Regulatory T cells: a possible promising approach to cancer recurrence induced by morphine.

Cancer recurrence is one of the most important causes of cancer-related deaths. In present, it has been revealed that there exist some factors especially opioids being able to affect the recovery of cancer patients in a long period. As the most commonly used potent analgesics in practice, morphine appears to be of crucial importance in the regulation of neoplastic tissues by modulating immune responses and promoting angiogenesis. Indeed, regulatory T cells have been shown to inhibit the response of the immune system to tumor and thereby to worsen prognoses. Some reliable evidences indicate that morphine acts directly on regulatory T cells through VEGFR 2 and opioid receptors present in, both of which play a vital role in the cancer recurrence. In addition, morphine might have a noticeable effect on regulatory T cells by regulating the function of some other immune cells or cytokines, TGF-β and IL-2 for instance. Thus, this paper speculates that morphine could induce cancer recurrence by disturbing the behavior of the regulatory T cells and provides a logical reasoning.