Mechanism of Borrelia immune evasion by FhbA-related proteins.

Immune evasion facilitates survival of Borrelia, leading to infections like relapsing fever and Lyme disease. Important mechanism for complement evasion is acquisition of the main host complement inhibitor, factor H (FH). By determining the 2.2 Å crystal structure of Factor H binding protein A (FhbA) from Borrelia hermsii in complex with FH domains 19-20, combined with extensive mutagenesis, we identified the structural mechanism by which B. hermsii utilizes FhbA in immune evasion. Moreover, structure-guided sequence database analysis identified a new family of FhbA-related immune evasion molecules from Lyme disease and relapsing fever Borrelia. Conserved FH-binding mechanism within the FhbA-family was verified by analysis of a novel FH-binding protein from B. duttonii. By sequence analysis, we were able to group FH-binding proteins of Borrelia into four distinct phyletic types and identified novel putative FH-binding proteins. The conserved FH-binding mechanism of the FhbA-related proteins could aid in developing new approaches to inhibit virulence and complement resistance in Borrelia.

SARS-CoV-2 variant with mutations in N gene affecting detection by widely used PCR primers.

While most of the spontaneous mutations in the viral genome have no functional, diagnostic, or clinical consequences, some have. In February 2021, we noticed in Southern Finland coronavirus disease 2019 cases where two commercial polymerase chain reaction (PCR) analyses failed to recognize the used N gene target but recognized the other target gene of severe acute respiratory syndrome coronavirus 2. Complete viral genome sequence analysis of the strains revealed several mutations that were not found at that time in public databases. A short 3 bp deletion and three subsequent single nucleotide polymorphisms in the N gene were found exactly at the site where an early published and widely used N gene-based PCR primer is located, explaining the negative results in the N gene PCR. Later the variant strain was identified as a member of the B.1.1.318 Pango lineage that had first been found from Nigerian samples collected in January 2021. This strain shares with the Beta variant the S gene E484K mutation linked to impaired vaccine protection, but differs from this variant in several other ways, for example by deletions in the N gene region. Mutations in the N gene causing diagnostic resistance and on the other hand E484K mutation in the causing altered infectivity warrants careful inspection on virus variants that might get underdiagnosed.

Dientamoeba fragilis - the most common intestinal protozoan in the Helsinki Metropolitan Area, Finland, 2007 to 2017.

BackgroundDespite the global distribution of the intestinal protozoan Dientamoeba fragilis, its clinical picture remains unclear. This results from underdiagnosis: microscopic screening methods either lack sensitivity (wet preparation) or fail to reveal Dientamoeba (formalin-fixed sample).AimIn a retrospective study setting, we characterised the clinical picture of dientamoebiasis and compared it with giardiasis. In addition, we evaluated an improved approach to formalin-fixed samples for suitability in Dientamoeba diagnostics.MethodsThis study comprised four parts: (i) a descriptive part scrutinising rates of Dientamoeba findings; (ii) a methodological part analysing an approach to detect Dientamoeba-like structures in formalin samples; (iii) a clinical part comparing demographics and symptoms between patients with dientamoebiasis (n = 352) and giardiasis (n = 272), and (iv) a therapeutic part (n = 89 patients) investigating correlation between faecal eradication and clinical improvement.ResultsThe rate of Dientamoeba findings increased 20-fold after introducing criteria for Dientamoeba-like structures in formalin-fixed samples (88.9% sensitivity and 83.3% specificity). A further increase was seen after implementing faecal PCR. Compared with patients with giardiasis, the symptoms in the Dientamoeba group lasted longer and more often included abdominal pain, cramping, faecal urgency and loose rather than watery stools. Resolved symptoms correlated with successful faecal eradication (p < 0.001).ConclusionsPreviously underdiagnosed, Dientamoeba has become the most frequently recorded pathogenic enteroparasite in Finland. This presumably results from improved diagnostics with either PCR or detection of Dientamoeba-like structures in formalin-fixed samples, an approach applicable also in resource-poor settings. Symptoms of dientamoebiasis differ slightly from those of giardiasis; patients with distressing symptoms require treatment.

Staphylococcal Ecb protein and host complement regulator factor H enhance functions of each other in bacterial immune evasion.

