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Mucorales fungi cause mucormycosis—an emerging, life-threatening, opportunistic infection with limited therapeutic options and incompletely understood pathogenesis1,6,7. The overall mortality of mucormycosis exceeds 50% and approaches 100% in patients with disseminated disease1. The clinical hallmark of mucormycosis is massive tissue necrosis induced by the fungus, which impedes antifungal agents from reaching the sites of infection and often necessitates radical, disfiguring surgery to control the disease1,8. In contrast to other fungal infections, mucormycosis predominantly affects an ever-expanding group of patients with metabolic abnormalities through incompletely characterized mechanisms1,4,5. Specifically, poorly controlled diabetes mellitus (DM), acidosis, acquired iron overload syndromes, malnutrition and immunometabolic dysregulation induced by COVID-19 uniquely predispose individuals to the development of mucormycosis1,2,3,4,6,8. Thus, uncharacterized metabolic host defence mechanisms may confer protective immunity against mucormycosis.

From the pathogen perspective, Mucorales senses cues in the tissue environment, triggering the expression of virulence factors that transform these saprophytic organisms into rapidly invasive and potentially lethal pathogens1. Specifically, the production of the potent mycotoxin mucoricin during Mucorales hyphae growth9 induces extensive tissue necrosis, whereas the binding of CotH invasins to specific host receptors promotes angioinvasion and fungal dissemination10,11,12. Notably, germinating spores of Mucorales evade phagocytosis and induce acute lethality within 24 h of pulmonary infection in immunocompetent mice13. Thus, it is essential to identify metabolic host effectors that prevent extracellular growth and modulate Mucorales pathogenicity during the early stages of infection.

Albumin selectively inhibits Mucorales

Human serum has important inhibitory effects against Mucorales, which remain molecularly unexplored14,15. We found that, compared with sera from healthy individuals, the ability of sera from patients with mucormycosis to inhibit hyphal growth of a clinical isolate of Rhizopus arrhizus var. delemar (hereafter, R. delemar) is almost completely lost (Fig. 1a). Albumin, the most abundant serum protein, regulates important physiological functions intravascularly, in the interstitial space, and on mucosal surfaces16. Furthermore, severe hypoalbuminaemia is a common finding in patients with diverse immunometabolic abnormalities predisposing for mucormycosis2,5,17. We therefore decided to comprehensively evaluate the physiological function of albumin against Mucorales.

Fig. 1: Selective antifungal activity of albumin against Mucorales.**

a, The length of germinating R. delemar spores cultured for 5 h in RPMI medium (n = 12), serum from healthy individuals (n = 5) or serum from patients with mucormycosis before initiation of antifungals (n = 4). Statistical analysis was performed using one-way analysis of variance (ANOVA) with Tukey’s post hoc test; *P = 0.0213, **P = 0.0079. Representative images are shown. Scale bar, 100 µm. b, Serum albumin concentrations at diagnosis in contemporaneously matched controls (n = 33), patients with pulmonary aspergillosis (n = 50) and patients with mucormycosis (n = 81). Statistical analysis was performed using one-way ANOVA with Tukey’s post hoc test; NS, not significant (P = 0.466). c, Kaplan–Meier survival curves for patients with mucormycosis presenting with severe hypoalbuminaemia (≤2.5 g dl−1) compared with all other mucormycosis cases from independent clinical cohorts in the USA (n = 81), India (n = 101) and France (n = 26). Survival differences were assessed using the log-rank (Mantel–Cox) test. MDACC, MD Anderson Cancer Center. d, The effect of albumin depletion from human serum of healthy donors on the growth of R. delemar (n = 15) and A. fumigatus (n = 6). Statistical analysis was performed using two-sided Wilcoxon matched-pairs signed-rank test; NS, P = 0.156. e, Coomassie blue staining of intact human serum, albumin-depleted serum and albumin-enriched eluates. Representative of three independent experiments. f, Germ-tube elongation of R. delemar spores cultured in medium with or without purified HSA, quantified by time-lapse microscopy. n = 50–200 spores per timepoint; two independent experiments, performed in triplicate. Data are mean ± s.d. Statistical analysis was performed using two-way ANOVA with Sidak’s multiple-comparisons test. g, Representative images of Calcofluor White (CFW)-stained R. delemar spores after 8 h of culture in medium with or without HSA. Scale bars, 50 µm. h, Inhibition of R. delemar growth by increasing concentrations of HSA or BSA. n = 4 independent experiments performed in triplicate. Data are mean ± s.d. i, Quantification of the growth of different Mucorales species and other pathogens cultured in medium with or without HSA. Representative of three independent experiments in triplicate. Full scientific names are given in the Methods. ****P < 0.0001 (b,d,f). The diagram in f was created using BioRender.

