Somatic diversification in the absence of antigen-driven responses is the hallmark of the IgM+IgD+CD27+ B cell repertoire in infants

Abstract

T cell-dependent immune responses develop soon after birth, whereas it takes 2 yr for humans to develop T cell-independent responses. We used this dissociation to analyze the repertoire diversification of IgM +IgD+CD27+ B cells (also known as "IgM memory" B cells), comparing these cells with switched B cells in children <2 yr of age, with the aim of determining whether these two subsets are developmentally related. We show that the repertoire of IgM+IgD +CD27+ B cells in the spleen and blood displays no sign of antigen-driven activation and expansion on H-CDR3 spectratyping, despite the many antigenic challenges provided by childhood vaccinations. This repertoire differed markedly from those of switched B cells and splenic germinal center B cells, even at the early stage of differentiation associated with μ heavy chain expression. These data provide evidence for the developmental diversification of IgM+IgD+CD27+ B cells, at least in very young children, outside of T cell-dependent and-independent immune responses. © 2008 Weller et al. The Rockefeller University Press.