Staphylococcus aureus is a major human pathogen causing more than a tenth of all septicemia cases and often superficial and deep infections in various tissues. One of the immune evasion strategies of S. aureus is to secrete proteins that bind to the central complement opsonin C3b. One of these, extracellular complement binding protein (Ecb), is known to interfere directly with functions of C3b. Because C3b is also the target of the physiological plasma complement regulator, factor H (FH), we studied the effect of Ecb on the complement regulatory functions of FH. We show that Ecb enhances acquisition of FH from serum onto staphylococcal surfaces. Ecb and FH enhance mutual binding to C3b and also the function of each other in downregulating complement activation. Both Ecb and the C-terminal domains 19-20 of FH bind to the C3d part of C3b. We show that the mutual enhancing effect of Ecb and FH on binding to C3b depends on binding of the FH domain 19 to the C3d part of C3b next to the binding site of Ecb on C3d. Our results show that Ecb, FH, and C3b form a tripartite complex. Upon exposure of serum-sensitive Haemophilus influenzae to human serum, Ecb protected the bacteria, and this effect was enhanced by the addition of the C-terminal domains 19-20 of FH. This finding indicates that the tripartite complex formation could give additional protection to bacteria and that S. aureus is thereby able to use host FH and bacterial Ecb in a concerted action to eliminate C3b at the site of infection.

Structural basis for complement evasion by Lyme disease pathogen Borrelia burgdorferi.

Borrelia burgdorferi spirochetes that cause Lyme borreliosis survive for a long time in human serum because they successfully evade the complement system, an important arm of innate immunity. The outer surface protein E (OspE) of B. burgdorferi is needed for this because it recruits complement regulator factor H (FH) onto the bacterial surface to evade complement-mediated cell lysis. To understand this process at the molecular level, we used a structural approach. First, we solved the solution structure of OspE by NMR, revealing a fold that has not been seen before in proteins involved in complement regulation. Next, we solved the x-ray structure of the complex between OspE and the FH C-terminal domains 19 and 20 (FH19-20) at 2.83 Å resolution. The structure shows that OspE binds FH19-20 in a way similar to, but not identical with, that used by endothelial cells to bind FH via glycosaminoglycans. The observed interaction of OspE with FH19-20 allows the full function of FH in down-regulation of complement activation on the bacteria. This reveals the molecular basis for how B. burgdorferi evades innate immunity and suggests how OspE could be used as a potential vaccine antigen.

Apolipoprotein A-I exerts bactericidal activity against Yersinia enterocolitica serotype O:3.

Apolipoprotein A-I (apoA-I), the main protein component of high density lipoprotein (HDL), is well recognized for its antiatherogenic, antioxidant, and antiinflammatory properties. Here, we report a novel role for apoA-I as a host defense molecule that contributes to the complement-mediated killing of an important gastrointestinal pathogen, Gram-negative bacterium Yersinia enterocolitica. We specifically show that the C-terminal domain of apoA-I is the effector site providing the bactericidal activity. Although the presence of the lipopolysaccharide O-antigen on the bacterial surface is absolutely required for apoA-I to kill the bacteria, apoA-I does not interact with the bacteria directly. To the contrary, exposure of the bacteria by serum proteins triggers apoA-I deposition on the bacterial surface. As our data show that both purified lipid-free and HDL-associated apoA-I displays anti-bacterial potential, apoA-I mimetic peptides may be a promising therapeutic agent for the treatment of certain Gram-negative infections.

[Intestinal parasite infections]. / Perä pörisee--bongaa parasiitti.

Symptoms in a diarrhea patient are most commonly due to a virus or a bacterium, but they may also be caused by a parasite. A long incubation period is typical of intestinal parasite infections, and in addition to diarrhea they cause prolonged symptoms such as abdominal pain and nausea. Parasitic cyst forms are secreted with feces and are highly tolerant against various environmental conditions. The infections are caught via fecally contaminated food or drink. The diagnosis is based on a formalin-fixed fecal parasitic specimen, leading to further investigations when necessary.

[Endemic cryptosporidiosis--underdiagnosed disease in Finland]. / Kotoperäinen kryptosporidioosi--alidiagnosoitu tauti.

