FUNCTIONS OF THE VARIOUS HUMAN-IGG FC-RECEPTORS IN MEDIATING KILLING OF TOXOPLASMA-GONDII

The three types of IgG FcR (Fc-gamma-RI, Fc-gamma-RII, Fc-gamma-RIII) on human leukocytes play an important role in elimination of antibody-coated infectious agents. To further understand the role of the different Fc-gamma-R in mediating this killing, we examined the ability of human myeloid and lymphoid cells to kill the protozoan Toxoplasma gondii in the presence of antitoxoplasma IgG or bispecific antibodies. Although human myeloid cells (monocytes, macrophages, neutrophils, and eosinophils) all lysed unsensitized T. gondii, killing by these cells was significantly enhanced by opsonization with antitoxoplasma rabbit IgG. Human lymphocytes, however, did not lyse T. gondii unless the parasites were coated with antibody. The role of antibody and Fc-gamma-R in mediating ADCC of T. gondii was then examined using bispecific antibodies made by chemically cross-linking Fab fragments of antitoxoplasma antibodies to Fab fragments of antibodies specific for human leukocyte surface Ag, including Fc-gamma-R. Thus, simultaneous binding of these bispecifics to parasites and effector cells allowed an evaluation of killing when T. gondii were targeted to each Ag independently. Bispecifics which targeted T. gondii to Fc-gamma-RI, II or III enhanced lysis by monocytes. However, similar results were obtained with bispecifics targeting T. gondii to non-Fc-gamma-R Ag (CD11b or beta-2-microglobulin) on monocytes. Likewise, polymorphonuclear leukocytes mediated significantly more lysis in the presence of bispecifics linking T. gondii to Fc-gamma-RII, Fc-gamma-RIII, or the two non-Fc-gamma-R Ag CD11b and beta-2-microglobulin. Thus, although human myeloid cells did not require antibody-Fc-gamma-R triggering to kill T. gondii, antibody appeared to enhance lysis by capturing and directing the parasites to the effector cell surface. Human lymphocytes, in contrast, mediated significant lysis of T. gondii only in the presence of bispecifics targeting T. gondii to Fc-gamma-RIII, indicating a requirement for specific triggering of Fc-gamma-RIII for killing by large granular lymphocytes. Consequently, using bispecifics to compare targeting to specific Ag, both non-Fc-gamma-R and Fc-gamma-R, allowed determination of the role of antibody-Fc-gamma-R interactions in T. gondii killing. In addition, these studies demonstrate the potential of bispecifics in determining the role of specific Ag in determining the role of specific Ag in killing of or infection by pathogens.