RAPID COMMUNICATION: TLR4 expressed but with reduced functionality on equine B lymphocytes.

Varying susceptibility exists among mammalian species to the development of potentially fatal endotoxemia due to gram-negative bacteria molecular component, lipopolysaccharide (LPS). Toll-like receptor 4 (TLR4) is responsible for LPS-associated immune response and is expressed on numerous immune cells including B lymphocytes. TLR4 is expressed in a functional form on mouse B lymphocytes, a species much less susceptible to endotoxemia compared with humans who are highly sensitive to endotoxin. Humans possess B lymphocytes that are not responsive to LPS. Likewise, horses are highly susceptible to endotoxemia but the expression and function of TLR4 on horse B lymphocytes is not known. Colic, the major cause of mortality in horses, is often complicated by resultant endotoxemia. The objective of this study was to determine the expression and function of TLR4 on equine B lymphocytes. Lymphocytes were isolated from peripheral blood mononuclear cells that were collected from six horses, and the expression and function of TLR4 was analyzed for each horse. Flow cytometry results indicate TLR4 is expressed on horse B lymphocytes but stimulation with LPS did not alter this expression (P = 0.99) compared with unstimulated B lymphocytes after 24 h. After 72 h of in vitro LPS stimulation, analysis of cell proliferation dye by flow cytometry demonstrated that equine B lymphocytes did not proliferate, while mouse B lymphocytes predictably did. Furthermore, the total number of LPS stimulated equine B lymphocytes did not significantly differ from unstimulated cells after 72 h of culture (P = 0.92). Horse lymphocytes exhibited no significant differences in the measured TLR4 signaling pathway genes (TLR4, IL-10, IL-6, IFNβ, and TNFα) when expression was compared with LPS stimulated vs. unstimulated cells. In conclusion, while TLR4 is expressed on horse B lymphocytes, it appears minimally responsive to LPS in vitro, similar to results seen in human B lymphocytes. While further studies are still needed, our work reveals a potential link between B lymphocyte TLR4 expression and endotoxin sensitivity.