Background
Trillions of microorganisms (including bacteria, archaea, viruses, phages, yeast, and fungi) residing in the gastrointestinal (GI) tract play a vital role in health and disease, and the intestinal microbiota, which has immense impact on nutrition, metabolism, physiology, and immune function of the host, is commonly referred to as a hidden metabolic organ of the body [
1‐
4]. The composition and function of intestinal microbiota in an individual remain stable, and the maintenance of microbiota homeostasis protects host from dysbiosis-related diseases [
5]. Intestinal microbiota as an immune modulator plays a pivotal role in the development and maintenance of a healthy immune system of host [
6]. The maternal microbiota drives and shapes early postnatal innate immune development [
7]; thereafter, an enduring mutualistic partnership developed during long-term host-microbiota interactions [
8].
Previous studies using germ-free (GF), antibiotic-treated (ABX), or selectively colonized mice have illustrated that maturation of the immune system depended on intestinal microbiota, while germ-free animals exhibited impaired immune development [
9,
10]. Intestinal microbes could regulate the development and function of a variety of immune cells including plasma cells secreting IgA, regulatory cells (Treg cells), T helper cells 17 (Th17 cells), natural killer cells (NK cells), dendritic cells, and mononuclear phagocytes [
11]. Alterations in the intestinal microbiota composition and its metabolites were not only linked to gastrointestinal diseases such as inflammatory bowel disease (IBD) [
12,
13] and irritable bowel syndrome (IBS) [
14], but also associated with obesity [
15,
16], nonalcoholic fatty liver disease (NAFLD) [
17], insulin sensitivity [
18], type 2 diabetes mellitus (T2DM) [
19], cancer [
20,
21], cardiovascular risk [
22,
23], central nervous system disease [
24], and allergic disease [
25]. There is a growing appreciation of a role for the host-microbiota interactions in human health and disease, as well as the effects of the metabolites and cellular or molecular components on the host immune system [
26].
Invasive candidiasis was defined as a group of infectious syndromes resulting from a variety of species of
Candida, and candidemia is the most commonly recognized syndrome.
Candida species were main causes of nosocomial bloodstream infections (BSIs); moreover, the major pathogen species of most candidemia cases were
Candida albicans [
27‐
29]. Besides, recent studies have reported a progressive shift into multidrug-resistant
Candida auris in the etiology of invasive candidiasis [
30,
31]. Invasive candidiasis was associated with prolonged hospital stay in the intensive care unit (ICU), higher healthcare cost, morbidity, and mortality [
32,
33]. The risk factors for invasive candidiasis mainly include parenteral nutrition, age, admission to the ICU, organ dysfunction, surgery, immuno-suppression due to chemotherapy and radiotherapy, biofilm formation by
Candida spp. and antifungal treatment strategies, indwelling central venous catheter (CVC), the usage of assisted ventilation, exposure to broad-spectrum antibiotics, and a gastrointestinal source of candidemia [
27,
33‐
35]. The dysbalance of intestinal microbiota caused by the use of broad-spectrum antibiotics and disruption of mucosal barriers due to surgery were seen as high risk factors for invasive candidiasis, and gastrointestinal colonization was considered as a common source of candidemia, suggesting a role of intestinal microbiota in invasive candidiasis [
29,
33,
36].
Candida albicans is a normal constituent of human intestinal, and intestinal commensal bacteria maintain immune responsiveness for host against invasive
C. albicans [
37], but the immuneregulatory role of intestinal microbiota in invasive candidiasis is unclear. In this study, we used a mouse model of invasive candidiasis combined with antibiotic cocktail pretreatment to investigate the role of intestinal microbiota in host immunity to invasive candidiasis. Mice after ABX-mediated depletion of intestinal microbiota showed impaired defense during invasive candidiasis. Treatment with rIL-17A or fecal microbiota transplantation (FMT) operation could improve survival of ABX mice after infection. Therefore, intestinal microbiota plays a protective role in invasive candidiasis via regulating IL-17A production.
Methods
Mice
Female C57BL/6 mice (6 to 8 weeks old) were purchased from Chongqing Medical University and were maintained in specific pathogen-free (SPF) facilities. These mice were housed in a temperature room with 12-h light-dark cycles and were given free access to autoclaved chow and water. All animal procedures were performed according to the protocol approved by the Institutional Animal Care and Use Committee’s guidelines of the Chongqing Medical University, Chongqing, China.
