The ontogeny and functional characteristics of human B-1 (CD5+ B) cells

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Abstract
We demonstrate that, on average, >90% of B lymphocytes In fetal spleen express CD5 at gestational ages of 17–23 weeks. Similarly, CD5+ B cells (B-1 cells) are the major B cell subset in umbilical cord blood. These findings depend on the optimization of fluorochrome conjugated anti-CD5 reagents for multiparameter fluorescent-activated cell sorter (FACS) analysis. From infancy through childhood the percentage of B-1 cells gradually diminishes in both spleen and peripheral blood. Stable adult levels, 25–35% of the total B cell population, are reached in late adolescence. The decrease in the percentage of B-1 cells in spleen Is accompanied by an Increase in conventional (CD5−) B cells, keeping the percentage of total B cells per mononuclear cells relatively constant. In contrast, in peripheral blood, the concentration of both B-1 cells and total B cells decreases, while T cells increase. At the functional level, we show that polyreactive IgM autoantibodies are produced by FACS-sorted CD5high B cells, but not by CD5− B cells from adolescent spleen. In contrast, fetal splenic CD5high and CD5− B cells appear functionally uniform, both producing IgM autoantibodies that are typical of B-1 cells. The apparent level of CD− B cells In fetal spleen, on average 10% of total B cells, may still result from limitations of our reagent. The prominence of B-1 cells In fetal spleen and cord blood, the gradual reduction of B-1 cells with Increasing age, and Its characteristic repertoire, all suggest a role for this cell type in immunologlcally Immature hosts.

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