Signal transduction events leading to the survival, differentiation, or apoptosis of cells of the innate or adaptive immune system must be properly coordinated to ensure the normal mounting and termination of immune responses. One of the key transcription factors in immune responses is nuclear factor kappaB (NF-kappaB), which has been the focus of intense investigation over the past two decades. With the identification of the CARMA1-BCL10-MALT1 complex and ongoing progress in understanding the molecular mechanisms connecting T cell and B cell receptor proximal signals to the IkappaB kinase (IKK) complex, a cohesive model of antigen receptor (AgR)-dependent signaling to NF-kappaB activation is beginning to emerge. In this review, we provide an overview of the current state of research into the mechanisms that regulate AgR-mediated NF-kappaB transcriptional activity, with particular focus on the events leading to activation of the IKK complex.

Threshold levels of individual NFAT factors appear to be critical for apoptosis induction in effector T cells. In these cells, the short isoform A of NFATc1 is induced to high levels due to the autoregulation of the NFATc1 promoter P1 by NFATs. P1 is located within a CpG island in front of exon 1, represents a DNase I hypersensitive chromatin site, and harbors several sites for binding of inducible transcription factors, including a tandemly arranged NFAT site. A second promoter, P2, before exon 2, is not controlled by NFATs and directs synthesis of the longer NFATc1/B+C isoforms. Contrary to other NFATs, NFATc1/A is unable to promote apoptosis, suggesting that NFATc1/A enhances effector functions without promoting apoptosis of effector T cells.

c Intracellular bacterial pathogens frequently inhibit host cell apoptosis to ensure survival of their host, thereby allowing bacterial propagation. The obligate intracellular pathogen Coxiella burnetii displays antiapoptotic activity which depends on a functional type IV secretion system (T4SS). Accordingly, antiapoptotic T4SS effector proteins, like AnkG, have been identified. AnkG inhibits pathogen-induced apoptosis, possibly by binding to the host cell mitochondrial protein p32 (gC1qR). However, the molecular mechanism of AnkG activity remains unknown. Here, we demonstrate that ectopically expressed AnkG associates with mitochondria and traffics into the nucleus after apoptosis induction, although AnkG lacks a predicted nuclear localization signal. We identified the p32 interaction region in AnkG and constructed an AnkG mutant (AnkG R22/23S ) unable to bind to p32. By using this mutant, we found that intracellular localization and trafficking of AnkG into the nucleus are dependent on binding to p32. Furthermore, we demonstrated that nuclear localization of AnkG but not binding to p32 is required for apoptosis inhibition. Thus, the antiapoptotic activity of AnkG is controlled by p32-mediated intracellular trafficking, which, in turn, seems to be regulated by host cell processes that sense stress.

Rhodococcus equi is a gram-positive intracellular pathogen that can cause severe bronchopneumonia in foals and AIDS patients. It has been reported that advanced infection of foals is characterized by tissue necrosis, coinciding with the presence of degenerate bacteria-laden macrophages. Here, we report that the possession of the VapA-expressing plasmid, which has been previously correlated with a high level of virulence for foals and mice, strongly increases cytotoxicity of R. equi for murine macrophage-like (J774E) cells. Isolates containing different, VapB-expressing plasmids are less virulent and also have a lower cytotoxic potential. Isogenic strains lacking either plasmid are avirulent and have a very low cytotoxic potential. We show, using fluorescenceactivated cell sorter analysis (annexin V/7-amino-actinomycin D and sub-G 1 -analysis), Western blotting [poly-(ADP-ribose) polymerase processing analysis], and electron microscopy (macrophage and nucleus morphologies) that the deaths of murine macrophages are the result of necrotic rather than apoptotic events. We demonstrate that the bacteria must be alive in order to act cytotoxic. Therefore, one effect of the virulenceassociated plasmids during infection with R. equi is the promotion of necrotic damage to the host.Rhodococcus equi is a nocardioform gram-positive coccobacillus and an important foal pathogen producing severe pyogranulomatous pneumonia in very young horses (24, 38). Infection usually occurs via the respiratory tract. In addition to being a foal pathogen, R. equi can infect AIDS patients, and more than 100 cases in which patients showed symptoms and histopathology similar to those seen in infected foals and leading to death in ϳ50% of the individuals have been documented (8,29,59). Furthermore, a low number of cases that occurred in immunocompetent humans with an ϳ11% mortality rate have been reported (29). In addition to their own importance in human and veterinary medicine, these bacteria have a close phylogenetic relationship to mycobacteria and, hence, to such important pathogens as Mycobacterium tuberculosis (causing human tuberculosis) and Mycobacterium leprae (causing leprosy). Members of both genera, Mycobacterium and Rhodococcus, are largely soil inhabitants and possess complex wax-like cell walls (4) enriched in lipoarabinomannans (41), in mycolic acids, and in mycolic-acid derived compounds such as cord factor (trehalose dimycolate). Also, both M. tuberculosis and R. equi can produce caseous granulomas and cavitary pneumonia, demonstrating not only phylogenetic but also pathogenetic relatedness. Not surprisingly, some human infections with rhodococci have been mistaken for tuberculosis (55).Due to its capability to survive and multiply in murine and equine macrophages, R. equi has been classified as a facultative intracellular bacterium (25). Accordingly, R. equi is found frequently in macrophages in vivo (24), although it does not seem to enter pulmonary epithelial cells. It has been reported that R. equi interferes with the matur...