Modulation of Fc-receptor expression and Fc-mediated phagocytosis in variants of a macrophage-like cell line

Variants with altered Fc-receptor mediated phagocytosis, some of which are corrected by the addition of cAMP, were isolated from the macrophage-like cell line, J774.2. Monoclonal mouse IgG2a and IgG2b were used to determine the Fc-receptor specificity of the defects. Most variants analyzed were defective in IgG2a-mediated phagocytosis. One variant had a general Fc-mediated phagocytosis defect. Variants selected in the presence of cAMP had different characteristics; one was unresponsive to stimulation of IgG2a- and IgG2b-mediated phagocytosis by cAMP; two showed selective defects in sensitivity of IgG2a phagocytosis to cAMP. A monomeric [(125)I] IgG2a binding assay was used to assess Fc-receptor expression. One variant that bound opsonized SRBC normally was selectively deficient in the binding of [(125)I] IgG2a. Scatchard analysis of the binding data showed that this variant expressed fewer than half the high affinity receptors of the parental line, but that the affinity of the receptors for IgG2a remained unchanged (Ka ~ 6 x 10⁶M^-1). Another of the nonphagocytic variants had no apparent defect in its Fc-receptors. Upon exposure to cAMP all lines analyzed, even a variant cell line whose phagocytosis was not correctable by cAMP, showed an increase in Fc-receptor expression. The results indicate that a) distinct classes of variants in the phagocytosis pathway can be isolated; b) IgG2a and IgG2b phagocytosis are separable at a point distal to Fc-receptor binding; c) there are steps subsequent to Fc-receptor binding that are limiting for phagocytosis and sensitive to cAMP; and d) cAMP affects Fc-receptor expression.