AUTHOR=Green Ryan S. , Izac Jerilyn R. , Naimi Waheeda A. , O'Bier Nathaniel , Breitschwerdt Edward B. , Marconi Richard T. , Carlyon Jason A. 

TITLE=Ehrlichia chaffeensis EplA Interaction With Host Cell Protein Disulfide Isomerase Promotes Infection

JOURNAL=Frontiers in Cellular and Infection Microbiology

VOLUME=10

YEAR=2020

URL=https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2020.00500

DOI=10.3389/fcimb.2020.00500

ISSN=2235-2988

ABSTRACT=<p><italic>Ehrlichia chaffeensis</italic> 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 (<italic>E. chaffeensis</italic> PDI ligand A) as an <italic>E. chaffeensis</italic> adhesin that interacts with host cell protein disulfide isomerase (PDI) to mediate bacterial entry into host cells. EplA is an outer membrane protein that <italic>E. chaffeensis</italic> expresses during growth in THP-1 monocytic cells. Canine sera confirmed to be positive for exposure to <italic>Ehrlichia</italic> spp. recognized recombinant EplA, indicating that it is expressed during infection <italic>in vivo</italic>. 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 <italic>E. chaffeensis</italic> 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 (EplA<sub>95−104</sub>) or outer membrane protein A amino acids 53-68 (OmpA<sub>53−68</sub>) reduced <italic>E. chaffeensis</italic> infection of THP-1 cells. Notably, TCEP rescued ehrlichial infectivity of bacteria that had been treated with anti-EplA<sub>95−104</sub>, but not anti-EcOmpA<sub>53−68</sub>. These results demonstrate that EplA contributes to <italic>E. chaffeensis</italic> 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 EplA<sub>95−104</sub> and EcOmpA<sub>53−68</sub> as novel ehrlichial receptor binding domains.</p>