CHAPTER 2 LITERATURE REVIEW
2.6 Interaction with the immune system
synonymous with the protrusions described by Hybiske and Stevens (2007). It is possible that different methods of release occur in vitro depending on the cell type, strain and culture conditions. Hybiske and Stephens (2007) report the occurrence of extrusion and lysis in cell monolayers infected with an OG strain and an LGV strain, while Todd and Caldwell (1985) report lysis as the only method of EB release with the LGV biovar.
These two biovars also show differences in pathogenicity in terms of the surface from which they are released. While the OG strains are released from the apical surface of their host cell, LGV strains may also be released from the basolateral surface of polarised cells (Wyrick et al 1989). This would enable the LGV strains to penetrate the epithelium to infect the underlying tissue and ultimately migrate to the lymph nodes.
2.5.4 Effect on neighbouring uninfected cells
The effect of C. trachomatis infection on neighbouring uninfected cells has not been widely investigated, however the organism does, either directly or indirectly, result in a pathogenic effect on uninfected neighbouring cells. Schöier et al (2001) and Greene et al (2004) demonstrated the presence of apoptotic uninfected cells in monolayers infected with C. trachomatis. This effect may be mediated indirectly since infected epithelial cells have been shown to secrete proinflammatory cytokines (Rasmussen et al 1997). The fact that extra-inclusion vesicles have been detected in the cytoplasm of infected cells (Giles et al 2006) indicates the possibility that chlamydia may affect neighbouring uninfected cells via a chlamydial molecule which is secreted by the host cell.
Chlamydia attempt to hide from the immune system, but both innate and adaptive immune responses are mounted. While the immune system identifies and eliminates pathogens, in the case of C. trachomatis the immune response contributes to tissue pathology and severity of disease (Roan and Starnbach 2008).
2.6.1 Innate immunity
Although they are not professional antigen presenting cells, epithelial cells of the genital tract possess features of innate immunity (Quale 2002). These cells can recognise antigens via the presence of MHC class I and II, mediate bacterial and viral killing, and respond to and secrete chemokines to recruit leukocytes (Quale 2002).
Chlamydia infection can induce the production of a range of proinflammatory cytokines, including interleukin-6 (IL-6), IL-8 and granulocyte-macrophage colony-stimulating factor (GM-CSF) (Rasmussen et al 1997), as well as IL-1 and tumour necrosis factor-α (TNF-α) (Dessus-Babus et al 2002). IL-8 recruits neutrophils to the site of infection (Taub et al, 1996) and these recruited cells secrete more cytokines such as TNF-α (Dessus-Babus et al 2002). Although TNF-α can restrict the growth of C. trachomatis, elevated levels of TNF- α and IL-6 may cause increased pathology (Darville et al 2003). NK cells migrate to the site of infection quickly where they lyse infected cells and produce interferon-γ (INF-γ) (Tseng and Rank 1998).
INF-γ has been shown to inhibit replication of all C. trachomatis serovars in vitro, but some serovars are more susceptible to INF-γ-induced effects than others (Morrison 2000).
INF-γ restricts chlamydial growth in the following ways:
• Up regulation of the IFN-γ-inducible nitric oxide (NO) synthase pathway (Igietseme et al 1997). NO has been shown to inhibit the intraepithelial replication of C. trachomatis serovars E, H and L2 (Igietseme et al 1997).
• Induction of indoleamine 2,3-dioxygenase (IDO) which catabolises tryptophan (Taylor and Feng 1991). A reduction in the intracellular tryptophan pools restricts chlamydia replication (Byrne et al 1986) and may induce a state of persistent infection (Beatty et al 1994a). Genital isolates of C. trachomatis are resistant to this method of INF-γ restriction due to functional tryptophan synthase enabling these strains to synthesise tryptophan from indole unlike the ocular trachoma strains (Caldwell et al 2003).
• Inhibition of sphingomyelin acquisition (serovar L2) (Nelson et al, 2005)
• Down regulation of intracellular ferritin, the protein responsible for iron storage by cells (Byrd and Horwitz, 1993). This may restrict the growth of C. trachomatis due to iron starvation (Freidank et al, 2001)
• Increase the phagocytic activity of macrophages by up regulation of MHC class II expression (Zhong and de la Maza, 1988)
The link between the innate and adaptive immune response are dendritic cells (DC) which process and present chlamydial antigens to T cells (Steele et al 2004).
