Marc Pellegrini

BH3-only Bcl-2 family member Bim is required for apoptosis of autoreactive thymocytes

During lymphocyte development, the assembly of genes coding for antigen receptors occurs by the combinatorial linking of gene segments. The stochastic nature of this process gives rise to lymphocytes that can recognize self-antigens, thereby having the potential to induce autoimmune disease. Such autoreactive lymphocytes can be silenced by developmental arrest or unresponsiveness (anergy), or can be deleted from the repertoire by cell death. In the thymus, developing T lymphocytes (thymocytes) bearing a T-cell receptor (TCR)–CD3 complex that engages self-antigens are induced to undergo programmed cell death (apoptosis), but the mechanisms ensuring this ‘negative selection’ are unclear. We now report that thymocytes lacking the pro-apoptotic Bcl-2 family member Bim (also known as Bcl2l11) are refractory to apoptosis induced by TCR–CD3 stimulation. Moreover, in transgenic mice expressing autoreactive TCRs that provoke widespread deletion, Bim deficiency severely impaired thymocyte killing. TCR ligation upregulated Bim expression and promoted interaction of Bim with Bcl-XL, inhibiting its survival function. These findings identify Bim as an essential initiator of apoptosis in thymocyte-negative selection.

FOXO3a-dependent regulation of Puma in response to cytokine/growth factor withdrawal

Puma is an essential mediator of p53-dependent and -independent apoptosis in vivo. In response to genotoxic stress, Puma is induced in a p53-dependent manner. However, the transcription factor driving Puma up-regulation in response to p53-independent apoptotic stimuli has yet to be identified. Here, we show that FOXO3a up-regulates Puma expression in response to cytokine or growth factor deprivation. Importantly, dysregulated Akt signaling in lymphoid cells attenuated Puma induction upon cytokine withdrawal. Our results suggest that Puma, together with another BH3 only member, Bim, function as FOXO3a downstream targets to mediate a stress response when PI3K/Akt signaling is down-regulated.

Shutdown of an acute T cell immune response to viral infection is mediated by the proapoptotic Bcl-2 homology 3-only protein Bim

We used mutant Fas-deficient (lpr) or Bim-deficient mice to investigate the role of the death receptor and Bcl-2-regulated apoptotic pathways in terminating a physiological T cell response to herpes simplex virus infection. In WT and lpr mice CD8+ antigen-specific T cells were deleted after viral clearance. In contrast, the immune response was not terminated in Bim-deficient mice despite viral clearance, and CD8+ antigen-specific T cells accumulated in the spleen. Thus, Bim is dispensable for viral clearance but is necessary for the death of activated T cells when immune responses are terminated. These findings have implications for the therapeutic manipulation of immune responses to infections and immunization.

IL-7 Engages Multiple Mechanisms to Overcome Chronic Viral Infection and Limit Organ Pathology

Understanding the factors that impede immune responses to persistent viruses is essential in designing therapies for HIV infection. Mice infected with LCMV clone-13 have persistent high-level viremia and a dysfunctional immune response. Interleukin-7, a cytokine that is critical for immune development and homeostasis, was used here to promote immunity toward clone-13, enabling elucidation of the inhibitory pathways underlying impaired antiviral immune response. Mechanistically, IL-7 downregulated a critical repressor of cytokine signaling, Socs3, resulting in amplified cytokine production, increased T cell effector function and numbers, and viral clearance. IL-7 enhanced thymic output to expand the naive T cell pool, including T cells that were not LCMV specific. Additionally, IL-7 promoted production of cytoprotective IL-22 that abrogated liver pathology. The IL-7-mediated effects were dependent on endogenous IL-6. These attributes of IL-7 have profound implications for its use as a therapeutic in the treatment of chronic viral diseases.

Adjuvant IL-7 antagonizes multiple cellular and molecular inhibitory networks to enhance immunotherapies

Identifying key factors that enhance immune responses is crucial for manipulating immunity to tumors. We show that after a vaccine-induced immune response, adjuvant interleukin-7 (IL-7) improves antitumor responses and survival in an animal model. The improved immune response is associated with increased IL-6 production and augmented T helper type 17 cell differentiation. Furthermore, IL-7 modulates the expression of two ubiquitin ligases: Casitas B-lineage lymphoma b (Cbl-b), a negative regulator of T cell activation, is repressed, and SMAD-specific E3 ubiquitin protein ligase-2 (Smurf2) is enhanced, which antagonizes transforming growth factor-β signaling. Notably, we show that although short term IL-7 therapy potently enhances vaccine-mediated immunity, in the absence of vaccination it is inefficient in promoting antitumor immune responses, despite inducing homeostatic proliferation of T cells. The ability of adjuvant IL-7 to antagonize inhibitory networks at the cellular and molecular level has major implications for immunotherapy in the treatment of tumors.

Hobit and Blimp1 instruct a universal transcriptional program of tissue residency in lymphocytes

Transcription factors define tissue T cells The immune system fights microbial invaders by maintaining multiple lines of defense. For instance, specialized memory T cells [resident memory T cells (Trms)] colonize portals of pathogen entry, such as the skin, lung, and gut, to quickly halt reinfections. Mackay et al. now report that in mice, Trms as well as other tissue-dwelling lymphocyte populations such as natural killer cells share a common transcriptional program driven by the related transcription factors Hobit and Blimp1. Tissue residency and retention of lymphocytes require expression of Hobit and Blimp1, which, among other functions, suppress genes that promote tissue exit. Science, this issue p. 459 Tissue-dwelling lymphocyte populations share a common transcriptional signature. Tissue-resident memory T (Trm) cells permanently localize to portals of pathogen entry, where they provide immediate protection against reinfection. To enforce tissue retention, Trm cells up-regulate CD69 and down-regulate molecules associated with tissue egress; however, a Trm-specific transcriptional regulator has not been identified. Here, we show that the transcription factor Hobit is specifically up-regulated in Trm cells and, together with related Blimp1, mediates the development of Trm cells in skin, gut, liver, and kidney in mice. The Hobit-Blimp1 transcriptional module is also required for other populations of tissue-resident lymphocytes, including natural killer T (NKT) cells and liver-resident NK cells, all of which share a common transcriptional program. Our results identify Hobit and Blimp1 as central regulators of this universal program that instructs tissue retention in diverse tissue-resident lymphocyte populations.

