Denise J. Lo

Selective Targeting of Human Alloresponsive CD8+ Effector Memory T Cells Based on CD2 Expression

Costimulation blockade (CoB), specifically CD28/B7 inhibition with belatacept, is an emerging clinical replacement for calcineurin inhibitor‐based immunosuppression in allotransplantation. However, there is accumulating evidence that belatacept incompletely controls alloreactive T cells that lose CD28 expression during terminal differentiation. We have recently shown that the CD2‐specific fusion protein alefacept controls costimulation blockade‐resistant allograft rejection in nonhuman primates. Here, we have investigated the relationship between human alloreactive T cells, costimulation blockade sensitivity and CD2 expression to determine whether these findings warrant potential clinical translation. Using polychromatic flow cytometry, we found that CD8+ effector memory T cells are distinctly high CD2 and low CD28 expressors. Alloresponsive CD8+CD2hiCD28− T cells contained the highest proportion of cells with polyfunctional cytokine (IFNγ, TNF and IL‐2) and cytotoxic effector molecule (CD107a and granzyme B) expression capability. Treatment with belatacept in vitro incompletely attenuated allospecific proliferation, but alefacept inhibited belatacept‐resistant proliferation. These results suggest that highly alloreactive effector T cells exert their late stage functions without reliance on ongoing CD28/B7 costimulation. Their high CD2 expression increases their susceptibility to alefacept. These studies combined with in vivo nonhuman primate data provide a rationale for translation of an immunosuppression regimen pairing alefacept and belatacept to human renal transplantation.

Biomarkers for kidney transplant rejection

The immune management of organ transplant recipients is imperfect. Beyond general dosing guidelines for immunosuppressive agents and clinical diagnostic tests for rejection or infection, there are few objective tools to determine the aggregate status of a patients alloimmune response or protective immune capacity. The lack of prognostic precision significantly contributes to patient morbidity and reduces long-term allograft survival after kidney transplantation. Noninvasive biomarkers that could serve as predictive tools or surrogate end points for rejection might help clinicians individualize immunosuppression and allow for early intervention, ideally prior to clinically evident organ dysfunction. Although the growing understanding of organ rejection has provided numerous candidate biomarkers, none has been confirmed in robust validation studies as sufficiently useful to guide clinical practice independent of traditional clinical methods. In this Review, the general characteristics of biomarkers and surrogate end points; current biomarkers under active clinical investigation; and the prominent barriers to the translation of biomarkers into clinical practice are discussed.

Chemokines and their Receptors in Human Renal Allotransplantation

Background. Chemokines and their receptors play a critical role in leukocyte trafficking, and inhibition of select chemokines has been shown to attenuate kidney disease and allograft rejection in animal models. Therefore, we evaluated chemokine and chemokine receptor transcripts in human renal allograft biopsies, correlating transcript levels with clinical course and immunohistochemical analysis to relate chemokine expression to relevant clinical human disease phenotypes. Methods. Renal biopsies were grouped as postreperfusion (n=10), stable function (n=10), subclinical (n=10) or acute rejection (n=17), or calcineurin inhibitor nephrotoxicity (n=9) based on clinical presentation and histopathologic assessment. Using quantitative real-time polymerase chain reaction analysis, chemokine transcripts were assessed relative to transcript levels in preprocurement biopsies from live donor kidneys (n=15). Results. Transcripts from several inflammatory chemokines (CCL3, CCL5, CXCL9, CXCL10, and CXCL11) and chemokine receptors (CCR5, CCR7, and CXCR3) were significantly increased in allografts with subclinical and clinical acute rejection, indicating a strong polarization toward a T-helper 1 effector phenotype during rejection. These transcripts also distinguished acutely rejecting allografts from allografts with nonrejection causes of renal dysfunction. Biopsies from patients with stable function without histologic evidence of rejection had increased chemokine transcript levels that were qualitatively similar but quantitatively reduced compared with rejecting allografts. Conclusions. This comprehensive evaluation of chemokines and their receptors in human renal transplantation defines associations between chemokine expression and clinical phenotypes, may have diagnostic utility, and highlights relevant pathways for therapeutic intervention.

Belatacept and Sirolimus Prolong Nonhuman Primate Renal Allograft Survival Without a Requirement for Memory T Cell Depletion

Belatacept is an inhibitor of CD28/B7 costimulation that is clinically indicated as a calcineurin inhibitor (CNI) alternative in combination with mycophenolate mofetil and steroids after renal transplantation. We sought to develop a clinically translatable, nonlymphocyte depleting, belatacept‐based regimen that could obviate the need for both CNIs and steroids. Thus, based on murine data showing synergy between costimulation blockade and mTOR inhibition, we studied rhesus monkeys undergoing MHC‐mismatched renal allotransplants treated with belatacept and the mTOR inhibitor, sirolimus. To extend prior work on costimulation blockade‐resistant rejection, some animals also received CD2 blockade with alefacept (LFA3‐Ig). Belatacept and sirolimus therapy successfully prevented rejection in all animals. Tolerance was not induced, as animals rejected after withdrawal of therapy. The regimen did not deplete T cells. Alefecept did not add a survival benefit to the optimized belatacept and sirolimus regimen, despite causing an intended depletion of memory T cells, and caused a marked reduction in regulatory T cells. Furthermore, alefacept‐treated animals had a significantly increased incidence of CMV reactivation, suggesting that this combination overly compromised protective immunity. These data support belatacept and sirolimus as a clinically translatable, nondepleting, CNI‐free, steroid‐sparing immunomodulatory regimen that promotes sustained rejection‐free allograft survival after renal transplantation.

