T-Cell Receptor/CD28-Mediated Activation of Human T Lymphocytes Induces Expression of Functional μ-Opioid Receptors

Opiates function as immunomodulators, partly by their effects on T cells. Opioids act via μ-, δ-, and κ-opioid receptors, among which the μ-type is of particular interest, because morphine-like opioids preferentially bind to it. Here we report that μ-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 μ-opioid receptor promoter was transactivated by CD3/28 stimulation. Transcriptional induction of the μ-opioid receptor gene was mediated by activator protein-1 (AP-1), nuclear factor-κB, and nuclear factor of activated T cells (NFAT). NFAT was found to bind to three sequences of the μ-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 inter-leukin-2 mRNA and protein in activated T cells was inhibited by morphine in cells, in which μ-opioid receptors had been induced by CD3/28 monoclonal antibodies (mAbs), and that this effect was blocked by the μ-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 μ-opioid receptor mRNA is translated into functional receptor protein. Furthermore, a μ-opioid receptor-enhanced green fluorescent protein-fusion protein was localized in membranes of Jurkat cells and internalized in response to [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin but not morphine. In conclusion, these data emphasize the role of opioids in the modulation of T lymphocyte signaling.