Acute diarrhea caused by Cryptosporidium-protozoan is rarely diagnosed in Finland. The infection is usually self-limited and does not require antimicrobial treatment. Cryptosporidiosis, like other intestinal parasite infections, is mostly associated with travelling, but may also cause large waterborne epidemics. Contact with infected calves may be a source of cryptosporidiosis also in Finland. Cryptosporidiosis should be considered in patients suffering from severe or long-lasting watery diarrhea. We describe three cases of cryptosporidiosis, originating from infected calves. These cases show that verification of the etiology of human cryptosporidiosis associated with calves may be difficult and demands collaboration of clinicians, laboratories and veterinarians.

Expression and sequence diversity of the complement regulating outer surface protein E in Borrelia afzelii vs. Borrelia garinii in patients with erythema migrans or neuroborreliosis.

Outer surface protein E (OspE) is a complement factor H-binding virulence factor of borrelial subspecies. It is usually absent from in vitro grown Borrelia garinii, although in vivo B. garinii causes neuroborreliosis (NB). We analyzed the presence and sequence spectrum of the ospE genes in vivo in Borrelia spirochetes. DNA samples from the skin, serum and cerebrospinal fluid (CSF) of patients with infections caused by Borrelia afzelii or B. garinii were studied, and anti-OspE antibodies in the corresponding patient sera were detected by IgG ELISA using recombinant OspE as an antigen. ospE genes were found in 20 of 23 erythema migrans (EM) skin biopsies with B. afzelii, in 2 EM skin biopsies with unknown underlying subspecies, in 5 of 9 EM biopsies with B. garinii, and in 1 of 4 CSF samples of NB patients with B. garinii infection. All OspE sequences from B. garinii samples were identical. In contrast, OspE of B. afzelii origin showed more variation. Anti-OspE antibodies were found in 8/21 (38.0%) sera from patients with B. afzelii-associated EM. In conclusion, our results indicate that all borrelial subspecies, but not necessarily all strains, causing human infections can carry ospE genes to protect themselves against complement attack in vivo.

Loa loa Microfilariae evade complement attack in vivo by acquiring regulatory proteins from host plasma.

Loa loa is a filarial nematode that infects humans. The adults live in subcutaneous tissues and produce microfilariae that live for several weeks in the blood circulation in order to be transmitted to another person via blood meals of a dipterian vector. As microfilariae live in continuous contact with plasma, it is obvious that they evade the complement system. We studied markers of complement activation and signs of complement regulation on Loa loa microfilariae in vivo. The microfilariae were isolated from anticoagulated blood samples of a Loa loa-infected Caucasian patient. C1q and some mannose-binding lectin but only a limited amount of C3b or C4b fragments and practically no C5 or C5b-9 were present on the microfilariae. The covalently microfilaria-bound C3 and C4 depositions were mainly inactive iC3b, C3c, and iC4b fragments indicating that microfilariae had regulated complement activation in vivo. Also, in vitro deposition of C3b onto the microfilariae upon serum exposure was limited. The patient-isolated microfilariae were found to carry the host complement regulators factor H and C4b-binding protein on the outermost layer, so called sheath. The microfilaria-bound factor H was functionally active. Binding of the complement regulators to the microfilariae was confirmed in vitro using (125)I-labeled factor H and C4b-binding protein. In conclusion, our study shows that Loa loa microfilariae block complement activation and acquire the host complement regulators factor H and C4b-binding protein in blood circulation. This is the first time that binding of complement regulators onto nonviral pathogens has been demonstrated to occur in humans in vivo.

Outer surface protein E antibody response and its effect on complement factor H binding to OspE in Lyme borreliosis.

Borrelia burgdorferi sensu stricto and B. afzelii, but not B. garinii, are able to escape complement attack by binding factor H via OspE proteins. Recent finding of ospE genes also in B. garinii isolates has raised the question whether, under in vivo-conditions, B. garinii also expresses OspE proteins and consequently induces an antibody response. We set up an IgG ELISA by using recombinant OspE as an antigen. Sixty percent of acute and 64% of convalescent 25 erythema migrans patient samples were positive for anti-OspE antibodies. Anti-OspE antibodies were also found in the sera (83.6%) and cerebrospinal fluids (36%) of patients with neuroborreliosis. Since B. garinii is the major causative agent of neuroborreliosis, the result suggests that OspE is expressed by B. garinii in vivo. Of the 10 acrodermatitis chronica atrophicans patients, 80% had anti-OspE antibodies. Anti-OspE antibody positive sera inhibited factor H binding to Borrelia more efficiently than normal control sera (65% vs. 33.7%). Our results indicate that Borrelia spirochetes, including B. garinii, can induce the production of anti-OspE antibodies. This implies that OspE protein is produced in vivo by B. garinii possibly enabling it to escape complement and cause a CNS infection.

