The Passenger Domain of Bartonella bacilliformis BafA Promotes Endothelial Cell Angiogenesis via the VEGF Receptor Signaling Pathway

Differential vasoproliferative traits of Bartonella henselae strains associated with autotransporter BafA variants

Bartonella henselae, a Gram-negative facultative intracellular bacterium, is the etiological agent of cat-scratch disease and also causes bacillary angiomatosis in immunocompromised individuals. Although the ability to promote vascular endothelial cell proliferation differs among Bartonella species, variations among strains within B. henselae remain unclear. Bartonella angiogenic factor A (BafA) and Bartonella adhesin A (BadA) have been identified as autotransporters of B. henselae that are involved in endothelial cell proliferation. Although strain-specific differences in the expression of BadA and the VirB/D4 type IV secretion system have been reported, BafA expression among B. henselae strains has yet to be examined. Therefore, the present study investigated the proliferation-promoting ability of 13 B. henselae strains from several sources in human umbilical vein endothelial cells (HUVECs). We identified BafA variants 1 and 2 based on the deduced amino acid sequences of its passenger domain. The recombinant proteins of both variants exhibited similar proliferation activity against HUVECs. However, BafA variant 2 strains showed cytotoxicity at a high bacterial inoculum in a direct coculture with HUVECs, which was attenuated in an indirect coculture. These strains, in contrast to BafA variant 1 strains, highly expressed BadA and exhibited bacterial aggregation. Based on a core genome SNP analysis of 50 B. henselae strains, the BafA variant types corresponded to clades 1-4. These results indicate that vasoproliferative traits differ among B. henselae clades based on the variant types. Therefore, this study provides a new conceptual framework in which the clades of B. henselae may predict their pathogenicity in humans.IMPORTANCEBartonella species including Bartonella henselae, Bartonella quintana, and Bartonella bacilliformis cause vasoproliferative lesions. Their proliferation-promoting ability in vascular endothelial cells differs among Bartonella species; however, it is unclear whether these differences exist among B. henselae strains. We herein showed that B. henselae strains exhibited variable proliferation-promoting ability and cytotoxicity in vascular endothelial cells, which corresponded to the bafA gene variants possessed by the strains. The expression levels of Bartonella angiogenic factor A (BafA) and Bartonella adhesin A, as well as the degree of proliferation-promoting ability and cytotoxicity in endothelial cells, varied among the strains. A core genome SNP analysis of strains using whole genome sequencing data divided B. henselae strains into four clades, with each clade corresponding to BafA variants 1-4. These results suggest the differential vasoproliferative potency of B. henselae, with potential implications in clinical management, including risk stratification and predictions of the clinical course.

Sneaky tactics: Ingenious immune evasion mechanisms of Bartonella

Gram-negative Bartonella species are facultative intracellular bacteria that can survive in the harsh intracellular milieu of host cells. They have evolved strategies to evade detection and degradation by the host immune system, which ensures their proliferation in the host. Following infection, Bartonella alters the initial immunogenic surface-exposed proteins to evade immune recognition via antigen or phase variation. The diverse lipopolysaccharide structures of certain Bartonella species allow them to escape recognition by the host pattern recognition receptors. Additionally, the survival of mature erythrocytes and their resistance to lysosomal fusion further complicate the immune clearance of this species. Certain Bartonella species also evade immune attacks by producing biofilms and anti-inflammatory cytokines and decreasing endothelial cell apoptosis. Overall, these factors create a challenging landscape for the host immune system to rapidly and effectively eradicate the Bartonella species, thereby facilitating the persistence of Bartonella infections and creating a substantial obstacle for therapeutic interventions. This review focuses on the effects of three human-specific Bartonella species, particularly their mechanisms of host invasion and immune escape, to gain new perspectives in the development of effective diagnostic tools, prophylactic measures, and treatment options for Bartonella infections.

Neurobartonelloses: emerging from obscurity!

BACKGROUND

Bartonella species are fastidious, intracellular bacteria responsible for an expanding array of human pathologies. Most are considered to be transmitted by direct inoculation with infected bodily fluids from a mammalian reservoir species or vector-transmitted through a variety of arthropod species and their excrement. However, there are mounting reports of infection in the absence of documented animal or vector contact. A variety of Bartonella species have been documented in conditions affecting both the peripheral and central nervous systems. More common conditions, including neuroretinitis, are often associated with Bartonella henselae. However, Bartonella quintana, the agent of trench fever, as well as emerging pathogens related to rodent reservoir species, B. grahamii and B. elizabethae, have also been documented. Encephalitis and encephalopathy, also most often associated with B. henselae, have been reported with B. quintana, B. washoensis (ground squirrels) and B. vinsonii subsp. vinsonii (voles) infections. Bartonella infections have also been associated with peripheral neuropathies, such as cranial nerve paresis and neuropathic pain, including infection with less commonly encountered species such as Bartonella koehlerae. Recently, molecular diagnostic testing revealed that DNA from Bartonella spp. was found to be more prevalent in blood of patients with neuropsychiatric disorders such as schizophrenia and psychoses compared to healthy controls.

