Cyclin-dependent kinase 9 expression and its association with CD8+ T cell infiltration in microsatellite-stable colorectal cancer

Programmed death 1 (PD-1)-targeted therapy has benefited patients with microsatellite instability-high metastatic colorectal cancer (mCRC). However, the efficacy of PD-1-targeted therapy is poor in patients with microsatellite-stable (MSS) mCRC. Therefore, it is imperative to explore additional co-inhibitory molecular signalling pathways to improve the efficacy of immunotherapy in MSS mCRC treatment. In the present study, the association between cyclin-dependent kinase 9 (CDK9) expression and the survival of patients with CRC was analysed using RNA sequencing data from 605 patients, including 121 cases of mortality, from human cancer datasets. Furthermore, 35 clinical MSS stage III-IV CRC specimens were collected to assess CDK9 protein expression by immunohistochemistry, and the frequency of tumor-infiltrating CD8(+) T cells was assessed by flow cytometry. The human cancer datasets demonstrated that upregulation CDK9 significantly shortened the survival of patients with stage II-IV colon cancer. Additionally, CDK9 mRNA expression was positively correlated with the expression levels of genes associated with immune evasion in the tumor. Notably, CDK9 was expression was upregulated in stage IV CRC compared with para-cancerous tissues and early-stage tumors. Interestingly, CDK9 expression was negatively associated with the infiltration of CD8(+) T cells at the tumor site. In addition, the expression levels of T-cell immunoglobulin mucin family member 3 and CD39, proteins associated with exhaustion, on tumor-infiltrating CD8(+) T cells were significantly elevated in patients with abnormal CDK9 expression levels. The present study demonstrated that CDK9 expression was negatively associated with CD8(+) T cell infiltration and positively associated with CD8(+) T cell exhaustion in MSS mCRC. In conclusion, CDK9 may be utilized to evaluate the prognosis and the immune-type of the tumor microenvironment in patients with MSS mCRC.