Antibiotic treatment
Mice were given autoclaved drinking water supplemented with ampicillin (0.5 mg/mL), gentamicin (0.5 mg/mL), metronidazole (0.5 mg/mL), neomycin sulfate (0.5 mg/mL), vancomycin (0.25 mg/mL), and sucralose (4 mg/mL) as previously described [
38,
39]. Antibiotic treatment was started 2–3 weeks prior to infection, and then continued for the duration of the experiment. Antibiotic treatment was withdrawn 24 h prior to fecal microbiota transplantation (FMT) procedure and was replaced with sterile water.
Systemic infection model of C. albicans
C. albicans strains SC5314 were cultured in yeast extract, peptone, and dextrose (YPD) medium at 30 °C for 18–24 h [
40,
41]. Mice were given an intravenous tail-vein injection of PBS or 2 × 10
5 colony-forming units (CFUs)
C. albicans. The weight of each mouse was then daily recorded. Survival was monitored daily for 14 days after
C. albicans intravenous challenge.
At day 3 after infection, the livers, kidneys, spleens, and lungs of mice were aseptically dissected, weighed, homogenized, and diluted with PBS, and then serial dilutions of each organ homogenate were plated on YPD agar plates containing penicillin and streptomycin. CFUs were enumerated after incubation for 48 h at 37 °C [
42].
Histopathology
Tissue histology and pathology scores were conducted according to previous studies [
43]. Briefly, mice were sacrificed at the designated time points, and the livers, kidneys, spleens, and lungs of mice were harvested, fixed in 4% formalin, embedded in paraffin, sectioned, and stained with hematoxylin and eosin (H&E), and then scored by a pathologist blinded for groups.
For immunohistochemically analysis, kidney sections made from ABX and conventional-housed (CNV) mice at day 3 after infection were formalin-fixed, paraffin-embedded, dewaxed with xylene, and rehydrated in alcohol series. Antigen retrieval, endogenous peroxidase activity blocking, and nonspecific binding site blocking were done before staining. Then sections were incubated with primary antibodies F4/80+ or Gr1+ (eBioscience) and mouse anti-rat secondary antibody (ZSBIO, Beijing, China), nuclei was stained using hematoxylin, and then color development was conducted by DAB Substrate Kit (ZSBIO). All procedures were done according to the manufacturer’s instructions.
Serum analysis
Mouse blood was harvested before tissue collection. Then, after centrifugation at 4000 rpm for 10 min at 4 °C, mouse serum was obtained and stored at − 80 °C for further analysis. The serum level of IL-17A were measured using commercially available enzyme-linked immunosorbent assay (ELISA) kits purchased from R&D Systems, while IFN-γ, TNF-α, IL-6, IL-10, IL-12, and IL-22 were assessed by ELISA kits from Neobioscience. The serum concentrations of ALT, AST, BUN, and creatinine of mice were determined by an automatic biochemical analyzer. All procedures were completed according to the manufacturer’s instructions.
Recombinant IL-17A (rIL-17A) treatment
Mouse rIL-17A protein was purchased from R&D Systems. ABX mice were injected intraperitoneally with a dose of 1 μg rIL-17A protein 8 h before C. albicans challenge, followed by a booster dose at 24 h after injection. In parallel, control ABX mice were injected solely with equivalent vehicle.
Administration of rIFN-γ protein
For in vivo rIFN-γ administration, ABX mice were intravenously injected with 5 μg rIFN-γ protein (R&D Systems) or equivalent vehicle at 8 h before and 24 h after C. albicans intravenous challenge.
Antibody-mediated neutralization of IL-10
We blocked the effects of IL-10 in ABX mice via intravenous injection with 5 μg anti-IL-10 antibodies (R&D Systems) at 2 h before and 24 h after C. albicans infection.
Fecal preparation and transplantation
The procedure of fecal microbiota transplantation was performed as described before [
44,
45]. In brief, fresh fecal pellets were collected directly from ten untreated female healthy mice and were pooled, mixed with sterile PBS, and homogenized immediately. The homogenate was centrifuged at 100
g for 5 min at 4 °C, and the supernatant was used for transplantation. After being switched to regular sterile drinking water, ABX mice in the transplantation group were reconstituted with 200 μl of such suspension by oral gavage 7 days before intravenous challenge, and subsequent 2 days, the reduplicative gavage was conducted, while ABX mice in control group were given 200 μl sterile PBS by the same way. Both groups were intravenously infected with
C. albicans at day 7 after first gavage operation.
Fecal bacteria quantification
Fresh feces sampled from uninfected ABX and CNV mice were weighed, homogenized, and serially diluted with PBS, then the dilutions of each sample were plated on blood agar plates in aerobic and anaerobic environments (BBL GasPak Plus system; BD Biosciences) respectively. After incubation for 24 h at 37 °C, CFUs were enumerated and the numbers of fecal bacteria colonies were expressed as CFUs/g feces.
