GRAFT-INFILTRATING T-HELPER CELLS, CD45RC PHENOTYPE, AND TH1/TH2-RELATED CYTOKINES IN DONOR-SPECIFIC TRANSFUSION-INDUCED TOLERANCE IN ADULT-RATS

Specific tolerance to LEW.1W (RT1(u)) heart allografts can be induced in adult LEW.1A (RT1(a)) rats by donor-specific blood transfusion (DST). We have previously shown that both rejected and tolerated grafts are heavily infiltrated by T lymphocytes, and that in both cases these T cells are capable of developing similar cytotoxic responses against donor cells in vitro; tolerance is therefore not due to the deletion of alloreactive T cells. At the same time, we found that the accumulation of IL-2 and IFN-gamma mRNA was decreased in tolerated grafts compared with rejected grafts, These results suggested that the induction of allograft tolerance in DST-treated animals could be mediated by anergy or suppression of graft-infiltrating Th1 cells, Although Th1 and Th2 clones have not yet been characterized in the rat, peripheral CD4(+) rat T cells can be divided into two populations, based on their expression of the isoform RC of the CD45 molecule, Upon activation, CD45RC(high) CD4(+) T cells produce IL-2 and IFN-gamma and are responsible for the induction of the graft-versus-host reaction, whereas CD45RC(low) CD4(+) T cells produce IL-4 in vitro and provide B cell help, In the present study, we show that heart allografts from both DST-treated and untreated rats were infiltrated by equivalent numbers of leukocytes, of which CD4(+) T cells also made up similar percentages. Among these CD4(+) T cells, we observed that in allografts from DST-treated recipients the CD45RC(high) population on day 5 was very significantly smaller (P = 0.004) than in the untreated group, while CD45RC(low) populations remained comparable. Moreover, using a new quantitative RT-PCR method, we found a dramatic reduction in the accumulation of IL-2, IFN-gamma, IL-10, IL-4, and IL-13 mRNA in hearts from DST-treated recipients compared with those of untreated recipients during the week following transplantation. These results show that in heart allografts from DST-treated recipients, despite phenotypic changes suggesting Th1 inhibition by Th2 imbalance, T helper function was inhibited as a whole, and that in vivo the phenotype CD4(+) CD45RC(low) does not always correlate with Th2-related cytokine-producing cells.