CD4⁺ T-cell-dependent immune damage of liver parenchymal cells is mediated by alloantibody

Background. Allogeneic hepatocytes initiate both CD4- and CD8-dependent rejection responses. The current studies address the hypothesis that acute damage of allogeneic liver parenchymal cells by the CD4-dependent pathway is alloantibody-mediated and examines immune conditions which promote activation of this pathway. Methods. The role of alloantibody in CD4-dependent hepatocyte rejection was evaluated by assessing hepatocyte (FVB/N, H-2^q) survival in CD8-depleted B-cell knockout (KO) (H-2^b) recipients and by monitoring hepatocyte survival in C57BL/6.SCID (H-2^b) recipients transfused with donor-reactive alloantibody. The development of donor-reactive alloantibody in C57BL/6 (H-2^b), CD8-depleted C57BL/6, CD8 KO (H-2^b), IFN-γ KO (H-2^b), perforin KO (H-2 ^b), and FasL mutant gld/gld (H-2^b) hepatocyte recipients was assessed. Results. Hepatocyte rejection in B-cell KO mice was significantly delayed by CD8⁺ T-cell depletion (median survival time [MST], 35 days) when compared to untreated (MST, 8 days) and CD4-depleted (MST, 10 days) recipient mice. Transfusion of donor-reactive alloantibody into SCID recipients with functional hepatocellular allografts was sufficient to precipitate rejection in a dose-dependent fashion. Donor-reactive alloantibody was minimal in the serum of C57BL/6 hepatocyte recipients, but was produced in significant quantities in hepatocyte recipients genetically deficient in or depleted of CD8⁺ T cells and in recipients with impaired cytotoxic effector mechanisms. In addition, recipients with defects in Th1 immunity, such as IFN-γ KO recipients, also produced readily detectable alloantibody. Conclusions. Collectively, these data support the hypothesis that acute immune damage of allogeneic hepatocytes by the CD4-dependent pathway is mediated by alloantibody and that this pathway is favored when Th1- or cell-mediated cytotoxic effector immune mechanisms are impaired.