Abstract
Anaplasma phagocytophilum transmembrane and surface proteins play a role during infection and multiplication in host neutrophils and tick vector cells. Recently, A. phagocytophilum Major surface protein 4 (MSP4) and Heat shock protein 70 (HSP70) were shown to be localized on the bacterial membrane, with a possible role during pathogen infection in ticks. In this study, we hypothesized that A. phagocytophilum MSP4 and HSP70 have similar functions in tick-pathogen and host-pathogen interactions. To address this hypothesis, herein we characterized the role of these bacterial proteins in interaction and infection of vertebrate host cells. The results showed that A. phagocytophilum MSP4 and HSP70 are involved in host-pathogen interactions, with a role for HSP70 during pathogen infection. The analysis of the potential protective capacity of MSP4 and MSP4-HSP70 antigens in immunized sheep showed that MSP4-HSP70 was only partially protective against pathogen infection. This limited protection may be associated with several factors, including the recognition of non-protective epitopes by IgG in immunized lambs. Nevertheless, these antigens may be combined with other candidate protective antigens for the development of vaccines for the control of human and animal granulocytic anaplasmosis. Focusing on the characterization of host protective immune mechanisms and protein-protein interactions at the host-pathogen interface may lead to the discovery and design of new effective protective antigens.
Highlights
Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae) is an emerging tick-borne intracellular bacterial pathogen in many regions of the world, but vaccines are not available for prevention of transmission and infection in humans and animals (Dumler et al, 2001; Severo et al, 2013; Stuen et al, 2013, 2015; Bakken and Dumler, 2015)
The subcellular localization of Major surface protein 4 (MSP4) and Heat shock protein 70 (HSP70) proteins was characterized in A. phagocytophilum purified from infected HL60 human promyelocytic leukemia cells, mock treated or surface digested with trypsin and loaded onto polyacrylamide gels for Western blot analysis using rabbit antibodies specific against recombinant proteins
The residue map showed that the majority of protein-protein contacts are formed between the β-sheets of MSP4 buried within the membrane and the C-terminus of HSP70, several contacts between the β-hairpin loops of MSP4 and the N-terminus of HSP70 are exposed extracellularly, and these residues may act as markers for mutational studies and antibody targeting (Supplementary Figure 1C)
Summary
Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae) is an emerging tick-borne intracellular bacterial pathogen in many regions of the world, but vaccines are not available for prevention of transmission and infection in humans and animals (Dumler et al, 2001; Severo et al, 2013; Stuen et al, 2013, 2015; Bakken and Dumler, 2015). A. phagocytophilum is recognized as a threat for human and animal health in Europe and the United States, its pathogenic and epidemic potential is neglected in tropical regions of the world (Heyman et al, 2010; Dugat et al, 2015). Prophylactic uses of tetracycline together with acaricide applications for tick control are the main measures to control A. phagocytophilum infection in endemic areas (Woldehiwet, 2006; Stuen et al, 2015) These control measures raise concerns about their impact on the environment and human health, and the selection of resistant pathogens and ticks (Woldehiwet, 2006; Stuen et al, 2015)
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