Targeting MLCK1 uncouples immune checkpoint inhibitor-induced colitis from antitumour immunity

Objective Immune checkpoint inhibitors (ICIs) have revolutionised cancer treatment and patients’ survival. However, ICIs also cause severe immune-related adverse events, notably colitis, resulting in ICIs therapy discontinuation and tumour immunotherapy failure. This study investigates long myosin light chain kinase 1 (MLCK1), a known regulator of tight junction and gut permeability, to elucidate the mechanisms underlying ICI-mediated colitis and identify approaches to reduce this toxicity. Design This study employed an integrated approach, using clinical samples, in vivo models and in vitro organoid systems. Biopsies from patients with ICIs colitis were profiled using single-cell RNA sequencing (scRNA-seq) and spatial transcriptomics. To recapitulate human ICIs colitis, we used a wild mouse microbiota (WildR) model, alongside various genetically modified and tumour-bearing models (including melanoma and MC38). Furthermore, mechanisms were investigated through organoid-immune cell co-cultures. Finally, surface plasmon resonance, microscale thermophoresis, full-spectrum flow cytometry, bulk RNA sequencing, immunostaining, ELISA and gut permeability assays were performed to comprehensively delineate the underlying molecular mechanism. Results Tight junction integrity was compromised in both human ICIs colitis and our WildR mouse model. We determined that this barrier dysfunction is driven by activation of the MLCK1-mediated leak pathway following ICI treatment. Using murine models, we identified tumour necrosis factor secreted by CD8 + and CD4 + T cells as an upstream regulator that induces colitis through this MLCK-dependent mechanism, as genetic deletion of MLCK preserved the tight junction structure and ameliorated the inflammation and ICIs colitis. Furthermore, a pharmacological screen identified the small molecule Epicatechin, which blocks MLCK1-FKBP8 interaction and inhibits the recruitment of MLCK1 to the perijunctional actomyosin ring and prevents the intestinal barrier loss. Finally, treatment with Epicatechin mitigated ICI-induced colitis without compromising the antitumour efficacy of the immunotherapy. Conclusions These findings suggest that MLCK1-dependent tight junction regulation is essential for ICIs colitis, positioning barrier restoration as a potential therapeutic strategy.