Regional immune zonation of the healthy human ileocolic mucosa and its disruption along the adenoma–carcinoma sequence

Colorectal carcinogenesis evolves through a stepwise adenoma–carcinoma sequence accompanied by changes in mucosal immune surveillance. Yet the spatial organisation of healthy ileocolic immunity, and its change during dysplastic progression, remain incompletely defined. We quantified immune cell density (cells/mm²) by compartment-resolved immunohistochemistry for CD3, CD8, CD20, and CD68 with HALO digital image analysis in 820 tissue sections from 57 individuals (14 controls, 16 small [≤ 1 cm] and 17 large [> 1 cm] adenomas, 10 carcinomas). In health, T cells were strongly zonated: enriched in the terminal ileum, decreasing toward the distal colon, and more abundant in stroma than epithelium. This zonation became progressively disrupted across the sequence. Lesional CD3 + density was higher in small than large adenomas. Normal-appearing mucosa showed two additional changes: ileal CD8 + depletion and expansion of right-colonic epithelial CD68 + macrophages. Both followed significant stage-trends. The epithelial CD68 + expansion remained significant after adjustment for age and sex but not after multiplicity correction; the ileal CD8 + change was no longer significant after either adjustment and is hypothesis-generating. These data establish a quantitative reference map of healthy ileocolic immune zonation and suggest that disruption of this spatial organisation, including within normal-appearing mucosa, may represent an early feature of colorectal tumorigenesis.