Linking the Human Gut Microbiome to Inflammatory Cytokine Production Capacity

Abstract

Gut microbial dysbioses are linked to aberrant immune responses, which are often accompanied by abnormal production of inflammatory cytokines. As part of the Human Functional Genomics Project (HFGP), we investigate how differences in composition and function of gut microbial communities may contribute to inter-individual variation in cytokine responses to microbial stimulations in healthy humans. We observe microbiome-cytokine interaction patterns that are stimulus specific, cytokine specific, and cytokine and stimulus specific. Validation of two predicted host-microbial interactions reveal that TNFα and IFNγ production are associated with specific microbial metabolic pathways: palmitoleic acid metabolism and tryptophan degradation to tryptophol. Besides providing a resource of predicted microbially derived mediators that influence immune phenotypes in response to common microorganisms, these data can help to define principles for understanding disease susceptibility. The three HFGP studies presented in this issue lay the groundwork for further studies aimed at understanding the interplay between microbial, genetic, and environmental factors in the regulation of the immune response in humans. PAPERCLIP.

Keywords: Human Functional Genomics Project; database of microbiome-cytokine associations; healthy human cohort; human gut microbiome; immunological profiles; inflammatory cytokine response; interindividual variation; metagenomics; microbial profiles; microbiome-host interactions.

Figure 1. Linking inter-individual variation in immune response to the gut microbiome in the 500FG cohort

(A) The 500FG cohort comprises 500 healthy adults from the Netherlands. Stool samples were collected and subjected to metagenomic sequencing to infer gut microbial profiles. Concurrent blood samples were collected to measure the inflammatory cytokine response in connection with various microbial stimulations. (B) Monocyte- (blue) [IL-6, TNFα, IL-1β] and lymphocyte-derived cytokines (green) [IFNγ, IL-17, IL-22] were measured in connection with three bacterial (LPS, B. fragilis, and S. aureus) and two fungal (C. albicans conidia and hyphae) stimulations in whole blood and/or PBMCs. (C) Schematic illustrating production kinetics of different cytokines in PBMCs. IL-17 and IL-22 increase steadily during a 7-day period, whereas IL-6 and TNFα production is maximal in the first 24 hr and decreases thereafter. IFNγ reaches its maximum at 4–5 days, and IL-1β reaches its maximum level at 24 hr, after which a plateau is attained. (Adapted from (Ruschen et al., 1992; van de Veerdonk et al., 2009)) See also Figure S1.

Figure 2. Healthy individuals show significant inter-individual variability in stimulated cytokine responses and in gut microbiota

(A) Inter-individual variation in cytokine response. Each color represents a type of stimulation, also indicated on the upper x-axis: C. albicans conidia, CA(C); C. albicans hyphae, CA(H); E. coli-derived lipopolysaccharide 100 ng, LPS; S. aureus, SA; and B. fragilis, BF. Cell type is indicated below the x-axis (whole blood, WB; peripheral blood mononuclear cells, PBMC). The y-axis specifies the cytokine response. Each cytokine was measured in a non-stimulated control for each cell type (RPMI, orange). Stars indicate significant differences in variation for each stimulatory response compared to controls, respectively for each cell type (Fligner-Killeen test, all p < 1e-15). (Note that B. fragilis-induced TNFα measurements were not considered for further analyses due to a small degree of variation across individuals.) Whole blood measurements are based on 456 individuals. TNFα, IL-1β, and IL-6 measurements in PBMCs (1 d) were available for 401 individuals; IFNγ, IL-17, and IL-22 measurements in PBMCs (7 d) are based on 462 individuals. (B) Taxonomic microbial profiles displaying phylum-level composition. Sample order was determined by the first principal component (PCA with Bray-Curtis distance). (C) Functional microbial profiles displaying the abundance of the ten most variable MetaCyc pathways (based on variance). Samples are in the same order as in (B). (D) The overall percentage of cytokine variation explained by species composition of the gut microbiome was 0.4–9.6%. To avoid overestimation due to species-species correlations, we represented the microbiota through the first 20 principal coordinates (PCoA with Bray-Curtis distance) which explain ∼50% of the variance. The cytokine variance explained by these principal coordinates was estimated through permutation ANOVA by summing over the significant contributions (p < 0.2). See also Figure S2.

