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 +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.