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We initially explored the associations between serum albumin levels and susceptibility to the development of pulmonary mucormycosis and its outcome in contemporaneous high-risk patients with haematological malignancy at a major tertiary care cancer centre in the United States (Supplementary Table 1). Notably, most patients who developed pulmonary mucormycosis had significantly lower albumin levels at diagnosis compared with control patients matched for the underlying disease who developed bacterial pneumonia or pneumonia caused by the major airborne human fungal pathogen Aspergillus fumigatus (Fig. 1b). Furthermore, patients with mucormycosis and very low albumin levels (≤2.5 g dl−1) had significantly lower survival rates compared with other patients with mucormycosis (Fig. 1c). These findings were independently validated through an analysis of serum albumin levels in a cohort of patients with pulmonary mucormycosis in a tertiary care centre from the Postgraduate Institute of Medical Education and Research (PGIMER), Chandigarh, India (Fig. 1c and Supplementary Table 1), who had DM and COVID-19 as main underlying risk factors, and a published cohort of patients with mucormycosis from France (Ambizygo Study18) who had different risk factors (Fig. 1c). Notably, multivariate survival analysis identified severe hypoalbuminaemia (≤2.5 g dl−1) as an independent predictor of poor outcome across all three clinical cohorts of patients with mucormycosis (Extended Data Tables 1–3).

We next analysed the functional relationship between the serum albumin concentration and antifungal activity in patients at high risk for mucormycosis. Notably, we detected a significant association between the degree of hypoalbuminaemia and the loss of inhibitory activity of serum against R. delemar hyphal formation in prospectively collected sera from patients with liver cirrhosis or haematological malignancy (Extended Data Fig. 1a and Supplementary Table 1).

To account for potential confounders related to the underlying disease, we performed albumin depletion in sera from healthy individuals using affinity chromatography19. Equilibration of Cibacron Blue chromatography columns was achieved with concentrated serum filtrate previously passed through a 50 kDa centrifugal filter unit, to ensure that the albumin-depleted flow-through was not diluted for other serum proteins. We assessed the effect of albumin depletion on the activity of serum against R. delemar and another major human respiratory fungal pathogen A. fumigatus7. Notably, albumin depletion resulted in a significant loss of antifungal activity of the serum selectively against R. delemar but not against A. fumigatus (Fig. 1d).

We next purified human albumin from the sera of healthy volunteers to directly evaluate its antifungal activity against Mucorales using a protocol based on affinity column chromatography19 (Fig. 1e and Extended Data Fig. 1b). We found no evidence of transferrin, a serum iron-transferring protein with important role in nutritional immunity against Mucorales20, in the albumin-containing eluted fractions by western blot analysis (Extended Data Fig. 1c). Notably, purified human albumin dissolved at physiological concentrations (around 3.5 g dl−1) in liquid culture medium had potent activity against R. delemar (Fig. 1f), an effect that was not observed with comparable concentrations of human IgG (Extended Data Fig. 1d). Moreover, purified albumin from different sources, including commercially available bovine serum albumin (BSA) and human serum albumin (HSA) used for therapeutic applications, inhibited Mucorales growth when added at physiologically relevant concentrations in culture medium (Fig. 1g,h). Notably, we found that albumin specifically blocks filamentous (hyphal) growth of Mucorales after the initial stage of isotropic growth (swelling) of fungal spores (Extended Data Fig. 2a). The antifungal activity of albumin was fully reversible after culture of inhibited Mucorales spores in fresh medium without albumin (Extended Data Fig. 2b). Albumin selectively inhibited a wide range of clinical isolates of Mucorales species at physiological serum concentrations (4.5 g dl−1), whereas it showed no significant activity against other major human bacterial or fungal pathogens (Fig. 1i and Extended Data Fig. 2c). Collectively, these results reveal the specialized activity of albumin against Mucorales and identify severe hypoalbuminaemia as an independent biomarker of poor outcome of mucormycosis.

