Analysis of spleen cell populations from old Lewis rats (>24 months) and from young Lewis rats (3 to 4 months) in a fluorescence-activated cell sorter indicated that with aging there is a loss of brightly stained Ia and Fc receptor- (FcR) positive cells. The density of AgB, Ia, and FcR was diminished on the surface of cells from old rats. The rate of capping of all three membrane proteins was slower on cells from old rats. Colchicine treatment allows capping of AgB with a single ligand only in young rats. Fluorescence photobleach recovery experiments (FPR) show that the fluidity of the lymphocyte membrane from old rats is diminished and the lateral diffusion of AgB is decreased. The colchicine and FPR experiments suggest that the changes in capping in old rats are due to, in part, alterations in membrane fluidity and cytoskeletal function.

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Publication No. 1808 from the Department of Immunopathology, Research Institute of Scripps Clinic, La Jolla, California 92037. This work was supported by National Institutes of Health Grant AI-07007, National Science Foundation Grant PCM 75-19594, and Contract 1001-CB-43874 from the National Cancer Institute.

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