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We have previously reported a polymorphism in the mitogenic effect of murine (m) IgGI anti-CD3 monoclonal antibodies. This polymorphism was genetically determined and could be attributed to polymorphism of the Fc receptor (FcR) for mIgGl present on human monocytes. We have now extended these studies by quantitating FcR expression on monocytes and cell lines by it recently developed EA rosette assay, using the erythrocyte-associated pseudoperoxidase activity. The data show that the polymorphism of the monocyte FcR for mIg2a-based on a quantitative rather than an absolute difference. Furthermore, this FcR is specific for mIgGI and does not bind mIgG2a nor mIgG2b nor surprisingly, human IgG. The expression of this FcR on cell lines correlates with their accessory function in IgG I anti-CD3-induced T cell proliferation mIgG2a can inhibit the rosetting of monocytes with erythrocytes sensitized with human IgG. The FcR detected by this rosette technique can interact with all four human IgG subclasses but not with mIgG1 or mIgG2b. The expression of this type of FcR in human cell lines correlates well with their ability to support mIgG2a anti-CD3-induced mitogenesis. These direct measurements of FcR expression support the concept that human monocytes have two independent FcR with affinity for mouse IgG: one receptor specific for mIgG2a (which also binds human IgG), and a second specific for mIgG1. [ABSTRACT FROM AUTHOR]

The Raji-cell test is one of the most widely used methods for the detection and quantitation of immune complexes. Immune complexes and not 7 S IgG bind via C3 to complement receptors on the cell membrane of the Raji cell, During sucrose gradient fractionation of human and murine systemic lupus erythematosus sera, with a high Raji cell-binding activity, we could not demonstrate immune complexes in these sera. Subsequent analysis showed that the major part of the Raji cell binding was used by 7 S IgG with an anti-DNA specificity. Blocking experiments with complement-bearing aggregated IgG revealed that complenicni and Fc receptors were not involved in the binding of these anti-DNA antibodies to Raji cells. We conclude that the Raji cell test is not suitable for the detection and quantitation of immune complexes in sera containing anti-DNA antibodies. [ABSTRACT FROM AUTHOR]

Anti-CD3 MoAbs arc widely used in T cell activation studies, and are effective in immunosuppressive therapy. We used a panel of mouse (m) anti-CD3 switch variant MoAbs of five different isotypes to study IL-6 release from accessory cells. Incubation of human (h) mononuclear cells with anti-CD3 MoAbs resulted in increased lL-6 levels with MoAbs of mIgGI and mIgG2a isotypes. with no effect of mIgG2b or mIgA. This suggested involvement of IgG Fc receptors (FcγR) in triggering IL-6 production. To evaluate the role of different FcγR molecules individually we used a panel of hFcγR-transfected mouse fibroblasts. and Jurkat T cells as a model. IL-6 secretion by CD32 transfectants expressing the hFcγRHa high-responder (HR) allelic form was triggered by mIgGl anti-CD3 MoAb, with no effect of four other isotypes. None of theanti-CD3 MoAbs induced IL-6 secretion by CD32 transfectants expressing either a variant of this receptor, containing only a single intracellular amino acid (CT-). the hFcγRIIa low-responder (LR) allelic form, or hFcγRIIb1. hFcγRI (CD64) transfectants exhibited IL-6 production after incubation with mIgG2a anti-CD3 MoAb, and to a lesser extent with mlgG2b. and mIgGI MoAb. Indirect involvement of T cells in triggering IL-6 secretion could be excluded by experiments in which transfectants were cultured with immobilized anti-CD3 MoAb. These data indicate that cross-linking of either hFcγRI, or hFcγRIIaHR by appropriate anti-CD3 MoAbs triggers IL-6 production of accessory cells, and not T cells. This may also take place in vivo during immunosuppressive therapy with anti-CD3 MoAbs. and related antibody-mediated immune responses. [ABSTRACT FROM AUTHOR]