Anaplasma marginale Outer Membrane Protein A Is an Adhesin That Recognizes Sialylated and Fucosylated Glycans and Functionally Depends on an Essential Binding Domain

Abstract

Anaplasma marginale causes bovine anaplasmosis, a debilitating and potentially fatal tick-borne infection of cattle. Because A. marginale is an obligate intracellular organism, its adhesins that mediate entry into host cells are essential for survival. Here, we demonstrate that A. marginale outer membrane protein A (AmOmpA; AM854) contributes to the invasion of mammalian and tick host cells. AmOmpA exhibits predicted structural homology to OmpA of A. phagocytophilum (ApOmpA), an adhesin that uses key lysine and glycine residues to interact with ␣2,3-sialylated and ␣1,3-fucosylated glycan receptors, including 6-sulfo-sialyl Lewis x (6-sulfo-sLe x ). Antisera against AmOmpA or its predicted binding domain inhibits A. marginale infection of host cells. Residues G55 and K58 are contributory, and K59 is essential for recombinant AmOmpA to bind to host cells. Enzymatic removal of ␣2,3-sialic acid and ␣1,3-fucose residues from host cell surfaces makes them less supportive of AmOmpA binding. AmOmpA is both an adhesin and an invasin, as coating inert beads with it confers adhesiveness and invasiveness. Recombinant forms of AmOmpA and ApOmpA competitively antagonize A. marginale infection of host cells, but a monoclonal antibody against 6-sulfo-sLe x fails to inhibit AmOmpA adhesion and A. marginale infection. Thus, the two OmpA proteins bind related but structurally distinct receptors. This study provides a detailed understanding of AmOmpA function, identifies its essential residues that can be targeted by blocking antibody to reduce infection, and determines that it binds to one or more ␣2,3-sialylated and ␣1,3-fucosylated glycan receptors that are unique from those targeted by ApOmpA.

Sequence and immunologic conservation ofAnaplasma marginaleOmpA within strains from Ghana as compared to the predominant OmpA variant

“…OmpA serves as an adhesion and invasion for A. marginale and as such is an important vaccine candidate [14]. Here we demonstrate that OmpA is remarkably conserved in A. marginale from geographically diverse regions and the minimal difference between OmpA sequences does not affect antibody binding.…”

“…Here we demonstrate that OmpA is remarkably conserved in A. marginale from geographically diverse regions and the minimal difference between OmpA sequences does not affect antibody binding. Additionally, no amino acid diversity is present in the experimentally determined binding domain [14]. In one study, recombinant OmpA failed to induce protective immunity against tick challenge in cattle[11].…”

“…Recently, ompA , annotated as Am854 in the A. marginale genome, was identified as an adhesin playing an essential role in A. marginale entry into mammalian and tick cells [14]. Consequently, this protein serves as a highly relevant vaccine candidate.…”

Outer Membrane Protein A Conservation among <em>Orientia tsutsugamushi</em> Isolates Suggests Its Potential as a Protective Antigen and Diagnostic Target

“…OmpA proteins of A. phagocytophilum and A. marginale, which are in the order Rickettsiales with O. tsutsugamushi, contribute to their abilities to bind and invade mammalian host cells [41,44]. We previously demonstrated that these two OmpA proteins' tertiary structures are highly similar and residues that are critical for adhesin function are conserved between them and are presented as part of surface exposed alpha helices [41,44,45] Figure 4C). This region is identical among all O. tsutsugamushi translated OmpA sequences in this study with the exception of residue 116 (Figure 3 and Figure 4).…”

“…Outer membrane protein A (OmpA; also referred to as peptidoglycan-associated lipoprotein) is conserved among most Gram-negative bacteria and contributes to the virulence of Gram-negative pathogens, especially their abilities to adhere to and invade host cells [38][39][40][41][42][43][44][45]. Antisera raised against entire OmpA proteins or specific binding domains thereof for Anaplasma spp., E. chaffeensis, and R. conorii inhibit bacterial invasion of host cells in vitro [38,41,42,44,45]. These Rickettsiales members express OmpA during infection of human patients and/or experimentally infected animals [38,44,46].…”