T cell receptor/CD28-mediated activation of human T lymphocytes induces expression of functional {micro}-opioid receptors.

Opiates function as immunomodulators, partly by their effects on T cells. Opioids act via -, -, and -opioid receptors, among which the -type is of particular interest, because morphine- like opioids preferentially bind to it. Here we report that -opioid receptor mRNA was induced after CD3/28-mediated activation of primary human T lymphocytes and Jurkat T cells, neither of which expresses the gene constitutively. Moreover, a reporter gene construct containing 2624 base pairs of the -opioid receptor promoter was transactivated by CD3/28 stimulation. Transcriptional induction of the -opioid receptor gene was mediated by activator protein-1 (AP-1), nuclear factor- B, and nuclear factor of activated T cells (NFAT). NFAT was found to bind to three sequences of the -opioid receptor promoter, located at nucleotides 1064, 785, and 486. Although the 486 element is in close proximity to a putative AP-1 site, there was no evidence for a combined AP-1/NFAT site. Furthermore, we demonstrated that the induction of interleukin- 2 mRNA and protein in activated T cells was inhibited by morphine in cells, in which -opioid receptors had been induced by CD3/28 monoclonal antibodies (mAbs), and that this effect was blocked by the -opioid receptor-specific antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2. CD3/28 mAb-induced interleukin-2 transcription was also inhibited by the opioids fentanyl and loperamide. This indicates that the induced -opioid receptor mRNA is translated into functional receptor protein. Furthermore, a -opioid receptor-enhanced green fluorescent protein-fusion protein was localized in membranes of Jurkat cells and internalized in response to [D-Ala2,NMe- Phe4,Gly5-ol]-enkephalin but not morphine. In conclusion, these data emphasize the role of opioids in the modulation of T lymphocyte signaling.