Assessment of interleukin 1 and interleukin 2 effects on cycling and noncycling murine thymocytes

Abstract

When mouse thymocytes are stimulated with PHA, the proliferative response is very low, unless the culture medium is enriched with interleukin 1 (IL-1)- or interleukin 2 (IL-2)-containing supernatants. Cytofluorometric analyses show, however, that PHA stimulation generates a significant number of cells with increased RNA content (transition from the G 0 to G 1 phase of the cell cycle). If IL-2 is added to such cultures, the activated cells complete their process of RNA synthesis and then enter the S phase. The use of IL-2-containing culture medium thus permits one to obtain a high correlation between the number of g 1 cells and [ 3H]thymidine incorporation ( r = 0.97). Enrichment with IL-1-containing supernatants also results in a statistically significant correlation ( r = 0.68), but the regression lines are markedly different for the two interleukins ( s = 20.3 for IL-2 and s = 9.2 for IL-1), when analyzed after 48 hr of incubation. These observations suggest that the G 1 phase must be divided into two subcompartments, G 1a and G 1b, the G 1a-G 1b transition being an IL-2-dependent event. If the number of G 1b cells is used to establish correlations with [ 3H]thymidine incorporation, all values fall on the same regression line, regardless of culture conditions and of the addition of interleukins. It is concluded that IL-2 regulates lymphocyte proliferation at the level of RNA synthesis (G 1a-G 1b transition) rather than that of DNA synthesis (G 1-S transition).