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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 (Mg2+‐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 Mg2+‐EGTA serum and from unlabeled cells incubated with 125I‐labeled C3 and Mg2+‐EGTA serum. Two‐dimensional sodium dodecyl sulfate‐polyacrylamide gel electrophoresis revealed 90,60 and ≦ 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− counterreceptors 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‐α and interferon‐γ. 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.