Behavior of helper T lymphocytes in cyclosporine-mediated long-term graft acceptance in the rat

(LEW X BN)F1 cardiac allografts are rejected acutely (7 days) in unmodified LEW rats, yet survive indefinitely following cyclosporine (CsA) treatment (15 mg/kg im for 7 days) or in T-cell-deprived (B) recipients. Using these models, the function of T helper cells (Th) in the maintenance phase of CsA-mediated long-term graft survival was examined. With monoclonal antibody immunoaffinity fractionation techniques, Th (W3/25+OX8-) were separated from spleens of CsA-treated hosts 3-4 weeks after grafting (CsA-Th), from specifically sensitized (s-Th), or from normal ungrafted (n-Th) rats. Adoptive transfer of 60 X 10(6) CsA-Th into B recipients produced rejection of donor-specific, but not third-party grafts in 21 +/- 7 days, comparable to s-Th (17 +/- 4 days), but faster than n-Th (4-5 weeks, P less than 0.025). CsA-Th recombined with T cytotoxic/suppressor phenotype (CsA-Tc/s, OX8+W3/25-) in numbers contained in 100 X 10(6) CsA-T cells were ineffectual, even when supplemented with exogenous interleukin 2-rich conditioned medium (IL-2CM); in contrast 100 X 10(6) s-T cells + IL-2CM inevitably caused acute rejection in B hosts (11 +/- 3 days). Increasing numbers of Th incrementally to 100 X 10(6) augmented the effectiveness of s-Th (rejection in 13 +/- 2 days), but did not improve potency of CsA-Th (20 +/- 2 days). Suppressor activity produced by small numbers of contaminating CsA-Tc/s (c. 0.4%, 4-5 X 10(5) cells in 100 X 10(6) CsA-Th) accounted for extended graft survival in B recipients, as this small number of CsA-Tc/s transferred into untreated syngeneic rats increased test graft survival to c. 16 days (P less than 0.001). IL-2 production by spleen cells, depressed during CsA treatment, returned to normal levels 2-3 weeks following drug withdrawal, whereas transfer of CsA-Th into B recipients induced a shift of IL-2 levels from dramatically depressed to normal, findings suggesting normal IL-2 production by CsA-Th. This report demonstrates that an unresponsive state in CsA-treated animals is achieved despite the presence of fully potent donor-specific Th. Active suppressor activity plays a critical role in the maintenance phase of graft survival in rats treated transiently with CsA.