Ehrlichia chaffeensis is an obligate intracellular bacterium that invades monocytes to cause the emerging and potentially severe disease, monocytic ehrlichiosis. Ehrlichial invasion of host cells, a process that is essential for the bacterium's survival and pathogenesis, is incompletely understood. In this study, we identified ECH_0377, henceforth designated as EplA (E. chaffeensis PDI ligand A) as an E. chaffeensis adhesin that interacts with host cell protein disulfide isomerase (PDI) to mediate bacterial entry into host cells. EplA is an outer membrane protein that E. chaffeensis expresses during growth in THP-1 monocytic cells. Canine sera confirmed to be positive for exposure to Ehrlichia spp. recognized recombinant EplA, indicating that it is expressed during infection in vivo. EplA antiserum inhibited the bacterium's ability to infect monocytic cells. The EplA-PDI interaction was confirmed via co-immunoprecipitation. Treating host cell surfaces with antibodies that inhibit PDI and/or thioredoxin-1 thiol reductase activity impaired E. chaffeensis infection. Chemical reduction of host cell surfaces, but not bacterial surfaces with tris(2-carboxyethyl)phosphine (TCEP) restored ehrlichial infectivity in the presence of the PDI-neutralizing antibody. Antisera specific for EplA C-terminal residues 95-104 (EplA₉₅₋₁₀₄) or outer membrane protein A amino acids 53-68 (OmpA₅₃₋₆₈) reduced E. chaffeensis infection of THP-1 cells. Notably, TCEP rescued ehrlichial infectivity of bacteria that had been treated with anti-EplA₉₅₋₁₀₄, but not anti-EcOmpA₅₃₋₆₈. These results demonstrate that EplA contributes to E. chaffeensis infection of monocytic cells by engaging PDI and exploiting the enzyme's reduction of host cell surface disulfide bonds in an EplA C-terminus-dependent manner and identify EplA₉₅₋₁₀₄ and EcOmpA₅₃₋₆₈ as novel ehrlichial receptor binding domains.

恰菲埃里希体是一种专性细胞内细菌，它侵入单核细胞，导致新出现的潜在严重疾病——单核细胞埃立克体病。埃里希体对宿主细胞的入侵是一种对细菌生存和发病机制至关重要的过程，目前尚不完全清楚。在这项研究中，我们确定了 ECH_0377，以下将其命名为 EplA (E. chaffeensisPDI 配体 A) 作为E. chaffeensis与宿主细胞蛋白二硫键异构酶 (PDI) 相互作用以介导细菌进入宿主细胞的粘附素。EplA 是一种外膜蛋白E. chaffeensis在 THP-1 单核细胞的生长过程中表达。犬血清证实暴露于埃立克体spp. 识别重组 EplA，表明它在感染期间表达体内. EplA 抗血清抑制细菌感染单核细胞的能力。通过免疫共沉淀证实了 EplA-PDI 相互作用。用抑制 PDI 和/或硫氧还蛋白-1 硫醇还原酶活性的抗体处理宿主细胞表面受损E. chaffeensis感染。在存在 PDI 中和抗体的情况下，用三 (2-羧乙基) 膦 (TCEP) 对宿主细胞表面进行化学还原，而不是细菌表面，可以恢复埃里希体的感染性。EplA C-末端残基 95-104 (EplA 95-104 ₎或外膜蛋白 A 氨基酸 53-68 (OmpA _53-68 ) 的特异性抗血清减少E. chaffeensisTHP-1 细胞感染。值得注意的是，TCEP 挽救了用抗 EplA 95-104 处理的细菌的埃里希体感染性_，但不是抗 EcOmpA _53-68。这些结果表明 EplA 有助于E. chaffeensis通过参与 PDI 并利用酶以 EplA C 末端依赖性方式减少宿主细胞表面二硫键来感染单核细胞，并将 EplA _95-104和 EcOmpA _{53-68 鉴定}为新的埃里希体受体结合结构域。