Pre-B cell loss in senescence coincides with preferential development of immature B cells characterized by partial activation and altered Vh repertoire

Senescent mice show decline in B lymphopoiesis marked by reduced pre-B cells. Analysis of bone marrow from aged (∼2 years old) BALB/c mice indicates that, in senescence, an increased proportion of immature B cells exhibit a CD43/S7+ surface phenotype. This results from continued production of new CD43/S7+ B cells in aged mice from their limited pre-B cell pool while production of CD43/S7− immature B cells is highly reduced. CD43/S7 is ordinarily observed on a minor subset of immature B cells in young mice and is indicative of their partial activation. Senescent immature B cells, both ex vivo and derived in vitro, also demonstrate increased expression of VhS107 concomitant with CD43/S7. These alterations in the phenotype and Vh repertoire among senescent immature B cells likely originate prior to surface Ig expression. In aged mice with depleted pre-B and immature B cells in vivo, pre-B and immature B cells exhibited increased apoptosis in vitro. Dexamethasone-induced apoptosis among B lineage cells in young adult mice also resulted in pre-B cell loss and increased expression of CD43/S7 and VhS107 among immature B cells similar to that observed spontaneously in aged mice. These results suggest that old age, possibly due to increased apoptosis, results in loss of pre-B cells and alterations in the phenotype and Vh repertoire of newly derived B cells.