Microglia protect fungi against copper starvation and promote brain infection

Abstract

Microglia provide protection against a range of brain infections, but how these glial cells respond to fungi is poorly understood. We investigated the role of microglia in the context of cryptococcal meningitis, the most common cause of fungal brain infections in humans. Using a series of transgenic- and chemical-based microglia depletion methods we found that, contrary to their protective role during other infections, microglia supported cryptococcal fungal brain infection. We show that microglia become hosts for intracellular fungal growth and are a site in which the fungus accesses the restricted micronutrient copper. We developed a reporter fungal strain to track copper starvation responses by the fungus and found that yeast were protected from copper starvation within microglia. Lastly, we show that stimulation of microglia with IFNγ causes restriction of phagosomal copper to intracellular fungi. These data provide a mechanistic explanation for why microglia depletion has a therapeutic effect in the context of this life-threatening fungal infection and is one of the few examples of microglia acting to promote infection. Our data demonstrate how tissue-resident phagocytes can support cryptococcal infections by acting as intracellular reservoirs and sites of microbial nutrient acquisition, and how these mechanisms may be blocked by IFNγ immunotherapy.

CSF1R inhibition by PLX5622 reduces pulmonary fungal infection by depleting MHCIIhiinterstitial lung macrophages

“…In the current work, we used an intranasal infection model where yeast infection disseminates from the lung to other organs following breakdown of pulmonary barrier immunity. We found that loss of microglia in the CNS equated to reduced fungal brain infection, validating our earlier observations in the acute infection model [21]. However, further examination of PLX5622-treated mice revealed that fungal infection was also reduced in the lung, which may have exaggerated the reduced fungal brain burden by reducing the rate of extrapulmonary dissemination.…”

“…We recently showed that microglia support C. neoformans brain infection by acting as a fungal reservoir during acute meningitis (intravenous infection) [21]. Loss of microglia in the PLX5622-treated mice may therefore cause a reduction in brain fungal burdens by removing this growth niche, in addition to reduced extrapulmonary dissemination from the lung, or the loss of microglia independently reduces brain fungal burden (as we previously observed in the intravenous infection model).…”

“…We treated wild-type mice with PLX5622 for 1 week and then performed an intranasal infection with C. neoformans prior to measuring fungal burdens in the lung at day 21 postinfection (Fig 3A). We found that PLX5622-treatment led to a significant reduction in lung fungal burden ( PLX5622 also did not directly have an antifungal effect, as yeast grown in PLX5622 (at concentrations previously measured in brain and serum of treated mice [24]) grew similarly to untreated yeast cultures (Fig S5 We recently showed that microglia support C. neoformans brain infection by acting as a fungal reservoir during acute meningitis (intravenous infection) [21]. Loss of microglia in the PLX5622-treated mice may therefore cause a reduction in brain fungal burdens by removing this growth niche, in addition to reduced extrapulmonary dissemination from the lung, or the loss of microglia independently reduces brain fungal burden (as we previously observed in the intravenous infection model).…”

“…Rodents deficient in CSF1R lack these brain macrophages [3, 5], and a recent case report illustrated that human mutations in CSF1R resulted in complete loss of microglia and a consequent lethal disruption to brain development [4]. We recently demonstrated that targeted microglia depletion reduced C. neoformans infection in the CNS in a model of acute meningitis that bypassed pulmonary immunity [21]. In the current work, we used an intranasal infection model where yeast infection disseminates from the lung to other organs following breakdown of pulmonary barrier immunity.…”

“…Tissue-resident macrophages are also an important reservoir for C. neoformans in both the brain and lung. We recently found that targeted microglia depletion could reduce fungal brain infection, since microglia shielded C. neoformans from copper starvation within the CNS [21]. In the lung, C. neoformans produces a secreted peptide called CPL-1 that drives arginase expression within interstitial macrophages to promote intracellular growth [22].…”