Lymphokine-activated killer cells and aging in mice: significance for defining the precursor cell.

The present study was undertaken to define the cell populations which mediate lymphokine-activated killer (LAK) cell activity in mice. Because old mice exhibit markedly decreased to nondetectable natural killer (NK) cell activity, this age-associated change provided an advantageous system to examine the contribution of NK and T cells to LAK activity. Spleen cells from either young (6-9 weeks) or old (20-26 months) mice were cultured with 1000 units/ml of recombinant interleukin 2 (rIL 2) for 3-5 days. The cells were then tested in a 51 Cr-release assay for their cytotoxicity against NK-resistant fresh tumor cells (MCA-102). The LAK activity exhibited by spleen cells from old mice following 5 days of culture was equivalent to that developed by spleen cells of young mice. This result was contrary to what would be anticipated if mature NK cells comprise the primary precursors of LAK activity, and required further elucidation. The Thy-1 and asialo GM1 (ASGM1) phenotypes of LAK precursor and effector cells were therefore examined by depletion techniques using the appropriate antibodies plus complement. The results using spleen cells harvested after 5 days of culture with rIL 2 showed that LAK effector cells which developed from spleen cells of both young and old mice were predominantly Thy-1+ (85.3% young; 91.8% old) and some coexpressed ASGM1. Spleen cells were treated prior to culture to study the precursor cells. Development of LAK activity by spleen cells from both young and old mice was greatly reduced by pretreatment with anti-ASGM1 plus complement. However, since spleen cells of old mice exhibit very low mature NK activity, these data suggest that the LAK precursors, at least in old mice, may be ASGM1+ NK precursor cells rather than mature ASGM1+ NK effector cells. In addition, treatment with anti-Thy-1 plus complement inhibited generation of a significant proportion of LAK activity only in the spleens of old mice, suggesting a qualitative difference in LAK precursor cells with age and supporting the heterogeneity of the cells which are capable of developing LAK activity.