Background Recent survivals of our pig-to-baboon kidney xenotransplants have been markedly shorter than the graft survivals we previously reported. The discovery of high levels of porcine CMV (pCMV) in one of the rejected xenografts led us to evaluate whether this reduction in graft survival might be due to the inadvertent introduction of pCMV into our GalT-KO swine herd. Methods Archived frozen sections of xeno-kidney grafts over the past 10 years were analyzed for the presence of pCMV, using real-time PCR. Three prospective pig-to-baboon renal transplants using kidneys from swine delivered by caesarian section (C-section) and raised in isolation were likewise analyzed. Results Kidney grafts from which 8 of the 18 archived samples were derived were found to be pCMV-negative, had a mean graft survival of 48.3 days and were from transplants performed before 2008. None had shown signs of DIC and were lost due to either proteinuria or infectious complications. In contrast, 10 of the archived samples were pCMV positive, were from kidney transplants with a mean graft survival of 14.1 days, had been performed after 2008 and had demonstrated early vascular changes and decreased platelet counts. Three prospective xenografts from swine delivered by C-section were pCMV negative and survived an average of 53.0 days. Conclusions Decreased survivals of GalT-KO renal xenografts in this laboratory correlate temporally with latent pCMV in the donor animals and pCMV in the rejected xeno-kidneys. Transmission of pCMV to swine offspring may be avoided by C-section delivery and scrupulous isolation of donor animals.

We previously reported life-supporting a1,3-galactosyltransferase knockout (GalTKO) thymokidney xenograft survival of .2 months in baboons. However, despite otherwise normal renal function, recipients developed proteinuria with morphologic changes (podocyte effacement), a condition that presents a major obstacle to long-term studies in this model. A recent clinical study showed that rituximab therapy after allogeneic transplant prevented proteinuria possibly associated with loss of sphingomyelin phosphodiesterase acid-like 3b . Here, we demonstrate that rituximab prevents the disruption of pig podocytes in an SMPDL-3b-dependent manner in vitro and the early development of proteinuria after xenogeneic kidney transplantation in baboons. Immunofluorescence showed SMPDL-3b expression in pig glomerular epithelium; immunoprecipitation demonstrated rituximab binding to SMPDL-3b in glomeruli. Culture of isolated pig podocytes with naive baboon sera, which has preformed antipig natural antibodies, reduced SMPDL-3b expression, disrupted podocyte morphology, and decreased podocyte proliferation, whereas pretreatment with rituximab prevented these effects. Six baboons received rituximab before transplantation to deplete B cells and again in the peri-transplant period; 18 baboons treated only before transplantation served as historical controls. The onset of post-transplant proteinuria was significantly delayed in a B cell-independent manner in the animals that received peri-transplant rituximab treatment. Although further optimization of this protocol is required, these data provide intriguing clues to the mechanisms of post-transplant proteinuria in xenogeneic kidney transplantation and a potential strategy for its prevention.

Anti-CD3 immunotoxins, which induce profound but transient T cell depletion in vivo by inhibiting eukaryotic protein synthesis in CD3+ cells, are effective reagents in large animal models of transplantation tolerance and autoimmune disease therapy. A diphtheria toxin based anti-porcine CD3 recombinant immunotoxin was constructed by fusing the truncated diphtheria toxin DT390 with two identical tandem single chain variable fragments (scFv) derived from the anti-porcine CD3 monoclonal antibody 898H2-6-15. The recombinant immunotoxin was expressed in a diphtheria-toxin resistant yeast Pichia pastoris strain under the control of the alcohol oxidase promoter. The secreted recombinant immunotoxin was purified sequentially with hydrophobic interaction chromatography (Butyl 650 M) followed by strong anion exchange (Poros 50 HQ). The purified anti-porcine CD3 immunotoxin was tested in vivo in four animals; peripheral blood CD3+ T cell numbers were reduced by 80% and lymph node T cells decreased from 74% CD3+ cells pretreatment to 24% CD3+ cells remaining in the lymph node following 4 days of immunotoxin treatment. No clinical toxicity was observed in any of the experimental swine. We anticipate that this conjugate will provide an important tool for in vivo depletion of T cells in swine transplantation models.

Post-transplant lymphoproliferative disease (PTLD) is a major complication of clinical organ and cell transplantation. Conditioning and immunosuppressive regimens that significantly impair T cell immunity, including depleting antibodies and calcineurin inhibitors, increase the risk of PTLD after transplantation. Swine PTLD has been shown to closely resemble human PTLD in morphology, histology, and viral-driven reactivation of B cells. Previously, we reported high incidences of PTLD after hematopoietic cell transplantation (HCT) in miniature swine recipients conditioned with thymic irradiation (TI) in addition to T cell depletion and cyclosporine A monotherapy after transplantation. Replacement of TI with 100 cGy of total body irradiation resulted in similar numbers of B cells early post-transplantation, greater numbers of T cells at day 0, and markedly decreased incidence of PTLD, suggesting that a threshold number of T cells may be necessary to prevent subsequent B cell proliferation and development of overt PTLD. Results from this large cohort of animals provide insight into the important effect of irradiation and T cell immunity on the incidence of PTLD after HCT and reinforce the pig model as a valuable tool for the study of PTLD and HCT.

Loss of chimerism is an undesirable outcome of allogeneic hematopoietic cell transplantation (HCT) after reduced-intensity conditioning. Understanding the nature of cellular and humoral immune responses to HCT after graft loss could lead to improved retransplantation strategies. We investigated the immunologic responses after graft loss in miniature swine recipients of haploidentical HCT that received reduced-intensity conditioning. After the loss of peripheral blood chimerism, antidonor cellular responses were present without detectable antidonor antibody. Reexposure to donor hematopoietic cells after graft loss induced a sensitized antidonor cellular response. No induced antidonor antibody response could be detected despite evidence of cellular sensitization to donor cells. In contrast, unconditioned animals exposed repeatedly to similar doses of haploidentical donor cells developed antidonor antibody responses. These results could have important implications for the design of treatment strategies to overcome antidonor responses in HCT and improve the outcome of retransplantation after graft loss.

Thymic involution is associated with age-related changes of the immune system. Utilizing our innovative technique of transplantation of a thymus as isolated vascularized graft in MHC-inbred miniature swine, we have previously demonstrated that aged thymi are rejuvenated after transplantation into juvenile swine. Here we have studied the role of insulin-like growth factor (IGF) and forkhead-box protein-N1 (FOXN1) as well as bone marrow (BM) in thymic rejuvenation and involution. We examined thymic rejuvenation and involution by means of histology and flow cytometry. Thymic function was assessed by the ability to induce tolerance of allogeneic kidneys. Aged thymi were rejuvenated in a juvenile environment, and successfully induced organ tolerance, while juvenile thymi in aged recipients involuted and had a limited ability to induce tolerance. However, juvenile BM inhibited the involution process of juvenile thymi in aged recipients. An elevated expression of both FOXN1 and IGF1 receptors (IGF-1R) was observed in juvenile thymi and rejuvenated thymi. Juvenile BM plays a role in promoting the local thymic milieu as indicated by its ability to inhibit thymic involution in aged animals. The expression of FOXN1 and IGF-1R was noted to increase under conditions that stimulated rejuvenation, suggesting that these factors are involved in thymic recovery.

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