The Fc Receptor for IgG (FcγRII; CD32) on human neonatal B lymphocytes

B cells express an Fc receptor for IgG (FcγRII; CD32) which is involved in feedback inhibition of antibody production. Engagement of FcγRII during ligation of the antigen receptor provides an inhibitory signal. FcγRII exists as several isoforms, with FcγRIIb (which carries an immunoreceptor tyrosine-based inhibition motif; ITIM) being predominant form on adult B cells. The inhibitory role of FcγRIIb may be unhelpful to the infant, since primary exposure to infectious agents is likely to be in the presence of maternal IgG. We hypothesized that neonatal B cells would be less susceptible to feedback inhibition by antibody, either through the expression of activation-competent FcγRII isoforms (FcγRIIa and FcγRIIc) or through reduced expression of the inhibitory FcγRIIb isoforms. Cord and adult B cells were examined for expression of FcγRII isoforms using monoclonal antibodies and RT-PCR. In vitro assays were performed to assess susceptibility of cord and adult cells to FcγRII-mediated suppression. Although there is no phenotypic difference in FcγRII expression (FcγRIIb predominating on both adult and cord B cells), FcγRIIb is expressed at lower levels on cord cells. This quantitative difference in FcγRIIb expression may explain the reduced susceptibility of cord B cells to antibody-mediated inhibition observed in these experiments