Cell signaling initiated by the B cell receptor is critical to normal 
development of B lymphocytes, most notably at the transitional B cell stage. 
Inhibition of this signaling pathway with the syk inhibitor, fostamatinib, has 
produced significant efficacy in lymphoid malignancies and autoimmune 
conditions. Here, we demonstrate that short-term use of fostamatinib impairs B 
lymphocyte development at the transitional stage without affecting mature B cell 
populations. Additionally, IL-10 producing B cells remained relatively constant 
throughout the treatment period. These findings provide insight into the 
mechanism of action of B cell receptor inhibition in autoimmune disease. As the 
development of agents targeting B cell receptor signaling proceeds, monitoring 
for long-term consequences as well as functional evaluation of B cell subsets 
may further improve our understanding of this rapidly growing class of novel 
agents.