# C-terminal Fragment Generated by HOIL-1 Cleavage Suppresses Inflammatory Responses of Myeloid Cells to Alleviate Colitis

## TL;DR

A protein fragment from HOIL-1 cleavage reduces gut inflammation by suppressing immune cell activity, offering a new treatment approach for inflammatory bowel disease.

## Contribution

The study reveals novel functions of the C-terminal HOIL-1 fragment in suppressing myeloid cell inflammation and its therapeutic potential in colitis.

## Key findings

- Uncleavable HOIL-1 in mice leads to more severe colitis with increased inflammatory cell infiltration.
- C-HOIL-1 inhibits NF-κB and STAT1 signaling while upregulating ARG1 to suppress macrophage inflammation.
- Lenti-C-HOIL-1 treatment in mice reduces DSS-induced intestinal inflammation.

## Abstract

Deciphering the molecular consequences of protein cleavage in inflammatory signaling is vital for defining the mechanisms of intestinal autoinflammation and identifying new therapeutic targets for inflammatory bowel disease (IBD). While it was previously established that HOIL-1 cleavage by MALT1 negatively regulates NF-κB activation and inflammatory responses in vitro, the pathophysiological role of HOIL-1 cleavage in regulating intestinal inflammation and the specific function of the resulting C-terminal fragment (C-HOIL-1) remained elusive. This study aimed to define the role of HOIL-1 cleavage and C-HOIL-1 in modulating gut inflammation.

To investigate the impact of HOIL-1 cleavage on intestinal inflammation, the global and myeloid-specific transgenic mouse models with uncleavable HOIL-1 (lacking C-HOIL-1) were established, and their disease phenotypes and immune profiles were characterized under DSS-induced colitis. Genetically engineered THP-1 monocytic cells expressing uncleavable HOIL-1 and C-HOIL-1 were constructed to elucidate the molecular mechanisms of C-HOIL-1 in regulating inflammatory signaling. Finally, Lenti-C-HOIL-1 was delivered to the colon of wild-type mice via enema to evaluate the therapeutic potential of C-HOIL-1 in controlling intestinal inflammation.

Mice with uncleavable HOIL-1 (lacking C-HOIL-1) present a more severe disease phenotype in DSS-induced colitis; specifically, the infiltration of inflammatory monocytes, M1-type macrophages, and neutrophils is significantly elevated in the colon. Mechanistically, we discover that C-HOIL-1 has novel biological functions in i) inhibiting NF-κB signaling, ii) interacting with STAT1 to down-regulate STAT1-mediated inflammatory signaling, and iii) up-regulating ARG1 expression. Collectively, these actions suppress the inflammatory responses in monocytes/macrophages, and impede the differentiation of M1-type macrophages. The pretreatment of Lenti-C-HOIL-1 to the colon of wild-type mice alleviates DSS-induced intestinal inflammation.

Our results define the pathophysiological role of HOIL-1 cleavage in colitis, and unveil new functions of C-HOIL-1 in regulating myeloid inflammatory responses. These findings provide a potential therapeutic strategy for controlling gut inflammation in IBD.