Chronic GVHD Is Associated with a BAFF Driven BCR-Activated B Cell Repertoire.

We recently showed that high levels of B cell Activating Factor (BAFF) after allogeneic hematopoietic stem cell transplantation (HSCT) correlate with chronic GVHD development. BAFF promotes survival and differentiation of human BCR-activated (CD27+) B cells into Ig-secreting cells (ISC). To examine whether BAFF drives ISC in cGVHD, CD27+ peripheral B subsets and plasma BAFF levels were examined in 57 patients ≥ 12 months after HSCT. Significantly higher BAFF levels were found in active and inactive cGVHD groups compared to control patients with no cGVHD. Patients with cGVHD had a relative reduction in naive B cells compared to controls, resulting in a preponderance of CD27+ B cells (Table 1). Both pre-germinal center [pre-GC (IgD+)] and post-GC (IgDLo) subsets had significantly increased CD27+ in active cGVHD (Table 2). When BAFF levels were compared with cell number, only active disease patients had a positive correlation between BAFF and plasmablasts (r=+0.51, p-value=0.013), suggesting CD27+ post-GC B cells are highly BAFF-dependent in cGVHD. Active and inactive cGVHD patients had a positive correlation between BAFF and number of CD27+ pre-GC cells (r=+0.5, r=+0.49, respectively, p-value=0.02 each). In vitro analysis of these infrequent B subpopulations was accomplished using a large-volume lymphocyte donation from an untreated active cGVHD patient. Plasmablast and pre-GC purified populations had distinctly higher apoptotic rates attenuated in vitro by BAFF. CD27+ pre-GC B cells were induced by BAFF to differentiate into post-GC plasmablasts, indicating that survival and differentiation effects likely explain the correlations between BAFF and pre and post-GC CD27+ B subsets in patients. Increased BAFF levels in patients with inactive disease also correlated with lower total B (r=−0.59, p=0.003) and memory subset numbers (r=−0.57, p=0.005) suggesting that simultaneous B cell turnover and reconstitution may occur as disease becomes inactive. Thus, peripheral CD27+ B cells differentiate into potentially alloreactive ISC in presence of excess BAFF without need for further antigen stimulation. These results implicate BAFF and BCR-activated B cells in cGVHD and provide a rationale for therapeutic use of BAFF antagonists in this disease.