Background: In this study, the mechanisms by which CD4+ T cells interact with the innate immune system in xenograft rejection were investigated.
Methods: Fetal pig pancreas (FPP) grafts were transplanted into female SCID mice. The FPP recipient SCID mice were reconstituted with exogenous leukocytes obtained from male BALB/c mice.
Results: Although nonreconstituted SCID recipients or recipients reconstituted with CD4+ T cell-depleted leukocytes showed indefinite FPP graft survival with very few macrophages infiltrating their grafts, reconstitution of SCID recipients with as few as 2x10(5) CD4+ T cells was sufficient to induce rapid xenograft rejection. CD4+ T cells secreted interferon-gamma but not interleukin-4 and initiated the activation and accumulation of macrophages and natural killer cells, that were responsible for the rapid graft destruction. Suppression of interferon-gamma prolonged graft survival and suppressed the macrophages and natural killer cell accumulation and activation.
Conclusions: These results demonstrate that CD4+ T cell-dependent cellular xenograft rejection was a result of macrophage and natural killer cell accumulation and activation, but was not mediated by eosinophils. Consistent with this was the finding that interferon-gamma but not interleukin-4 was in part responsible for mediating this effect.