Injection of soluble protein antigen into animals causes abortive proliferation of the responding T cells. Immunological adjuvants boost T cell responses at least in part by increasing the survival of activated T cells during and after the initial proliferative phase of their clonal expansion. To understand how adjuvants promote T cell survival, we used gene microarrays to analyze gene expression in T cells activated either with antigen alone or in the presence of two different adjuvants. Among the genes whose expression was increased by both adjuvants was the IkappaB family member Bcl-3. Retroviral infection experiments showed that expression of Bcl-3 increased survival of activated T cells in vitro and in vivo. Adjuvants may therefore improve survival of activated T cells via induction of Bcl-3.

Interleukin-7 is widely accepted as a major homeostatic factor involved in T cell development. To assess the IL-7 responsiveness of thymocytes involved in selection processes, we used a new sensitive flow cytometry-based assay to detect intracellular phosphorylation of STAT-5 induced by IL-7 in defined mouse thymocyte subsets. Using this method, we found the earliest thymocyte subset (CD4−CD8−CD25−CD44+) to contain both IL-7-responsive and nonresponsive cells. Transition through the next stages of development (CD4−CD8−CD25+CD44+ and −) was associated with responsiveness of all thymocytes within these populations. Passage of thymocytes through β-selection resulted in a significant reduction in IL-7 sensitivity. In the next phases of development (TCR− and TCRlowCD69−), thymocytes were completely insensitive to the effects of IL-7. STAT-5 phosphorylation in response to IL-7 was again observed, however, in thymocytes involved in the positive selection process (TCRlowCD69+ and TCRintermediate). As expected, CD4 and CD8 single-positive thymocytes were responsive to IL-7. These findings delineate an IL-7-insensitive population between the β-selection and positive selection checkpoints encompassing thymocytes predicted to die by neglect due to failure of positive selection. This pattern of sensitivity suggests a two-signal mechanism by which survival of thymocytes at these checkpoints is governed.