Background Chlamydia trachomatis (CT) is an obligate intracellular parasite which causes STD and trachoma. Despite major research into Chlamydial pathogenesis and host immune responses, immuno-protection has been hampered by the incomplete understanding of protective immunity in the genital tract. Characterised vaccine candidates in general have shown variable efficacy ranging from no protection to partial protection. It is therefore a research priority to identify novel Chlamydial antigens that may elicit protective immune responses.

Objectives The goal of the present study was to assess the seroprevalence to pkn1and DNA j following natural CT infection in human. The prospects of pkn1 as a Type third secretion substrate and DNA j a non surface Chlamydial protein as potential antigen, prompted us to explore the immunogenic potential of both protein.

Methods pkn, DNA j and ompA were cloned in bacterial expression vector pTrcHis. Ni+-NTA affinity chromatography was used to purify the recombinant proteins. Antigenic stretches of Pkn1, DNA j and OmpA were identified using Bcepred web server, designed for identification of subunit vaccine candidate by Bioinformatics Centre of IMTECH Chandigarh, India. To validate the bioinformatics based analysis, sera of human patient were used to determine seroreactivity of pkn1 and DNA j proteins. OmpA was used as a positive control during the study.

Results Present study showed a high seroprevalence of antibodies against Pkn1 and OmpA (p<0.001) in sera of humans infected with CT. while, no antibodies were observed for DNA j. Our studies have shown an association between release of TNF-α and IFN-y levels upon stimulation of PBMC with Pkn1 and OmpA. Cytokine expression profiling (IL-1ß, TNF-α, IL-2 , IL10 and IFN-y) of Human PBMCs in response to Pkn1 stimulation demonstrate for the first time that Pkn1 is a novel immunodominant Chlamydial antigen that is capable of influencing both Th1 and Th2 immune responses by stimulating the release of both Humoural and Cell-mediated regulatory cytokines.

Conclusions Our study demonstrated strong serological responses to Pkn1 and major outer membrane in natural human infection suggesting the role of pkn1 in immune response. Studies are in progress to check how the immunomodulation by Pkn1 alters host-pathogen interactions.