Toll-like receptor signaling in small intestinal epithelium promotes B-cell recruitment and IgA production in lamina propria

Several lines of evidence support a role for Toll-like receptor (TLR) signaling to protect the intestine from pathogenic infection. We hypothesized that TLR signaling at the level of the intestinal epithelium is critical for mucosal immune responses.

Several lines of evidence support a role for Toll-like receptor (TLR) signaling to protect the intestine from pathogenic infection. We hypothesized that TLR signaling at the level of the intestinal epithelium is critical for mucosal immune responses.

TLR signaling in the intestinal epithelial cells significantly elevated the production of IgA in the intestine. This effect was mediated by TLR-induced expression of a specific set of chemokines and cytokines that promoted both recruitment of B cells into the lamina propria and IgA class switching of B cells.

We generated transgenic mice that express a constitutively active form of TLR4 in the intestinal epithelium (V-TLR4 mice). Lamina propria cellularity was evaluated by immunostaining and flow cytometry. Immunoglobulin (Ig) A levels in the stool and serum were measured by enzyme-linked immunosorbent assay. Chemokine and cytokine expression were analyzed by quantitative polymerase chain reaction and enzyme-linked immunosorbent assay.

V-TLR4 transgenic mice reproduced normally and had a normal life span. Constitutive activity of TLR4 in the intestinal epithelium promoted recruitment of B cells and an increase in fecal IgA levels. Intestinal epithelial cells of V-TLR4 mice expressed higher levels of CCL20 and CCL28, chemokines known to be involved in B-cell recruitment, and of a proliferation-inducing ligand (APRIL), a cytokine that promotes T-cell-independent class switching of B cells to IgA. The changes in B-cell numbers and IgA levels were blocked by simultaneous expression in intestinal epithelial cells of M3, a herpes virus protein that binds and inhibits multiple chemokines. Conclusions: TLR signaling in the intestinal epithelial cells significantly elevated the production of IgA in the intestine. This effect was mediated by TLR-induced expression of a specific set of chemokines and cytokines that promoted both recruitment of B cells into the lamina propria and IgA class switching of B cells.