Pathogenic yeasts Cryptococcus neoformans and Candida albicans produce immunomodulatory prostaglandins

Abstract

Enhanced prostaglandin production during fungal infection could be an important factor in promoting fungal colonization and chronic infection. Host cells are one source of prostaglandins; however, another potential source of prostaglandins is the fungal pathogen itself. Our objective was to determine if the pathogenic yeasts Cryptococcus neoformans and Candida albicans produce prostaglandins and, if so, to begin to define the role of these bioactive lipids in yeast biology and disease pathogenesis. C. neoformans and C. albicans both secreted prostaglandins de novo or via conversion of exogenous arachidonic acid. Treatment with cyclooxygenase inhibitors dramatically reduced the viability of the yeast and the production of prostaglandins, suggesting that an essential cyclooxygenase like enzyme may be responsible for fungal prostaglandin production. A PGE series lipid was purified from both C. albicans and C. neoformans and was biologically active on both fungal and mammalian cells. Fungal PGE(x) and synthetic PGE(2) enhanced the yeast-to-hypha transition in C. albicans. Furthermore, in mammalian cells, fungal PGE(x) down-modulated chemokine production, tumor necrosis factor alpha production, and splenocyte proliferation while up-regulating interleukin 10 production. These are all activities previously documented for mammalian PGE(2). Thus, eicosanoids are produced by pathogenic fungi, are critical for growth of the fungi, and can modulate host immune functions. The discovery that pathogenic fungi produce and respond to immunomodulatory eicosanoids reveals a virulence mechanism that has potentially great implications for understanding the mechanisms of chronic fungal infection, immune deviation, and fungi as disease cofactors.

FIG. 1

Secretion of arachidonic acid metabolites by C. neoformans. Strain 24067E was grown to stationary phase (72 h) and was incubated with 0.5 μCi of [³H]arachidonic acid (³H-AA) in 1 ml of SDB at 37°C for 30 min. The supernatant was analyzed by HPLC, and peaks were identified on the basis of their coelution with authentic standards. Pc, prostacyclin.

FIG. 2

Effect of cyclooxygenase inhibitors on prostaglandin production and survival of C. neoformans and C. albicans. Cultures of C. neoformans strain H99 and C. albicans strain CHN1 were grown for 24 h at 25°C. Indomethacin (1.0 mM dissolved in DMSO) was added, and the cultures were then incubated for an additional 24 h. Control cultures contained DMSO alone. The numbers in parentheses represent percent inhibition of PGE₂ compared to control (A) and percent killing compared to initial CFU per milliliter measured before the addition of inhibitor (B). The results represent the averages (± standard errors) of two independent experiments. ∗, P < 0.05 compared to control.

FIG. 3

Effects of exogenous prostaglandins on C. albicans germ tube formation. C. albicans was incubated in FCS with either buffer, commercially available PGE₂, or affinity-purified fungal PGE_x. Ca, C. albicans; Cn, C. neoformans. The results represent the averages (± standard errors) of three independent experiments. ∗, P < 0.05 compared to buffer alone.

FIG. 4

Effect of fungal PGE₂ on mitogen-induced lymphocyte proliferation in murine splenocytes. Ca, C. albicans; Cn, C. neoformans. Splenocytes were harvested from CBA/J mice and cultured in the presence of mitogen and either medium, commercially available PGE₂, or affinity-purified fungal PGE_x. The results shown are the means (± standard errors) from two or three experiments. ∗, P < 0.05 compared to buffer alone.

FIG. 5

Effect of fungal PGE₂ on chemokine production by a human bronchial epithelial cell line. Ca, C. albicans; Cn, C. neoformans. Human epithelial cell line A549 was cultured in the presence of either medium, PGE₂, or affinity-purified fungal PGE_x for 24 h. For IL-6 induction, TNF-α (10 ng/ml) was added to the cultures. The IL-8 and IL-6 concentrations were measured by ELISA. The results shown are the means (± standard errors) from two or three experiments. ∗, P < 0.05 compared to buffer alone.

FIG. 6

Effect of fungal PGE₂ on mitogen-induced cytokine production in murine splenocytes. Ca, C. albicans; Cn, C. neoformans. Splenocytes were harvested from CBA/J mice and cultured in the presence of ConA and either medium, PGE₂, or affinity-purified fungal PGE_x. The supernatants were harvested after 24 h, and the cytokines were measured by ELISA. The results shown are the means from two or three experiments. ∗, P < 0.05 compared to buffer alone.