CD8A and neoplasm: When the pH of the tumor microenvironment decreases to a value specific to tumor masses (pH 6-6.5), human and murine tumor-specific CD8+ T lymphocytes lose their immune response to tumor cells, manifested by impaired cytotoxic activity and reduced cytokine secretion, decreased expression of IL-2Rα (CD25) and T cell receptor (TCR), and weakened activation of signal transducer and activator of transcription 5 (STAT5) and extracellular signal-regulated kinase (ERK).