Nitric oxide (NO) is an intercellular messenger that performs a number of functions, including neurotransmission, vasodilatation, inhibition of platelet aggregation, and modulation of leukocyte adhesion. NO has recently been shown to act as a potent cytotoxic effector molecule as well as to play an important role in the pathogenesis of organ-specific autoimmunity. NO may also modulate the immune response by interfering with Th1/Th2 balance in autoimmune diseases. This review will discuss the role of NO and nitric oxide synthase (NOS) in pathophysiologic and therapeutic implications in various autoimmune diseases with particular reference to T helper-1 (Th1) and T helper-2 (Th2) cytokines.

The methionine-enkephalin (Met-enkephalin, Tyr-Gly- Gly-Phe-Met) analogs Tyr-D-Ala-Gly-MePhe-Met NHC₃H₇- iso (1) and Tyr-D-Ala-Gly-MePhe-Gly-NHC₃H₇-iso (2) have been shown to enhance human T cell proliferation in in vitro treatment. Their immunomodulatory activities were completely blocked by naloxone, an opioid antagonist. Now we demonstrate that a selective δ-opioid receptor antagonist, ICI-174,864, completely blocks enhancement of T cell proliferation by analogs (1) and (2). The T cell-stimulatory effect was only partially inhibited by the µ-receptor-selective antagonist, β-funaltrexamine hydrochloride. The κ-opioid receptor antagonist, nor-binaltorphimine dihydrochloride, showed no effect on T cell-proliferation stimulated by analogs (1) and (2). These observations suggest that analogs (1) and (2) of Met-enkephalin stimulate T cell proliferation predominantly via δ-opioid receptor present on T cells.

Objective: As a follow-up to our earlier studies on immunomodulation with opioid peptides, we synthesized and evaluated immunomodulatory activity of four peptidomimetic compounds, i.e. Tyr-NH-C(Me)₂-CH₂-O-Phe-NH₂ (1), Tyr-NH-C₆H₅-(o)-CH₂-CH₂-O-Phe-NH₂ (2), Tyr-NH-CH₂-CH₂-O-Phe-NH₂ (3) and Tyr-NH-CH(D-Et)-CH₂-O-Phe-NH₂ (4). Methods: These compounds were synthesized in solution phase and evaluated for their immunomodulatory properties in vitro by mixed lymphocyte reaction (MLR), proliferation of opioid receptor-expressing cells, production of tumor necrosis factor-α (TNF-α) and nitric oxide. Results: This study shows the immunosuppressive potential of synthetic peptidomimetic compound 3. This compound inhibited two-way MLR and suppressed the proliferation of the µ-opioid receptor expressing human embryonic kidney cells HEK 293 in vitro. Inhibition of MLR by compound 3 was reversed by naloxone (opioid receptor antagonist) and β-funaltrexamine hydrochloride (µ-opioid receptor antagonist). The immunosuppressive effect of compound 3 was further demonstrated by inhibition of TNF-α and nitric oxide production in lipopolysaccharide-stimulated human PBMCs and mouse macrophage cells RAW 264.7, respectively. Conclusion: These observations suggest that compound 3 inhibits MLR through µ-opioid receptor present on cells.

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In continuation to our earlier studies with peptidomimetic opioid compounds, we have further investigated immunosuppressive properties of one of our peptidomimetic compound (Tyr-NH-CH2-CH2-O-Phe-NH2) using peripheral blood mononuclear cells (PBMCs) of healthy volunteers. Peptidomimetic compound was evaluated for its effect on anti-CD3 and recombinant human interleukin-2 (rhIL-2) stimulated lymphocyte proliferation in vitro and lipopolysaccharide (LPS) induced activation of mitogen activated protein kinase (MAPK, pp42/44) in mouse macrophage cells (RAW 264.7). Our results show the immunosuppressive potential of synthetic peptidomimetic compound. This compound significantly inhibited anti-CD3 and rhIL-2 stimulated lymphocyte proliferation in vitro. However, this peptidomimetic compound did not show any effect on LPS induced MAPK activation. These observations suggest that above peptidomimetic compound has potential to inhibit immune responses mediated by lymphocytes.