B lymphocyte selection and age-related changes in VH gene usage in mutant alicia rabbits

Young Alicia rabbits use V(H)a-negative genes, V(H)x and V(H)y, in most VDJ genes, and their serum Ig is V(H)a negative. However, as Alicia rabbits age, V(H)a2 allotype Ig is produced at high levels. We investigated which VH gene segments are used in the VDJ genes of a2 Ig-secreting hybridomas and of a2 Ig(+) B cells from adult Alicia rabbits. We found that 21 of the 25 VDJ genes used the a2-encoding genes, V(H)4 or V(H)7; the other four VDJ genes used four unknown V-H gene segments. Because V(H)4 and V(H)7 are rarely found in VDJ genes of normal or young Alicia rabbits, we investigated the timing of rearrangement of these genes in Alicia rabbits. During fetal development, V(H)4 was used in 60-80% of nonproductively rearranged VDJ genes, and V(H)x and V(H)y together were used in 10-26%, These data indicate that during B lymphopoiesis V(H)4 is preferentially rearranged. However, the percentage of productive V(H)x- and V(H)y-utilizing VDJ genes increased from 38% at day 21 of gestation to 89% at birth (gestation day 31), whereas the percentage of V(H)4-utilizing VDJ genes remained at 15%, These data suggest that during fetal development, either V(H)4-utilizing B-lineage cells are selectively eliminated, or B cells with V(H)x- and V(H)y-utilizing VDJ genes are selectively expanded, or both. The accumulation of peripheral V(H)4-utilizing a2 B cells with age indicates that these B cells might be selectively expanded in the periphery. We discuss the possible selection mechanisms that regulate V-H gene segment usage in rabbit B cells during lymphopoiesis and in the periphery.