Susceptibility of TLR4-defective C3H/HeJ mice to Coccidioides posadasii infection.

Coccidioides posadasii is one of the two fungal pathogens that cause coccidioidomycosis. The inhalation of air-borne arthroconidia leads to the formation of endospore-forming spherules in the lungs and pulmonary infection. In severe condition, the endospores are disseminated to other non-pulmonary organs in the body. The Toll-like receptors (TLR) expressed by a number of immune and non-immune cells can significantly impact the host defense and susceptibility to C. posadasii infection. In this study, we infected TLR4-defective C3H/HeJ mice with a sublethal dose of C. posadasii and studied fungal dissemination, mortality and humoral response. We also measured IL-12 cytokine secreted by C. posadasii-infected dendritic cells. We found that the C3H/HeJ mice were equally susceptible to C. posadasii as compared to C3H/OuJ mice which have intact TLR4. No significant changes were observed in pulmonary fungal load, survival and humoral response. The blockade of TLR4 did not affect C. posadasii-induced IL-12 secretion. However, the fungal counts were 10 times less in spleens of C3H/HeJ mice as compared to C3H/OuJ mice (P<0.05). Our results suggest that the TLR4 may not be involved in inducing protective host defense against C. posadasii, but it appears to be critical for fungal dissemination.