Understanding the roles of the microbiome in autoimmune rheumatic diseases

Integration of Metabolomics and 16S Ribosomal RNA Sequencing to Elucidate the Pathogenesis of Ankylosing Spondylitis

OBJECTIVE

Despite growing interest in the gut microbiota and blood metabolome in patients with ankylosing spondylitis (AS), its role remains poorly understood. Here, we investigate how microbial and metabolic alterations contribute to AS.

METHODS

Fecal microbiome data from 40 AS patients were compared with those from 40 healthy controls (HCs) using 16S ribosomal RNA (rRNA) gene sequencing. The plasma metabolic profiles were analyzed and integrated with the microbiota data to identify biological characteristics specific to AS.

RESULTS

AS patients showed significant enrichment of specific genera, including Megamonas, Elusimicrobium, Dysgonomonas, Ruminococcus_gauvreauii_group, and unclassified_Prevotellaceae. Pathways with the most differentially expressed metabolites included bile secretion; neomycin, kanamycin, and gentamicin biosynthesis; and arachidonic acid metabolism. Positive correlations between Megamonas and Elusimicrobium and metabolites such as piribedil, l-cystathionine, and crocetin dialdehyde suggested microbial enrichment in AS patients.

CONCLUSIONS

A disrupted gut microbiota and altered metabolites are present in AS patients. Integrating microbiome and metabolomic data reveals significant disruptions in AS patients, improving our understanding of its pathogenesis.

Intersection of the microbiome and immune metabolism in lupus

Systemic lupus erythematosus is a complex autoimmune disease resulting from a dysregulation of the immune system that involves gut dysbiosis and an altered host cellular metabolism. This review highlights novel insights and expands on the interactions between the gut microbiome and the host immune metabolism in lupus. Pathobionts, invasive pathogens, and even commensal microbes, when in dysbiosis, can all trigger and modulate immune responses through metabolic reprogramming. Changes in the microbiota's global composition or individual taxa may trigger a cascade of metabolic changes in immune cells that may, in turn, reprogram their functions. Factors contributing to dysbiosis include changes in intestinal hypoxia, competition for glucose, and limited availability of essential nutrients, such as tryptophan and metal ions, all of which can be driven by host metabolism changes. Conversely, the accumulation of some host metabolites, such as itaconate, succinate, and free fatty acids, could further influence the microbial composition and immune responses. Overall, mounting evidence supports a bidirectional relationship between host immunometabolism and the microbiota in lupus pathogenesis.

Dietary tryptophan and genetic susceptibility expand gut microbiota that promote systemic autoimmune activation

Tryptophan modulates disease activity and the composition of microbiota in the B6.Sle1.Sle2.Sle3 (TC) mouse model of lupus. To directly test the effect of tryptophan on the gut microbiome, we transplanted fecal samples from TC and B6 control mice into germ-free or antibiotic-treated non-autoimmune B6 mice that were fed with a high or low tryptophan diet. The recipient mice with TC microbiota and high tryptophan diet had higher levels of immune activation, autoantibody production and intestinal inflammation. A bloom of Ruminococcus gnavus (Rg), a bacterium associated with disease flares in lupus patients, only emerged in the recipients of TC microbiota fed with high tryptophan. Rg depletion in TC mice decreased autoantibody production and increased the frequency of regulatory T cells. Conversely, TC mice colonized with Rg showed higher autoimmune activation. Overall, these results suggest that the interplay of genetic and tryptophan can influence the pathogenesis of lupus through the gut microbiota.