CD28 Negative T Cells: Is Their Loss Our Gain?

CD28 is a primary costimulation molecule for T cell activation. However, during the course of activation some T cells lose this molecule and assume a CD28‐independent existence. These CD28− T cells are generally antigen‐experienced and highly differentiated. CD28− T cells are functionally heterogeneous. Their characteristics vary largely on the context in which they are found and range from having enhanced cytotoxic abilities to promoting immune regulation. Thus, CD28 loss appears to be more of a marker for advanced differentiation regardless of the cytotoxic or regulatory function being conducted by the T cell. CD28− T cells are now being recognized as playing significant roles in several human diseases. Various functional CD28− populations have been characterized in inflammatory conditions, infections and cancers. Of note, the recent introduction of costimulation blockade‐based therapies, particularly those that inhibit CD28–B7 interactions, has made CD28 loss particularly relevant for solid organ transplantation. Certain CD28− T cell populations seem to promote allograft tolerance whereas others contribute to alloreactivity and costimulation blockade resistant rejection. Elucidating the interplay between these populations and characterizing the determinants of their ultimate function may have relevance for clinical risk stratification and personal determination of optimal posttransplant immune management.

Inhibition of αvβ6 Promotes Acute Renal Allograft Rejection in Nonhuman Primates

The integrin αvβ6 activates latent transforming growth factor‐β (TGF‐β) within the kidney and may be a target for the prevention of chronic allograft fibrosis after kidney transplantation. However, TGF‐β also has known immunosuppressive properties that are exploited by calcineurin inhibitors (CNIs); thus, the net benefit of αvβ6 inhibition remains undetermined. To assess the acute impact of interference with αvβ6 on acute rejection, we tested a humanized αvβ6‐specific monoclonal antibody (STX‐100) in a randomized, double‐blinded, placebo‐controlled nonhuman primate renal transplantation study to evaluate whether αvβ6 blockade alters the risk of acute rejection during CNI‐based immunosuppression. Rhesus monkeys underwent renal allotransplantation under standard CNI‐based maintenance immunosuppression; 10 biopsy‐confirmed rejection‐free animals were randomized to receive weekly STX‐100 or placebo. Animals treated with STX‐100 experienced significantly decreased rejection‐free survival compared to placebo animals (p = 0.049). Immunohistochemical analysis confirmed αvβ6 ligand presence, and αvβ6 staining intensity was lower in STX‐100‐treated animals (p = 0.055), indicating an apparent blockade effect of STX‐100. LAP, LTBP‐1 and TGF‐β were all decreased in animals that rejected on STX‐100 compared to those that rejected on standard immunosuppression alone, suggesting a relevant effect of αvβ6 blockade on local TGF‐β. These data caution against the use of αvβ6 blockade to achieve TGF‐β inhibition in kidney transplantation.

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

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

Selective Targeting of High-Affinity LFA-1 Does Not Augment Costimulation Blockade in a Nonhuman Primate Renal Transplantation Model.

Costimulation blockade (CoB) via belatacept is a lower‐morbidity alternative to calcineurin inhibitor (CNI)‐based immunosuppression. However, it has higher rates of early acute rejection. These early rejections are mediated in part by memory T cells, which have reduced dependence on the pathway targeted by belatacept and increased adhesion molecule expression. One such molecule is leukocyte function antigen (LFA)‐1. LFA‐1 exists in two forms: a commonly expressed, low‐affinity form and a transient, high‐affinity form, expressed only during activation. We have shown that antibodies reactive with LFA‐1 regardless of its configuration are effective in eliminating memory T cells but at the cost of impaired protective immunity. Here we test two novel agents, leukotoxin A and AL‐579, each of which targets the high‐affinity form of LFA‐1, to determine whether this more precise targeting prevents belatacept‐resistant rejection. Despite evidence of ex vivo and in vivo ligand‐specific activity, neither agent when combined with belatacept proved superior to belatacept monotherapy. Leukotoxin A approached a ceiling of toxicity before efficacy, while AL‐579 failed to significantly alter the peripheral immune response. These data, and prior studies, suggest that LFA‐1 blockade may not be a suitable adjuvant agent for CoB‐resistant rejection.

Pancreas transplantation in unconventional recipients.

Purpose of reviewAdvances in surgical technique and immunosuppression have significantly improved outcomes after pancreas transplantation, and as a result pancreas transplants increasingly are being performed for indications other than type 1 diabetes mellitus. This review summarizes the current literature on pancreas transplantation in unconventional recipient populations. Recent findingsAn increasing body of work suggests that pancreas transplantation can be performed with good outcomes in patients with type 2 diabetes mellitus and those 50 years of age and older. Obesity appears detrimental to patient and pancreas graft survival, and bariatric surgery prior to transplantation may be of increasing interest and relevance. There are limited data yielding mixed outcomes on pancreas transplantation in patients with HIV or hepatitis C virus. However, rapidly improving antiviral therapies are prolonging survival in patients with HIV and chronic hepatitis C virus infections and may increase the number of candidates available for pancreas transplantation in these populations in the future. SummaryDespite limited literature in these patient populations, pancreas transplantation may be a viable treatment option for endocrine pancreas failure in appropriately selected patients regardless of disease cause or age.