Leukocyte integrin-dependent and antibody independent cytotoxicity of macrophage against allografts

Macrophages (M phi s), but not T cells, infiltrating into the rejection sit e of either i.p. allografted Meth A (H-2(d)) fibrosarcoma cells in C57BL/6 (B6) (H-2(b)) mice or BALB/c (H-2d) skin onto B6 mice are cytotoxic against allografts with H-2(d) specificity. To determine the mechanisms of specifi c killing of allografts by allograft-induced M phi (AIM), we raised approxi mate to 5,000 rat monoclonal antibodies (mAbs) against AIM and selected thr ee of them (R1-73, R2-40 and R1-34), each of which inhibited cytotoxic acti vity against allografts in a dose-dependent manner. The antigens recognized by R1-73, R2-40 and R1-34 mAbs were defined by immunoprecipitation and Wes tern blot analyses as CD11(a), CD18 and CD11(b), respectively; and the allo grafts expressed CD54, a ligand of CD11(a), or CD11(b), suggesting leukocyt e integrin-dependent killing. Although Ab-dependent cellular cytotoxicity h as been recognized as a mechanism of specific killing by M phi s, the infil tration of AIM into the rejection site of allografts far (approximate to 6 days) preceded the appearance of serum IgG Ab specific for the allograft, A IM exhibiting full cytotoxic activity against allografts was also induced i n the transplantation site of Fc gamma receptor knockout [(B6x129) F-1] mic e as well as B10.D2 (H-2 compatible with allograft) and B6-xid (X-linked im munodeficiency with B cell-specific defect) strains of mice. In the latter two strains of mice, the levels of serum IgG Ab to the allograft were negli gible, Moreover the cytotoxic activity of AIM against allografts was not af fected by pretreatment of the cells with anti-mouse IgG serum, suggesting A b-independent cytotoxicity.