EFFECT OF COMPLEMENT TYPE-1 RECEPTOR PRE- BINDING TO NEUTROPHILS ON FC MEDIATED PHAGOCYTOSIS

As a result of IgG-Fc interaction, sheep erythrocytes sensitized with IgG (E-IgG) are constitutively phagocytosed by resting PMN, although i n low numbers. The low efficiency of the system can be improved by ops onization with C3b, since the combination of C3b and IgG makes a power ful opsonic signal. It is accepted that the mechanism of C3b enhanceme nt of IgG mediated phagocytosis is achieved by increasing the adherenc e of the targets to the phagocyte. Recently, a non-opsonic role for C3 b enhancement of IgG mediated phagocytosis has been proposed, in cultu red human monocytes. These cells, when adhered on glass surfaces preco ated with C3b, showed a marked increase in E-IgG internalization. The effect was dose dependent and it was reproduced utilizing a monoclonal antibody against CR1 (C3b receptor). In the present work we studied t he existence of this phenomenon in resting neutrophils and in neutroph ils stimulated with two kinds of agents: a phorbol ester (PDBu), which activates the CR1 and fMLP, which increases the expression of this re ceptor. In previous experiments we determined that the adherence of re sting neutrophils on different concentrations of iC3 (which binds CR1 and exerts the same effect that C3b), did not increase the phagocytosi s of the E-IgG, using as a control neutrophils adhered on the same con centrations of human serum albumin (HSA) (data not shown). When neutro phils (PMN) were activated with different doses of PDBu, stimulation o f Fc mediated phagocytosis was induced reaching a maximun effect at 5 ng/ml, while at higher concentrations there was a progressive decline of stimulation, and with a dose of 20 ng/ml, the phagocytosis dropped below the base-line level (Fig. 1A). This finding is in contrast with previous reports that have only shown inhibition. PDBu activated cultu red monocytes but not PDBu activated PMN increased their FcR mediated phagocytosis when plated on iC3 covered surfaces (Fig. 1 B, Fig. 2). T he failure of PDBu activated PMN to increase their phagocytic activity was not related to the amount of sensitizing antibody or the time of stimulation with the phorbol ester (Fig. 1C). The PMN stimulation with different concentrations of the chemotactic peptide fMLP, simultaneou sly to the adherence onto iC3, also failed to enhance phagocytosis (Fi g. 3). Our data suggest that the communication between CR1 and FcR is different in neutrophils and monocytes. This difference may correlate with distint activation mechanisms of CR1 and/or the kind of FcR found in each cellular type.