The clinical significance of PD-L1 in colorectal cancer

Colorectal cancer (CRC) is one of the most frequently encountered neoplasms and has a high rate of morbidity and mortality. Recent findings showing that tumor immune evasion is an important mechanism underlying propagation of a cancer have changed the landscape of medical oncology through identification of Programmed-Death receptor 1 and its ligand (PD-1 and PD-L1) as novel targets for oncological immune therapies. PD-1 is primarily expressed on peritumoral lymphocytes and when activated, it suppresses its immune functions. Conversely, PD-L1 is primarily expressed on the tumor infiltrating front with the purpose of deregulating physiological cytotoxic immune responses. Numerous studies have linked PD-L1 overexpression to specific adverse clinicopathological features, such as poor differentiation, lymphovascular invasion and worse overall survival in CRC patients. Nevertheless, there is no concrete evidence showing which patients may exhibit the maximal beneficial effects of PD-1/PD-L1 blockade therapy, and how these novel molecular targets may be optimally integrated into therapeutic regimens for management of CRC patients with resectable and generalized disease.