Development of gut-homing receptors on circulating B cells during infancy.

B cell gut-homing is mainly mediated by α4β7, CCR9 and CCR10. We here studied the expression of these receptors on B cells from cord blood and from peripheral blood at 1, 4, 18 and 36 months of age in a prospective cohort of Swedish infants. The proportion of all B cells expressing α4β7 as well as the fraction of CCR10+ B cells expressing α4β7 was highest in early infancy. Nearly all naïve B cells in all age groups expressed α4β7, whereas the expression on class-switched B cells decreased with age. Moreover, the proportion of both IgA+ and IgG+ B cells expressing α4β7, CCR9 and CCR10 were higher during the first months when compared to adults. In conclusion, the high fraction of circulating IgA+ and IgG+ B cells expressing CCR9 and CCR10 in the first months of life indicates activation of naïve B cells in the gut, coinciding with bacterial colonization.