Xenograft rejection of fetal porcine islet‐like cell clusters in the rat: effects of active and passive immunization

Abstract

The process of islet xenograft rejection is still poorly understood. To elucidate further possible mechanism(s) involved in xenograft rejection, the effect of different immunization protocols was investigated. Fetal porcine islet-like cell clusters (ICCs) were transplanted under the kidney capsule in otherwise untreated rats, rats pre-immunized by s.c. injections of ICCs and in rats passively immunized with immune serum. The rejection process was evaluated with regard to antibody and complement deposition in the graft, as well as to morphology and phenotype of the infiltrating cells. In otherwise untreated animals, a moderate perigraft mononuclear cell infiltrate was seen after 3 days. Graft destruction became evident on day 6 with marked intragraft infiltration by macrophages (ED1 positive), whereas T cells were in the minority and mainly located in the perigraft area. In contrast to the findings in non-immunized rats, the rejection process in pre-immunized rats was characterized by marked intragraft infiltration by macrophages 3 days after transplantation. Moreover, both T cells and macrophages heavily infiltrated the adjacent kidney parenchyma, and major histocompatibility complex (MHC) class II expression in surrounding kidney tubular cells was concomitantly enhanced. Syngeneic rat islets mixed with porcine ICCs escaped the rejection process in non-immunized rats but were affected in pre-immunized animals. Thus, the specificity of the rejection process in non-immunized animals seems to be lost in pre-immunized animals. The early macrophage infiltration was also accelerated in rats passively immunized with immune serum, but no early switch from perigraft to intragraft infiltration or subsequent cellular infiltration in the adjacent kidney parenchyma was seen. Circulating xenoreactive antibodies of the IgG isotype increased after transplantation in normal and otherwise untreated rats. No distinct IgG deposition in the ICC xenografts was observed until day 12 after transplantation in untreated rats, whereas perigraft deposition of IgG was found 1 day after transplantation in pre-immunized rats and in rats given immune serum. No deposition of complement was observed within the ICC xenograft in any of the groups during the observation period. The dependence on T cells, the massive infiltration of macrophages with a unique phenotype, the cellular distribution, and the loss of specificity (bystander killing) of the rejection process in immunized rats suggest that ICC xenograft rejection shares some of its main characteristics with a delayed type hypersensitivity-like (DTH) immune response.

A Distinct Th1 Immune Response Precedes the Described Th2 Response in Islet Xenograft Rejection

“…It may be speculated that this balanced immune response is lost when an animal is rechallenged with the same antigen, leading to a fully mature DTH reaction characterized by the well-known dominance of Th1-associated cytokines and a pronounced bystander effect. Our previous observations regarding the specificity and intensity in rejection of an ICC xenograft in nonimmunized compared with immunized animals support this notion (20).…”

“…However, it should be noted that the DTH reaction is per definition an immune response observed after previous immunization and generation of memory T-cells, and that almost nothing is known about the immunological mechanisms orchestrating the reaction or its specificity at the time when the antigen is first encountered. Interestingly, both the tempo and strength of the ICC xenograft rejection is increased, whereas the specificity of the rejection is lost in animals previously immunized with pig islets (20). Based on the temporal correlation between immunohistological findings and cytokine profiles in this xenograft model, it is argued that cellular rejection is caused by Th1 CD4 ϩ T-cell-mediated activation of macrophages, i.e., a DTH reaction.…”

Control of In Vivo Collateral Damage Generated by T Cell Immunity

“…In transplant models, using a mixture of syngeneic and allogeneic or xenogeneic islets (5,6) or with allophenic skin grafts (7,8), results supported the concept of selective killing of specific target cells. In contrast, other skin transplant models have shown significant bystander damage to adjacent syngeneic cells (9), and bystander injury occurred in mixed xenogeneic and syngeneic islets in primed recipients (10). Importantly, it is not yet clear how bystander damage is mediated in vivo and, conversely, what mechanisms restrain this form of collateral tissue injury.…”

Organogenetic tolerance

“…In previous studies that did not employ transplantation of E28 pig pancreatic primordia prior to implantation of islets or islet cell clusters [14][15][16] complete destruction of grafts was evident within two weeks. Pre-immunization of rats by subcutaneous injection of porcine islet cell clusters accelerates the rejection of islet probes.…”

“…6) may reflect the second cellular mechanism described by Korsgren 23 in the context of engraftment of a cell component of porcine islets to which rats have been rendered clusters transplanted subsequently beneath the renal capsule. 16 In contrast, prior transplantation of E28 pig pancreatic primordia in mesentery enables survival of an insulin-expressing cell component of porcine islets implanted subsequently in rat kidneys (Fig. 3).…”