Decline, in aging mice, of the anti-2,4,6-trinitrophenyl (TNP) cytotoxic T cell response attributable to loss of Lyt-2-, interleukin 2-producing helper cell function.

The in vitro generation of cytotoxic T lymphocytes (CTL) specific for 2,4,6-trinitrophenyl (TNP)-modified syngeneic spleen cells is found to be almost invariably depressed in apparently healthy 18-month-old mice of the long-lived (BALB/c x C57BL/6)F1 hybrid strain. Studies of CTL production from Lyt-2+ thymus cells have suggested that pre-killer cells may require, for maturation into effectors, the presence of a soluble helper factor, interleukin 2 (IL2), produced by Lyt-2- cells which are themselves devoid of pre-CTL activity. We have therefore developed a petri-dish adherence technique for separating spleen cells into Lyt-2+ and Lyt-2- populations in order to test for helper and pre-killer activity independently. Pre-CTL function is measured by stimulating Lyt-2+ cells in the presence of exogenous IL2. Helper cell activity is tested by adding Lyt-2- cells to "indicator" populations of Lyt-2+ pre-CTL. Estimation of IL2 levels in medium conditioned by unfractionated, TNP-self-stimulated splenocytes provides a second measurement of helper cell function. Mice 18 months of age, when compared to 4 month-old controls, are found to retain nearly all of their pre-CTL activity, but to have lost sufficient helper cell activity to account for the decline in unseparated spleen cell cultures. Older mice also produce lower IL2 levels.