ISOTYPE-SPECIFIC CROSS-LINKING OF SELECT HUMAN FC-GAMMA-R ISOFORMS TRIGGERS RELEASE OF IL-6

Anti-CD3 MoAbs are widely used in T cell activation studies, and are e ffective in immunosuppressive therapy. We used a panel of mouse (m) an ti-CD3 switch variant MoAbs of five different isotypes to study IL-6 r elease from accessory cells. Incubation of human (h) mononuclear cells with anti-CD3 MoAbs resulted in increased IL-6 levels with MoAbs of m IgG1 and mIgG2a isotypes, with no effect of mIgG2b or mIgA. This sugge sted involvement of IgG Fc receptors (FcgammaR) in triggering IL-6 pro duction. To evaluate the role of different FcgammaR molecules individu ally we used a panel of hFcgammaR-transfected mouse fibroblasts, and J urkat T cells as a model. IL-6 secretion by CD32 transfectants express ing the hFcgammaRIIa high-responder (HR) allelic form was triggered by mIgG1 anti-CD3 MoAb, with no effect of four other isotypes. None of t he anti-CD3 MoAbs induced IL-6 secretion by CD32 transfectants express ing either a variant of this receptor, containing only a single intrac ellular amino acid (CT-), the hFcgammaRIIa low-responder (LR) allelic form, or hFcgammaRIIb1. hFcgammaRI (CD64) transfectants exhibited IL-6 production after incubation with mIgG2a anti-CD3 MoAb, and to a lesse r extent with mIgG2b, and mIgG1 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 dat a indicate that cross-linking of either hFcgammaRI, or hFcgammaRIIa(HR ) by appropriate anti-CD3 MoAbs triggers IL-6 production of accessory cells, and not T cells. This may also take place in vivo during immuno suppressive therapy with anti-CD3 MoAbs, and related antibody-mediated immune responses.