Evidence That Marginal Zone B Cells Possess an Enhanced Secretory Apparatus and Exhibit Superior Secretory Activity

Marginal zone B (MZB) cells are the first splenic B cells to initiate Ab secretion against polysaccharide-encapsulated Ags in vivo. This swift MZB cell response can be reproduced in vitro as LPS treatment induces Ab secretion in as little as 12 h. Conversely, in vitro LPS treatment of splenic follicular B (FOB) cells results in Ab secretion after 2-3 days. The basis for these distinct response kinetics is not understood. We performed ex vivo analysis of resting and LPS-stimulated murine MZB and FOB cells and found that MZB cells express higher levels of the LPS TLR complex RP105/MD-1 and respond to much lower concentrations of LPS than do FOB cells. Furthermore, increasing doses of LPS do not accelerate the kinetics by which FOB cells transition into Ab secretion. Ultrastructural analysis of resting cells demonstrated that rough endoplasmic reticulum is more abundant in MZB cells than in FOB cells. Additionally, RT-PCR and immunoblot analyses revealed that numerous endoplasmic reticulum resident chaperones and folding enzymes are expressed at greater levels in resting MZB cells than in resting FOB cells. Although both LPS-stimulated MZB and FOB cells increase expression of these factors, MZB cells exhibit a more rapid increase that correlates with accelerated kinetics of Ab secretion and higher per cell output of secreted IgM. These data indicate that MZB cells are equipped for exquisite sensitivity to bacterial components like LPS and poised for rapid, robust Ab production, making MZB cells ideally suited as frontline defenders in humoral immunity.