Multiple death signals influence mitochondria during apoptosis, yet the critical initiating event for mitochondrial dysfunction in vivo has been unclear. tBID, the caspase-activated form of a "BH3-domain-only" BCL-2 family member, triggers the homooligomerization of "multidomain" conserved proapoptotic family members BAK or BAX, resulting in the release of cytochrome c from mitochondria. We find that cells lacking both Bax and Bak, but not cells lacking only one of these components, are completely resistant to tBID-induced cytochrome c release and apoptosis. Moreover, doubly deficient cells are resistant to multiple apoptotic stimuli that act through disruption of mitochondrial function: staurosporine, ultraviolet radiation, growth factor deprivation, etoposide, and the endoplasmic reticulum stress stimuli thapsigargin and tunicamycin. Thus, activation of a "multidomain" proapoptotic member, BAX or BAK, appears to be an essential gateway to mitochondrial dysfunction required for cell death in response to diverse stimuli.Members of the "BH3-domain-only" subset of BCL-2 family proteins connect proximal death signals to the core apoptotic pathway (1-5). After activation of CD95 (Fas) or TNFR1 death receptors, BID is cleaved and activated to p15 tBID (6-8), which, in a model system using purified mitochondria, serves as a death ligand that induces the oligomerization of BAK (9) and BAX (10). tBID does not cause release of cytochrome c from purified Bakdeficient mitochondria, suggesting that interaction of tBID and BAK is a critical event at least in vitro (9).Here we assess whether BAK is required for tBID-induced apoptosis of intact, whole cells by using a retroviral vector to express tBID in murine embryonic fibroblasts (MEFs). Numerous cells with shrunken, apoptotic morphology were detected in Bak-deficient as well † To whom correspondence should be addressed.

Cell-mediated (type-1) immunity is necessary for immune protection against most intracellular pathogens and, when excessive, can mediate organ-specific autoimmune destruction. Mice deficient in Eta-1 (also called osteopontin) gene expression have severely impaired type-1 immunity to viral infection [herpes simplex virus-type 1 (KOS strain)] and bacterial infection (Listeria monocytogenes) and do not develop sarcoid-type granulomas. Interleukin-12 (IL-12) and interferon-gamma production is diminished, and IL-10 production is increased. A phosphorylation-dependent interaction between the amino-terminal portion of Eta-1 and its integrin receptor stimulated IL-12 expression, whereas a phosphorylation-independent interaction with CD44 inhibited IL-10 expression. These findings identify Eta-1 as a key cytokine that sets the stage for efficient type-1 immune responses through differential regulation of macrophage IL-12 and IL-10 cytokine expression.

Osteopontin (Opn) is important for T helper type 1 (T H 1) immunity and autoimmunity. However, the role of this cytokine in T H 2-mediated allergic disease as well as its effects on primary versus secondary antigenic encounters remain unclear. Here we demonstrate that OPN is expressed in the lungs of asthmatic individuals and that Opn-s, the secreted form of Opn, exerts opposing effects on mouse T H 2 effector responses and subsequent allergic airway disease: pro-inflammatory at primary systemic sensitization, and anti-inflammatory during secondary pulmonary antigenic challenge. These effects of Opn-s are mainly mediated by the regulation of T H 2-suppressing plasmacytoid dendritic cells (DCs) during primary sensitization and T H 2-promoting conventional DCs during secondary antigenic challenge. Therapeutic administration of recombinant Opn during pulmonary secondary antigenic challenge decreased established T H 2 responses and protected mice from allergic disease. These effects on T H 2 allergic responses suggest that Opn-s is an important therapeutic target and provide new insight into its role in immunity.

