Mechanism of action of anti-IL-2R monoclonal antibodies. ART-18 prolongs cardiac allograft survival in rats by elimination of IL-2R+ mononuclear cells

ART-18, a mouse IgG1 mAb recognizing the IL-2 binding domain of the rat p55 subunit IL-2R molecule, prevents graft rejection in various experimental models, although its mechanism of action in vivo, like that of anti-IL-2R mAb generally, remains elusive. These studies were designed to define whether IL-2R+ T effector cells were actually eliminated or their function merely inhibited by comparing directly the in vitro and in vivo efficacy of intact ART-18 and its F(ab)/F(ab')2 fragments. Addition of each mAb preparation profoundly suppressed MLR set up between naive LEW responders and x-radiated BN stimulators, suggesting that mAb fragments retained Ag binding functions in vitro. However, both ART-18 F(ab) and F(ab')2 were ineffectual in vivo as judged by their inability to affect acute (8 days) rejection of (LEW X BN)F1 cardiac allografts in LEW recipients (graft survival ca. 11 and 9 days, respectively, compared to ca. 21 days after therapy with intact ART-18, p less than 0.001). The sera levels of ART-18 and ART-18 F(ab')2 were 4 to 5 micrograms/ml, but only less than 0.5 micrograms/ml of F(ab) could be detected. The therapeutic failure of ART-18 fragments was unrelated to potential host sensitization, as rat antimouse F(ab) or F(ab')2 serum IgG titers remained in the same range as those against intact ART-18. The role of the Fc portion of Ig in the mode of action of ART-18 was then tested further by flow microfluorimetry analysis of host mononuclear spleen cells and immunoperoxidase stains of the graft infiltrate. IL-2R+ cells were abundant in rats treated with ART-18 fragments, comparable to acutely rejecting controls. In contrast, IL-2R expression was abolished in animals undergoing ART-18 therapy. The elimination of IL-2R+ cells is required to prolong cardiac allograft survival in rats after IL-2R targeted treatment with ART-18 mAb.