We have previously shown that tolerance of kidney allografts across a full major histocompatibility complex (MHC) barrier can be induced in miniature swine by a 12-day course of high-dose tacrolimus. However, that treatment did not prolong survival of heart allografts across the same barrier. We have now tested the effect of cotransplanting an allogeneic heart and kidney from the same MHC-mismatched donor using the same treatment regimen. Heart allografts (n = 3) or heart plus kidney allografts (n = 5) were transplanted into MHC-mismatched recipients treated with high-dose tacrolimus for 12 days. As expected, all isolated heart allografts rejected by postoperative day 40. In contrast, heart and kidney allografts survived for >200 days with no evidence of rejection on serial cardiac biopsies. Heart/kidney recipients lost donor-specific responsiveness in cell-mediated lympholysis and mixed-lymphocyte reaction assays, were free of alloantibody and exhibited prolonged survival of donor, but not third-party skin grafts. Late (>100 days) removal of the kidney allografts did not cause acute rejection of the heart allografts (n = 2) and did not abrogate donor-specific unresponsiveness in vitro. While kidney-induced cardiac allograft tolerance (KI-CAT) has previously been demonstrated across a Class I disparity, these data demonstrate that this phenomenon can also be observed across the more clinically relevant full MHC mismatch. Elucidating the renal element(s) responsible for KICAT could provide mechanistic information relevant to the induction of tolerance in recipients of isolated heart allografts as well as other tolerance-resistant organs.

Donor-specific antibodies (DSA) contribute to renal allograft loss. However, biomarkers to guide clinical management of DSA post-transplant or detect humoral alloimmune responses before alloantibodies develop are not available. Circulating T follicular helper (Tfh) cells are CD4 + CXCR5 + Tfh-like cells in the blood that have been associated with alloantibodies in transplant recipients, but whether they precede antibody formation for their evaluation as a predictive biomarker in transplantation is unknown. To evaluate the ability of circulating Tfh (cTfh) cells to predict DSA, we utilized murine transplant models to determine the temporal relationship between cTfh cells, germinal center (GC) formation, and DSA development. We observed that donor-reactive CD4 + CXCR5 + cTfh cells expand following allotransplantation. These cTfh cells were equivalent to graft-draining lymph node-derived Tfh cells in their ability to provide B cell help for antibody production. Circulating Tfh cell expansion and differentiation into ICOS + PD-1 + cells temporally correlated with GC alloreactivity and preceded the generation of DSA in instances of modified and unmodified alloantibody formation. Importantly, delayed costimulation blockade initiated after the detection of ICOS + PD-1 + cTfh cells prevented DSA. These findings suggest that cTfh cells could serve as a biomarker for humoral alloreactivity prior to the detection of alloantibodies and inform therapeutic approaches to prevent DSA.

Donor-specific antibodies (DSAs) are a barrier to improved long-term outcomes after kidney transplantation. Costimulation blockade with CTLA4-Ig has shown promise as a potential therapeutic strategy to control DSAs. T follicular helper (Tfh) cells, a subset of CD4 T cells required for optimal antibody production, are reliant on the CD28 costimulatory pathway. We have previously shown that selective CD28 blockade leads to superior allograft survival through improved control of CD8 T cells relative to CTLA4-Ig, but the impact of CD28-specific blockade on CD4 Tfh cells is unknown. Thus, we identified and characterized donor-reactive Tfh cells in a murine skin transplant model and then used this model to evaluate the impact of selective CD28 blockade with an anti-CD28 domain antibody (dAb) on the donor-specific Tfh cell-mediated immune response. We observed that the anti-CD28 dAb led to superior inhibition of donor-reactive CXCR5 PD-1 Tfh cells, CD95 GL7 germinal center B cells and DSA formation compared with CTLA4-Ig. Interestingly, donor-reactive Tfh cells differentially upregulated CTLA4 expression, suggesting an important role for CTLA4 in mediating the superior inhibition observed with the anti-CD28 dAb. Therefore, selective CD28 blockade as a novel approach to control Tfh cell responses and prevent DSA after kidney transplantation warrants further study.

Because extraanatomic bypass grafting can provide long-term results equivalent to aortorenal bypass grafting, the choice among techniques for RAR in patients with diffuse atherosclerosis should be based on both technical and operative safety considerations, rather than adherence to aortorenal bypass grafting as an inherently superior technique.

Kidney allografts possess the ability to enable a short course of immunosuppression to induce tolerance of themselves and of cardiac allografts across a full-MHC barrier in miniature swine. However, the renal element-(s) responsible for kidney-induced cardiac allograft tolerance (KICAT) are unknown. Here we investigated whether MHC disparities between parenchyma versus hematopoietic-derived “passenger” cells of the heart and kidney allografts affected KICAT. Heart and kidney allografts were co-transplanted into MHC-mismatched recipients treated with high-dose tacrolimus for 12 days. Group 1 animals (n=3) received kidney and heart allografts fully MHC-mismatched to each other and to the recipient. Group 2 animals (n=3) received kidney and heart allografts MHC-matched to each other but MHC-mismatched to the recipient. Group 3 animals (n=3) received chimeric kidney allografts whose parenchyma was MHC-mismatched to the donor heart. Group 4 animals (n=3) received chimeric kidney allografts whose passenger leukocytes were MHC-mismatched to the donor heart. Five of six heart allografts in Groups 1 and 3 rejected <40 days. In contrast, heart allografts in Groups 2 and 4 survived >150 days without rejection (p<0.05). These data demonstrate that KICAT requires MHC-matching between kidney allograft parenchyma and heart allografts, suggesting that cells intrinsic to the kidney enable cardiac allograft tolerance.