Background.

Skin grafts from mice that are deficient in the expression of both class I and class II major histocompatibility complex (MHC) antigens are rejected rapidly by normal recipients.

Methods.

To determine the mechanism of this rejection, MHC-deficient skin grafts were placed on recipients with different degrees of antigenic disparity and on recipients depleted of selected T cell subpopulations. In addition, the recipient's T cells were examined in vitro for their responses before and after graft rejection.

Results.

The results indicate that (1) CD4+ cells provide help for this rejection by recognizing donor antigens presented by recipient class II antigens, and (2) CD8+ cells can participate as effector cells, recognizing residual class I antigens expressed by the MHC-dificient grafts.

Conclusions.

The primary conclusion from these studies is that the supposedly MHC-deficient mice actually do have sufficient class I antigen expression to cause skin graft rejection. This finding prevents the use of these mice to answer definitively the question of whether grafts entirely lacking MHC antigens would be rejected. However, these studies do illustrate two important (although previously recognized) features of allogeneic skin graft rejection:(1) that rejection can be initiated by help provided entirely through the indirect pathway, and (2) that help provided through the indirect pathway is available for effector T cells sensitized directly by donor cells. However, the results from these and other studies suggest that indirect effector mechanisms would probably be able to destroy truly MHC-deficient grafts under some circumstances.