Sterile triggers drive joint inflammation in TNF- and IL-1β-dependent mouse arthritis models

Intestinal epithelial expression of human TNF is sufficient to induce small bowel inflammation and sacroiliitis, mimicking human spondyloarthritis

OBJECTIVES

Gut and joint disease commonly co-occur in SpA. Up to 50% of patients with SpA show signs of subclinical gut inflammation, of which 10% develops IBD. However, the mechanisms underlying this gut-joint axis are still unclear. Here we investigated the hypothesis that restricted expression of a pro-inflammatory cytokine in the intestine may trigger the onset of combined gut and joint inflammation.

METHODS

Intestinal expression of human TNF (hTNF) was achieved by driving hTNF gene expression using the rat FAPB2 promoter, creating a new animal model, TNFgut mice, that expresses hTNF in the proximal intestinal tract. Intestinal-specific TNFgut mice were examined for pathological changes in the intestine and extra-intestinal tissues by means of histology, reverse transcription PCR (RT-PCR) and flow cytometry, along with 16S sequencing on stools.

RESULTS

Local expression of hTNF in the epithelium of the small intestine induces a pro-inflammatory state of the proximal intestinal tract, with epithelial alterations and induction of members of the S100 family, as well as local upregulation of Th17 and Treg, but no obvious signs of dysbiosis. Curiously, TNFgut mice develop sacroiliitis (P <0.05) in addition to small bowel inflammation (P <0.05). However, no signs of peripheral arthritis or enthesitis could be documented.

CONCLUSION

Intestinal expression of hTNF is sufficient to initiate a pro-inflammatory cascade culminating in small bowel inflammation and sacroiliitis. Thus, gut-derived cytokines are sufficient to induce SpA.

New thoughts on the gut-immune axis of arthritis

Arthritis is associated with varying degrees of intestinal inflammation and microbiota dysbiosis, leading to the 'gut-joint axis hypothesis' in which intestinal and joint inflammation are suggested to be interconnected through immune-microbiota interactions. While clinical observations support this, causality remains uncertain. Rodent models have provided insights into potential mechanisms by uncovering microbial influences and immune pathways that either connect or uncouple gut and joint inflammation. Based on recent findings, we propose the 'immune hypersensitivity hypothesis' whereby central immune hyper-reactivity can independently drive joint inflammation via local sterile triggers, and gut inflammation via microbial triggers. We argue that this suggests a more nuanced role of the microbiota in arthritis pathogenesis that varies according to the predominant immune mechanisms in disease subtypes. We explore gut-immune interactions in arthritis, highlight ongoing challenges, and propose future research directions.

A guide to germ-free and gnotobiotic mouse technology to study health and disease

The intestinal microbiota has major influence on human physiology and modulates health and disease. Complex host-microbe interactions regulate various homeostatic processes, including metabolism and immune function, while disturbances in microbiota composition (dysbiosis) are associated with a plethora of human diseases and are believed to modulate disease initiation, progression and therapy response. The vast complexity of the human microbiota and its metabolic output represents a great challenge in unraveling the molecular basis of host-microbe interactions in specific physiological contexts. To increase our understanding of these interactions, functional microbiota research using animal models in a reductionistic setting are essential. In the dynamic landscape of gut microbiota research, the use of germ-free and gnotobiotic mouse technology, in which causal disease-driving mechanisms can be dissected, represents a pivotal investigative tool for functional microbiota research in health and disease, in which causal disease-driving mechanisms can be dissected. A better understanding of the health-modulating functions of the microbiota opens perspectives for improved therapies in many diseases. In this review, we discuss practical considerations for the design and execution of germ-free and gnotobiotic experiments, including considerations around germ-free rederivation and housing conditions, route and timing of microbial administration, and dosing protocols. This comprehensive overview aims to provide researchers with valuable insights for improved experimental design in the field of functional microbiota research.

Myeloid A20 is critical for alternative macrophage polarization and type-2 immune-mediated helminth resistance

Background

Protective immunity against intestinal helminths requires induction of robust type-2 immunity orchestrated by various cellular and soluble effectors which promote goblet cell hyperplasia, mucus production, epithelial proliferation, and smooth muscle contractions to expel worms and re-establish immune homeostasis. Conversely, defects in type-2 immunity result in ineffective helminth clearance, persistent infection, and inflammation. Macrophages are highly plastic cells that acquire an alternatively activated state during helminth infection, but they were previously shown to be dispensable for resistance to Trichuris muris infection.

Methods

We use the in vivo mouse model A20myel-KO, characterized by the deletion of the potent anti-inflammatory factor A20 (TNFAIP3) specifically in the myeloid cells, the excessive type-1 cytokine production, and the development of spontaneous arthritis. We infect A20myel-KO mice with the gastrointestinal helminth Trichuris muris and we analyzed the innate and adaptive responses. We performed RNA sequencing on sorted myeloid cells to investigate the role of A20 on macrophage polarization and type-2 immunity. Moreover, we assess in A20myel-KO mice the pharmacological inhibition of type-1 cytokine pathways on helminth clearance and the infection with Salmonella typhimurium.

Results

We show that proper macrophage polarization is essential for helminth clearance, and we identify A20 as an essential myeloid factor for the induction of type-2 immune responses against Trichuris muris. A20myel-KO mice are characterized by persistent Trichuris muris infection and intestinal inflammation. Myeloid A20 deficiency induces strong classical macrophage polarization which impedes anti-helminth type-2 immune activation; however, it promotes detrimental Th1/Th17 responses. Antibody-mediated neutralization of the type-1 cytokines IFN-γ, IL-18, and IL-12 prevents myeloid-orchestrated Th1 polarization and re-establishes type-2-mediated protective immunity against T. muris in A20myel-KO mice. In contrast, the strong Th1-biased immunity in A20myel-KO mice offers protection against Salmonella typhimurium infection.

Conclusions

We hereby identify A20 as a critical myeloid factor for correct macrophage polarization and appropriate adaptive mucosal immunity in response to helminth and enteric bacterial infection.