Histological diversity of anti-PD1-induced colitis.

AIM: Histological data on anti-PD1-associated colitis are limited, while the colitis subtypes are still not clearly defined and different terms are being used. The aim of the study was to explore the histopathology of anti-PD1-induced colitis.

METHODS AND RESULTS: Colonic biopsies from 9 patients under anti-PD1 agents presenting diarrhea were examined. Histological evaluation revealed colitis of mild to moderate severity in almost all cases. Four distinct dominant histological patterns were identified with nearly the same incidence: Ulcerative colitis (UC)-like (n=2), GVHD-like (n=2), collagenous-like (n=3) and a mixed colitis pattern combining features of microscopic and UC-like colitis (n=2). The latter was additionally characterized by high crypt epithelium apoptosis and cryptitis with mixed inflammatory infiltrate. Thickening of the subepithelial band of collagen, detachment of the surface epithelium and increased apoptosis of the crypt epithelium were commonly encountered features, irrespective of colitis subtype. CD4/CD8 ratio was lower in the "combined" and higher in the GVHD-like subtype.

CONCLUSIONS: Anti-PD1-induced colitis is expressed by different patterns of injury which share distinct histological hallmarks harboring diagnostic value, while a "combined" colitis subtype is being established. The histological alterations are indicative of mucosa barrier damage after antΙ-PD1 treatment and its participation in the pathogenetic process.