Memory lymphocytes of young and old C57BL/6 mice express high levels of class I major histocompatibility complex (H-2 Kb) protein.

Memory lymphocytes play a central role in the secondary immune response. The concentration of memory lymphocytes increases with age. A high level of cell surface CD44 is a marker of memory lymphocytes compared to naïve lymphocytes which express a low level of CD44. Major histocompatibility complex (MHC) class I protein expression also increases with age. To explore a possible correlation between the expression of CD44 and MHC class I protein (Kb), peripheral blood lymphocytes from 27 C57BL/6 mice ranging in age from 3 months to 33 months were isolated by Ficoll-Hypaque gradient centrifugation. Single and double indirect immunofluorescence assays were then performed with rat IgG anti-CD44 and/or mouse IgG anti-Kb as first antibodies, and phycoerythrin (PE) labeled goat anti-rat IgG and/or fluorescein (FITC) labeled goat anti-mouse IgG as second antibodies. Cells were then analyzed by using a FACScan flow cytometer. As expected, the percentage of lymphocytes expressing high levels of CD44 (memory cells) increased significantly with age and the expression of Kb increased significantly with age. Interestingly, the expression of Kb in lymphocytes expressing high levels of CD44 (memory cells) was 72% more than in cells expressing low levels of CD44 (naive cells) regardless of age.