Checkpoint blockade therapy triggers tumor-specific immune responses in a variety of cancer types. We presumed that rectal cancer patients could have become sensitive to immunotherapy after receiving neoadjuvant chemoradiotherapy (nCRT). In this study, we report immune alternation in post-nCRT patients compared with pretreatment conditions from gene-expression omnibus (GEO) data. Whole-exome sequencing of 14 locally advanced rectal cancer (LARC) patient samples showed that nCRT induced new mutations compared with the paired pretreatment biopsies, evidenced by appearance of a neoantigen landscape. An association was identified between mutation burden and enrichment of immune activation-related pathways. Animal experiment results further demonstrated that radiotherapy enhanced the efficacy of anti-PD-1. Mutation burden and the neoantigens of LARC patients were associated with response to nCRT. The mRNA expression profiling of 66 pretreatment biopsy samples from LARC patients showed that immune activation-related pathways were enriched in response to nCRT. PD-L1 expression was negatively correlated with disease-free survival in the CD8-low expression patient group who received nCRT in a cohort of 296 samples. Thus, nCRT was able to alter immune function in LARC patients, which may be associated with the appearance of neoantigens. Neoantigens could make rectal cancer patients potential candidates to receive checkpoint blockade immunotherapy, and mutation burden could be a useful biomarker to stratify patients into responding and nonresponding groups for immunotherapy. (C) 2018 AACR.