Overexpression of the proto-oncogene c-myc has been implicated in the genesis of diverse human tumours. c-Myc seems to regulate diverse biological processes, but its role in tumorigenesis and normal physiology remains enigmatic. Here we report the generation of an allelic series of mice in which c-myc expression is incrementally reduced to zero. Fibroblasts from these mice show reduced proliferation and after complete loss of c-Myc function they exit the cell cycle. We show that Myc activity is not needed for cellular growth but does determine the percentage of activated T cells that re-enter the cell cycle. In vivo, reduction of c-Myc levels results in reduced body mass owing to multiorgan hypoplasia, in contrast to Drosophila c-myc mutants, which are smaller as a result of hypotrophy. We find that c-myc substitutes for c-myc in fibroblasts, indicating they have similar biological activities. This suggests there may be fundamental differences in the mechanisms by which mammals and insects control body size. We propose that in mammals c-Myc controls the decision to divide or not to divide and thereby functions as a crucial mediator of signals that determine organ and body size.

Activation-induced cell death (AICD) of lymphocytes is an important mechanism of self-tolerance. In CD4+ T cells, AICD is mediated by the Fas pathway and is enhanced by IL-2. To define the mechanisms of this pro-apoptotic action of IL-2, we analyzed CD4+ T cells from wild-type and IL-2-/- mice expressing a transgenic T cell receptor. T cells become sensitive to AICD after activation by antigen and IL-2. IL-2 increases transcription and surface expression of Fas ligand (FasL) and suppresses transcription and expression of FLIP, the inhibitor of apoptosis. The ability of IL-2 to enhance expression of a pro-apoptotic molecule, FasL, and to suppress an inhibitor of Fas signaling, FLIP, likely accounts for the role of this cytokine in potentiating T cell apoptosis.

The effector cytokine interferon γ (IFN-γ) may play a role in T cell homeostasis. We have examined the requirement for IFN-γ in one mechanism that regulates T cell expansion and survival, activation-induced cell death (AICD). CD4+ T cells lacking IFN-γ or the Stat1 transcription factor are resistant to AICD. IFN-γ is required for the production of caspases, and retrovirus-mediated expression of caspase-8 restores the sensitivity of Stat1-deficient T cells to AICD. In vitro, IFN-γ limits the expansion of T cells that are stimulated through their antigen receptors. Thus, IFN-γ may function to control the expansion and persistence of T cells by promoting caspase-8–dependent apoptosis.

Recently, immunization techniques in which DNA constructs are introduced directly into mammalian tissue in vivo have been developed. In theory, gene inoculation should result in the production of antigenic proteins in a natural form in the immunized host. Here we present the use of such a technique for the inoculation of mice with a human immunodeficiency virus type 1 (HIV-1) envelope DNA construct (pM160). Mice were injected intramuscularly with pM160 and were subsequently analyzed for their anti-HIV envelope immune responses. Antisera collected from inoculated animals reacted with the recombinant HIV-1 envelope in ELISA and immunoprecipitation assays. The antisera also contained antibodies that were able to neutralize HIV-1 infection and inhibit HIV-1-mediated syncytium formation in vitro. Furthermore, splenic lymphocytes derived from pM160-inoculated animals demonstrated HIV-envelope-specific proliferative responses. The gene inoculation technique mimics features of vaccination with live attenuated viruses and, therefore, may ultimately prove useful in the rapid development of safe and efficacious vaccines as it provides for production of relevant antigen in vivo without the use of infectious agents.