CD8+ T Cells Negatively Regulate IL-4–Dependent, IgG1-Dominant Posttransplant Alloantibody Production

Abstract

We have previously reported that CD8+ T cells significantly influence Ab production based on the observation that posttransplant alloantibody levels in CD8-deficient murine hepatocyte transplant recipients are markedly enhanced. However, the precise mechanisms contributing to enhanced alloantibody production in the absence of CD8+ T cells is not understood. We hypothesized that alloactivated CD8+ T cells inhibit Ab production by skewing toward a proinflammatory cytokine profile, whereas when these cells are absent, an anti-inflammatory cytokine profile shifts the alloimmune response toward alloantibody production. To investigate this possibility, alloantibody isotype profiles were examined in CD8-deficient and wild-type hepatocyte recipients. We found that IgG1 (IL-4–dependent isotype) was the dominant alloantibody isotype in wild-type recipients as well as in CD8-deficient recipients, although the amount of alloantibody in the latter group was substantially higher. Utilizing real-time PCR we found that CD4+ T cells from wild-type recipients significantly upregulated IFN-γ but not IL-4 mRNA. In contrast, in the absence of CD8+ T cells, CD4+ T cells switched to significantly upregulate IL-4 mRNA, while IFN-γ was downregulated. IL-4 knockout mice do not produce any posttransplant alloantibody. However, adoptive transfer of wild-type CD4+ T cells into CD8-depleted IL-4 knockout mice restores high alloantibody levels observed in CD8-depleted wild-type recipients. This suggests that IL-4–producing CD4+ T cells are critical for posttransplant alloantibody production. Additionally, this CD8-mediated regulation of posttransplant alloantibody production is IFN-γ–dependent. Further elucidation of the mechanisms by which CD8+ T cells influence Ab production will significantly contribute to development of therapies to manipulate humoral responses to Ag.