IgG-dependent activation of human mast cells following up-regulation of FcγRI by IFN-γ

Abstract

It has been reported that FcγRI is up-regulated on human mast cells (huMC) by IFN-γ and aggregation of this receptor using mouse F(ab′)₂ specific for receptor-bound, mouse anti-CD64 F(ab′)₂ results in activation. To determine whether huMC can similarly be stimulated by aggregation of FcγRI-bound human IgG, IFN-γ-treated, CD34⁺-derived, cultured uMC were sensitized with human immunoglobulins and activation was evaluated following addition of antibodies specific for each IgG isotype. Degranulation was also examined following simultaneous IgG- and IgE-dependent aggregation of FcγRI and FcϵRI. Activation of IFN-γ-treated huMC sensitized with 100 ng/ml IgG₁ resulted in 40% β-hexosaminidase (β-hex) release; minimal degranulation was observed using IgG₂, IgG₃ or IgG₄. IgG₁-dependent activation led to PGD₂ and LTC₄ generation as well as elevated cytokine production, most notably TNF–α. Preincubation of cells with F(ab′)₂ from CD64-specific clones 10.1 and 32.2 reduced IgG₁-mediated β-hex release by 46% and 74%, respectively. While IgG-dependent cell stimulation induced half-maximal degranulation by 11 min, IgE-dependent activation resulted in half maximal responses within 1 min. Simultaneous activation of huMC via FcγRI and FcϵRI led to additive degranulation using suboptimal concentrations of IgG₁ and IgE. Activation of huMC thus may occur via monomeric IgG and FcγRI thereby providing a novel paradigm for huMC recruitment into inflammation.