Effects on human and nonhuman primate immune response of a new rat anti-CD2 monoclonal antibody

Background. Nonhuman primate models are highly clinically relevant in trans plantation. The development of immunosuppressive tools or a tolerogenic reg imen for primate models therefore represents an important goal of transplan tation immunological research. Hence, we have developed a rat monoclonal an tibody (mAb) that recognizes the CD2 molecule (LO-CD2b) on both human and n onhuman primate cells. Methods. The LO-CD2b mAb has been characterized by flow cytometry, E-rosett ing inhibition, and Western blotting. In vitro inhibition of immune respons es by LO-CD2b was assessed after both mitogenic and allogeneic stimulation in mixed lymphocyte reactions (MLR). Several LO-CD2b dose and time response s were tested. In vivo, peripheral and lymph node T-cell depletion was exam ined both by flow cytometry and immunohistology in 10 baboons that received intravenous injection of LO-CD2b at different doses and time courses. Xeno sensitization (anti-rat) was assessed by ELISA. Renal allograft survival wa s followed in two baboons treated with iterative LO-CD2b injections. Results, In vitro, LO-CD2b binds a lymphocyte antigenic determinant of 52 k Da that is recognized by other well-characterized anti-CD2 mAbs (T11, Leu5b ), LO-CD2b recognized natural killer CD2+ cells. Administration of 200 ng/m l LO-CD2b almost completely inhibited human and baboon mitogenic stimulatio n. Allogeneic baboon and human MLR were completely inhibited by the additio n of LO-CD2b (at 312 ng/ml) on the day of the initiation of culture; when a dded after 1 or 2 days, LO-CD2b still provided a significant MLR inhibition (>50%). Incubation of LO-CD2b with baboon peripheral blood mononuclear cel ls produced very low cytokine levels (interferon-gamma, tumor necrosis fact or-a, interleukin 2), In secondary MLR, baboon peripheral blood mononuclear cells previously incubated with LO-CD2b were unable to respond to a second allogeneic stimulation but were able to react to mitogens, In vivo, within the first hour after LO-CD2b, injection (at 0.15, 0.5, and 2 mg/kg), an 85 -90% peripheral depletion of CD2+ cells was observed. A partial T-cell depl etion in inguinal lymph nodes was seen after 1 week. The mechanism of perip heral T-cell depletion could have been antibody-dependent cell cytotoxicity or opsonization but was complement independent. Iterative LO-CD2b injectio ns (12 days at 0.35 mg/kg) slightly prolonged the renal allograft survival in two baboons. Conclusion. LO-CD2b is a nonactivating rat anti-CD2 mAb able to strongly in hibit both mitogenic and allogeneic responses in human and nonhuman primate s. In vivo, LO-CD2b provides a rapid peripheral T-cell depletion, which is reversible within days after the cessation of injections. This rat mAb repr esents a very important tool for in vivo experimental investigation in nonh uman primates because it similarly reacts against human T cells in vitro.