CD8A and colorectal carcinoma: In response to tumours, naive CD4 + T cells can differentiate into four main groups of cells—Th1, Th2, Th17 and T regulatory (Treg) cells—based on cytokine production and regulatory transcription factors.34,35 Some studies have revealed that Th1 promotes CD8 + T-cell-mediated adaptive immunity,36 that Th1 and cytotoxic gene levels are associated with a good prognosis,17 and that CRC patients with high Th17 expression have a poor prognosis.