TNF-α is critical for antitumor but not antiviral T cell immunity in mice

TNF-alpha antagonists are widely used in the treatment of inflammatory and autoimmune diseases, but their use is associated with reactivation of latent infections. This highlights the importance of TNF-alpha in immunity to certain pathogens and raises concerns that critical aspects of immune function are impaired in its absence. Unfortunately, the role of TNF-alpha in the regulation of T cell responses is clouded by a myriad of contradictory reports. Here, we show a role for TNF-alpha and its receptors, TNFR1 and TNFR2, specifically in antitumor immunity. TNF-alpha-deficient mice exhibited normal antiviral responses associated with strong inflammation. However, TNF-alpha/TNFR1-mediated signals on APCs and TNF-alpha/TNFR2 signals on T cells were critically required for effective priming, proliferation, and recruitment of tumor-specific T cells. Furthermore, in the absence of TNF-alpha signaling, tumor immune surveillance was severely abrogated. Finally, treatment with a CD40 agonist alone or in combination with TLR2 stimuli was able to rescue proliferation of TNF-alpha-deficient T cells. Therefore, TNF-alpha signaling may be required only for immune responses in conditions of limited immunostimulatory capacity, such as tumor surveillance. Importantly, these results suggest that prolonged continuous TNF-alpha blockade in patients may have long-term complications, including potential tumor development or progression.

Loss of Bim Increases T Cell Production and Function in Interleukin 7 Receptor–deficient Mice

Interleukin (IL)-7 receptor (R) signaling is essential for T and B lymphopoiesis by promoting proliferation, differentiation, and survival of cells. Mice lacking either IL-7 or the IL-7Rα chain have abnormally low numbers of immature as well as mature T and B lymphocytes. Transgenic expression of the apoptosis inhibitor Bcl-2 rescues T cell development and function in IL-7Rα–deficient mice, indicating that activation of a proapoptotic Bcl-2 family member causes death of immature and mature T cells. BH3-only proteins such as Bim, which are distant proapoptotic members of the Bcl-2 family, are essential initiators of programmed cell death and stress-induced apoptosis. We generated Bim/IL-7Rα double deficient mice and found that loss of Bim significantly increased thymocyte numbers, restored near normal numbers of mature T cells in the blood and spleen, and enhanced cytotoxic T cell responses to virus infection in IL-7Rα−/− mice. These results indicate that Bim cooperates with other proapoptotic proteins in the death of IL-7–deprived T cell progenitors in vivo, but is the major inducer of this pathway to apoptosis in mature T cells. This indicates that pharmacological inhibition of Bim function might be useful for boosting immune responses in immunodeficient patients.

Fatal Hepatitis Mediated by Tumor Necrosis Factor TNFα Requires Caspase-8 and Involves the BH3-Only Proteins Bid and Bim

Apoptotic death of hepatocytes, a contributor to many chronic and acute liver diseases, can be a consequence of overactivation of the immune system and is often mediated by TNFalpha. Injection with lipopolysaccharide (LPS) plus the transcriptional inhibitor D(+)-galactosamine (GalN) or mitogenic T cell activation causes fatal hepatocyte apoptosis in mice, which is mediated by TNFalpha, but the effector mechanisms remain unclear. Our analysis of gene-targeted mice showed that caspase-8 is essential for hepatocyte killing in both settings. Loss of Bid, the proapoptotic BH3-only protein activated by caspase-8 and essential for Fas ligand-induced hepatocyte killing, resulted only in a minor reduction of liver damage. However, combined loss of Bid and another BH3-only protein, Bim, activated by c-Jun N-terminal kinase (JNK), protected mice from LPS+GalN-induced hepatitis. These observations identify caspase-8 and the BH3-only proteins Bid and Bim as potential therapeutic targets for treatment of inflammatory liver diseases.

CD4 T cells, lymphopenia, and IL-7 in a multistep pathway to autoimmunity

There are many inhibitory mechanisms that function at the cellular and molecular levels to maintain tolerance. Despite this, self-reactive clones escape regulatory mechanisms and cause autoimmunity in certain circumstances. We hypothesized that the same mechanisms that permit T cells to expand during homeostatic proliferation may inadvertently promote autoimmunity under certain conditions. One major homeostatic cytokine is IL-7, and studies have linked it or its receptor to the development of multiple sclerosis and other autoimmune diseases. We show in a model of β-islet cell self-reactivity that the transfer of activated autoreactive CD4 T cells can prime and expand endogenous autoreactive CD8 T cells in a CD28- and CD40-dependent manner through the licensing of dendritic cells. Despite this, mice do not develop diabetes. However, the provision of exogenous IL-7 or the physiological production of IL-7 associated with lymphopenia was able to profoundly promote the expansion of self-reactive clones even in the presence of regulatory T cells. Autoimmune diabetes rapidly ensued with CD4 help and the subsequent activation of CD8 T cells, which contributed to disease progression. With the advent of many biologicals targeting TNFα, IL-6, and IL-1 and their effective use in the treatment of autoimmune diseases, we propose that IL-7 and its receptor may be promising targets for biological agents in the treatment of autoimmunity.