Immune evasion by Treponema pallidum subspecies pallidum (T. pallidum) has been attributed to antigenic variation of its putative outer-membrane protein TprK. In TprK, amino acid diversity is confined to seven variable (V) regions, and generation of sequence diversity within the V regions occurs via a non-reciprocal segmental gene conversion mechanism where donor cassettes recombine into the tprK expression site. Although previous studies have shown the significant role of immune selection in driving accumulation of TprK variants, the contribution of baseline gene conversion activity to variant diversity is less clear. Here, combining longitudinal tprK deep sequencing of near clonal Chicago C from immunocompetent and immunosuppressed rabbits along with the newly developed in vitro cultivation system for T. pallidum, we directly characterized TprK alleles in the presence and absence of immune selection. Our data confirm significantly greater sequence diversity over time within the V6 region during syphilis infection in immunocompetent rabbits compared to immunosuppressed rabbits, consistent with previous studies on the role of TprK in evasion of the host immune response. Compared to strains grown in immunocompetent rabbits, strains passaged in vitro displayed low level changes in allele frequencies of TprK variable region sequences similar to that of strains passaged in immunosuppressed rabbits. Notably, we found significantly increased rates of V6 allele generation relative to other variable regions in in vitro cultivated T, pallidum strains, illustrating that the diversity within these hypervariable regions occurs in the complete absence of immune selection. Together, our results demonstrate antigenic variation in T. pallidum can be studied in vitro and occurs even in the complete absence of immune pressure, allowing the T. pallidum population to continuously evade the immune system of the infected host.

中文翻译：

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体内和体外传播的梅毒螺旋体亚种的纵向 TprK 分析。苍白球揭示了在没有免疫压力的情况下抗原变体的积累。

梅毒螺旋体亚种梅毒 (T. pallidum) 的免疫逃避归因于其假定的外膜蛋白 TprK 的抗原变异。在 TprK 中，氨基酸多样性被限制在七个可变 (V) 区域，并且 V 区域内序列多样性的产生是通过非互惠的片段基因转换机制发生的，在该机制中，供体盒重组到 tprK 表达位点。尽管之前的研究表明免疫选择在推动 TprK 变体积累中的重要作用，但基线基因转换活动对变体多样性的贡献尚不清楚。在这里，将来自免疫活性和免疫抑制兔的近克隆 Chicago C 的纵向 tprK 深度测序与新开发的梅毒螺旋体体外培养系统相结合，我们在存在和不存在免疫选择的情况下直接表征了 TprK 等位基因。我们的数据证实，与免疫抑制兔相比，免疫活性兔在梅毒感染期间 V6 区域内的序列多样性随时间显着增加，这与之前关于 TprK 在逃避宿主免疫反应中的作用的研究一致。与在免疫活性兔中生长的菌株相比，体外传代的菌株在 TprK 可变区序列的等位基因频率方面表现出低水平变化，类似于在免疫抑制兔中传代的菌株。值得注意的是，我们发现在体外培养的苍白球 T 株中，相对于其他可变区，V6 等位基因生成率显着增加，说明这些高变区内的多样性发生在完全没有免疫选择的情况下。