DNA extraction and 16S ribosomal RNA gene sequencing
Fresh fecal pellets (about 150 mg per mouse) were collected from non-infected ABX and CNV mice and frozen at − 80 °C within 2 h after sampling until analysis. Microbial DNA was extracted using the E.Z.N.A.® Soil DNA Kit (Omega Bio-tek, Norcross, GA, USA), following the standard procedures [
46]. The V3-V4 hypervariable regions of the bacteria’s 16S rRNA genes were amplified with barcoded primers 338F (5′-ACTCCTACGGGAGGCAGCAG-3′) and 806R (5′-GGACTA-CHVGGGTWTCTAAT-3′). The purified amplicons were pooled in equimolar and paired-end sequenced (2 × 300) on an Illumina MiSeq platform (Illumina, San Diego, USA) [
47].
Microbial analysis
The sequencing raw reads were demultiplexed and filtered using QIIME (version 1.9.1) [
48]. Operational taxonomic units (OTUs) were clustered with 97% similarity cutoff by UPARSE (version 7.1), and chimeric sequences were identified and removed by UCHIME [
49]. The taxonomy of each 16S rRNA gene sequence was analyzed by RDP Classifier algorithm against the Silva (SSU128) 16S rRNA database using confidence threshold of 70% [
50]. Rarefaction analysis and alpha-diversity calculations were performed on the OTU table, and beta-diversity was measured by computing unweighted UniFrac and was visualized by principal coordinate analysis (PCoA) according to the matrix of distance [
51]. The linear discriminant analysis (LDA) effect size (LEfSe) was performed using LEfSe program, and an effect size threshold (on the logarithmic LDA score) of 2.0 was used [
52].
Statistical analysis
Data were expressed as the mean ± SEM. Statistical analyses were performed using GraphPad Prism 7.0 or SPSS 21.0 software. Details of individual tests are included in the figure legends. Comparisons between two groups of data were analyzed by Mann-Whitney U test. The weight loss over time after infection in comparison to starting weight between groups was analyzed using two-way ANOVA. Survival analysis over time after infection was assessed by the Kaplan-Meier analysis followed by log-rank tests. A two-tailed P values less than 0.05 was considered statistically significant.
Discussion
In the recent years, the role of intestinal microbiota in host immunity and human disease has gained more and more attention. Intestinal microbiota has an effect on the development of the immune system and the susceptibility to infectious diseases [
60]. In this study, we demonstrated a protective role for intestinal microbiota in host resistance against invasive candidiasis. Mice after depletion of intestinal microbiota exhibited impaired defense against invasive candidiasis, expressed as significantly increased weight loss, mortality, tissue fungal burden, and tissue damage, compared with the CNV mice. The serum level of IL-17A in ABX mice was significantly decreased after infection, while IL-17A played a protective role in the resistance to
C. albicans infection [
61]. IL-17A treatment could improve survival of ABX mice after invasive candidiasis. However, as aberrant IL-17 could lead to systemic inflammation as well as autoinflammatory conditions, the potential harms should be concerned when rIL-17A was used as a potential therapy, which requires further study.
Previous studies have shown that commensal bacteria could affect the function of host immune cells. For example, macrophages from ABX mice exhibited decreased expression of genes associated with antiviral immunity, defective responses to type I and type II IFNs, and impaired capacity to limit viral replication [
39]. Effective host defense against fungi required Th1- and Th17-mediated immunity, whereas Th2 type responses were generally associated with adverse outcomes, and Th17 responses have been shown to be even more important than Th1 cell responses in antifungal immunity [
55]. Our study found that ABX mice exhibited a decreased Th1 type T cell responses (indicated by decreased IFN-γ) and Th17 cell response (indicated by decreased IL-17A and IL-22) and increased Th2-type responses (indicated by increased IL-10 and IL-6). Intestinal microbiota exerted protective effect through regulating IL-17 production during invasive candidiasis.
