Background: Polyclonal antithymocyte globulins have been assumed to deplete or sequester immunocompetent T cells. We investigated the hypothesis that anti-human thymocyte globulin (ATGAM)-mediated immunosuppression is delivered via nondepletive, immunologically specific actions as a consequence of simultaneous engagement of multiple T cell receptors.
Methods: Purified T cells obtained from healthy volunteers or renal transplant recipients receiving their first dose of ATGAM were evaluated for proliferative responses and cell-mediated lympholysis. ATGAM binding and receptor expression were determined by flow cytometry. Cytokines and ATGAM levels were measured by enzyme-linked immunosorbent assay.
Results: ATGAM-treated T cells showed significant dose-dependent inhibition of proliferation in vitro at concentrations comparable to those measured in patients. Effectors raised after ATGAM treatment failed to develop cytotoxicity. Supernatant interleukin (IL)-2 levels in ATGAM-treated cultures were significantly reduced (P<0.01 vs. control). IL-4 was not significantly altered. In vivo studies confirmed significant ATGAM-mediated inhibition of proliferative responses. Concanavalin A and OKT3-driven proliferation were reduced 30-60% by ATGAM. Flow cytometry showed that ATGAM recognized multiple cell surface receptors and resulted in markedly increased IL-2R and CD28 expression in the absence of proliferation, demonstrating partial T-cell activation. ATGAM synergized with phorbol myristate acetate to produce strong proliferation, which suggests that it provides a calcium-based signal resulting in anergy.
Conclusions: ATGAM recognizes and cross-links multiple cell surface receptors and costimulator molecules on human T cells. Simultaneous engagement by ATGAM in the context of allogeneic or mitogenic stimulation leads to partial T-cell activation and anergy.