Epo Engages a Tnfr-13c Pathway to Advance for Primary Bone Marrow (Pro)Erythroblast Development

Abstract 4238

In late stage erythroblasts, EPO can increase levels of Bclx, Bcl2 and/or Mcl1 anti-apoptotic factors. Proerythroblasts, however, are a key EPO target (and exhibit sharp dependence on EPO for growth, and survival). In these progenitors, however, Bclx, Bcl2 and Mcl1 are not prime EPO/EPOR targets. Via transcriptome-based analyses of EPO response circuits in developmentally staged primary bone marrow proerythroblasts (which we now analyze and present at a global level) an atypical TNF receptor, Tnfrsf13c proved to be among the top 1% of EPO/EPOR induced factors. Within lymphoid lineages, Tnfrsf13c is a known receptor for BAFF ligand; and BAFF is an essential mediator of B-cell survival and development. Possible effects of BAFF (a bone marrow stromal cell surface ligand) on primary erythroid cell formation therefore were assessed. Notably, limited BAFF exposure (15 hours) inhibited apoptosis; increased erythroid cell numbers; and enhanced the formation of late-stage Ter119^pos erythroblasts. Specifically, cytoprotection by BAFF rivaled that afforded by EPO; cell numbers were enhanced 140% (in 15 hr); and frequencies of Ter119^pos erythroblasts were enhanced to 200% of controls. In keeping with Tnfrsf13c's role as an EPOR target, each of the above effects further proved to depend upon proerythroblast exposure to EPO. With regards to Tnfrsf13c expression, analyses using primary erythroid progenitors with knocked-in minimal EPOR alleles indicated dependence for EPO- induction upon JAK2, STAT5 as well as EPOR C-terminal coupled pathways. Studies overall reveal a novel EPOR action route within primary proerythroblasts as a Tnfrsf13c/BAFF pathway (which engages non-canonical NF-kappaB molecular mechanisms).