Importance of Factors H and I for the Adherence of C3b-Coated Erythrocytes to Cells

Abstract

The role of cell membrane-associated human factor H for the binding of cell-bound Cab to complement receptor-carrying (CR +) cells was investigated. Pretreatment of CR + cells with antibodies to factor H inhibited the adherence of Cab-coated red cells to human tonsil lymphocytes (TL) and peripheral blood monocytes (Mø). The Cab receptor reactivity of human polymorphonuclear leucocytes (PMN) was not influenced and the one of Raji lymphoblastoid cells only slightly influenced; iC3b and Cad receptor reactivity was in no case affected. When diisopropylfluorophosphate (DFP) in a concentration of 0.1 mM was present during pretreatment of the CR + cells with anti H, the antibodies gained the capacity to inhibit the adherence of Cab-coated erythrocytes to Raji cells; this effect was dose-dependent with respect to DFP. In contrast, there was no influence of DFP on the inhibition pattern of anti H in the case of TL and Mø. The adherence of Cab-coated erythrocytes to PMN remained unaffected by anti H antibodies in the presence of DFP. Polyclonal as well as monoclonal antibodies directed against human factor I inhibited the binding of Cab cells to Raji cells but not to TL. Additionally, when anti I and anti H antibodies were both present, Cab receptor reactivity of Raji cells was inhibited to a larger extent than with either antibody alone; again, TL remained unaffected. Results obtained by washing the Raji cells before and after treatment with anti H and anti I suggest that the respective antibodies act on factor H primarily on the level of the cell membrane and on factor I in the fluid phase. The results presented indicate that factor H may, to a certain extent, be involved in the binding process of particle-bound Cab to CR + cells. However, regarding lymphocytes and Raji cells, different mechanisms have to be proposed for the adherence of Cab-coated erythrocytes. In the case of Raji cells, we believe that membrane-associated proteases are involved in the binding of EAC14°23b to the CR + cell. It may be that a conversion of Cab to iC3b occurs after the initial adherence of Cab-coated erythrocytes to Raji cells.