2.6.2 Adaptive Immunity
A B cell response is mounted against chlamydia. Although it doesn’t prevent subsequent infection, B cell deficient mice are slightly more susceptible to subsequent infection than
wild type mice (Su et al 1997, Williams et al 1997). Although antibodies can neutralize extracellular organisms, both CD4+ and CD8+ T cells are required for eliminating intracellular chlamydia (Starnbach et al 1994, Su and Caldwell 1995, Roan and Starnbach 2006). The CD4+ T cells recognize chlamydial antigen bound to MHC class II on the surface of antigen presenting cells (Roan and Starnbach 2006), while the CD8+ T cells recognise chlamydial antigen bound to MHC class I molecules presented on the surface of the infected cells (Cresswell et al 2005).
C. trachomatis has been shown to interfere with antigen presentation. Serovar L2 was shown to down regulate INF-γ-inducible MHC class II by degrading upstream stimulatory factor (USF)-1 (Zhong et al 1999). This in turn prevents the induction of class II transactivator (CIITA) which is required for expression of INF-γ-induced MHC class II molecules (Zhong et al 1999). The same strain of C. trachomatis LGV biovar has been shown to suppress both the constitutive and INF-γ-induced expression of MHC class I molecules on infected cells (Zhong et al 2000). This is mediated by the degradation of RFX5 by chlamydial proteasome-like activity in the host cell cytoplasm (Zhong et al 2001). Decreased levels of antigen presentation of MHC class I and II molecules would limit the ability of CD8+ and CD4+ T cells to recognise infected cells and professional antigen presenting cells respectively (Roan and Starnbach 2008). This may also increase the likelihood of NK cell mediated lysis which has been shown in vitro (Hook et al, 2004).
It is possible the MHC class I molecule down regulation may also be mediated by deubiquitinases (DUB) encoded by the C. trachomatis genome (Misaghi et al. 2006). If transcribed these proteins could target host cell ubiquitin thereby interfering with antigen presentation (Roan and Starnbach 2008).
CD4+ T cells exist as two clones, T helper type (Th) 1 and Th2. The Th1 cells produce INF-γ which assists in the clearance of chlamydia (Perry et al 1997). Th2 cells enhance antibody production but do not aid clearance of the organism (Perry et al 1997, Hawkins et al 2002). When the Th1 response is insufficient and the organism is not cleared, sustained high levels of inflammatory cytokines can cause tissue damage (Perfettini et al, 2003a).
CD4+ T cells also play a role in the activation of B cells and CD8+ T cells (Roan and Starnbach 2008).
Like CD4+ T cells, CD8+ T cells can produce INF-γ to mediate intra-cellular kill of the organism. It is only these chlamydia-specific CD8+ T cells capable of producing INF-γ which have this effect (Roan et al., 2006). CD8+ T cells also recognize chlamydia-infected cells via presentation of chlamydial antigens on MHC class I molecules and kill these cells (Starnbach et al., 1994). Theoretically this should aid in removal of the organism, but this has not been shown in vivo (Roan and Starnbach 2008). This may be due to the presence of small numbers infectious EB relatively early in infection. Lysis of the host cell would enable EBs to enter a new host and continue its replication cycle. Phagocytosis of EB by antigen presenting cells does not guarantee death of the organism. Opsonised C. trachomatis EB which enter host cells by Fc-mediated endocytosis have been shown to escape lysosomal death and undergo a normal replication cycle (Scidmore et al. 1996).
Activation and proliferation of CD4+ and CD8+ T cells occurs in the nearby lymph nodes which drain antigen from the genital mucosa. These cells develop their INF-γ producing capacity prior to migration to the site of infection where they aid in chlamydial clearance (Roan and Starnbach 2006; Roan et al., 2006).
The persistent dormant form of the organism may contribute to disease. Reversible persistence can be induced by INF-γ (Beatty et al, 1993). The organism remains dormant until this cytokine is removed, then reactivates and stimulates inflammation and immune cells. Alternating cycles of the persistent dormant form followed by an inflammatory immune response may result in chronic chlamydia-induced disease and tissue damage (Roan and Starnbach 2008).