METHODS

A systematic literature search was conducted on PubMed, Google Scholar and Web of Science. Search terms included Bartonella and specific neurological conditions and focused on peer-reviewed case reports published after 2012 pursuant to a prior review, with limited exceptions for conditions not previously covered. Published diagnostic testing, serology, molecular testing or pathology, were necessary for inclusion, except for one case which had clinical and epidemiological evidence consistent with diagnosis along with follow-up.

RESULTS

Neurobartonelloses included neuralgic amyotrophy, complex regional pain syndrome, chronic inflammatory demyelinating polyneuropathy, cranial nerve paralysis, Guillain-Barré syndrome, peripheral vasculitic polyneuropathy, acute transverse myelopathy, neuroretinitis, encephalitis/encephalopathy, cerebral vasculitis/aneurysm and neuropsychiatric conditions.

CONCLUSIONS

The breadth of reported symptoms and clinical syndromes associated with an increasing number of Bartonella species continues to expand. Increased clinical awareness of this important zoonotic pathogen is necessary to advance One Health among the medical and veterinary communities.

Advancements in understanding the molecular and immune mechanisms of Bartonella pathogenicity

Bartonellae are considered to be emerging opportunistic pathogens. The bacteria are transmitted by blood-sucking arthropods, and their hosts are a wide range of mammals including humans. After a protective barrier breach in mammals, Bartonella colonizes endothelial cells (ECs), enters the bloodstream, and infects erythrocytes. Current research primarily focuses on investigating the interaction between Bartonella and ECs and erythrocytes, with recent attention also paid to immune-related aspects. Various molecules related to Bartonella's pathogenicity have been identified. The present review aims to provide a comprehensive overview of the newly described molecular and immune responses associated with Bartonella's pathogenicity.

Comparison of Biological Activities of BafA Family Autotransporters within Bartonella Species Derived from Cats and Rodents

Bartonella species are hemotropic, facultative intracellular bacteria, some of which cause zoonoses, that are widely disseminated among many mammals, including humans. During infection in humans, vascular endothelial cells play a crucial role as a replicative niche for Bartonella, and some are capable of promoting vascular proliferation. Along with well-studied pathogenic factors such as a trimeric autotransporter adhesin BadA or VirB/D4 type IV secretion system, bacteria-secreted protein BafA is also involved in Bartonella-induced vasoproliferation. Genes encoding BafA orthologs have been found in the genomes of most Bartonella species, but their functionality remains unclear. In this study, we focused on three cat-derived zoonotic species (B. henselae, B. koehlerae, and B. clarridgeiae) and two rodent-derived species (B. grahamii and B. doshiae) and compared the activity of BafA derived from each species. Recombinant BafA proteins of B. henselae, B. koehlerae, B. clarridgeiae, and B. grahamii, species that also cause human disease, induced cell proliferation and tube formation in cultured endothelial cells, while BafA derived from B. doshiae, a species that is rarely found in humans, showed neither activity. Additionally, treatment of cells with these BafA proteins increased phosphorylation of both vascular endothelial growth factor receptor 2 and extracellular signal-regulated kinase 1/2, with the exception of B. doshiae BafA. Differential bafA mRNA expression and BafA secretion among the species likely contributed to the differences in the cell proliferation phenotype of the bacteria-infected cells. These findings suggest that the biological activity of BafA may be involved in the infectivity or pathogenicity of Bartonella species in humans.

The autotransporter BafA contributes to the proangiogenic potential of Bartonella elizabethae

Bartonella elizabethae is a rat-borne zoonotic bacterium that causes human infectious endocarditis or neuroretinitis. Recently, a case of bacillary angiomatosis (BA) resulting from this organism was reported, leading to speculation that B. elizabethae may also trigger vasoproliferation. However, there are no reports of B. elizabethae promoting human vascular endothelial cell (EC) proliferation or angiogenesis, and to date, the effects of this bacterium on ECs are unknown. We recently identified a proangiogenic autotransporter, BafA, secreted from B. henselae and B. quintana, which are recognized as Bartonella spp. responsible for BA in humans. Here, we hypothesized that B. elizabethae also harbored a functional bafA gene and examined the proangiogenic activity of recombinant B. elizabethae-derived BafA. The bafA gene of B. elizabethae, which was found to share a 51.1% amino acid sequence identity with BafA of B. henselae and 52.5% with that of B. quintana in the passenger domain, was located in a syntenic region of the genome. The recombinant protein of the N-terminal passenger domain of B. elizabethae-BafA facilitated EC proliferation and capillary structure formation. Furthermore, it upregulated the receptor signaling pathway of vascular endothelial growth factor, as observed in B. henselae-BafA. Taken together, B. elizabethae-derived BafA stimulates human EC proliferation and may contribute to the proangiogenic potential of this bacterium. So far, functional bafA genes have been found in all BA-causing Bartonella spp., supporting the key role BafA may play in BA pathogenesis.