THE CD4+ EFFECTOR T CELL REQUIRES IFN-γ TO MEDIATE ACUTE CARDIAC ALLOGRAFT REJECTION

Abstract

61 While generally regarded as a helper cell, CD4+ T cells can also act as direct effector cells in acute cardiac allograft rejection. Previous studies show that the CD4+ T cell is sufficient to mediate acute cardiac rejection. However, the distinct mechanism(s) of CD4-mediated rejection has not been clarified. One potential mechanism is via elaboration of pro-inflammatory cytokines such as IFN-γ. This study tests the hypothesis that CD4-mediated acute cardiac allograft rejection is IFN-γ-dependent. Methods: Heterotopic allografts from IFN-γ receptor (IFN-γ R) deficient 129 (H-2b) mice were transplanted into immune-deficient BALB/c SCID (H-2d SCID) mice. SCID mice with established grafts then underwent adoptive transfer of 107 purified CD4+ T cells from BALB/c (H-2d) donors. This group was compared to a control group of C.B-17 SCID mice (H-2d SCID) recipients of IFN-γ R+ C57BL/6 (H-2b) hearts with similar adoptive transfer of BALB/c CD4+ T cells. CD4+ T cell purification was performed by negative selection on immunoaffinity columns (Biotex). CD4 purity was verified by flow cytometry to contain <0.5% contaminating CD8+ T cells or B220+ B cells both prior to adoptive transfer and on day +30 following adoptive transfer. Hearts were monitored for rejection by daily palpation. Results: Purified CD4 T cells failed to trigger rejection of IFN-γ R deficient heart allografts established in SCID mice (n=4, 3/4 grafts >50 days, p=.006 vs. controls; one recipient died on day 42 with a functioning graft). Conversely, CD4 T cells triggered acute rejection of IFN-γ R+ heart allografts in SCID recipients (survival of 11.5 days; n=10, days 8, 9, 9, 9, 10, 10, 11, 11, 16, 22). Conclusion: IFN-γ receptor deficiency on the cardiac allograft provides protection from CD4-mediated acute allograft rejection, suggesting that IFN-γ is a primary effector cytokine in CD4+ T cell-mediated acute rejection. Thus, while previous studies have found that IFN-γ−/− mice acutely reject cardiac allografts, rejection mediated by CD4+ effector T cells appears to require IFN-γ. Taken together, cardiac allograft immunity may have both IFN-γ dependent and IFN-γ independent pathways of rejection.