Fc-receptor variants of a mouse macrophage cell line

Abstract

Variants of the J774 mouse macrophage [tumor] cell line that lack immunologically important membrane receptors were isolated. After mutagenesis, variants were selected in a metrizamide gradient that separated cells heavily rosetted with sheep erythrocytes (E) coated with rabbit anti-E Ig[immunoglobulin]G (EIgG) from poorly rosetted cells. Stable variants that exhibited altered binding were found with a frequency of < 10-7, and 5 clones were studied in detail. The variants failed to bind E opsonized with a monoclonal mouse IgG2b anti-E antibody but bound monomeric IgG2a normally when compared to the parental J774 line (Ka 4.degree. C = .apprxeq. 1 .times. 108 M-1; .apprxeq. 2 .times. 105 sites per cell). This demonstrates the independence of the receptor for mouse IgG2b complexes (FcRII) from the trypsin-sensitive receptor for mouse IgG2a monomer (FcRI). The variants bound an average of 10-15 EIgG per cell, compared to > 20 per cell for J774. After trypsinization, 3 variants bound only 3-5 EIgG per cell; the J774 line was not affected by this treatment. Monomeric IgG2a could inhibit the binding of soluble rabbit IgG-antigen complexes to the variants but not to the parent line. E coated with IgM and complement (EIgMC) were bound poorly by all the variants, relative to the J774 parent. Rabbit IgG complexes are bound by FcRI and FcRII on mouse macrophages. The impairment of EIgMC rosetting in the variants suggests that the C3b [b fragment of complement component 3] receptor and FcRII are related.