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Lack of IL 2-dependent proliferation despite significant expression of high-affinity IL 2 receptor on murine cytolytic T lymphocyte clones late after antigenic stimulation.

In this study, we examine the expression of IL 2 receptors on class I and class II MHC-restricted, influenza-specific murine T lymphocyte clones at early (day 3) and late (day 8 to day 12) times after antigenic stimulation. IL 2 receptor expression on the three clones examined increases to peak levels early and subsequently decays 10-fold to 50-fold during this time period, as evidenced by monoclonal anti-IL 2 receptor antibody binding. However, in IL 2 binding site studies these clones retain high levels of high-affinity IL 2 receptors (46 to 97% of day 3 levels) at the later time points, despite their inability to proliferate in response to IL 2 in the form of supernatant from Con A-stimulated rat splenocytes. Considerable clone to clone variation is seen in binding site affinity for IL 2, whereas no significant change in binding affinity for IL 2 is exhibited for a particular clone as a function of time after activation, suggesting little structural change in the IL 2 receptor with time. Contrary to this finding, Con A-stimulated murine splenocytes exhibit a sharp decay in IL 2 receptor expression reaching 25% of peak (day 3) levels by day 10 after stimulation by lectin, with a significant decrease in the average binding site affinity for IL 2 in the population. This change in affinity in the Con A-stimulated population may, however, reflect a selection with time for lymphocytes with lower affinity for IL 2. To elucidate where the potential block may be that prevents these CTL clones from proliferating in an IL 2-dependent manner, the ability of the cells to internalize bound IL 2 at late times after activation is examined. All three clones late after activation are able to internalize bound IL 2 with efficiencies equivalent to that seen at day 3. Additionally, two of the three clones late after activation are able to upregulate expression of IL 2 receptors in response to picamolar concentrations of IL 2, indicating that the receptors on these clones are able to transmit a signal, although insufficient to induce proliferation of the cells. These observations strongly suggest that for the CTL clones examined, at late times after antigenic stimulation, engagement of IL 2 by the high-affinity receptor is not a sufficient signal to induce cells to transit through the cell cycle.