Activation of Human Monocyte Tumoricidal Activity by C-reactive Protein1

We examined the effect of purified human C-reactive protein (CRP) on induction of human peripheral blood monocyte (Mo)-mediated cytotoxicity (CTX) and oxidative metabolism. Exposure of Mo to acute phase serum levels of CRP in vitro resulted in dose-dependent expression of CTX against human tumor cell lines. Nonneoplastic human fibroblasts and glial cells were not affected by ('HP-exposed Mo, and treatment of Mo monolayers with anti-Leu lib (a natural killer marker) and comple ment did not abrogate or diminish CTX. Tumoricidal activity was ob served after 20-44 h of Mo exposure to CRP, and after 48-72 h of coculture with radiolabeled target tumor cells. Mo exposed to CRP for 48 h also demonstrated elevated Superoxide aniÃ3n production when challenged with phorbol myristate acetate. Unlike CTX induced by lipopolysaccharide, CRP-induced CTX was completely inhibited by preincubation of CRP with phosphorylcholine, a CRP ligand, at a concentration of 5.5 molecules phosphorylcholine per molecule CRP. Further, when Mo medium (which contained 5% human AB serum) was preincubated with immobilized CRP, exposure of Mo to CRP in such medium did not result in CTX. In contrast, LPS-induced CTX was not affected. CRPinduced Mo CTX was observed, however, when Mo were exposed to CRP in medium preincubated with phosphorylcholine-treated immobi lized CRP, suggesting that an active serum component which complexed with CRP was not removed. These findings indicate that one of the functions of the acute phase protein, CRP, may be to activate Mo and that the process may require a CRP-binding serum component.