Differential kappa-opioid receptor expression on mouse lymphocytes at varying stages of maturation and on mouse macrophages after selective elicitation.

The combination of indirect immunofluorescent labeling and flow cytometry has proven to be a sensitive method for labeling of the kappa-opioid receptor on mouse thymocytes. In the present study, this labeling procedure was applied, along with phenotypic analysis, to mature immune cell populations to determine whether kappa-opioid receptor expression is present after immune cell maturation. Unfixed primary splenocytes from 6- to 8-week-old C57BL/6ByJ male mice were incubated with the fluorescein-containing, kappa-selective ligand fluorescein-conjugated 2-(3, 4-dichlorophenyl)-N-methyl-N-[1-(3-aminophenyl)-2-(1-pyrrolidinyl)eth yl]acetamide (FITC-AA). Amplification of FITC-AA binding to the kappa-opioid receptor was attained by adding a biotin-conjugated antifluorescein antibody, followed by extravidin-R-phycoerythrin. It has been shown previously that greater than 60% of immature thymocytes (CD4(+)/CD8(+)) demonstrated specific kappa-opioid receptor labeling. However, the present report shows that less than 25% of either T-helper or T-cytotoxic splenic lymphocytes expressed the kappa-opioid receptor. Likewise, only 16% of all splenic B lymphocytes were labeled for the kappa-opioid receptor. These findings demonstrate a decrease in kappa-opioid receptor expression on maturation of mouse lymphocytes. Interestingly, resident peritoneal macrophages showed a greater magnitude of specific receptor labeling, compared with either thymocytes or splenocytes, and approximately 50% of the resting Mphi expressed the kappa-opioid receptor. However, elicitation of Mphi with thioglycollate resulted in the complete loss of the expression of this receptor. Taken together, these findings demonstrate the diversity in the expression of the kappa-opioid receptor on immune cells at varying stages of differentiation, with preferential expression demonstrated by resident, peritoneal macrophages.