Identification of IgG subclasses and C-reactive protein in lupus nephritis: the relationship between the composition of immune deposits and FCgamma receptor type IIA alleles.

OBJECTIVE: To characterize the subclass composition of IgG deposited in lupus glomeruli, to examine its relationship to allelic polymorphisms of IgG receptors (Fcgamma receptors [FcgammaR]), and to determine whether C-reactive protein (CRP), a ligand for FcgammaRIIa, is present in these immune deposits.
METHODS: Renal biopsy samples from 80 patients with lupus nephritis were examined by light microscopy and indirect immunofluorescence with IgG-subclass-specific monoclonal antibodies. FcgammaRIIA genotypes were determined using allele-specific polymerase chain reaction. Immunostaining for CRP was performed on lupus and nonlupus glomerulonephritis specimens.
RESULTS: IgG2 and IgG3 were the predominant subclasses in immune deposits in all World Health Organization classes of nephritis. The frequency of genotypes containing the low-binding IgG2 allele, FcgammaRIIa-R131, was significantly greater than expected in patients with class III or class IV nephritis and in patients with intense IgG2 deposition. CRP, a ligand with particular affinity for FcgammaRIIa-R131, was consistently present in the renal immune deposits of lupus nephritis specimens.
CONCLUSION: FcgammaRIIA genes are associated with proliferative renal disease and may contribute to disease pathogenesis. FcgammaRIIa-R131, the variant with low affinity for IgG2, has high affinity for CRP. Thus, FcgammaRIIa-R131 may contribute to impaired removal of circulating immune complexes, as well as efficiently triggering phagocyte activation and the release of inflammatory mediators within glomeruli.