Characterization of a serum inhibitor of MLC reactions.

Serum from a healthy multigravida female was found to inhibit most MLC combinations tested. Attempts to characterize the inhibitory factor have revealed that it is not lymphocytotoxic, destroyed by heat or directed against HL-A specificities. The inhibitory activity segregates with the 7S IgG fraction, is effective in very small concentrations and consistently fails to inhibit the MLC combinations between cells from the serum donor and those from one other unrelated individual. The blocking effect on MLC occurs during the first 6–9 hr of incubation. Inhibitory activity persists in the donor's serum 16 months after delivery of her last child. An attempt to utilize the inhibitory serum to minimize an expected graft-versus-host (GVH) reaction following bone marrow transplantation to the serum donor's daughter with marrow from the child's father resulted in neither GVH nor a `take' of the graft. The possible effects of the serum on the grafted cells are discussed. While a recognition site for the inhibitory effect remains unknown, it appears to involve a T-cell receptor.