Journal of Fungi Review Fungal Melanin and the Mammalian Immune System Sichen Liu 1, Sirida Youngchim 2 , Daniel Zamith-Miranda 1,3 and Joshua D. Nosanchuk 1,3,* 1 Department of Medicine, Albert Einstein College of Medicine, Bronx, NY 10461, USA; siliu@montefiore.org (S.L.); [email protected] (D.Z.-M.) 2 Department of Microbiology, Faculty of Medicine, Chiang Mai University, Chiang Mai 50200, Thailand; [email protected] 3 Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, NY 10461, USA * Correspondence: [email protected] Abstract: Melanins are ubiquitous complex polymers that are commonly known in humans to cause pigmentation of our skin. Melanins are also present in bacteria, fungi, and helminths. In this review, we will describe the diverse interactions of fungal melanin with the mammalian immune system. We will particularly focus on Cryptococcus neoformans and also discuss other major melanotic pathogenic fungi. Melanin interacts with the immune system through diverse pathways, reducing the effectiveness of phagocytic cells, binding effector molecules and antifungals, and modifying complement and antibody responses. Keywords: melanin; fungus; yeast; immune response 1. Introduction Citation: Liu, S.; Youngchim, S.; Melanins are a family of structurally complex dark pigment polymer present in all bio- Zamith-Miranda, D.; Nosanchuk, J.D. logical kingdoms [1]. The polymer is made up of covalently linked indoles, but the overall Fungal Melanin and the Mammalian structure is granular; however, detailed structures are not well characterized because it is a Immune System. J. Fungi 2021, 7, 264. collection of polymers with mixed pre-indole structures [2]. Moreover, melanins are amor- https://doi.org/10.3390/jof7040264 phous and are not suitable for study by crystallography or cryo-electron microscopy, which has led to complicated efforts to solve the structure of these natural pigments. Mammalian Academic Editors: Thomas Lehrnbecher and Hector melanin biogenesis happens by oxidation of L-tyrosine via tyrosinase in melanocytes, M. Mora-Montes which are neural crest-derived dendritic cells [3,4]. However, melanins in bacteria, fungi, and helminths are produced through the polyketide synthase (PKS) pathway or catalyzed Received: 22 February 2021 by phenoloxidase [5]. This review will focus on melanin in fungi and its function in Accepted: 26 March 2021 pathogen–host interaction with a particular emphasis on the immune system. Published: 31 March 2021 2. Melanin Synthesis Publisher’s Note: MDPI stays neutral Fungi synthesize melanin via two main pathways, namely 1,8-dihydroxynaphthalene with regard to jurisdictional claims in (DHN) and L-3,4-dihyroxyphenylalanine (L-DOPA) (Figure1 and Table1). In the DHN published maps and institutional affil- pathway, 1,3,6,8-tetrahydroxynaphthalene (1,3,6,8-THN) is first synthesized through PKS, iations. which is a multi-domain enzyme complex that produces polyketides. Polyketides are prod- ucts derived from acetyl-CoA or propionyl-CoA with malonyl-CoA or methylmalonyl-CoA. The condensation reactions are driven by decarboxylation, yielding a beta-keto functional group [6]. This is then followed by reduction and dehydration reactions that eventually pro- Copyright: © 2021 by the authors. duce DHN. It is the polymerization of DHN that leads to the formation of melanin [5,7,8]. Licensee MDPI, Basel, Switzerland. Polyketides are a class of secondary metabolites mainly produced in bacteria, fungi, and This article is an open access article plants, and they serve very different purposes in our society [9]. Polyketides such as distributed under the terms and macrolide, tetracycline, and amphotericin antimicrobials serve tremendous value, while conditions of the Creative Commons aflatoxin can be lethal to mammals [10,11]. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). J. Fungi 2021, 7, 264. https://doi.org/10.3390/jof7040264 https://www.mdpi.com/journal/jof J. Fungi 2021, 7, x FOR PEER REVIEW 2 of 15 J. Fungi 2021, 7, 264 2 of 15 FigureFigure 1. 1.CurrentCurrent knowledge knowledge of of melanin melanin synthetic synthetic pathways pathways in in fungi. fungi. Table 1.Table Representative 1. Representative fungi and fungi the and types the of types melanin of melanin they produce they produce [12–15]. [12 –15]. Species Isolate Environment Melanin Types Species Isolate Environment Melanin Types Aspergillus fumigatus Clinical DHN and pyo-melanin Aspergillus niger Aspergillus fumigatusIndustrial fermentation ClinicalDHN DHN and L-DOPA and pyo-melanin Blastomyces dermatitidis Aspergillus niger Clinical Industrial fermentation DHN and L-DOPA Candida Albicans Blastomyces dermatitidisClinical ClinicalL-DOPA DHN Cryptococcus neoformans Clinical L-DOPA Candida