal model with which to investigate mechanisms of antibody-mediated rejection and novel therapeutic approaches for its prevention and treatment.

CD4 T Cell Help via B Cells Is Required for Lymphopenia-Induced CD8 T Cell Proliferation

Ab-mediated lymphoablation is commonly used in solid organ and hematopoietic cell transplantation. However, these strategies fail to control pathogenic memory T cells efficiently and to improve long-term transplant outcomes significantly. Understanding the mechanisms of T cell reconstitution is critical for enhancing the efficacy of Ab-mediated depletion in sensitized recipients. Using a murine analog of anti-thymocyte globulin (mATG) in a mouse model of cardiac transplantation, we previously showed that peritransplant lymphocyte depletion induces rapid memory T cell proliferation and only modestly prolongs allograft survival. We now report that T cell repertoire following depletion is dominated by memory CD4 T cells. Additional depletion of these residual CD4 T cells severely impairs the recovery of memory CD8 T cells after mATG treatment. The CD4 T cell help during CD8 T cell recovery depends on the presence of B cells expressing CD40 and intact CD40/CD154 interactions. The requirement for CD4 T cell help is not limited to the use of mATG in heart allograft recipients, and it is observed in nontransplanted mice and after CD8 T cell depletion with mAb instead of mATG. Most importantly, limiting helper signals increases the efficacy of mATG in controlling memory T cell expansion and significantly extends heart allograft survival in sensitized recipients. Our findings uncover the novel role for helper memory CD4 T cells during homeostatic CD8 T cell proliferation and open new avenues for optimizing lymphoablative therapies in allosensitized patients.