Multiplexed Immunofluorescence Imaging Reveals an Immune Rich Tumor Microenvironment in Mucinous Rectal Cancer Characterized by Increased Lymphocyte Infiltration and Enhanced PD-1 Expression

Dis Colon Rectum. 2022 Nov 17. doi: 10.1097/DCR.0000000000002624. Online ahead of print.

Abstract

Background: Mucinous rectal cancer is associated with a higher incidence of microsatellite instability, and a poorer response to neoadjuvant chemoradiotherapy compared to other subtypes of rectal adenocarcinoma. Immune checkpoint inhibitors are an emerging family of anti-cancer therapeutics associated with highly variable outcomes in colorectal cancer. Though the immune landscape of mucinous rectal cancer has not been fully explored, the presence of mucin is thought to act as a barrier preventing immune cell infiltration.

Objective: The aim of this study was to determine the immune properties of mucinous rectal cancer and investigate the degree of lymphocyte infiltration in this cohort.

Design: This is a retrospective cohort study which involved, multiplexed immunofluorescence staining of tumor microarrays.

Settings: Samples originated from a single university teaching hospital.

Patients: Our cohort included 15 cases of mucinous and 43 cases of non-mucinous rectal cancer.

Main outcome measures: Immune cells were classified and quantified. Immune cell counts were compared between mucinous and non-mucinous cohorts. Immune marker expression within tumor epithelial tissue was evaluated to determine degree of lymphocyte infiltration.

Results: Cytotoxic ( p = 0.022), and regulatory T-cells ( p = 0.010) were found to be overrepresented in the mucinous cohort compared to the non-mucinous group. PD-1 expression was also found to be significantly greater in the mucinous group ( p = 0.001). CD3 ( p = 0.001) and CD8 ( p = 0.054) expression within tumor epithelium was also higher in the mucinous group, suggesting adequate immune infiltration despite the presence of mucin. Microsatellite instability status was not found to be a predictor of immune marker expression in our analysis.

Limitations: The relatively small size of the cohort.

Conclusion: Mucinous rectal cancer is associated with an immune rich tumor microenvironment, which was not associated with microsatellite instability status. See Video Abstract at http://links.lww.com/DCR/C65 .