Albicans Clinical L-DOPA Histoplasma capsulatum Clinical DHN and L-DOPA Paracoccidioides brasiliensis Cryptococcus neoformansClinical ClinicalDHN and L-DOPA L-DOPA Fonsecaea monophora Histoplasma capsulatumClinical ClinicalDHN and DHN L-DOPA and L-DOPA Fonsecaea pedrosoi Paracoccidioides brasiliensisClinical ClinicalDHN and L-DOPA Sporothrix schenckii Clinical DHN Fonsecaea monophora Clinical DHN and L-DOPA Abbreviations: DHN is 1,8-dihydroxynaphthalene and L-DOPA is L-3,4-dihyroxyphenylalanine. Fonsecaea pedrosoi Clinical DHN TheSporothrix L-DOPA schenckii pathway is similar to mammalianClinical melanin biosynthesis, DHN where the pathwayAbbreviations: typically DHN isuses 1,8-dihydroxynaphthalene either L-DOPA or andtyro L-DOPAsine as is startingL-3,4-dihyroxyphenylalanine. molecules. If the pathway starts with tyrosine, tyrosinase will perform a two-step oxidation, turning tyrosine into dopaquinone.The L-DOPA Similarly, pathway laccase is is similar the enzy to mammalianme responsible melanin for converting biosynthesis, L-DOPA where into the dopaquinone.pathway typically Dopaquinone uses either is then L-DOPA turned or into tyrosine leucodopachrome as starting molecules. (cyclodopa) If the and pathway then oxidizedstarts with to dopachrome. tyrosine, tyrosinase Dopachrome will perform then goes a two-step through oxidation,tautomerization turning to tyrosineform dihy- into droxyindoles,dopaquinone. which Similarly, are simultaneously laccase is the enzyme oxidized responsible and polymerized for converting to produce L-DOPA DOPA into melanindopaquinone. [8]. Dopaquinone is then turned into leucodopachrome (cyclodopa) and then oxidized to dopachrome. Dopachrome then goes through tautomerization to form di- 3.hydroxyindoles, Cryptococcus neoformans which are simultaneously oxidized and polymerized to produce DOPA melanin [8]. C. neoformans is one of the most well studied pathogens for melanization (Figure 2). Cryptococcus3. Cryptococcus neoformans neoformans is unique among pathogenic fungi as it solely relies on the L- DOPA pathway and requires exogenous phenolic substrates to form melanin. After the C. neoformans is one of the most well studied pathogens for melanization (Figure2). polysaccharide capsule, melanin is the second most important virulence factor in C. neofor- Cryptococcus neoformans is unique among pathogenic fungi as it solely relies on the L-DOPA mans as it has been calculated as contributing to 14% of the pathogen’s total virulence [16]. pathway and requires exogenous phenolic substrates to form melanin. After the polysac- This notable aspect of cryptococcal melanization has led to extensive study of this polymer charide capsule, melanin is the second most important virulence factor in C. neoformans as init Cryptococcus has been calculated spp. [17]. as Other contributing species to can 14% prod of theuce pathogen’s melanin using total virulenceendogenous [16 ].com- This poundsnotable or aspect exogenous of cryptococcal substrates, melanization and some can has produce led to extensive more than study one oftype this of polymer melanin in [12,18,19].Cryptococcus spp. [17]. Other species can produce melanin using endogenous compounds or exogenous substrates, and some can produce more than one type of melanin [12,18,19]. J. Fungi 2021, 7, x FOR PEER REVIEW 3 of 15 J. Fungi 2021, 7, 264 3 of 15 FigureFigure 2. Cryptococcus 2. Cryptococcus neoformans melanin neoformans “ghosts” obtainedmelanin from “ghosts” lungs of infected obtained mice as described from lungs of infected mice as de- inscribed [20]. The redin arrows[20]. showThe bud red scars arrows on the melanin. show bud scars on the melanin. In the early 1980s, Kwon-Chung et al. produced melanin-deficient strains (Mel−) via UV irradiation and observed that the mutants lacked virulence as the cells were cleared − from mouseIn the organs, early whereas 1980s, wild-type Kwon-Chung (WT) cells expanded et in al numbers,. produced especially melanin-deficient in the strains (Mel ) via brainUV [ 21irradiation]. The same study and showed observed that Mel− mutants that the were defectivemutants in the lacked active transport virulence as the cells were cleared system for diphenolic compounds and phenoloxidase, which is the first enzyme needed infrom the L-DOPA mouse melanogenesis organs, pathway.whereas A subsequentwild-type study (WT) confirmed cells that expanded the loss of in numbers, especially in the phenoloxidasebrain [21]. activity The same was responsible study forshowed the Mel− mutantthat Mel phenotype− mutants [22]. Williamson were defective in the active transport next discovered that laccase, a phenoloxidase, was encoded by the lac1 gene [23]. Laccase wassystem linked toforC. neoformansdiphenolicvirulence