Albumin-bound FFAs inhibit Mucorales

To examine whether the inhibitory activity of albumin requires direct interaction of the protein with fungal cells, we filtered culture medium containing inhibitory concentrations of albumin (4.5 g dl−1) and assessed the antifungal activity of the culture filtrate (flow-through). Notably, we found that the flow-through of albumin retains full inhibitory activity against R. delemar (Extended Data Fig. 3a). Nutritional immunity is an essential host defence mechanism against Mucorales1,13. In view of the ability of albumin to bind a wide range of compounds, endogenous molecules, iron and other transition metals21, we investigated the possibility that albumin inhibits Mucorales through depletion of essential nutrients from the culture medium. We therefore analysed the components of regular RPMI culture medium and found that the presence of albumin resulted in a significant reduction in the concentration of certain amino acids (Extended Data Fig. 3b and Supplementary Table 2). However, supplementation experiments with amino acids and different combinations of nutrients contained in RPMI medium did not affect the inhibitory activity of albumin flow-through against R. delemar (Extended Data Fig. 3c).

We next considered the possibility that the release of an inhibitory molecule bound to albumin could account for the antifungal activity of the albumin-rich culture filtrate. Serum albumin acts as the main shuttle of non-esterified middle- and long-chain fatty acids (FFAs) in extracellular fluids22 and FFAs possess antimicrobial properties23. We therefore performed fractionation of lipid-containing elutions from a BSA-containing culture filtrate and functionally characterized the inhibitory activity against R. delemar. We identified a fraction with significant inhibitory activity against Mucorales; this fraction contained high amounts of caprylic acid (C8:0) as determined using gas chromatography–mass spectrometry (GC–MS) and electrospray ionization high-resolution MS (ESI-HRMS) analysis (Extended Data Fig. 4a–c). We also confirmed that purified caprylic acid dissolved in ethanol has potent inhibitory activity against Mucorales at concentrations lower than those contained in the BSA filtrate (Extended Data Fig. 4d).

To further explore the physiological relevance of our findings, we performed lipidomic profiling in purified human albumin isolated from healthy donors before and after filtration (Fig. 2a). The albumin filtrate retained full inhibitory activity against Mucorales spores (Fig. 2a) and contained physiological middle- and long-chain FFAs (Fig. 2b and Supplementary Table 3). Furthermore, we found that purified serum FFAs dissolved in ethanol exerted potent anti-Mucorales activity at physiologically relevant concentrations24 (Fig. 2c). We also found that a broad range of FFAs of various carbon chain lengths and degrees of saturation display potent antifungal activity against Mucorales (Fig. 2d). Importantly, purified human albumin after charcoal treatment for the removal of bound FFAs25, or commercially available BSA free of fatty acids (FFA-free BSA), had no activity against Mucorales (Fig. 2e). Furthermore, charcoal-treated BSA complexed with physiologically relevant concentrations of oleic acid (OA) dissolved the FFA and fully restored its activity against Mucorales (Fig. 2f). Experiments with fluorescent-labelled albumin further demonstrated that, although albumin avidly binds to the fungal cell wall, it is not internalized by Mucorales spores (Fig. 2g). Collectively, these findings demonstrate that albumin binds to, dissolves, shuttles and facilitates the release of physiological FFAs to optimize their antifungal activity.

