Elevated BCR signaling and decreased survival of Lyn-deficient transitional and follicular B cells

The Src-family tyrosine kinase Lyn negatively regulates BCR signaling and also myeloid cell activity. Mice deficient in Lyn have substantially decreased numbers of peripheral B cells, despite spontaneously producing IgG anti-DNA antibodies. Here, we examine the mechanism underlying the B-cell depletion in these mice. Lyn-deficient B cells were out-competed by WT B cells in mixed BM chimeras at two steps, at the T1 to T2 transitional maturation stage in the spleen and again between the T2 or T3 stage and the mature follicular B-cell population. Lyn-deficient T2 and follicular B cells expressed elevated levels of the pro-apoptotic factor Bim and deletion of Bim restored splenic B cells of Lyn-deficient mice to close to WT numbers. Lyn-deficient T2 and later stage B cells also had changes in cell surface phenotype consistent with increased in vivo BCR signaling. Similarly, an increased proportion of T2 and follicular B cells had elevated basal intracellular free calcium levels. Overall, these observations suggest that increased BCR signaling is responsible for increased death of weakly self-reactive Lyn-deficient B cells both at the T2 stage and additionally as these cells mature to follicular B cells.