Evolution of antigenic diversity in the tick-transmitted bacterium Borrelia afzelii: a role for host specialization?

Antigenic diversity in pathogenic microbes can be a result of at least three different processes: diversifying selection by acquired immunity, host-pathogen coevolution, and/or host specialization. Here, we investigate if host specialization drives diversity at ospC (which encodes an immunodominant surface protein) in the tick-transmitted bacterium Borrelia afzelii. We determined prevalence and infection intensity of ospC strains in naturally infected wild mammals (rodents and shrews) by 454 amplicon sequencing in combination with qPCR. Neither prevalence nor infection intensity of specific ospC strains varied in a species-specific manner (i.e. there were no significant ospC × host species interactions). Rankings of ospC prevalences were strongly positively correlated across host species. Rankings of ospC infection intensities were correlated more weakly, but only in one case significantly less than 1. ospC prevalences in the studied mammals were similar to those in ticks sampled at the study site, indicating that we did not miss any mammal species that are important hosts for specific ospC strains. Based on this we conclude that there is at best limited host specialization in B. afzelii, and that other processes are likely the main drivers of ospC diversity.

病原微生物的抗原多样性（antigenic diversity）可由至少三种不同过程促成：获得性免疫（acquired immunity）介导的多样化选择、宿主-病原体共进化（host-pathogen coevolution），以及宿主特化（host specialization）。本研究旨在探究宿主特化是否会驱动蜱传病原菌阿氏疏螺旋体（Borrelia afzelii）中编码免疫显性表面蛋白的ospC基因的多样性。我们通过454扩增子测序结合定量聚合酶链反应（qPCR），测定了自然感染的野生哺乳动物（啮齿类与鼩鼱类）体内ospC菌株的感染率与感染强度。特定ospC菌株的感染率与感染强度均未呈现物种特异性差异（即未检测到显著的ospC×宿主物种互作）。不同宿主物种间，ospC菌株感染率的排序呈显著正相关；而ospC菌株感染强度的排序相关性较弱，且仅在一例中其相关系数显著小于1。本研究涉及的哺乳动物体内的ospC感染率，与研究地点采集的蜱体内的ospC感染率相近，这表明我们未遗漏对特定ospC菌株而言属于关键宿主的哺乳动物物种。基于上述结果，我们得出结论：阿氏疏螺旋体的宿主特化程度至多有限，而其他过程更可能是ospC基因多样性的主要驱动因素。