Rapid expansion of Treg cells protects from collateral colitis following a viral trigger

Regulatory T (Treg) cells, characterized by the expression of the transcription factor Foxp3, are critical for maintaining immune homeostasis and preventing autoimmunity. While genetic factors can increase susceptibility to autoimmune diseases, environmental triggers like viral infections are often necessary to initiate disease. This study investigates the role of Treg cells in the context of viral infection and its impact on the development of colitis, a type of inflammatory bowel disease (IBD). The researchers hypothesized that viral infections, particularly those inducing a strong type I interferon (IFN) response, might disrupt the Treg cell compartment, leading to immune dysregulation and subsequent autoimmune inflammation. The specific research question focuses on how viral infection affects the Treg cell population and whether this disruption contributes to the onset of colitis. Understanding this interaction is vital for developing effective therapeutic strategies for autoimmune diseases triggered by viral infections. The significance of this study lies in its potential to unravel the complex interplay between viral infection, immune regulation, and the pathogenesis of IBD. This research could contribute to the development of novel therapeutic targets for IBD and other autoimmune diseases.

The existing literature highlights the essential role of Foxp3⁺ Treg cells in maintaining immune tolerance and suppressing autoimmunity. A deficiency in Treg cells or their impaired function is a hallmark of several autoimmune diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), and IBD. While genetic predisposition plays a role, environmental factors, such as viral infections, are often implicated as triggers. Viral infections have been linked to an increased risk of developing IBD later in life. Type I IFNs are crucial components of the antiviral response, but their involvement in exacerbating autoimmune diseases is also well-established. Previous studies have shown that long-term IFN therapy can trigger autoimmune conditions. However, the precise mechanisms by which viral infections and type I IFNs impact the Treg cell compartment and contribute to autoimmunity remain incompletely understood. This study builds upon previous research by investigating the impact of viral infection on Treg cells in the context of colitis development.

The study utilized lymphocytic choriomeningitis virus (LCMV) infection as a model to investigate the effects of viral infection on the Treg cell compartment. Different LCMV strains, inducing either acute or chronic infections, were used to assess the impact of varying IFN responses. The researchers employed flow cytometry to analyze the TCR Vβ repertoire of Treg and conventional T cells during and after infection. RNA sequencing was used to further analyze TCR usage. Vβ5-deficient mice were generated and used to determine the functional role of Vβ5⁺ Treg cells in controlling antiviral immunity and preventing colitis. In vitro suppression assays were conducted to assess the suppressive capacity of CXCR3⁺Vβ5⁺ Treg cells. In vivo experiments included adoptive cell transfers to study the conversion of conventional T cells into iTreg cells. Intestinal permeability was assessed using a FITC-dextran feeding assay. The impact of antibiotic treatment on colitis development was also evaluated. Finally, human samples were analyzed to determine the relevance of the findings to human IBD. Detailed methods for animal infections, flow cytometry, cell sorting, adoptive cell transfer, in vitro Treg cell cultures, foci-forming assays, statistical evaluation, and data analysis are provided in the paper. Ethical approval for animal experiments and human studies was obtained.

1. **Transient Treg Loss and Compensation:** LCMV infection caused a transient type I IFN-dependent loss of Treg cells, rapidly compensated by an expansion of induced Treg (iTreg) cells, predominantly Vβ5⁺. This compensation was dependent on type I IFN signaling, as it was absent in Ifnar1-/- mice. 2. **Vβ5⁺ iTreg Cells in Colitis Prevention:** Vβ5⁺ iTreg cells, although fully functional and suppressive, were not essential for controlling the antiviral response. However, their absence resulted in severe colitis in Vβ5-deficient mice, highlighting their critical role in suppressing microbiota-dependent activation of CD8⁺ T cells. 3. **Increased Intestinal Permeability:** LCMV infection, regardless of Vβ5⁺ Treg cell presence, increased intestinal permeability, allowing gut microbes to trigger colitogenic CD8⁺ T cell responses. This colitis was prevented by antibiotic treatment, confirming the role of microbiota. 4. **Superior Conversion of Vβ5⁺ T Cells to iTreg Cells:** Vβ5⁺ T cells exhibited enhanced proliferation and a higher intrinsic TCR signal compared to other T cells, contributing to their superior conversion into iTreg cells under suboptimal conditions. 5. **Human Relevance:** In humans, the Vβ2⁺ Treg cell population shows a similar overrepresentation in IBD patients with inactive disease, suggesting conserved mechanisms between mouse and human.

This study provides crucial insights into the interplay between viral infection, Treg cell dynamics, and the pathogenesis of colitis. The findings demonstrate that while type I IFN responses effectively control viral infection, they simultaneously impair the Treg cell compartment. The rapid conversion and expansion of a specific T cell subset (Vβ5⁺ in mice, Vβ2⁺ in humans) into iTreg cells is vital for restoring immune homeostasis and preventing colitis. The increased intestinal permeability resulting from viral infection represents a critical factor triggering a colitogenic response in the absence of sufficient Treg cell replenishment. The parallels between mouse models and human IBD patients with inactive disease strengthen the translational relevance of these findings. These results contribute significantly to understanding the complex interactions between the immune system, the microbiota, and viral infections in the development of autoimmune diseases.

This study reveals the crucial role of iTreg cells in preventing virus-induced colitis. The type I interferon-mediated transient reduction of Treg cells is compensated by the expansion of specific Vβ chains in T cells, preventing autoimmunity. The study demonstrates the importance of a robust Treg replenishment mechanism for maintaining intestinal homeostasis during viral infection and emphasizes the translational relevance of the findings to human IBD. Future research should focus on exploring the specific mechanisms driving the enhanced conversion of these T cells into iTreg cells and investigating the potential therapeutic applications of targeting these pathways.

The study primarily focuses on LCMV infection as a model for viral triggers of colitis. The generalizability of the findings to other viral infections and autoimmune diseases requires further investigation. The mechanism by which increased intestinal permeability contributes to colitis could be explored in greater detail. While human data supports the relevance of the mouse model findings, larger cohort studies are needed to confirm the association between Vβ2⁺ Treg cells and inactive IBD.