Kidney transplantation remains limited by toxicities of calcineurin inhibitors (CNIs) and steroids. Belatacept is a less toxic CNI alternative, but existing regimens rely on steroids and have higher rejection rates. Experimentally, donor bone marrow and sirolimus promote belatacept’s efficacy. To investigate a belatacept-based regimen without CNIs or steroids, we transplanted recipients of live donor kidneys using alemtuzumab induction, monthly belatacept and daily sirolimus. Patients were randomized 1:1 to receive unfractionated donor bone marrow. After 1 year, patients were allowed to wean from sirolimus. Patients were followed clinically and with surveillance biopsies. Twenty patients were transplanted, all successfully. Mean creatinine (eGFR) was 1.10±0.07mg/dl (89±3.56ml/min) and 1.13±0.07 mg/dl (and 88±3.48 ml/min) at 12- and 36-months, respectively. Excellent results were achieved irrespective of bone marrow infusion. Ten patients elected oral immunosuppressant weaning, seven of whom were maintained rejection-free on monotherapy belatacept. Those failing to wean were successfully maintained on belatacept-based regimens supplemented by oral immunosuppression. Seven patients declined immunosuppressant weaning and three patients were denied weaning for associated medical conditions; all remained rejection-free. Belatacept and sirolimus effectively prevent kidney allograft rejection without CNIs or steroids when used following alemtuzumab induction. Selected, immunologically low-risk patients can be maintained solely on once monthly intravenous belatacept.

Tacrolimus impairs allo- and viral-specific T cell responses. Belatacept, a costimulation-based alterative to tacrolimus, has emerged with a paradoxical picture of less complete control of alloimmunity with concomitant impaired viral immunity limited to viral-naïve patients. To reconcile these signatures, bulk population and purified memory and naïve lymphocytes from Cytomegalovirus (CMV) seropositive (n=10) and seronegative (n=10) volunteers were studied using flow cytometry, interrogating proliferation (CFSE dilution) and function (intracellular cytokine staining) in response to alloantigens or CMV-pp-65 peptides. As anticipated, T cells from CMV-experienced, but not naïve, individuals responded to pp-65 with a small percentage of their repertoire (<2.5%) consisting predominantly of mature, polyfunctional (expressing IFN-γ, TNF-α, and IL-2) T effector memory cells. Both CMV naïve and experienced individuals responded similarly to alloantigen with a substantially larger percentage of the repertoire (up to 48.2%) containing proportionately fewer polyfunctional cells. Tacrolimus completely inhibited responses of CMV- and allo-specific T cells regardless of their maturation. However, belatacept’s effects were decreasingly evident in increasingly matured cells, with minimal effect on viral-specific triple cytokine producers and CD28 negative allospecific cells. These data indicate that belatacept’s immunosuppressive effect, unlike tacrolimus’s, wanes on progressively developed effector responses, and may explain the observed clinical effects of belatacept.

CD80 and CD86 (also known as B7-1 and B7-2, respectively) are both ligands for the T cell costimulatory receptors CD28 and CD152. Both CD80 and CD86 mediate T cell costimulation, and as such, have been studied for their role in promoting allograft rejection. In this study we demonstrate that administering monoclonal antibodies specific for these B7 ligands can delay the onset of acute renal allograft rejection in rhesus monkeys. The most durable effect results from simultaneous administration of both anti-B7 antibodies. The mechanism of action does not involve global depletion of T or B cells. Despite in vitro and in vivo evidence demonstrating the effectiveness of the anti-B7 antibodies in suppressing T cell responsiveness to alloantigen, their use does not result in durable tolerance. Prolonged therapy with murine anti-B7 antibodies is limited by the development of neutralizing antibodies, but that problem was avoided when humanized anti-B7 reagents are used. Most animals develop rejection and an alloantibody response although still on antibody therapy and before the development of a neutralizing antibody response. Anti-B7 antibody therapy may have use as an adjunctive agent for clinical allotransplantation, but using the dosing regimens we used, is not a tolerizing therapy in this non-human primate model.

These results show that treatment with the CD154-specific monoclonal antibody, hu5C8, greatly delays the onset of acute skin allograft rejection.

CD154 is a cell surface molecule expressed on activated T cells that binds to CD40, an activating molecule on APCs. Its blockade has been shown to prevent allograft rejection, presumably by interrupting interactions between T cells and APCs. It is known that activated human platelets express and shed CD154 and can induce APC activation and other immune processes in vitro. Here we show that platelet-derived CD154 is sufficient to initiate cardiac allograft rejection independent of any cellular source of this molecule. CD154-KO mice reject cardiac allografts after receiving CD154-expressing human platelets or recombinant CD154 (rCD154) trimers. Treatment with the human CD154-specific mAb 5c8 specifically prevents this induced rejection. Soluble trimers, but not platelets, induce rejection when infused temporally remote from the surgical procedure, suggesting that surgically induced platelet activation is required for CD154 release. Allograft rejection can thus be instigated by activated platelets through CD154. These data implicate platelets as a proximal component of acquired alloimmunity, providing insight into the mechanisms of allograft rejection and the physiological response to trauma in general.