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.