Negative impact of Aspergillus galactomannan and DNA detection in the diagnosis of fungal rhinosinusitis.

A proportion of patients with chronic rhinosinusitis, especially if nasal polyps are present, have a diagnosis of fungal rhinosinusitis. The diagnosis is difficult to establish because the symptoms and clinical and radiological signs are non-specific. Also current diagnostic methods, i.e. histology, fungal staining and culture, are insensitive. The performance of the Aspergillus galactomannan (GM) ELISA and real-time PCR for Aspergillus fumigatus mitochondrial DNA was evaluated for the detection of Aspergillus in sinus mucus samples from 25 patients with chronic rhinosinusitis with nasal polyposis. The results were compared with those from nasal lavage fluid from 19 healthy volunteers. Seven patients (28 %) were diagnosed as having fungal rhinosinusitis according to the presence of filaments in histology or direct microscopy using Calcofluor white. All fungal rhinosinusitis patients were negative in the GM ELISA. GM ELISA was positive in five patients whose samples were negative using conventional methods and A. fumigatus PCR. Two out of seven patients with fungal rhinosinusitis were positive by A. fumigatus PCR: one also had a positive A. fumigatus culture, and one had hyphae consistent with Aspergillus in histology. One additional patient had a weak positive PCR result, but other fungal tests were negative. In control subjects, the GM ELISA was positive in 21 %, whereas direct microscopy, culture and A. fumigatus PCR were negative in all samples. Direct microscopy and culture together with histology remain pivotal in defining fungal rhinosinusitis diagnosis. A. fumigatus PCR may have additional value in allowing the diagnosis to be made sooner, whereas the GM ELISA is not reliable in diagnosing Aspergillus infection of the paranasal sinuses.

Crystal structure of a tripartite complex between C3dg, C-terminal domains of factor H and OspE of Borrelia burgdorferi.

Complement is an important part of innate immunity. The alternative pathway of complement is activated when the main opsonin, C3b coats non-protected surfaces leading to opsonisation, phagocytosis and cell lysis. The alternative pathway is tightly controlled to prevent autoactivation towards host cells. The main regulator of the alternative pathway is factor H (FH), a soluble glycoprotein that terminates complement activation in multiple ways. FH recognizes host cell surfaces via domains 19-20 (FH19-20). All microbes including Borrelia burgdorferi, the causative agent of Lyme borreliosis, must evade complement activation to allow the infectious agent to survive in its host. One major mechanism that Borrelia uses is to recruit FH from host. Several outer surface proteins (Osp) have been described to bind FH via the C-terminus, and OspE is one of them. Here we report the structure of the tripartite complex formed by OspE, FH19-20 and C3dg at 3.18 Å, showing that OspE and C3dg can bind simultaneously to FH19-20. This verifies that FH19-20 interacts via the "common microbial binding site" on domain 20 with OspE and simultaneously and independently via domain 19 with C3dg. The spatial organization of the tripartite complex explains how OspE on the bacterial surface binds FH19-20, leaving FH fully available to protect the bacteria against complement. Additionally, formation of tripartite complex between FH, microbial protein and C3dg might enable enhanced protection, particularly on those regions on the bacteria where previous complement activation led to deposition of C3d. This might be especially important for slow-growing bacteria that cause chronic disease like Borrelia burgdorferi.

Staphylococcal protein Ecb impairs complement receptor-1 mediated recognition of opsonized bacteria.

