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Objective Regulatory B cells that inhibit immune responses through interleukin-10 (IL-10) secretion (B10 cells) have been characterized in adult subjects with autoimmune disease. The aim of this study was to characterize B10 cells in individuals across the entire age range of normal human development and changes in their frequency and numbers in children with autoimmunity. Methods The phenotype and numbers of B10 cells in blood were examined in healthy individuals and children with autoimmunity, using flow cytometry. B10 cell function was assessed by measuring the effect of B cell-derived IL-10 on interferon-γ (IFNγ) expression by CD4+ T cells. Serum cytokine levels were measured by enzyme-linked immunosorbent assay. Results The frequency of B10 cells transiently increased during childhood, when up to 30% of B cells were competent to produce IL-10, compared with the low frequencies in healthy newborns (3-4%) and adults (7-9%). The surface phenotype of B10 cells in children revealed age-dependent variability. B10 cells from children were distinct from proinflammatory cytokine-producing B cells and down-regulated IFNγ production by CD4+ T cells in vitro. Compared with age-matched healthy controls, children with autoimmunity had lower numbers and frequencies of B10 cells (decreased by 39% and 48%, respectively), higher IFNγ levels, and lower IL-21 levels in serum. IFNγ inhibited, whereas IL-21 promoted, B cell IL-10 competence in vitro. Conclusion B10 cells, a functionally defined cell subset with a variable surface phenotype reflective of overall B cell development, transiently expand during childhood. B10 cell frequencies and numbers were decreased in children with autoimmunity, which may be explained in part by alterations in serum IFNγ and IL-21 that differentially regulate B10 cell development. [ABSTRACT FROM AUTHOR]