Analysis of the Th1/Th2 Paradigm in Transplantation: Interferon-γ Deficiency Converts Th1-Type Proislet Allograft Rejection to a Th2-Type Xenograft-Like Response

The rejection mechanisms for fetal proislet allografts and pig proislet xenografts in mice are characterized by different intragraft cytokine mRNA profiles and cellular responses. Allograft rejection is predominantly CD8 T-cell-dependent and is associated with a Th1-type cytokine pattern (i.e., IFN-γ, IL-2 but no IL-4 or IL-5 mRNA). In contrast, xenograft rejection is CD4 T-cell-dependent and is accompanied by a strong Th2-type response (i.e., enhanced expression of IL-4 and IL-5 mRNA) and by marked eosinophil accumulation at the graft site. We have now examined and compared the regulatory role of IFN-γ in both proislet allograft and xenograft rejection processes. The histopathology and intragraft cytokine mRNA profile of BALB/c (H-2^d) proislet allografts were examined in IFN-γ-deficient and wild-type C57BL/6J recipient mice. The survival of pig proislet xenografts was also assessed in IFN-γ –/– and wild-type hosts. Both proislet allografts and xenografts were acutely rejected in IFN-γ –/– and wild-type mice. Unlike the conventional allograft reaction, which lacks eosinophil infiltration, the rejection of proislet allografts in IFN-γ-deficient hosts correlated with intragraft expression of IL-4 and IL-5 mRNA (i.e., a Th2-type response) and eosinophil recruitment. The rejection of proislet allografts and xenografts can therefore occur by IFN-γ-independent pathways; IFN-γ, however, regulates the pathology of the allograft reaction but not the xenograft response. The immune destruction of proislet allografts is not prevented by Th2 cytokine gene expression; instead, the latter correlated with the recruitment of unconventional inflammatory cells (eosinophils), which may play an accessory role in effecting graft injury. Significantly, the Th1-to-Th2-like switch resulted in the novel conversion of an allograft rejection reaction into a xenograft-like rejection process.