Expression of Rickettsia Adr2 protein in E. coli is sufficient to promote resistance to complement-mediated killing, but not adherence to mammalian cells

Abstract

Bacteria exposed to host serum are subject to the antibacterial effects to the complement system. However, pathogenic microorganisms have evolved mechanisms of evading this immune attack. We have previously demonstrated that at least two R. conorii antigens, RC1281/Adr1 and OmpB β-peptide, contribute to the evasion of complement-mediated killing by binding the complement regulatory proteins vitronectin and factor H. RC1282/Adr2, a protein related to Adr1, is predicted to share similar structural features, suggesting that this protein may also contribute to evasion of complement-mediated killing. Interestingly, the R. prowazekii Adr1 and Adr2(RP828) proteins were originally found to interact with host cell surface proteins, suggesting their putative roles as adhesins in this pathogenic rickettsial species. In this study, we expressed both R. conorii and R. prowazekii Adr2 on the surface of a non-adherent, serum-sensitive strain of E. coli to examine the potential role of this protein to mediate evasion of complement-mediated killing and adherence to host cells. We demonstrate that, similar to R. conorii Adr1, R. conorii and R. prowazekii Adr2 are sufficient to mediate serum resistance and to promote interaction with the host complement regulator vitronectin. Furthermore, we demonstrate that expression of Adr2 in a non-adherent strain of E. coli is insufficient to mediate adherence to cultured mammalian endothelial cells. Together, our data demonstrate that the R. conorii and R. prowazekii Adr2 protein does not participate in the interactions with mammalian cells, but rather, participates in the evasion of killing by complement.

Molecular Camouflage of Plasmodium falciparum Merozoites by Binding of Host Vitronectin to P47 Fragment of SERA5

“…Among them, in Streptococcus pneumoniae serotype 3 strains, a central core of factor H-binding inhibitor of complement (Hic) (amino acid 151–201), a member of the pneumococcal surface protein C (PspC) family, interacts with VTN preventing the formation of the terminal complement complex 40 . In Rickettsia , RC1281/Adr1 and RC1282/Adr2 contributed to evasion of complement-mediated killing by binding to VTN and factor H 43 . The central region of Hic (the predicted binding site of VTN in the pneumococcal Hic molecule) 40 and the predicted surface exposed loops of the highly conserved outer membrane protein Adr2 in Rickettsia 43 bear no structural similarity with the far end C-terminal 18 residues of peptide-15 using the BLAST algorithm at NCBI ( https://blast.ncbi.nlm.nih.gov/Blast.cgi ).…”

“…In Rickettsia , RC1281/Adr1 and RC1282/Adr2 contributed to evasion of complement-mediated killing by binding to VTN and factor H 43 . The central region of Hic (the predicted binding site of VTN in the pneumococcal Hic molecule) 40 and the predicted surface exposed loops of the highly conserved outer membrane protein Adr2 in Rickettsia 43 bear no structural similarity with the far end C-terminal 18 residues of peptide-15 using the BLAST algorithm at NCBI ( https://blast.ncbi.nlm.nih.gov/Blast.cgi ). Furthermore, although the impact of VTN glycosylation in SE36 binding needs to be clarified in future studies, oligosaccharide residues in VTN were also not required for binding of UspA2 and Hic in Moraxella catarrhalis and Streptococcus pneumoniae , respectively 40 , 44 .…”

Immunity against the Obligate Intracellular Bacterial Pathogen Rickettsia australis Requires a Functional Complement System

“…1C ). This result demonstrates that, like other pathogenic rickettsial species, R. australis is resistant to bacteriolytic complement attack in naive serum ( 14 – 16 , 20 ) and that these bacteria are capable of acquiring the serum regulator vitronectin (see Fig. S1 in the supplemental material).…”

“…A recent report demonstrated that patients undergoing infection with the obligate intracellular bacterium R. conorii present with elevated levels of activated complement proteins ( 13 ). Additionally, rickettsial species express surface proteins that are sufficient to mediate resistance to the antibacterial effects of serum complement ( 14 – 16 , 20 ). We reasoned that while pathogenic Rickettsia species can activate complement, this class of pathogens have evolved mechanisms to resist complement-mediated killing during infection.…”

“…This finding suggests that the complement system is activated in vivo during a Rickettsia infection and that complement may play a role in the immune response to this class of pathogens. Conversely, analysis of the interaction between Rickettsia conorii and in vitro serum identified a series of molecular interactions that ultimately prevent complement-mediated killing of the bacteria ( 14 – 16 ). These apparently conflicting findings suggest that complement activation in a naive mammal is unlikely to contribute to protective immunity against rickettsial infection.…”