Figure 3. Significant correlations between gut microbial abundances and cytokine responses to stimulations

(A) Three microbiome-cytokine interaction patterns were observed. Each colored cell represents a significant interaction between a specific microbial feature (y-axis) and a stimulus-induced cytokine response (x-axis). Interaction pattern 1 (IP1) refers to stimulus-specific interactions, where the same microbial feature is associated with several cytokines in connection with the same stimulation. IP2 describes cytokine-specific interactions, regardless of stimulation. IP3 refers to interactions that are cytokine- as well as stimulation-specific. (B and C) Summary of species (B) and genus (C) associations with cytokine responses using Spearman correlation with Benjamini-Hochberg FDR correction (α ≤ 0.2). All species/genera were required to be detected in ≥ 3% of all samples (corresponding to ≥ 14 samples). Only species/genera significantly associated with at least one cytokine response are displayed. C. albicans conidia, CA(C); C. albicans hyphae, CA(H); lipopolysaccharide 100 ng, LPS; B. fragilis, BF; S. aureus, SA; whole blood, WB; peripheral blood mononuclear cells, PBMC. (D) A stimulus-specific association of Coprococcus comes was observed for IL-1β and IL-6 in connection with C. albicans hyphae stimulation. C. comes was also negatively correlated with S. aureus-induced IL-22 production. All displayed cell types are PBMCs. See also Tables S1–S4 and Figure S3.

Figure 4. Significant correlations between functional potential of the gut microbiome and cytokine responses

Functional summary for MetaCyc pathways (A) and Gene Ontology (GO) categories (B) using Spearman correlation with Benjamini-Hochberg FDR correction (α ≤ 0.2). All functional categories were required to occur in ≥ 3% of the samples (≥ 14 samples). C. albicans conidia, CA(C); C. albicans hyphae, CA(H); lipopolysaccharide 100ng, LPS; B. fragilis, BF; S. aureus, SA; whole blood, WB; peripheral blood mononuclear cells, PBMC. For MetaCyc pathways and GO categories, respectively, higher level functional categories are indicated on the left side. See also Tables S5–S8.

Figure 5. Immunomodulatory effects of tryptophol and palmitoleic acid

(A) L-tryptophan degradation to tryptophol (MetaCyc PWY 5081) was negatively associated with IFNγ in connection with S. aureus, LPS, and C. albicans conidia in whole blood. (B) Whole blood was stimulated with LPS, C. albicans conidia, or S. aureus in the presence or absence of tryptophan (50 µg/mL) or tryptophol (0.25 mM) and IFNγ concentrations were measured in supernatants for 4 donors. Group differences are expressed as percentage change compared to RPMI (tryptophan) or ethanol (tryptophol) and analyzed by paired t-test. *p < 0.05. Mean values for RPMI controls in pg/mL: RPMI: 8, LPS: 370, C. albicans conidia: 115, S. aureus: 405. Mean values for ethanol controls in pg/mL: RPMI: 8, LPS: 219, C. albicans conidia: 131, S. aureus: 91. (C) Palmitoleic acid biosynthesis (MetaCyc PWY 6282) in PBMCs was positively correlated with C. albicans conidia-induced TNFα production. (D) Cis-vaccenate biosynthesis (MetaCyc PWY 5973) was positively correlated with IFNγ (in whole blood) and TNFα (in PBMCs) in connection with C. albicans conidia stimulation. (E) PBMCs from 5–6 healthy volunteers were stimulated with C. albicans conidia in the presence or absence of palmitoleic acid and cis-vaccenate (cis-vaccenic acid). Cytokine concentrations were measured in supernatants. Group differences are expressed as percentage change compared to ethanol or DMSO measurements and analyzed by paired t-test. ***p < 0.001. Mean values in pg/mL: TNFα (DMSO: 10,000, ethanol: 10,000, palmitoleic acid: 1,504, cis-vaccenate: 9,957; IFNγ (DMSO: 1,370, ethanol: 1,571, palmitoleic acid: 976, cis-vaccenate: 2,241). See also Figure S4.

Figure 6. Environmental, host genetic and gut microbial factors impact human cytokine responses

The impact of host environmental factors (ter Horst et al., 2016), host genetics (Li et al., 2016), and the gut microbiome (this study) on stimulus-induced cytokine responses was assessed in three complementary studies of the HFGP. While gender and seasonality were the main environmental factors affecting the response of monocyte-derived cytokines, age was associated with IL-22 and IFNγ production. Further, 17 independent loci were implicated in specific differential cytokine responses of monocyte- and Th-derived cytokines. Lastly, gut microbial functions were more influential on cytokine production than taxonomic features, where the strongest effects were observed for the stimulus-induced IFNγ and TNFα responses.