A human factor H-binding protein of Bartonella bacilliformis and potential role in serum resistance

Bartonella bacilliformis is a Gram-negative bacterium and etiologic agent of Carrión’s disease; a potentially life-threatening illness endemic to South America. B. bacilliformis is a facultative parasite that infects human erythrocytes (hemotrophism) and the circulatory system, culminating in a variety of symptoms, including a precipitous drop in hematocrit, angiomatous lesions of the skin (verruga peruana) and persistent bacteremia. Because of its specialized niche, serum complement imposes a continual selective pressure on the pathogen. In this study, we demonstrated the marked serum-resistance phenotype of B. bacilliformis, the role of factor H in serum complement resistance, and binding of host factor H to four membrane-associated polypeptides of ∼131, 119, 60 and 43 kDa by far-western (FW) blots. The ∼119-kDa protein was identified as ABM44634.1 by mass spectrometry; a protein annotated as a 116.5-kDa outer membrane autotransporter (encoded by the BARBAKC583_1133 locus). We designated the protein as factor H-binding protein A (FhbpA). FhbpA possesses three structural motifs common to all autotransporter proteins (i.e., a signal peptide, autotransporter β-barrel domain and passenger domain). Recombinant FhbpA passenger domain, but not the recombinant autotransporter domain, was able to bind human factor H when analyzed by FW blots. Phylogenetic analyses of the passenger domain suggest that it is well-conserved among Bartonella autotransporters, with closest matches from Bartonella schoenbuchensis. Transcriptomic analyses of B. bacilliformis subjected to conditions mimicking the sand fly vector or human host, and infection of human blood or vascular endothelial cells showed maximal expression of fhbpA under human-like conditions and during infection of blood and endothelial cells. Expression during HUVEC infection was significantly higher compared to all other conditions by DESeq2. Surface binding of serum factor H by FhbpA is hypothesized to play a protective role against the alternative pathway of complement fixation during B. bacilliformis infection of the human host. Author Summary B. bacilliformis is a bacterial pathogen that colonizes the circulatory system of humans, where it can cause a life-threatening illness unless treated. Serum complement is a major effector of innate humoral immunity and a significant obstacle that must be evaded for successful survival and colonization by pathogens, especially those residing in the vasculature. In this study, we examined the serum complement resistance phenotype of B. bacilliformis and identified four membrane-associated proteins that bind serum factor H; a protein used by the host to protect its own tissues from complement activation. One of the proteins was identified by mass spectrometry, characterized, and designated factor H-binding protein A (FhbpA). FhbpA is a predicted autotransporter, and we determined that the translocated “ passenger” domain of the protein is responsible for binding factor H. We also determined that expression of the fhbpA gene was highest during infection of human blood and especially vascular endothelial cells or under conditions that simulate the human host. The results suggest that FhbpA binding of host serum factor H protects the bacterium against complement activation during infection.