Chlamydiae may induce host cell apoptosis either directly or via induction of the host inflammatory response. Chlamydial induction of apoptosis generally occurs late in the developmental cycle [Schoier et al., 2001] and may be important for achieving host cell rupture and the release of infectious chlamydial elementary bodies. Stenner-Liewen and colleagues described a C. trachomatis protein that interacted with mammalian death receptors to induce apoptosis following gene transfection  [Stenner-Liewen et al., 2002].  The protein had homologues in other Chlamydiaceae and was termed the CADD protein [ "Chlamydia protein Associating with Death Domains"]. The protein interacted with Death Domains of the TNF family of receptors, ie with  TNFR1, Fas, DR4 and DR5, but not with the respective downstream adaptors. These CADD effects were blocked by caspase inhibitors. The CADD was expressed late in the C. trachomatis developmental  cycle, co-localizing with Fas in the region of the inclusion. It was suggested that CADD might modulate apoptosis pathways in infected cells. 

Perfettini et al., 2002 described a different mechanism of chlamydial-induced apoptosis which was independent of caspase. The mechanism involved over expression of the proapoptotic Bcl-2 family member, Bax, which was activated in C. psittaci-infected cells and translocated from the cytosol to the mitochondria. Bcl-2 itself blocked both caspase-dependent and caspase-independent apoptosis. Bax and mitochondria are ideally located to sense stress-related metabolic changes emanating from the interior of an infected cell. C. muridarum induces a much higher level of apoptosis 24 hours after infection in normal (Bax +/+) compared with Bax deficient (Bax -/-) cells. In Bax deficient  mice, C. muridarum infection was less efficient and of less duration compared with normal mice and was associated with much higher levels of interferon gamma and other pro-inflammatory cytokines plus increased granulomas 3 weeks post infection and increased cell death by necrosis. The Bax activation precursor, Bid, was not involved in apoptosis during chlamydial infection, suggesting that Bax activation is initiated in the host cell cytoplasm, rather than via the Fas or TNRF1 death receptors. Activation might be due to infection-related stress responses or due to chlamydial proteins injected into the cytoplasm via type 3 secretion mechanisms [Ojcius et al., 2002]. In either case, Bax-dependent induction of apoptosis in the intact animal was advantageous to the chlamydiae.  Intravaginal infection with C. muridarum is cleared more quickly from Bax -/- knock out mice than from wild type mice  and this is associated with enhanced secretion of proinflammatory cytokines and more severe pathology. It was suggested that chlamydia-induced apoptosis via Bax contributes to chlamydial propagation and decreases inflammation [Perfettini et al., 2003].

Nitric oxide and activated oxygen radicals may also play a role in apoptosis induced by C. trachomatis [Freidank et al., 2002]. 

Jendro et al., 2003 found that C. trachomatis infected macrophages induce T cell apoptosis, a mechanism which may help perpetuate chronic chlamydial infection. Induction of apoptosis involved both direct cell to cell contact and indirect humoral mechanisms mediated by tumour necrosis factor alpha.

DNA micro array studies of C. trachomatis-infected HeLa cells or chondrocytes found that 18 of 1,176 cellular genes were up regulated after 24 hours of infection, including the glycoprotein 130 family members IL-11 and LIF, the chemokine gene MIP2-alpha, various transcription factor genes,  adhesion molecule genes such as ICAM1 and, of relevance here, the apoptosis-related genes IEX-1L and MCL-1 [Hess et al., 2001]. These effects were independent of chlamydial lipopolysaccharide. Xia et al., 2003  prepared complementary DNA (cDNA) probes from the messenger RNA of HeLa cells infected with C. trachomatis. The mRNA produced was identified by hybridisation to a high-density human DNA microarray of 15,000 genes / expressed sequence tags. C. trachomatis altered host cell transcription at both the early and middle phases of its developmental cycle. Altered transcripts included factors inhibiting apoptosis as well as factors regulating cell differentiation, the cytoskeleton, transcription, and proinflammatory cytokines. This further demonstrates the ability of chlamydiae to modulate gene transcription including apoptosis [See: 14-3-3 mediated transduction].

