The Tumour Immune Microenvironment as a Predictor of the Response to Neoadjuvant Therapy in Rectal Cancer

Background: Treatment response to neoadjuvant therapy in rectal cancer exhibits a considerable degree of interpatient heterogeneity. Select components of the tumour immune microenvironment have been identified as predictive biomarkers of therapeutic response, for which more evidence is required for future clinical prediction models. Aim: The research aimed to identify key tumour immune microenvironment biomarkers predictive of the response to neoadjuvant therapy through the systematic appraisal of existing literature. Methods: A structured search was performed across PubMed, Ovid Embase, and Cochrane databases to retrieve primary studies investigating the association between the tumour immune microenvironment and pathological complete response (pCR) or tumour regression grade (TRG) in patients with rectal cancer. Studies were screened against predefined inclusion and exclusion criteria. Results: Fifteen studies satisfied the inclusion criteria, with cohorts ranging between 24 and 298 participants with predominantly stage II–III disease. Considerable heterogeneity was observed in both types and methods of quantification of biomarkers. Biomarkers assessed in pretreatment biopsies included tumour-infiltrating lymphocytes (TILs), investigated by subtype (cluster of differentiation (CD)8+, CD4+, forkhead box protein 3+ (FOXP3)) or as a composite measure, as well as programmed death-ligand 1 (PD-L1), PD-1+, natural killer (NK) cells, CD163+, and CD68+. Findings showed that high densities of TILs—particularly the CD8+ subtype—consistently correlated with improved tumour regression. FOXP3+ and CD163+ were inconsistently associated with reduced treatment response. NK cells and CD68+ cells were less frequently investigated and yielded non-significant findings. Conclusions: CD8+ TILs have the potential to serve as predictive biomarkers of therapeutic response to neoadjuvant treatment in patients with rectal cancer. Inconsistent findings with FOXP3+ Tregs and CD163+ macrophages reinforce the need for their further investigation.