Fig. 2: Physiological FFAs mediate the antifungal activity of albumin against Mucorales.

a, Schematic of the experimental workflow for generating flow-through and isolating albumin from human serum using a 3 kDa molecular weight cut-off (MWCO) centrifugal filter (top). Bottom, representative bright-field images from three independent experiments showing R. delemar spores cultured for 5 h in minimal medium alone or in medium supplemented with isolated HSA or albumin flow-through. FT, flow-through; HD, healthy donor; ND, not detectable. Scale bar, 50 µm. b, Lipidomic profiling of FFAs in isolated HSA and corresponding flow-through obtained as in a. c, Antifungal activity of the major serum FFAs against R. delemar. The red-striped areas indicate physiological serum concentrations of each FFA. Data are mean ± s.d., representative of n = 3 independent experiments. IC50, half-maximum inhibitory concentration. d, The dose-dependent inhibitory effect of short-, medium- and long-chain FFAs on R. delemar growth. e, Representative images of R. delemar spores cultured for 5 h in medium supplemented with isolated HSA or charcoal-stripped albumin (left). Scale bar, 50 µm. Right, quantification of R. delemar growth inhibition by BSA, FFA-free BSA or charcoal-stripped BSA. n = 4 independent experiments, performed in triplicate. Data are mean ± s.d. Statistical analysis was performed using one-way ANOVA with Tukey’s multiple-comparison test; NS, P = 0.574. f, The inhibitory effects of charcoal-stripped BSA, OA or charcoal-stripped BSA reconstituted with OA on R. delemar growth. Data are a representative example of* n* = 3 independent experiments, shown as the mean ± s.d., performed in triplicate. Statistical analysis was performed using one-way ANOVA with Tukey’s multiple-comparison test. g, Representative fluorescence microscopy images from three independent experiments showing CFW-labelled R. delemar spores cultured for 6 h in the presence of FITC-labelled albumin, performed in duplicate. Scale bar, 10 µm. ****P < 0.0001 (e,f). The diagram in a was created using BioRender.

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Albumin protects FFAs from oxidation

We next performed targeted lipidomic profiling of the sera of patients with mucormycosis and matched controls to analyse the FFA composition and identify abnormalities associated with the loss of antifungal activity in the serum. Notably, we found that the sera of patients with mucormycosis contained a significantly greater proportion of oxidized forms of FFAs than the sera of control patients, who were matched for the underlying disease without infection or those who have been diagnosed with invasive aspergillosis (Fig. 3a and Supplementary Table 3). Furthermore, the sera of patients susceptible to mucormycosis due to underlying cirrhosis or haematological malignancy contained high amounts of oxidized FFAs, which was directly proportional to the severity of hypoalbuminaemia, particularly in cirrhosis (Fig. 3b,c). Importantly, in patients with cirrhosis, the degree of serum FFA oxidation was strongly correlated with the loss of inhibitory activity against Mucorales (Fig. 3d), whereas this association was less pronounced in the sera of patients with haematological malignancy. Collectively, these findings suggested that oxidized FFAs display attenuated activity against Mucorales.

Fig. 3: Albumin protects FFAs from oxidation and preserves their antifungal activity against Mucorales.

