Transcriptional and immunological analysis of the putative outer membrane protein and vaccine candidate TprL ofTreponema pallidum
Authors: Haynes Austin M. , Fernandez Mark , Romeis Emily , Mitjà Oriol , Konda Kelika A. , Vargas Silver K. , Eguiluz Maria , Caceres Carlos F. , Klausner Jeffrey D. , Giacani Lorenzo 
Published: September 2020

Background

An effective syphilis vaccine should elicit antibodies toTreponema pallidumsubsp.pallidum(T. p. pallidum) surface antigens to induce pathogen clearance through opsonophagocytosis. Although the combination of bioinformatics, structural, and functional analyses ofT. p. pallidumgenes to identify putative outer membrane proteins (OMPs) resulted in a list of potential vaccine candidates, still very little is known about whether and how transcription of these genes is regulated during infection. This knowledge gap is a limitation to vaccine design, as immunity generated to an antigen that can be down-regulated or even silenced at the transcriptional level without affecting virulence would not induce clearance of the pathogen, hence allowing disease progression.

Principal findings

We report here thattp1031, theT. p. pallidumgene encoding the putative OMP and vaccine candidate TprL is differentially expressed in severalT. p. pallidumstrains, suggesting transcriptional regulation. Experimental identification of thetprLtranscriptional start site revealed that a homopolymeric G sequence of varying length resides within thetprLpromoter and that its length affects promoter activity compatible with phase variation. Conversely, in the closely related pathogenT. p.subsp.pertenue, the agent of yaws, where a naturally-occurring deletion has eliminated thetprLpromoter region, elements necessary for protein synthesis, and part of the gene ORF,tprLtranscription level are negligible compared toT. p. pallidumstrains. Accordingly, the humoral response to TprL is absent in yaws-infected laboratory animals and patients compared to syphilis-infected subjects.

Conclusion

The ability ofT. p. pallidumto stochastically varytprLexpression should be considered in any vaccine development effort that includes this antigen. The role of phase variation in contributing toT. p. pallidumantigenic diversity should be further studied.

Author Summary

Syphilis is still an endemic disease in many low- and middle-income countries and has been resurgent in high-income nations for almost two decades now. In endemic areas, syphilis still causes significant morbidity and mortality in patients, particularly when its causative agent, the bacteriumTreponema pallidumsubsp. pallidumis transmitted to the fetus during pregnancy. Although there are significant ongoing efforts to identify an effective syphilis vaccine to bring into clinical trials within the decade in the U.S., such efforts are partially hindered by the lack of knowledge on transcriptional regulation of many genes encoding vaccine candidates. Here, we start addressing this knowledge gap for the putative outer membrane protein (OMP) and vaccine candidates TprL, encoded by thetp1031gene. As we previously reported for other putative OMP-encoding genes of the syphilis agent,tprLtranscription level appears to be affected by the length of a homopolymeric sequence of guanosines (Gs) located within the gene promoter. This is a mechanism known as phase variation and often involved in altering the surface antigenic profile of a bacterial pathogen to facilitate immune evasion and/or adaptation to the host milieu.