Cytokines in Spondyloarthritis and Inflammatory Bowel Diseases: From Pathogenesis to Therapeutic Implications

Spondyloarthritis and inflammatory bowel diseases are chronic immune disorders of the joints and the gut that often coexist in the same patient, increasing the burden of each disorder, worsening patients' quality of life, and influencing therapeutic strategies. Genetic predisposition, environmental triggers, microbiome features, immune cell trafficking, and soluble factors such as cytokines contribute to the pathogenesis of both articular and intestinal inflammation. Most of the molecular targeted biological therapies developed over the last two decades were based on evidence that specific cytokines may be involved in these immune diseases. Despite pro-inflammatory cytokine pathways sharing the pathogenesis of both articular and gut diseases (i.e., tumor necrosis factor and interleukin-23), several other cytokines (i.e., interleukin-17) may be differently involved in the tissue damage process, depending on the specific disease and the organ involved in inflammation, making difficult the identification of a therapeutic plan that is efficacious for both inflammatory manifestations. In this narrative review, we comprehensively summarize the current knowledge on cytokine involvement in spondyloarthritis and inflammatory bowel diseases, underlining similarities and differences among their pathogenetic pathways; finally, we provide an overview of current and potential future treatment strategies to simultaneously target both articular and gut immune disorders.

Bifidobacterium pseudocatenulatum-Mediated Bile Acid Metabolism to Prevent Rheumatoid Arthritis via the Gut-Joint Axis

Early intervention in rheumatoid arthritis (RA) is critical for optimal treatment, but initiation of pharmacotherapy to prevent damage remains unsatisfactory currently. Manipulation of the gut microbiome and microbial metabolites can be effective in protecting against RA. Thus, probiotics can be utilized to explore new strategies for preventing joint damage. The aim of this study was to explore the metabolites and mechanisms by which Bifidobacterium pseudocatenulatum affects RA. Based on 16S rRNA sequencing and UPLC-MS/MS assays, we focused on bile acid (BA) metabolism. In a collagen-induced arthritis (CIA) mouse model, B. pseudocatenulatum prevented joint damage by protecting the intestinal barrier and reshaped gut microbial composition, thereby elevating bile salt hydrolase (BSH) enzyme activity and increasing the levels of unconjugated secondary BAs to suppress aberrant T-helper 1/17-type immune responses; however, these benefits were eliminated by the Takeda G protein-coupled receptor 5 (TGR5) antagonist SBI-115. The results suggested that a single bacterium, B. pseudocatenulatum, can prevent RA, indicating that prophylactic administration of probiotics may be an effective therapy.

The Role of Dietary Fiber in Rheumatoid Arthritis Patients: A Feasibility Study

Short-chain fatty acids are microbial metabolites that have been shown to be key regulators of the gut-joint axis in animal models. In humans, microbial dysbiosis was observed in rheumatoid arthritis (RA) patients as well as in those at-risk to develop RA, and is thought to be an environmental trigger for the development of clinical disease. At the same time, diet has a proven impact on maintaining intestinal microbial homeostasis. Given this association, we performed a feasibility study in RA patients using high-fiber dietary supplementation with the objective to restore microbial homeostasis and promote the secretion of beneficial immunomodulatory microbial metabolites. RA patients (n = 36) under routine care received daily high-fiber bars or cereals for 28 days. Clinical assessments and laboratory analysis of immune parameters in blood and stool samples from RA patients were done before and after the high-fiber dietary supplementation. We observed an increase in circulating regulatory T cell numbers, favorable Th1/Th17 ratios, as well as decreased markers of bone erosion in RA patients after 28 days of dietary intervention. Furthermore, patient-related outcomes of RA improved. Based on these results, we conclude that controlled clinical studies of high-fiber dietary interventions could be a viable approach to supplement or complement current pharmacological treatment strategies.

It Takes "Guts" to Cause Joint Inflammation: Role of Innate-Like T Cells

Innate-like T cells such as invariant natural killer T (iNKT) cells and mucosal-associated T (MAIT) cells, characterized by a semi-invariant T cell receptor and restriction toward MHC-like molecules (CD1 and MR1 respectively), are a unique unconventional immune subset acting at the interface of innate and adaptive immunity. Highly represented at barrier sites and capable of rapidly producing substantial amounts of cytokines, they serve a pivotal role as first-line responders against microbial infections. In contrast, it was demonstrated that innate-like T cells can be skewed toward a predominant pro-inflammatory state and are consequently involved in a number of autoimmune and inflammatory diseases like inflammatory bowel diseases and rheumatic disorders, such as spondyloarthritis (SpA) and rheumatoid arthritis. Interestingly, there is link between gut and joint disease as they often co-incide and share certain aspects of the pathogenesis such as established genetic risk factors, a critical role for pro-inflammatory cytokines, such as TNF-α, IL-23, and IL-17 and therapeutic susceptibility. In this regard dysregulated IL-23/IL-17 responses appear to be crucial in both debilitating pathologies and innate-like T cells likely act as key player. In this review, we will explore the remarkable features of iNKT cells and MAIT cells, and discuss their contribution to immunity and combined gut-joint disease.