Crohn’s disease (CD) is a long-term condition that causes inflammation in the digestive system, often making life challenging for those affected. What makes CD particularly complex is its variability—symptoms and treatment responses can differ greatly between individuals. To better understand this condition, scientists have turned to cutting-edge technologies, including single-cell RNA sequencing (scRNA-seq), to unravel its molecular mysteries.

Researchers at the Wellcome Sanger Institute have introduced IBDverse, the largest collection of single-cell RNA data ever compiled for Crohn’s disease. This dataset analyzed over 1.1 million individual cells from the terminal ileum (a part of the small intestine) of 111 CD patients and 232 healthy individuals. This massive effort aims to identify the specific cells, genes, and biological pathways involved in CD.

Single-cell expression atlas of the terminal ileum in healthy controls and patients with Crohn’s Disease

(a) Terminal ileum biopsies from 343 individuals were dissociated to single cells on ice in Hank’s balanced salt solution (HBSS) containing Bacillus Licheniformis (BL) protease. This was followed by a brief incubation in collagenase and then red blood cell (RBC) lysis buffer. Single cell suspensions were then profiled with the Chromium 10X 3’ kit (Methods). (b) Centre: Uniform Manifold Approximation and Projection (UMAP) of ∼216k cells from the atlas cohort that meet quality control criteria (Methods), with eight colors representing the primary cell populations. Surrounding the central UMAP, these major populations are further subdivided into 57 distinct cell subtypes.

Key Findings from IBDverse

1. Epithelial Cell Changes
   • Epithelial cells line the gut and play a critical role in protecting it from harm. In CD patients, these cells exhibited significant changes, particularly an increase in a protein complex called MHC-I. This change was driven by interferons, which are immune system signals typically released during inflammation.
   • Interestingly, even after visible inflammation had subsided, these changes persisted in the epithelial progenitor cells (the ones that give rise to new epithelial cells).
2. The Role of ITGA4+ Macrophages
   • Macrophages are immune cells that can either help resolve inflammation or, in some cases, worsen it. Researchers identified a specific type of macrophage, known as ITGA4+ macrophages, as major contributors to inflammation in CD.
   • These cells were highly active in pathways related to immune signaling, such as the JAK/STAT pathway, and produced inflammatory molecules like IL-6, IL-12, and IL-23.
3. Linking Genetics to Disease
   • By analyzing genetic heritability, the study connected certain immune cells, such as inflammatory monocytes and macrophages, to a person’s risk of developing CD. This highlights the dual roles of resident (naturally present) and recruited (brought in during inflammation) immune cells in driving the disease.

Why These Findings are Important

IBDverse offers a treasure trove of data that paints a clearer picture of how Crohn’s disease develops and persists at the cellular and molecular levels. Understanding the involvement of specific cell types and pathways provides valuable insights for developing targeted therapies. For instance:

• The persistence of epithelial cell changes could guide the creation of treatments that focus on repairing the gut lining even after inflammation subsides.
• Targeting ITGA4+ macrophages and their signaling pathways (like JAK/STAT) could lead to new anti-inflammatory drugs for CD.

Looking Ahead

This comprehensive study represents a significant step forward in Crohn’s disease research, offering new opportunities to tailor treatments based on the underlying molecular drivers of the condition. The IBDverse dataset not only enhances our understanding of CD but also sets the stage for future research aimed at alleviating the burden of this challenging disease.

Availability – the expression atlas of gastrointestinal cell types is accessible at https://www.ibdverse.info/.