Staphyloccus aureus is a major human pathogen leading frequently to sepsis and soft tissue infections with abscesses. Multiple virulence factors including several immune modulating molecules contribute to its survival in the host. When S. aureus invades the human body, one of the first line defenses is the complement system, which opsonizes the bacteria with C3b and attract neutrophils by release of chemotactic peptides. Neutrophils express Complement receptor-1 [CR1, CD35) that interacts with the C3b-opsonized particles and thereby plays an important role in pathogen recognition by phagocytic cells. In this study we observed that a fraction of S. aureus culture supernatant prevented binding of C3b to neutrophils. This fraction consisted of S. aureus leukocidins and Efb. The C-terminus of Efb is known to bind C3b and shares significant sequence homology to the extracellular complement binding protein [Ecb). Here we show that S. aureus Ecb displays various mechanisms to block bacterial recognition by neutrophils. The presence of Ecb blocked direct interaction between soluble CR1 and C3b and reduced the cofactor activity of CR1 in proteolytic inactivation of C3b. Furthermore, Ecb could dose-dependently prevent recognition of C3b by cell-bound CR1 that lead to impaired phagocytosis of NHS-opsonized S. aureus. Phagocytosis was furthermore reduced in the presence of soluble CR1 [sCR1). These data indicate that the staphylococcal protein Ecb prevents recognition of C3b opsonized bacteria by neutrophil CR1 leading to impaired killing by phagocytosis and thereby contribute to immune evasion of S. aureus.

Binding of the complement inhibitor C4b-binding protein to Lyme disease Borreliae.

The Lyme disease spirochetes, Borrelia burgdorferi sensu stricto, Borrelia afzelii and Borrelia garinii, are tick-borne pathogens that can cause chronic disseminated infections. To survive in the human host borreliae need to evade the immune system. It is already well known that B. burgdorferi ss. and B. afzelii bind the complement (C) alternative pathway inhibitor factor H from serum using OspE and CRASP-1/Bba68 proteins to escape C attack. In the presence of natural antibodies and in chronic infections, when specific antibodies develop, borreliae have to protect themselves from antibody-induced classical pathway C attack. In this study we demonstrate binding of the classical pathway inhibitor, C4b-binding protein (C4bp), to three genospecies of B. burgdorferi sensu lato. Binding was strongest to B. garinii, which has been found to be the weakest factor H binder. The bacteria bound both purified (125)I-labeled C4bp and C4bp from serum. Unlabeled C4bp competed for binding with (125)I-C4bp, whereas BSA had no effect. Binding was salt-sensitive and inhibited by C4b and partially by heparin. C4bp maintained its cofactor activity for factor I in cleaving C4b when bound to the bacterial surface. Ligand blotting analysis of whole cell lysates and fractionated outer cell membranes of the bacteria suggested one major receptor of approximately 43 kDa (P43) for C4bp in B. garinii and B. burgdorferi sensu stricto. Binding of C4bp may thus allow Lyme disease borreliae to escape activation of the classical C pathway and allow chronic infections in humans even in the presence of specific antibodies.

Prevention and monitoring of invasive fungal infections in pediatric patients with cancer and hematologic disorders.

BACKGROUND: The occurrence of invasive fungal infection (IFIs) in a pediatric hematology/oncology unit after renovation of the ventilation system, and initiating routine azole antifungal prophylaxis was monitored. In addition, the value of serial screening for Aspergillus galactomannan (GM) for diagnosing invasive aspergillosis was assessed. PROCEDURE: A total of 98 consecutive high-risk pediatric patients were prospectively surveyed for signs of IFI and weekly monitored for serum GM. The data was not made available to treating physicians. RESULTS: Only 2 patients had proven and 27 possible IFI based on the European Organization for Research and Treatment of Cancer/Mycoses Study Group definitions. The incidence of proven IFI was 1/31 (3.2%) in the allogeneic stem cell transplant (SCT) (Aspergillus spp), 0/26 in the autologous SCT, and 1/60 (1.6%) in the induction therapy group (C. krusei). GM was detected at least in one tested sample in 12/98 patients (12.2%), in five patients in two or more sequential samples. In the latter group, IFI was proven in one patient and could not be excluded in the others. Four of the five patients belonged to the 31 allogeneic and one to the 26 autologous SCT patients. In patients with only one positive GM test none developed signs of IFI and only one received empirical amphotericin B. CONCLUSIONS: With the currently used preventative and prophylactic measures, IFI is uncommon in children with high-risk for infection. Regular screening for GM could be useful among allogeneic SCT patients and two positive samples should prompt further investigative procedures and pre-emptive antifungal therapy.

Regulation of complement activation at the C3-level by serum resistant leptospires.

