Anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis involves innate and adaptive immune cells in the induction of autoimmunity and in autoimmune effector responses. Most Fcgamma receptors (FcgammaRs) activate immune cells, but FcgammaRIIB, found in humans and mice on B cells and innate cells, is an inhibitory receptor. Here we tested whether endogenous FcgammaRIIB negatively regulates autoreactivity and effector responses in experimental anti-myeloperoxidase (MPO) glomerulonephritis, using wild-type and FcgammaRIIB(-/-) mice. After MPO immunization, FcgammaRIIB(-/-) mice developed higher MPO-ANCA titers and increased anti-MPO T-cell responses. Transfer of FcgammaRIIB-deficient dendritic cells loaded with a nephritogenic MPO peptide (MPO409-428) into wild-type mice induced stronger autoimmunity than dendritic cells derived from wild-type mice. Transferring anti-MPO antibodies into lipopolysaccharide-primed mice resulted in increased glomerular neutrophil accumulation and injury in FcgammaRIIB(-/-) mice, showing a role for FcgammaRIIB in suppressing neutrophil activation. Inducing active autoimmunity to MPO followed by triggering T cell-mediated glomerular injury by transfer of sub-nephritogenic doses of lipopolysaccharide and anti-MPO antibodies resulted in more disease in FcgammaRIIB(-/-) mice. Thus, endogenous FcgammaRIIB negatively regulates anti-MPO autoimmunity and glomerulonephritis by dendritic cells, B cells, and neutrophils to limit MPO-ANCA production, T-cell responses, and neutrophil activation.