The functional similarities between C-reactive protein (CRP) and IgG raised the question as to whether human phagocytes are stimulated by CRP in the same way as by binding of antigen-complexed or aggregated IgG to their Fc receptors. Studies with the use of highly purified 125I-labeled CRP showed specific and saturable binding to human polymorphonuclear leukocytes (PMN) with a KD of 10.5 +/- 5.7 X 10(-8) M only when carried out in heat-inactivated plasma. The number of specific binding sites per cell was estimated at 1 to 3 X 10(6). Competitive inhibition of CRP binding by antigen-complexed or aggregated IgG suggests CRP binding sites to be associated with PMN Fc receptors. Only when assayed in heat-inactivated plasma did CRP binding induce adherence of cells to tissue culture dishes. However, no metabolic and potentially cytotoxic stimulation of PMN was detected during CRP plasma-dependent attachment to surfaces: induction of aggregation, release of secondary granule constituents, and activation of the hexose monophosphate pathway were not observed. These results imply that CRP-PMN interactions is dependent on an additional factor present in heat-inactivated plasma and is followed only by a complement-independent increase in PMN attachment to surfaces. Because CRP was found to be deposited at sites of tissue injury, the CRP-mediated adherence of PMN may be an important step in localizing an inflammatory focus.

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