Prevention of Murine Graft-Versus-Host Disease and Bone Marrow Alloengraftment Across the Major Histocompatibility Barrier After Donor Graft Preincubation With Anti-LFAl Immunotoxin

We have investigated the effects of the in vitro depletion of LFAl positive cytolytic T lymphocytes, natural killer (NK) cells, and monocytes on the afferent phase of graft-versushost disease (GVHD). Lethal GVHD was induced across the murine major histocompatibility complex by injecting C57BL/6 (H-2b) bone marrow (BM) cells (a source of stem cells) and splenocytes (S) (a source of T cells) into lethally irradiated B1O.BR (H-27 recipients. Because anti-LFAl does not bind complement (C’) effectively, we conjugated antiLFAl a chain monoclonal antibody (MoAb) to ricin toxin A chain (RTA) as a means of facilitating target cell elimination. A 2-hour preincubation of C57BL/6 bone marrow/spleen (BMS) with anti-LFAl-RTA in the presence of ammonium chloride (a potentiator of immunotoxin toxicity), but not a control immunotoxin (IT), reduced CTL activity by greater than 2 logs, significantly reduced NK cell activity, and prevented B1O.BR mice from developing GVHD. Depletion of target cells by toxin-labeled-MoAb and not the blockade of the LFAl molecule by the anti-LFAl MoAb accounted for our results, because incubating cells with IT in the absence of a potentiator had no effect on GVHD prevention. In contrast, C57Bt/6 recipients of C3H BMS grafts only partially bene-