Transcriptional analysis of the tpr genes in Treponema pallidum subsp. pallidum (referred to here as simply T. pallidum) has been limited to date, and yet the expression of members of this gene family is likely relevant to the pathogenesis of syphilis. Recently, immunological studies and semiquantitative mRNA analysis led to the hypothesis of the modulation of tpr gene transcription during infection and suggested that various strains of T. pallidum might differentially express these genes. In this study we developed a real-time amplification assay to quantify the tpr mRNAs with respect to the 47-kDa lipoprotein message and to compare transcript levels among four different strains of T. pallidum. In addition, we analyzed the lymphocyte responsiveness pattern toward the Tpr antigens in late experimental syphilis to identify tpr genes that had been expressed during the course of infection. The T-cell response has been implicated in clearance of treponemes from early lesions, and some of the Tprs were identified as strong targets of the cellular immune response. We show that message for many of the tpr genes can be detected in treponemes harvested at the peak of early infection. Interestingly, tprK seems to be preferentially expressed in almost every strain, and it is uniformly the target of the strongest cellular immune response. These studies demonstrate the differential expression of certain tpr genes among strains of T. pallidum, and further studies are needed to explore the relationship between tpr gene expression and the clinical course of syphilis in infected individuals. Copyright © 2007, American Society for Microbiology. All Rights Reserved.