Restoring the intestinal microbiota by FMT could enhance the defense ability and the expression of IL-17A in ABX mice against invasive candidiasis. In addition, the intestinal microbiota diversity of ABX mice was significantly reduced, and the intestinal microbiota structure of ABX mice was significantly deviated from the CNV mice. The complicated interactions between intestinal microbiota and host immune system play a key role in controlling gut barrier [
62]. Gut microbes were recognized and monitored by the innate immune system with pattern recognition receptors (PRRs), a kind of recognition molecule which could recognize one or more pathogen-associated molecular patterns (PAMPs), such as toll-like receptors (TLRs), NOD-like receptors (NLRs), and C-type lectin receptors (dectin-1, dectin-2) [
63]. TLR-2 is vital for murine defense against
C. albicans [
64];
Lactobacillus crispatus could promote epithelial cell defense against
C. albicans via TLR-2 and TLR-4 [
65]. Dectin-1 mediates recognition of
C. albicans [
66]. Dectin-1 could modify microbiota to regulate the homeostasis of intestinal immunity [
67]. Dectin-1 could recognize β-(1,3)-glucan and mediate downstream Syk kinase signaling resulting in the secretion of IL-6 and IL-23, while IL-6 and IL-23 could promote IL-17 expression in T cells [
68]. Therefore, the immune communication is essential for intestinal microbiota to protect against invasive candidiasis. A previous study has indicated that commensal bacteria eradication enhanced protection against disseminated
C. albicans infection [
38], which is in contrast with our present work. The following two points may explain this difference. Firstly, different strain types (standard vs recombinant) and inoculation dosage (2 × 10
5 VS 5 × 10
4 CFUs) of SC5314 were used in two studies. Secondly, variations in intestinal microbiota of two studies may result in a different phenotype due to confounding factors in the experimental setup, such as maternal effects, cage effects, mouse vendors, and housing conditions (diet, light, stress factors) [
69]. Moreover, some studies have shown that intestinal commensal microbiota could provide protective effects on host defense [
70,
71] and prevent
C. albicans colonization and dissemination from the gut [
72]. Besides, the number of anaerobic bacteria and abundance of Lactobacillus were all significantly reduced in ABX mice. However, a decrease in anaerobic bacteria could promote overgrowth of
Candida glabrata [
73],
Lactobacillus crispatus could modulate epithelial cell defense against
C. albicans [
65], clinical strains of
Lactobacillus could influence the growth and expression of
C. albicans virulence factors [
58], and the metabolites of
L. gasseri and
L. crispatus could downregulate biofilm formation-related genes of
C. albicans, thus inhibiting biofilm formation of
C. albicans [
59]. These previous studies suggest that we should explore in detail the specific mechanisms by which members of the intestinal microbiota communicate with the host immune system and identify specific species as different members of intestinal microbiota elicit different immune responses relating to itself or its metabolites. In view of different individuals having different structures of intestinal microbiota, so do mice and humans, pinpointing the immune response initiated by specific bacteria could help to apply the findings into human research. Given this, we characterized the microbiota structure and discrepant genera of non-infected ABX and CNV mice in the intestine.
In addition, numerous metabolites produced by the microbiota affect host metabolism mostly by combining with specific membranes or nuclear receptors of host [
74] and act as extracellular signaling molecules to activate cell-surface G-protein-coupled receptors (GPCRs) [
75]. Notably, except for the role of intestinal microbiota in the development and activation of the host immune system, the adaptive immunity also in turn has a predominant effect on regulating gut microbiota’s composition and diversity [
76]. Thereby, the pivotal communications network between intestinal microbiota and host immune are considerably intricate and delicate. Hence, investigation for the specific role in host immune of bacteria and its metabolites in intestine will be an enormous but a worthwhile task.
There are a few important limitations to this study. Firstly, antibiotic treatment in mouse model could cause alterations in the gut including depletion of the microbiota, direct effects of antibiotics on host tissues, and the effects of remaining antibiotic-resistant microbes [
77], whose impact on our study is difficult to estimate. Although our model has shown that the routine biochemical indicators, weight curve, and renal pathology of ABX mice were consistent with CNV mice, future studies with a shorter duration of antibiotics are still needed to be performed. Secondly, as the intravenous inoculation of Candida might likely not as clinically relevant as gut colonization preceding systemic infection, further study using mouse model of invasive candidiasis via GI
C. albicans colonization is needed. Finally, this work depicted the influence of intestinal microbiota on invasive candidiasis, while how the intestinal microbiota communicates with the cytokines in circulatory system during invasive candidiasis remains to be elucidated.
Conclusions
In conclusion, this study characterized the protective role of intestinal microbiota in limiting immunopathology and improving survival ability of host against invasive candidiasis, and we revealed that IL-17 plays an important role in intestinal microbiota-mediated protection against invasive candidiasis. Therefore, these data enlarge our understanding of intestinal microbiota, host immunity, and human diseases. Future studies are required to figure out the mechanisms involved in microbiota-mediated protection against invasive candidiasis, such as the location and function of Th17 cells, the pathway by which intestinal microbiota communicates with IL-17A and other factors (IFN-γ, IL-12, IL-22), the interaction between these cytokines, and the verification of specific species working during invasive candidiasis.
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