The Granuloma Response Controlling Cryptococcosis in Mice Depends on the Sphingosine Kinase 1-Sphingosine 1-Phosphate Pathway

Cryptococcus neoformans is a fungal pathogen that causes pulmonary infections, which may progress into life-threatening meningitis. In commonly used mouse models of Cryptococcus neoformans infections, fungal cells are not contained in the lungs, resulting in dissemination to the brain. We have previously reported the generation of an engineered Cryptococcus neoformans strain (Cryptococcus neoformans Delta gcs1) which can be contained in lung granulomas in the mouse model and have shown that granuloma formation is dependent upon the enzyme sphingosine kinase 1 (SK1) and its product, sphingosine 1-phosphate (S1P). In this study, we have used four mouse models, CBA/J and C57BL6/J (both immunocompetent), Tg epsilon 26 (an isogenic strain of strain CBA/J lacking T and NK cells), and SK-/- (an isogenic strain of strain C57BL6/J lacking SK1), to investigate how the granulomatous response and SK1-S1P pathway are interrelated during Cryptococcus neoformans infections. S1P and monocyte chemotactic protein-1 (MCP-1) levels were significantly elevated in the bronchoalveolar lavage fluid of all mice infected with Cryptococcus neoformans Delta gcs1 but not in mice infected with the Cryptococcus neoformans wild type. SK1(-/-) mice did not show elevated levels of S1P or MCP-1. Primary neutrophils isolated from SK1(-/-) mice showed impaired antifungal activity that could be restored by the addition of extracellular S1P. In addition, high levels of tumor necrosis factor alpha were found in the mice infected with Cryptococcus neoformans Delta gcs1 in comparison to the levels found in mice infected with the Cryptococcus neoformans wild type, and their levels were also dependent on the SK1-S1P pathway. Taken together, these results suggest that the SK1-S1P pathway promotes host defense against Cryptococcus neoformans infections by regulating cytokine levels, promoting extracellular killing by phagocytes, and generating a granulomatous response.