Age-associated B cells (ABC) cause reduced surrogate light chain expression and loss of B cell precursors in old age (HEM2P.269)

Abstract

Abstract Age-associated B cells (ABC, CD21/35- CD23-) accumulate in spleen and bone marrow of ~2 yrs. old mice and induce TNF-α-dependent apoptosis of pro-B cells. The remaining B cell precursors in aged mice exhibit reductions in surrogate light chain (SLC) and compromise of the preBCR checkpoint. Notably, the residual pro-B cells in aged bone marrow are resistant to ABC/TNF-α-mediated apoptosis. SLC is associated with cadherin 17 on the surface of pro-B cells. Like SLC-low aged pro-B cells, pro-B cells from λ5 SLC-deficient young adult mice were also resistant to TNF-α-induced apoptosis. This implies that expression of the SLC/cadherin 17 complex is required for normal susceptibility to apoptosis in pro-B cells. Resistance to apoptosis in aged pro-B cells and λ5-deficient young pro-B cells coincided with increased phosphorylation of the pro-apoptotic protein Bim, a modification that tags Bim for degradation. We hypothesize that, in old mice, expanded pro-inflammatory ABC within the bone marrow cause apoptosis of pro-B cells. However, this cell death occurs preferentially in pro-B cells which express relatively high levels of SLC. The remaining pro-B cells in aged mice are consequently skewed to lower SLC expression. We suggest that ABC, by inducing the loss of “high SLC” pro-B cells, deviate B lymphopoiesis into an ”SLC low pathway” with inefficient B cell production and altered B cell antibody repertoire.