IL-4/IL-13-Dependent Alternative Activation of Macrophages but Not Microglial Cells Is Associated with Uncontrolled Cerebral Cryptococcosis

Both interleukin (IL)-4- and IL-13-dependent Th2-mediated immune mechanisms exacerbate murine Cryptococcus neoformans-induced bronchopulmonary disease. To study the roles of IL-4 and IL-13 in cerebral cryptococcosis, IL-4 receptor α-deficient (IL-4Rα^−/−), IL-4-deficient (IL-4^−/−), IL-13-deficient (IL-13^−/−), IL-13 transgenic (IL-13^T/+), and wild-type mice were infected intranasally. IL-13^T/+ mice displayed a higher fungal brain burden than wild-type mice, whereas the brain burdens of IL-4Rα^−/−, IL-4^−/−, and IL-13^−/− mice were significantly lower as compared with wild-type mice. On infection, 68% of wild-type mice and 88% of IL-13-overexpressing IL-13^T/+ mice developed significant cerebral lesions. In contrast, only a few IL-4Rα^−/−, IL-4^−/−, and IL-13^−/− mice had small lesions in their brains. Furthermore, IL-13^T/+ mice harbored large pseudocystic lesions in the central nervous system parenchyma, bordered by voluminous foamy alternatively activated macrophages (aaMphs) that contained intracellular cryptococci, without significant microglial activation. In wild-type mice, aaMphs tightly bordered pseudocystic lesions as well, and these mice, in addition, showed microglial cell activation. Interestingly, in resistant IL-4^−/−, IL-13^−/−, and IL-4Rα^−/− mice, no aaMphs were discernible. Microglial cells of all mouse genotypes neither internalized cryptococci nor expressed markers of alternative activation, although they displayed similar IL-4Rα expression levels as macrophages. These data provide the first evidence of the development of aaMphs in a central nervous system infectious disease model, pointing to distinct roles of macrophages versus microglial cells in the central nervous system immune response against C. neoformans.The opportunistic pathogenic yeast Cryptococcus neoformans causes life-threatening fungal infections of most internal organs including the central nervous system (CNS), primarily in patients affected by immunodeficiency syndromes such as AIDS.¹ The pathogenesis of cryptococcosis is not fully understood, however, especially in cases of different levels of immunocompetence. It is generally accepted that the fungus first invades the respiratory system, where it leads to relatively mild or asymptomatic bronchopneumonia in the immunocompetent.^2,3,4,5 Fungemia with generalization of the infection may result from reduced immunological control mechanisms.^6,7,8,9 Invasion of the CNS with subsequent development of meningoencephalitis is the major cause of death during cryptococcosis.^10,11The precise reaction pattern of recruited inflammatory cells, especially monocytes/macrophages, due to fungal invasion of the CNS parenchyma has been addressed mainly via analysis of helper T cell (Th)1 responses.¹² In this context, in addition to protective Th1-driven immune responses, the role of Th2 cytokines has gained interest recently.¹³ The major Th2 cytokines interleukin (IL)-4 and IL-13 act via the IL-4Rα chain together with the γ_c chain or the IL-13Rα1/2 chains, and regulate macrophage functional status.¹⁴ IL-4 has been shown to be detrimental in murine models of systemic and pulmonary cryptococcosis,^{6,15,16,17,18} and we have recently illustrated the role of IL-13 in inducing the formation of alternatively activated macrophages (aaMphs) in murine pulmonary cryptococcosis.¹⁹The activation phenotype of macrophages may critically influence the regulatory mechanisms by which inflammation and infection in the CNS are controlled. According to the current paradigm, classically activated macrophages are primed by interferon-γ and produce tumor necrosis factor, IL-1, oxygen and nitrogen radicals,²⁰ thereby producing proinflammatory cytokines that regulate the Th1 immune response. In contrast, aaMph²¹ develop in response to Th2 cytokine stimulation such as IL-4 and IL-13 and are characterized by expression of genes associated with endocytosis and tissue repair such as arginase-1, mannose receptor (CD206), found-in-inflammatory-zone (FIZZ), and chitinase 3-like 3 (YM1) and largely fail to produce nitric oxide (NO) due to their induction of arginase.²² As such, they are thought to be involved in tissue repair and remodeling,^22,23 in protection against diet-induced obesity,^24,25 and schistosomiasis,²⁶ but they may also elicit adverse tissue processes such as pulmonary or liver fibrosis.^{27,28,29,30,31} In particular, their development renders the host vulnerable to infection with pathogens where macrophage activation and killing functions are required.³²In murine models of pulmonary C. neoformans infection, aaMph have been shown to be associated with uncontrolled lung infection.^18,19 The role of aaMph versus classically activated macrophage in the CNS due to pulmonary infection with the neurotropic pathogen C. neoformans has not been defined yet. In this study, we aimed to characterize the morphology and functional status of CNS macrophages in cerebral cryptococcosis following intranasal infection of susceptible wild-type and IL-13-transgenic BALB/c mice. Moreover, using mice unable to produce IL-4 or IL-13 or respond to both (IL-4Rα^−/− mice), we show that abrogation of CNS aaMph development is associated with controlled infection.