a, The relative concentrations of oxidized FFAs in sera from matched control patients (n = 6), patients with invasive aspergillosis (n = 6) and mucormycosis (n = 18). NS, P = 0.973; **P = 0.0061, **P = 0.0031. b, The relative concentrations of oxidized FFAs in sera from healthy controls (n = 21), patients with cirrhosis (n = 18) and patients with haematological malignancies (n = 20). NS, P = 0.576; *P = 0.0402, **P = 0.0066. c,d, Correlation between oxidized serum FFAs and albumin levels (c) or antifungal activity (d) of sera from patients with cirrhosis (n = 18) and haematological malignancies (n = 20) against R. delemar. Statistical analysis was performed using two-sided linear regression. e, Inhibitory effects of increasing concentrations of OA and oxidized OA on R. delemar growth.* n* = 2 independent experiments performed in triplicate. Data are mean ± s.d. f, Serum lipids from healthy individuals were oxidized and tested for antifungal activity. n = 13. Statistical analysis was performed using the two-sided Wilcoxon matched-pairs signed-rank test; ***P = 0.0002. g, CFW-labelled R. delemar spores cultured for 3 h with control or pre-oxidized C11-BODIPY (left). Scale bars, 8 µm. Right, quantification of C11-BODIPY mean fluorescence intensity (MFI) at 510 nm and 590 nm (n = 269–979 spores, three experiments). NS, P = 0.769. Em., emission. h, GC–MS analysis of non-oxidized OA in OA and BSA-conjugated OA before and after microwave oxidation. n = 3, triplicates. **P = 0.0041. i, R. delemar spores cultured for 5 h with oxidized OA or oxidized BSA–OA. Scale bar, 100 µm. Quantification of germling length is shown. n = 202–237 spores, three experiments. Statistical analysis was performed using two-sided Mann–Whitney U-tests. j, Area under the curve (AUC) quantification of caprylic acid in mock-treated and glycosylated BSA. n = 3, triplicates. Statistical analysis was performed using two-sided unpaired t-tests. k, The inhibitory effects of mock-treated and glycosylated BSA on R. delemar growth (n = 4, triplicates). Statistical analysis was performed using two-sided Mann–Whitney U-tests; *P = 0.0286. For i–k, data are mean ± s.d. For a, b, g and i, statistical analysis was performed using one-way ANOVA with Tukey’s test. ****P < 0.0001 (g–j).

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To directly evaluate the antifungal activity of oxidized FFAs, we oxidized OA—a major physiological serum FFA—and assessed the effect on antifungal activity. We found that the oxidation of OA resulted in a greater than 100-fold decrease in its inhibitory activity against R. delemar (Fig. 3e). To establish a causal relationship between FFA oxidation and the loss of anti-Mucorales activity in serum, we isolated serum lipids from healthy individuals and assessed the effects of oxidation on their antifungal properties. Importantly, oxidation of serum lipids resulted in a significant decrease in their inhibitory properties against R. delemar spores (Fig. 3f).

As FFA oxidation diminishes their uptake by mammalian cells26, we evaluated the internalization of oxidized FFAs by fungal cells. We initially found that the uptake of a long-chain FFA fluorescent analogue C12-BODIPY by Mucorales spores is energy dependent, as it was abolished at 0 °C (Extended Data Fig. 5a). We next evaluated uptake of C11-BODIPY581/591 (hereafter, C11-BODIPY), a ratiometric reporter of lipid peroxidation that shifts its fluorescence after oxidation from red (around 590 nm; reduced) to green (around 510 nm; oxidized)27, in R. delemar spores with or without previous in vitro oxidation. Pre-oxidation almost completely abolished C11-BODIPY uptake by R. delemar, as indicated by a marked reduction in both reduced and oxidized fluorescence within fungal cells (Fig. 3g and Extended Data Fig. 5b). Furthermore, we measured OA uptake by R. delemar cells by fluorescence labelling with Nile Red lipid dye28. Notably, we detected a substantial degree of OA uptake by R. delemar spores within a few hours in culture, which was profoundly reduced after OA oxidation (Extended Data Fig. 5c,d).

Given the well-established antioxidant properties of albumin21, we reasoned that the binding of FFAs to albumin can protect them from oxidation. We therefore performed chemical oxidation of OA dissolved in ethanol or after complexation with albumin and assessed the degree of oxidation. We found that albumin significantly protected OA from oxidation (Fig. 3h) and retained its antifungal properties against Mucorales (Fig. 3i).

Albumin glycation induced by DM, a major predisposing factor for mucormycosis1, results in the dissociation of FFAs from their binding sites and increased oxidation29 We therefore performed in vitro glycation of albumin (BSA) and assessed its ability to inhibit Mucorales. BSA glycation led to near-complete dissociation of bound FFA (Fig. 3j and Extended Data Fig. 5e–g), resulting in significant loss of albumin antifungal activity (Fig. 3k). Collectively, these studies reveal that albumin protects FFAs from oxidation, which results in the loss of their antifungal activity against Mucorales. Moreover, we identified FFA oxidation as a prominent abnormality in mucormycosis sera.