Activin-A is a pleiotropic cytokine that participates in developmental, inflammatory, and tissue repair processes. Still, its effects on T helper (Th) cell–mediated immunity, critical for allergic and autoimmune diseases, are elusive. We provide evidence that endogenously produced activin-A suppresses antigen-specific Th2 responses and protects against airway hyperresponsiveness and allergic airway disease in mice. Importantly, we reveal that activin-A exerts suppressive function through induction of antigen-specific regulatory T cells that suppress Th2 responses in vitro and upon transfer in vivo. In fact, activin-A also suppresses Th1-driven responses, pointing to a broader immunoregulatory function. Blockade of interleukin 10 and transforming growth factor β1 reverses activin-A–induced suppression. Remarkably, transfer of activin-A–induced antigen-specific regulatory T cells confers protection against allergic airway disease. This beneficial effect is associated with dramatically decreased maturation of draining lymph node dendritic cells. Therapeutic administration of recombinant activin-A during pulmonary allergen challenge suppresses Th2 responses and protects from allergic disease. Finally, we demonstrate that immune cells infiltrating the lungs from individuals with active allergic asthma, and thus nonregulated inflammatory response, exhibit significantly decreased expression of activin-A's responsive elements. Our results uncover activin-A as a novel suppressive factor for Th immunity and a critical controller of allergic airway disease.

Recent studies indicate that early T lymphocyte activation 1 (Eta-1), also known as osteopontin, is a cytokine contributing to the development of Th1 immunity. In the present report, the role of Eta-1 in experimental autoimmune encephalomyelitis (EAE), a disease associated with Th1 immunity, was examined by analysis of disease progression in Eta-1-deficient (Eta-1−/−) mice. Although incidence and onset of peptide-induced EAE were found to be similar in Eta-1−/− and Eta-1+/+ mice, Eta-1−/− mice displayed significantly lower mean maximal clinical score and faster recovery without spontaneous relapses. Accordingly, decreased inflammatory infiltration and demyelination were observed in the spinal cords of Eta-1−/− mice. Furthermore, in comparison to Eta-1+/+, Eta-1−/− CD4+ T cells had reduced expression of IFN-γ and TNF-α upon ex vivo restimulation. Taken together, these results suggest that Eta-1 may sustain autoimmune responses by assisting in maintenance of Th1 immunity during EAE.

Cardiomyocyte death due to desmin deficiency leads to inflammation and subsequent overexpression of a series of remodelling modulators. Among them, OPN seems to be a major regulator of des(-/-) adverse myocardial remodelling and it functions at least by potentiating galectin-3 up-regulation and secretion.

The clinical association between viral infection and onset or exacerbation of autoimmune disorders remains poorly understood. Here, we examine the relative roles of molecular mimicry and nonspecific inflammatory stimuli in progression from infection to autoimmune disease. Murine herpes virus 1 (HSV-1 KOS) infection triggers T cell-dependent autoimmune reactions to corneal tissue. We generated an HSV-1 KOS point mutant containing a single amino acid exchange within the putative mimicry epitope as well as mice expressing a TCR transgene specific for the self-peptide mimic to allow dissection of two pathogenic mechanisms in disease induction. These experiments indicate that viral mimicry is essential for disease induction after low-level viral infection of animals containing limited numbers of autoreactive T cells, while innate immune mechanisms become sufficient to provoke disease in animals containing relatively high numbers of autoreactive T cells.

Recent studies have associated osteopontin (OPN) with allergic inflammation; however, its role in human asthma remains unclear. The aim of this study was to measure OPN levels in the serum, bronchoalveolar lavage fluid (BALF) and bronchial tissue of healthy controls and asthmatics, identify cellular sources of OPN and examine possible correlations between OPN expression, disease severity and airway remodelling.Serum samples were obtained from 35 mild-to-moderate asthmatics, 19 severe asthmatics and 17 healthy controls in the steady state and in cases of exacerbation. Of these subjects, 29 asthmatics and nine controls underwent bronchoscopy with endobronchial biopsy and BALF collection. OPN expression was determined by ELISA and immunohistochemistry/immunofluorescence. Reticular basement membrane thickness and goblet cell hyperplasia were also determined.Serum and BALF OPN levels were significantly increased in all asthmatics in the steady state, whereas serum levels decreased during exacerbations. OPN was upregulated in the bronchial tissue of all patients, and expressed by epithelial, airway and vascular smooth muscle cells, myofibroblasts, T-lymphocytes and mast cells. OPN expression correlated with reticular basement membrane thickness and was more prominent in subepithelial inflammatory cells in severe compared to mild-to-moderate asthma.OPN expression is upregulated in human asthma and associated with remodelling changes, and its subepithelial expression correlates with disease severity.