Distribution of mucosal PD-1 expressing T cells in patients with colitis of different etiologies.

Background: Immunotherapy, targeting programmed death-1 (PD-1) enhances antitumor T-cell activity in patients with malignancies. Blocking PD-1 or its ligand may lead to fulminant colitis as serious adverse event in these patients. Since little is known of the presence and role of PD-1T cells in colitis of different etiologies, we determined PD-1T cells in mucosal specimens of patients with inflammatory bowel disease, infectious colitis (InfC), immunotherapy-related colitis (ImC) and healthy controls (HC).

Methods: Newly diagnosed patients with ulcerative colitis (UC,  = 73), Crohn's disease (CD,  = 50), InfC ( = 5), ImC ( = 8) and HC ( = 8) were included. Baseline inflamed colonic biopsies were studied with immunohistochemistry and flowcytometry.

Results: Using immunohistochemistry, PD-1 was not present on lymphocytes in the epithelium of all patients, nor in HC. The percentage PD-1 of all lymphocytes in the lamina propria was 40% in UC, 5% in InfC, 3% in ImC and 0% in HC. Flowcytometry showed significant higher percentages of PD-1T cells in inflamed biopsy specimens of UC patients (22%) compared to all other groups: CD patients (13%), InfC (12%), ImC (5%) and HC (6%).

Conclusion: There are relevant differences in distribution and frequencies of mucosal PD-1 T-cell subsets in patients with UC, CD, InfC and ImC, supporting the hypothesis that these types of colitis are driven by different immunological pathways. The increased numbers of PD-1 and PD-L1 lymphocytes in the colonic mucosa of UC patients suggest that the PD-1/PD-L1 pathway might be more activated in UC than in CD.