SIMILAR LEVELS OF GRANZYME-A AND PERFORIN MESSENGER-RNA EXPRESSION INREJECTED AND TOLERATED HEART ALLOGRAFTS IN DONOR-SPECIFIC TOLERANCE IN RATS

Congenic LEW.1W (RT1u) heart grafts in LEW.1A (RT1a) recipient rats ar e rejected within 15+/-6 days. Tolerance (> 100 days) can be induced b y pretransplant donor-specific blood transfusions. In both cases, the graft is heavily infiltrated by recipient cells, and class I and class II molecules of the major histocompatibility complex are strongly exp ressed. Moreover, T lymphocytes extracted from both tolerated and reje cted grafts are similarly cytotoxic in vitro against donor cells. Howe ver, it cannot be excluded that this cytotoxicity does not operate in vivo. To answer this important question, we have studied the expressio n of granzyme A and perforin mRNA expression, in situ, by Northern blo tting. Our data show that the two corresponding mRNAs accumulate with the same kinetic and at the same level in rejected and tolerated graft s. These results strongly suggest that infiltrating cells are cytotoxi c in vivo and that a 'cellular rejection'' does occur in the ''tolerat ed'' grafts. In addition, these findings show that cytotoxic T lymphoc ytes are not sufficient to impair graft survival and lead to a revalua tion of the relevance, in general, terms of granzyme and perforin expr ession as a correlate of rejection.