Anti-CD2 (T, p50) intact ricin immunotoxins for GVHD-prophylaxis in allogeneic bone marrow transplantation

We evaluated the inhibitory effects of two immunotoxins (IT) synthesized by linking two different anti-CD2 (T, p50) murine monoclonal antibodies (MoAb) to intact ricin (R). Pretreatment with 1000 ng ml^-1 35.1-R or OKT 11_a R inhibited PHA-induced T-cell proliferation by 93% and 86%, respectively. At this IT concentration generation of alloreactive cytotoxic T-cells (CTL) was inhibited by more than 99% by either IT. 35.1-R and OKT 11_a were minimally toxic to natural killer (NK) effectors or pluripotent bone marrow progenitor cells (CFU-GEMM). Blocking experiments suggested that 35.1-R and OKT 11_a-R might recognize different epitopes of the CD2 (T, p50) surface determinant. Our findings show that anti-CD2 IT may be useful for T-cell depletion in allogeneic bone marrow transplantation. We compared TU3, an equimolar mixture of T101 [anti-CD5]-R, UCHT-1 [anti-CD3]-R and 35.1 [anti-CD2]-R with the TUT-cocktail (a mixture of T101-R, UCHT-1-R and TA-1 [antiCDwl8]-R. TUT is currently under evaluation in Phase 1 clinical trials as a T-cell depletion regimen for GVHD prophylaxis. TU3 was as effective as TUT-cocktail in inhibition of PHA response and CTL generation but unlike TUT spared NK effectors. Cocktails of immunotoxins directed against subpopulations of lymphocytes may be useful (a) for more effective anti-GVHD strategies, and (b) to circumvent problems of graft failure/rejection associated with current purgation regimens.