Cryptococcus neoformans melanin and virulence: mechanism of action.
Black melanin-like pigments are produced by several neurotropic fungi, including Cryptococcus neoformans. Pigment production is associated with virulence. In media containing phenolic substrates such as L-dopa, C. neoformans cells become black as a result of pigment accumulation. Pigmented and nonpigmented C. neoformans cells were studied with transmission electron microscopy and electron spin resonance (ESR) spectroscopy. Transmission electron microscopy showed electron-dense cell walls, and ESR spectroscopy revealed a stable free-radical population in pigmented cells. The ESR signals of pigmented cells were increased by light, alkaline pH, and Zn2+ and decreased by acid pH, indicating that the black pigment was a type of melanin. A mutant deficient in melanin synthesis (mel) generated by UV radiation lacked ESR-detectable radicals, was less virulent for mice, was more susceptible to killing by nitrogen- and oxygen-derived radicals, and had 100-foldless phenoloxidase activity than the parent strain. The interaction of melanized C. neoformans, nonmelanized C. neoformans, and the hypomelanotic mel mutant with J774.16 murine macrophage-like cells was studied. Melanized cells were more resistant to antibody-mediated phagocytosis and the antifungal effects of murine macrophages than nonmelanized cells. Small increases in the intensity of the ESR signals of melanized cells in solutions containing chemically generated oxygen- and nitrogen-derived radicals indicated electron transfer to or from melanin. Melanin appears to contribute to virulence by protecting fungal cells against attack by immune effector cells.
Wang
Y
Y
Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
Aisen
P
P
Casadevall
A
A
D000818:Animals; D001483:Base Sequence; D003455:Cryptococcus neoformans / Q000737:chemistry / Q000472:pathogenicity*; D017931:DNA Primers / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D008264:Macrophages / Q000382:microbiology; D008543:Melanins / Q000737:chemistry*; D051379:Mice; D008805:Mice, Inbred A; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009584:Nitrogen / Q000737:chemistry; D010587:Phagocytosis; D017382:Reactive Oxygen Species / Q000737:chemistry
D017931:DNA Primers; D005609:Free Radicals; D008543:Melanins; D017382:Reactive Oxygen Species; D009584:Nitrogen