Homologous C3 deposition and homotypic cell adhesion in a human myeloid cell line, P39.

It has been accepted that decay-accelerating factor (DAF) and membrane cofactor protein (MCP) on human cells block C3 deposition, thereby preventing homologous complement attack. In this study, we discovered that a human myeloid cell line, P39, was a target for human C3 even though it expressed normal DAF and MCP. This homologous C3 deposition was induced by serum containing Mg2+ and EGTA (Mg(2+)-EGTA serum) selectively on a P39 subline [P39(+)cells] having the capacity to form cell aggregates. Another P39 subline [P39(-)cells] growing as a separated form did not induce homologous C3 deposition. Multiple C3 fragments, C3b and C3bi, were fixed on P39(+) cells and a significant amount of C5a was released. Several distinct C3 fragment-membrane acceptor molecule complexes were immunoprecipitated with anti-C3c antibody from surface-labeled P39(+) cells treated with Mg(2+)-EGTA serum and from unlabeled cells incubated with 125I-labeled C3 and Mg(2+)-EGTA serum. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed 90, 60 and < or = 40 kDa C3 acceptors on P39(+) cells. On these cells, some of the C3 bound to these acceptors remained in the form of C3b, which can form C3/C5 convertases. P39(+) cells differed phenotypically from P39(-) cells in that P39(+) cells expressed intercellular adhesion molecule-1 (ICAM-1), whereas P39(-) cells did not. Both cells were lymphocyte function-associated antigen 1 (LFA-1)+/CR3- counter-receptors for ICAM-1. However, homotypic cell adhesion was not completely inhibited by antibodies against ICAM-1 and LFA-1, suggesting that the homotypic cell aggregation of P39(+) cells is due only in part to ICAM-1 and LFA-1. In addition, C3 deposition, the expression of ICAM-1, and cell aggregation were enhanced by both tumor necrosis factor-alpha and interferon-gamma. Although the principal causative mechanisms remain obscure, C3 deposition and cell adhesion appear in parallel in this cell line and may be involved in the modulation of cell-mediated immune reactions.