FFAs target pathogenicity of Mucorales

We next investigated the physiological importance of albumin during in vivo infection with Mucorales after pulmonary infection of immunocompetent mice with swollen fungal spores13. Specifically, we allowed dormant spores of R. delemar to grow in culture medium with or without physiological concentrations (4.5 g dl−1) of albumin for around 4 h and performed intratracheal (i.t.) infection of the mice (Fig. 4a). We found that albumin pre-exposure rendered R. delemar spores almost completely avirulent in vivo (Fig. 4b). Notably, pre-exposure to albumin did not inhibit the in vivo germination of Mucorales during the early stages of infection in the lungs (Extended Data Fig. 6a). Instead, albumin pre-exposure completely abrogated massive tissue necrosis and tissue invasion induced by germinating fungal spores, as evidenced by staining for active caspase-3 in sections of the lung (Fig. 4c), and lung fungal burden determined by quantitative PCR (qPCR; Fig. 4d) and histopathology (Fig. 4e). These findings suggest a predominant effect of albumin in attenuating the virulence of Mucorales.

Fig. 4: Albumin-bound FFAs target Mucorales pathogenicity by inhibiting protein synthesis.

a, Schematic of i.t. instillation of dormant or swollen R. delemar spores. b, Survival of C57BL/6 mice infected i.t. with 2.5 × 106 dormant (n = 6), control-swollen (n = 18) or albumin-swollen (n = 12) spores as in a. Statistical significance was determined using the log-rank (Mantel–Cox) test. c, Representative lung histopathology on day 1 after infection with control- or albumin-swollen spores, stained for active caspase-3 (left), haematoxylin and eosin (H&E; middle) and Grocott’s methenamine silver (GMS; right). Scale bars, 100 µm. d, The fungal burden determined by qPCR, expressed as spores per g of lung tissue. n = 7 mice. Statistical analysis was performed using a two-sided unpaired t-test; *P = 0.0331. eq., equivalent. e, Quantification of GMS-stained R. delemar hyphae in lung tissue. n = 8 control-swollen and n = 7 albumin-swollen mice. Statistical analysis was performed using two-sided Mann–Whitney U-tests; ***P = 0.0006. HPF, high-power field. f, Workflow of RNA-seq analysis of dormant and swollen spores cultured as in a for 3 h and 6 h. g, Differentially expressed genes (DEGs) in albumin-swollen versus control-swollen spores. The red and blue dots indicate upregulated and downregulated genes, respectively. FC, fold change. h, Enriched GOs among DEGs identified by gene set enrichment analysis after culture of spores in albumin versus medium for 3 h. NES, normalized enrichment score. i, Representative fluorescence images of R. delemar spores cultured for 5 h in RPMI medium alone or supplemented with BSA (4.5 g dl−1) or caprylic acid (2 mM). Protein synthesis was assessed by OP-puro incorporation and 5-FAM-azide staining. Scale bars, 20 µm. j, Quantification of 5-FAM-azide MFI. n = 943–1,191 spores per group, three independent experiments. Statistical analysis was performed using one-way ANOVA with Tukey’s test. k, Differential mRNA expression of virulence-related genes after incubation in medium with or without albumin for 3 or 6 h.* n* = 3 biologically independent samples. Data show log2-transformed normalized counts per million (CPM) values. l, Confocal images of mucoricin expression in CFW-labelled control- and albumin-swollen R. delemar spores after 3 h (left). n = 107 control-swollen and n = 70 albumin-swollen spores, three experiments. Scale bars, 10 µm. Right, quantification of the mucoricin fluorescence intensity. Statistical analysis was performed using two-sided Mann–Whitney U-tests. Data are mean ± s.d. ****P < 0.0001 (j,l). The diagrams in a and** f** were created using BioRender.

