Antigenic Modulation in Vitro.

Abstract

Abstract The acquisition of resistance by TL antibody-sensitized mouse RADA1 leukemia cells and A/J thymocytes to cytolysis in the presence of fresh TL antiserum and guinea pig C (TL antigenic modulation) required a heat-labile mouse serum factor or factors. Most mouse strains had high levels of serum-modulating activity, but the B6 strain was extremely low, and the AKR/J strain moderately low, in activity. The modulating capacity of mouse serum was also destroyed by treatment with zymosan, an immune complex, and cobra venom factor, and was partially restored to heated serum by freeze-thawing. Human C component C3, and to a lesser extent C4, promoted modulation of cells presentitized with heated TL antiserum and cobra venom factor-treated normal serum. Similar sensitivity of modulating activity of human C3 and of mouse serum to temperatures below 37°C indicated that an analogous mouse C component, mouse C3, was required for modulation. Successful modulation by C2-deficient human serum and susceptibility of mouse serum activity to zymosan, immune complex, and cobra venom factor treatment indicated involvement of the alternative pathway of C activation. All modulating activity in mouse serum and in human C3 preparations could be absorbed onto and eluted from RADA1 cells and thymocytes. Immunofluorescence analysis indicated specific binding of mouse or human C3 to the cell surface only under conditions promoting modulation. Modulating activity of human C3, unlike that of mouse serum, was not destroyed by heating, and modulation was achieved with the IgG fraction of TL antiserum and heated human C3, suggesting a contribution of factor B-like, C3-cleaving activity by the cells, resulting in deposition of C3 on the cell surface. Intercalation of C3 into aggregated TL antigen-antibody complexes may be required to achieve modulation.