**5. CD8 T cells**

CD8 T cells are associated with MHC I. MHC I is expressed in all nucleated cells. Cytosolic proteins, which may originate from intracellular pathogens, are degraded by the proteasome. The degradation product peptides are loaded into the binding groove of MHCI at the end of the process involving TAP and a chaperone protein, tapasin. The MHC I-peptide complex is then exported to the surface of the cell [72]. TCRs on CD8 T cells recognize the endogenous antigen presented on MHC I. The results of this recognition are the expression of various effector cytokines, including IFN-γ, and the release of cytotoxic granzyme and perforin molecules that can lead to target cell death [97]. Because they can kill infected cells, CD8 T cells are thought to play an important role in the immune response to intracellular pathogens.

The role of CD8 T cells in chlamydial infections is controversial. While a broader CD8 T cell response was expected against Chlamydia, an intracellular pathogen, it was determined that the CD8 T cell response against *C. muridarum* in the genital tract was much lower than CD4 cells in mouse experiments. Despite their small number, CD8 T cells are known to migrate to the site of infection, and both human and mouse CD8 T cells have been shown to destroy Chlamydia-infected cells [98]. It has been shown that CD8 T cells recognizing trachomatis proteins class I accessible protein-1 (Cap1) and cysteine-rich protein A (CrpA) can kill target cells in an antigen-dependent manner

#### *Immune Response to Chlamydia DOI: http://dx.doi.org/10.5772/intechopen.110799*

[99, 100]. However, there is evidence to suggest that CD8 T cells are not essential for clearance of Chlamydia. It has been observed in past studies that CD8−/− and perforin-deficient mice clear the infection at the same rate as wild-type mice [82, 84]. Murthy and colleagues have also shown in a more recent study that CD8 T knockout mice exhibited similar clearances of C. muridarum as wild-type mice following vaginal exposure. In the same study, less hydrosalpinx formation in CD8 T knockout mice is remarkable in terms of the relationship between pathogenesis and CD8 T [101]. It has been suggested that they are primarily responsible for the immunopathology associated with chlamydial Infection [102]. The association of CD8 T cells with pathogenesis has also been reported in a macaque model [103]. Although CD8 T cells are not critical for *C. trachomatis* elimination and may even cause chlamydial sequelae, antigen-specific CD8 T cell clones can localize to the genital tract and contribute to clearance of infection through IFN-γ production [98].

During *C. pneumoniae* infection, it is observed that the infection progresses rapidly in the absence of CD8 T cells. Unlike *C. trachomatis* infection, CD8 T cells have been suggested to play a very important role in protection against *C. pneumonia* [104].
