Role Of α₂-Adrenoreceptors In Mediating The Modulation Of T Lymphocyte Function

Objective: Previous work in our laboratory has shown thatα-andβ-adrenoreceptors exist on lymphocytes from the functional profiles, and the adrenoreceptors participate in regulating lymphocyte function. However, there is still no direct evidence to show the expression and synthesis ofα-adrenoreceptors (α-AR) by lymphocytes and also roles ofα-AR subtypes in mediating the modulation of lymphocyte function are less known. Therefore, in the present study we on the one hand revealed the expression of two kinds ofα-AR subtypes,α₁-andα₂-AR mRNA, and on the other hand investigated roles of the two kinds ofα-AR subtypes in mediating the regulation of T lymphocyte function and relevant intracellular signal-transduction mechanisms involved in the roles in order to provide more evidence for neuro-endocrine-immune modulatory network. Methods: Lymphocytes were separated from mesenteric lymph nodes of rats. T cells were purified by using a nylon wool column. The purity and viability of the purified T lymphocytes were determined by using E-rosette forming test and trypan blue exclusion, respectively. Reverse transcription-polymerase chain reaction (RT-PCR) was used to measure the expression ofα₁-AR mRNA andα₂-AR mRNA in both resting T cells and activated T cells. Colorimetric assay of methylthiazol tetrazolium bromide (MTT) was employed to measure lymphocyte proliferation induced by concanavalin A (Con A) after the lymphocytes from mesenteric lymph nodes of rats were treated with phenylephrine, a selectiveα₁-AR agonist, clonidine, a selectiveα₂-AR agonist, yohimbine, aα₂-AR antagonist, U-73122, a phospholipase C (PLC) inhibitor, and chelerythrine, a protein kinase C (PKC) inhibitor, respectively. Simultaneously, interleukin-4 (IL-4) levels in the culture supernatants of the Con A-stimulated and drug-treated lymphocytes were detected by enzyme linked immunosorbent assay (ELISA). Results: The purity of T cells that were separated was significantly higher than that of T cells that were not separated, but the viability of the purified T cells did not remarkably decrease compared with that of non-purified T cells. T cells expressed bothα₁-andα₂-AR mRNA, but the expression ofα₁-andα₂-AR mRNA was both significantly higher in the Con A-activated lymphocytes than in the resting lymphocytes. Phenylephrine (10^-9 to 10^-6 mol/L) had no significant effect on the Con A-induced lymphocyte proliferation. However, clonidine (10^-9 to 10^-6 mol/L) dramatically inhibited the proliferation of lymphocytes induced by Con A. The inhibition of lymphocyte proliferation by clonidine (10^-9 or 10^-6 mol/L) could be completely blocked by yohimbine (10^-10 to 10^-5 mol/L) and be partly blocked by U-73122 or chelerythrine (10^-10 to 10^-7 mol/L). Clonidine (10^-9 mol/L) reduced IL-4 production and release by Con A-stimulated lymphocytes. Yohimbine, U-73122 and chelerythrine could all reverse the IL-4 reduction induced by clonidine. Conclusions: T lymphocytes can express two kinds of subtypes ofα-AR,α₁-AR andα₂-AR, and the expression increases when the T cells are activated.α₁-AR could not participate in the regulation of lymphocyte proliferation, butα₂-AR actively mediates the modulation of T cell function and exerts a suppressive effect. The inhibited T cell function byα₂-AR participation may be mediated by intracellular PLC and PKC pathways in the T cells. Thus, our present findings suggest a neuro-endocrine-immune regulatory mechanism may exist in the body, i.e., norepinephrine and epinephrine released by the sympathetic nerves and adrenal medulla modulate immune function viaα₂-AR and its corresponding intracellular signal transduction pathways.