Membrane fluidity and cholesterol in thymus and spleen cells from mice treated with immunomodulatory drugs.

We have used spin labeling, fluorescence polarization, and chemical analysis to characterize membrane properties of thymocytes from mice treated with immunomodulatory drugs. The number of thymocytes was reduced 90-95% by treatment of 6-9 week old mice with hydrocortisone acetate (HCA) or methylprednisolone (both 125 mg/kg) or with cyclophosphamide (250 mg/kg). Electron spin resonance (esr) examination of thymocytes labeled with 5-nitroxyl stearic acid indicated that the membranes of cells remaining after treatment with any of these drugs were more rigid than those from saline-treated controls. The total cholesterol/phospholipid (C/PL) molar ratio of the HCA-resistant thymocytes was twice that of the control mice. Treatment of mice with other immunomodulatory drugs, cyclophosphamide, cytosine arabinoside (Ara-C), 2-amino-5-bromo-6-phenyl-4-(3H)-pyrimidinone (ABPP) and 15(S)-methyl prostaglandin E1 (15(S)-methyl PGE1), also altered the C/PL ratio in thymocytes and, in some cases, in spleen cells. Fluorescence polarization measurements of thymocytes labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH) did not reveal the differences between cells from HCA-and saline-treated mice that were detected by spin labeling and chemical analysis. Our results indicate that the greater rigidity detected by spin labeling of hydrocortisone-resistant thymocytes may be due, at least in part, to greater membrane cholesterol content. Of the methods employed, chemical analysis was the most sensitive in revealing drug-induced alterations in thymocyte populations.