Immune response and cytokine-determined apoptosis probably mainly occurs via caspase and can be blocked by chlamydiae [Wahl et al., 2001].  The ability of C. pneumoniae to block apoptosis in THP-1 cells is dependent on protein synthesis [Carratelli et al., 2002]. Specifically, Wahl et al 2001 demonstrated that C. pneumoniae-induced inhibition of apoptosis in the human monocytic cell line Mono Mac 6 is due to the expression of the inhibitor of apoptosis 2 (c -IAP2) family of proteins via  NF-κB/REL transcription factors. 

Inhibition of host cell apoptosis might lead to persistent chlamydial infection. Dean & Powers, 2001 compared apoptotic responses among HeLa 229 cells acutely and persistently infected or mock infected with C. trachomatis serovar A/HAR-13. Persistent infection was induced by gamma interferon treatment. Cells were treated with the apoptosis inducers etoposide or staurosporine at 24-h intervals, then assayed for apoptosis. From the 24- to 120- hour time points, C. trachomatis - infected cultures were 87 and 90% viable for etoposide and staurosporine treatment, respectively, produced no DNA ladder characteristic of apoptosis, and there was no release of cytochrome c from mitochondria. In contrast, mock-infected cells were 22 and 37% viable for etoposide (P = 0.0001) and staurosporine (P = 0.01), respectively, and displayed the features of apoptosis. Resistance to apoptotic stimuli was identical for both acute and persistent infections but expression of hsp60 was upregulated in persistent infection. It was suggested that chlamydial disregulation of apoptosis and the ensuing persistence of organisms offer an alternative pathogenic mechanism for chlamydial scarring characteristic of  trachoma and infertility.

Interferon gamma concentrations that induce the formation of aberrant, persistent chlamydiae inhibit apoptosis caused by C. muridarum infection. Apoptosis was inhibited preferentially in cells which contained the aberrant chlamydiae. This inhibitory effect was related to the ability of interferon gamma to deplete host cells of tryptophan, an essential amino acid for chlamydiae, as the inhibition of apoptosis could be partially reversed by exogenous tryptophan. Apoptotic cells were observed in the genital tracts of wild-type mice infected with C. muridarum, and a significantly larger number of apoptotic cells was detected in infected interferon gamma-deficient mice. Thus interferon gamma may contribute to the pathogenesis of persistent chlamydial infections in vivo by preventing the apoptosis of infected cells [Perfettini et al., 2002b].

Roblin et al., 2002 found that spontaneous apoptosis occurred 3 to 4 days post infection in 50 - 80% of C. pneumoniae - infected A-549 alveolar carcinoma cells compared to only 5 - 14% of uninfected cells. Spontaneous apoptosis was seen in only 1 - 5% of A-549 cells infected over a long period with C. pneumoniae. These chronically infected cells also showed less sensitivity to sorbitol-induced apoptosis. These preliminary data suggested that chronic infection with C. pneumoniae, at least in this model, is associated with a substantial inhibition of apoptosis.

Neureiter et al., 2003 reported a slightly increased rate of apoptosis in C. pneumoniae (but not H. pylori) infected carotid atherosclerotic plaque, presumably because of T cell infiltration.

For methodology in studying chlamydial apoptosis see Perfettini et al., 2002.

[MEW comment: There are both pro and anti apoptotic stimuli and it is sometimes difficult to distinguish the wheat from the chaff. In vitro, the choice of experimental system may explain some of the observed differences.  To study the overall importance of apoptosis there is a need for more studies  at whole animal level of the kind performed by Perfettini et al].

See also: Chlamydial peturbation of cell mediated immune responses.

See also: Role of N-cadherin

[MEW] August 2003

NEXT: Chlamydial heat shock proteins in immunopathology.

Gao, L. Y. & Kwaik, Y. A. (2000). The modulation of host cell apoptosis by intracellular bacterial pathogens. Trends in Microbiology 8, 306 - 313. [Review].

Neureiter, D., Heuschmann, P., Stintzing, S., Kolominsky-Rabas, P., Barbera, L., Jung, A., Ocker, M., Maass, M., Faller, G. & Kirchner, T. (2003). Detection of Chlamydia pneumoniae but not of Helicobacter pylori in symptomatic atherosclerotic carotids associated with enhanced serum antibodies, inflammation and apoptosis rate. Atherosclerosis 168, 153 - 162.