Chlamydial protease-like activity factor--insights into immunity and vaccine development

Abstract

Chlamydia trachomatis is a Gram-negative obligate intracellular pathogen that remains the leading cause of bacterial sexually transmitted disease worldwide, despite the availability of efficacious antimicrobial therapy. Given that chlamydial infections cause severe pathological sequelae in the upper genital tract, a licensed vaccine to prevent infection and disease would be an ideal solution. Chlamydial protease-like activity factor (CPAF) is a protein secreted in considerable amounts into the cytosol of infected cells and released into the extracellular milieu upon cellular lysis, which therefore is accessible to the host immune system. This is further substantiated by the observation that CPAF is immunodominant among other antigens in Chlamydia sero-positive humans. The efficacy of vaccination with CPAF against genital chlamydial challenge has been evaluated extensively in the murine model. This review will discuss important insights into the potential of CPAF as a component of an anti-chlamydial vaccine.

Fig. 1

rCPAF+IL-12 vaccination enhances the resolution of genital chlamydial infection. Animals (10 mice/group) were treated with three doses of rCPAF+IL-12, rCPAF, IL-12, or PBS (Mock). One month following the final vaccination, mice were challenged intravaginally with 5 × 10⁴ IFU of C. muridarum. On the days indicated following challenge, chlamydial shedding was measured; the numbers of chlamydial IFU recovered from vaginal swabs were determined. Each symbol represents an individual animal. The number(s) of animals in each group that had resolved the infection are indicated on the X-axis. An asterisk indicates that there is a significant difference between the groups indicated (P < 0.05, as determined by the Kruskall-Wallis test). The results are representative of three independent experiments. (Reproduced with permission from Infection and Immunity; 2007, 75: 666–676).

Fig. 2

Proposed model for the importance of CD4⁺ T cells in rCPAF-mediated anti-chlamydial immunity. (A) The infectious EBs enter the host cell and are contained within the endosome. At this stage, the infection can be prevented by neutralizing antibodies against surface structures on the EB. (B) The infection is at an advanced stage with chlamydial reticulate bodies (RBs) that are actively multiplying and producing chlamydial proteins, including CPAF. At this juncture, most chlamydial proteins are walled-off from the host by the sturdy inclusion membrane, whereas abundant amounts of CPAF localize to the cytosol of the infected cell. The activation of either CD4⁺ or CD8⁺ T cells is minimal since CPAF actively downregulates of MHC I and II expression. (C) The rupture of the infected cell releases the intracellular contents into the extracellular matrix. At this stage, antibodies that bind to the surface of EB can neutralize infectivity, whereas others (i.e. anti-CPAF antibodies) cannot. (D) The extruded antigens are taken up by neighboring host cells in endosomes. Apart from the infectious EB, various chlamydial proteins (such as CPAF), RBs, and inclusion membrane proteins also are endocytosed. Antibody-bound antigens may be taken up more efficiently via Fc-receptor mediated processes at this stage. Due to the abundant amounts of CPAF, compared to other chlamydial antigens, relatively more molecules are taken up into the endosomes. (E) While the infecting EB progresses through the developmental cycle, the endocytosed proteins are processed and presented via the exogenous MHC II pathway to CD4⁺ T cells; due to the greater uptake, more CPAF epitopes are processed and presented. Although MOMP is expressed abundantly on surface of chlamydial EBs and RBs, only MOMP on the RBs or dead EBs, but not on live EBs, have the opportunity to be processed and presented by MHC II pathways. The surface presentation of MHC II molecules loaded with these chlamydial antigens occurs before CPAF-mediated MHC down-regulation begins within this cell. The activated CD4⁺ T cells secrete IFNγ which aborts the chlamydial developmental cycle, leading to (F) elimination of infection. Antigen presentation to CD4⁺ T cells may happen more efficiently in macrophages and dendritic cells, but the secreted IFNγ can act in a paracrine fashion to eliminate infection from epithelial cells. This model explains the predominant role of CD4⁺ T cells, followed by antibody and then CD8⁺ T cells, in rCPAF vaccination-induced immunity against genital chlamydial infection.