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We next performed RNA-sequencing (RNA-seq) analysis during the in vitro growth of R. delemar to examine the molecular mechanism of action of albumin on the pathogenicity program of the fungus (Fig. 4f). Volcano plot analysis revealed that albumin treatment differentially modulated the expression of a large number of genes within 3 h (Fig. 4g). Enrichment analysis (Gene Ontology (GO) (Fig. 4h) and KEGG (Extended Data Fig. 6b,c and Supplementary Table 3)) showed broad downregulation of protein-synthesis pathways, accompanied by induction of oxidative stress responses and lipid metabolism. To test whether albumin-bound FFAs directly impact fungal protein synthesis, we measured global translation in R. delemar using an assay based on incorporation of O-propargyl-puromycin (OP-puro) into nascent polypeptide chains, detected by click-chemistry with a fluorescent azide30. Both BSA and purified caprylic acid almost completely inhibited OP-puro incorporation in germinating R. delemar spores within 2 h of treatment (Fig. 4i,j). In view of the pronounced inhibitory effect of albumin-bound FFAs on protein synthesis, we analysed the transcriptional response of all characterized virulence factors of Mucorales, including CotH invasins1,10, the mycotoxin mucoricin9 and genes regulating the iron assimilation program of the fungus1. Notably, we identified mucoricin and CotH3 invasin10 as the genes most significantly downregulated by albumin at 6 h of R. delemar growth (Fig. 4k). We also found that pre-exposure of Mucorales to the albumin culture filtrate almost completely abolished the expression of mucoricin and CotH3 on the surface of swollen spores, as evidenced by immunostaining (Fig. 4l and Extended Data Fig. 6d); similarly, exposure of R. delemar spores to purified FFAs blocked mucoricin protein expression (Extended Data Fig. 6e). Finally, the silencing of mucoricin in R. delemar (mucoricin RNA interference (RNAi) strain9) resulted in a significant decrease in the pathogenicity of swollen spores after pulmonary infection of immunocompetent mice (Extended Data Fig. 6f). Overall, these results reveal that albumin-bound FFAs inhibit protein synthesis to modulate pathogenicity during in vivo fungal growth.

Albumin deficiency promotes mucormycosis

We next used a humanized model of albumin knockout (KO) transgenic mice to genetically validate the role of albumin in host defence against Mucorales31. These transgenic mice are double KO for albumin and the neonatal Fc receptor (FcRn), which regulates the recycling of albumin, and transgenic for human FcRn. The expression of human FcRn results in a prolonged half-life of human albumin following systemic administration31. We found that albumin KO (Alb*−/−) mice were highly susceptible to disseminated and pulmonary R. delemar infection (Fig. 5a). However, Alb*−/− mice displayed comparable susceptibility to control Alb*+/+* mice after pulmonary infection with A. fumigatus in the neutropenic model of invasive aspergillosis (Fig. 5b and Extended Data Fig. 7a) or disseminated bloodstream infection with Candida albicans (Fig. 5c). Importantly, prophylactic or pre-emptive therapeutic administration of purified, FFA-free human albumin fully restored the resistance of Alb*−/−* mice to pulmonary mucormycosis (Fig. 5d). Histopathological sections of lungs from neutropenic Alb*−/−* versus Alb*+/+* mice after pulmonary infection with R. delemar revealed invasive fungal growth (Fig. 5e). Alb*−/−* exhibited a trend toward higher pulmonary fungal burden (Extended Data Fig. 7b) and significantly higher expression of mucoricin on germinating hyphae, as evidenced by immunostaining of the tissue (Fig. 5f). Notably, albumin significantly attenuated mucoricin-induced cytotoxicity in epithelial cells ex vivo (Extended Data Fig. 7c). Sera (Fig. 5g) and bronchoalveolar fluid (Fig. 5h) obtained from Alb*−/−* mice exhibited significant loss of inhibitory activity against R. delemar growth, and serum lipids from these mice were less inhibitory towards Mucorales (Fig. 5i). Lipidomic analyses of serum FFAs revealed a significantly lower amount of non-oxidized FFAs in Alb*−/−* mice compared with in Alb*+/+* mice, particularly at 72 h of infection (Extended Data Fig. 7d,e and Supplementary Table 3), consistent with previous reports32. Alb*−/−* sera also contained a significantly higher proportion of oxidized FFAs, although their absolute levels were comparable to those in Alb*+/+* mice (Extended Data Fig. 7d,f and Supplementary Table 3). Consistent with these findings, albumin administration restored the inhibitory activity of *A