Abstract
ROLE OF CD8+ T CELLS IN CARDIAC ALLOGRAFT REJECTION IN WILD-TYPE AND CD28-DEFICIENT MICE
CD4+ but not CD8+ T cells are known to play a major role in cardiac allograft rejection in wild-type mice. In addition, blockade of the CD28 costimulatory pathway by CTLA4-Ig has been shown to promote tolerance in wild-type recipients. However, mice deficient in CD28 expression by homologous recombination are fully capable of rejecting cardiac allografts. To determine whether the mechanism of rejection is similar in wild-type and CD28-deficient mice, we have used antibodies to deplete CD4+ and CD8+ T cells following heterotopic abdominal transplantation of fully mismatched cardiac allografts. Depletion of CD4+ T cells 2 days before transplantation of a BALB/c heart resulted in long-term allograft survival (greater than 100 days) in both wild-type and CD28-deficient C57BL/6 mice. Depletion of CD8+ T cells did not prolong graft survival in wild-type mice, confirming the importance of CD4+ T cells in this model. Surprisingly, depletion of CD8+ T cells in CD28-deficient mice resulted in prolonged allograft survival greater than 100 days. These results led to 2 different hypotheses. First, the mechanism of cardiac allograft rejection may be different in CD28-deficient mice than in wild-type mice. Alternatively, lack of CD28 on CD4+ T cells may reveal a role for CD8+ T cells which was not normally appreciated in wild-type mice because CD4+ T cells were sufficient to promote rejection. To distinguish between these possibilities, we depleted CD4+ T cells at day 4 following transplantation rather than prior to it. This regimen failed to prolong cardiac allograft survival in wild-type mice. However, when depletion of CD8+ T cells was combined with this delayed anti-CD4 treatment, heart allografts survived greater than 100 days. These results reveal a role for CD8+ T cells in allograft rejection in wild-type mice when CD4+ T cells are only present during the initial phase of acute allograft rejection. As an alternative approach, wild-type mice were treated with a blocking anti-CD40L antibody, inasmuch as CD40L has been shown to play a major role in the costimulation of CD4+ but not, or to a lesser degree, of CD8+ T cells. This treatment induced moderate prolongation of allograft survival in wild-type mice (MST 39 days, as compared with 8 days in untreated mice). However, when depleting anti-CD8 mAb was administered simultaneously to anti-CD40L mAb, long-term survival of greater than 100 days was again achieved in wild-type mice. Taken together these data support a role for CD8+ T cells for cardiac allograft rejection in both wild-type and CD28-deficient mice and indicate that CD4+ T cells usually drive the initial rejection process. In conclusion, CD4+ T cells play a crucial role in cardiac allograft rejection in wild-type mice. However, when the function of CD4+ T cells is impaired by lack of CD28, blockade of CD40L, or late depletion using anti-CD4 mAb at day 4, CD8+ T cells become essential for acute allograft rejection to occur.