Leptospires are spirochetes that are transmitted to humans through contacts with wild or domestic animals or via an exposure to contaminated soil or water. In this study we have compared the serum-sensitivity of five pathogenic strains of leptospires (L. interrogans) to an environmental isolate (strain Patoc). Different levels of sensitivities to human serum were seen. Interestingly, the most sensitive strain was the non-pathogenic Patoc strain. The fully and intermediately resistant strains have been isolated from human patients. Testing was performed in the absence of specific antibodies, and killing was found to be dependent on the complement system. The serum sensitive Patoc strain was killed in human serum within minutes, whereas the most resistant strains tolerated serum up to 4h. We also tested the deposition of the complement components C3, C5, C6, C8 and C5b-9 to the surfaces of the sensitive and resistant strains of Leptospira by immunofluorescence microscopy and ELISA. C3 was deposited on both the sensitive and resistant strains, but the terminal complement components were detected only on the surface of the complement-sensitive strain. The complement resistant and intermediate strains were found to bind more factor H from human serum than the complement sensitive strain. Thus, binding of this major alternative complement pathway inhibitor is related to serum resistance in Leptospira spirochetes.

Expression of complement factor H binding immunoevasion proteins in Borrelia garinii isolated from patients with neuroborreliosis.

The Lyme disease-pathogen Borrelia burgdorferi binds the complement inhibitor factor H (FH) to its outer surface protein E- (OspE) and BbA68-families of lipoproteins. In earlier studies, only serum-resistant strains of the genospecies B. burgdorferi sensu stricto or B. afzelii, but not serum-sensitive B. garinii strains, have been shown to bind FH. Since B. garinii often causes neuroborreliosis in man, we have readdressed the interactions of B. garinii with FH. B. garinii 50/97 strain did not express FH-binding proteins. By transforming the B. garinii 50/97 strain with an OspE-encoding gene from complement-resistant B. burgdorferi (ospE-297), its resistance to serum killing could be increased. OspE genes were detected and cloned from the B. garinii BITS, Pistoia and 40/97 strains by PCR and sequencing. The deduced amino acid sequences differed in an N-terminal lysine-rich FH-binding region from OspE sequences of resistant strains. Recombinant B. garinii BITS OspE protein was found to have a considerably lower FH-binding activity than the B. burgdorferi sensu stricto 297 OspE protein P21 (P21-297). Unlike bacteria that had been kept in culture for a long time, neurovirulent B. garinii strains from neuroborreliosis patients were found to express approximately 27-kDa FH-binding proteins. These were not recognized by polyclonal anti-OspE or anti-BbA68 antibodies. We conclude that B. garinii strains carry ospE genes but have a decreased expression of OspE proteins and a reduced ability to bind FH, especially when grown for prolonged periods in vitro. Recently isolated neuroinvasive B. garinii strains, however, can express FH-binding proteins, which may contribute to the virulence of neuroborreliosis-causing B. garinii strains.

Onchocerca volvulus microfilariae avoid complement attack by direct binding of factor H.

The filarial parasite Onchocerca volvulus is the causative agent of river blindness. The adult worms produce microfilariae (mf), which are responsible for the disease pathogenesis; mf activate the complement system, but the activation stops before the formation of terminal complement complexes. Because of the arrest of complement activation, this study analyzed binding of the main alternative pathway regulator, factor H (fH), to the mf. The mf bound fH after incubation in nonimmune human serum or with purified radiolabeled fH. In the presence of factor I, mf-bound fH promoted the cleavage of complement 3 molecule b (C3b) to iC3b. An analysis with recombinant constructs of fH showed that the C-terminal short consensus repeats (SCRs) 8-20 of fH bound to mf, whereas the N-terminal SCRs 1-7 containing the complement-regulatory domains in SCRs 1-5 did not. Thus, mf of the nematode O. volvulus may evade human complement by binding fH and by promoting inactivation of C3b into iC3b.

Complement inhibitor factor H binding to Lyme disease spirochetes is mediated by inducible expression of multiple plasmid-encoded outer surface protein E paralogs.

Borrelia burgdorferi spirochetes can circumvent the vertebrate host's immune system for long periods of time. B. burgdorferi sensu stricto and B. afzelii, but not B. garinii, bind the complement inhibitor factor H to protect themselves against complement-mediated opsonophagocytosis and killing. We found that factor H binding and complement resistance are due to inducible expression of a wide repertoire of outer surface protein E (OspE) lipoproteins variably called OspE, p21, ErpA, and ErpP. Individual Borrelia strains carry multiple plasmid-encoded OspE paralogs. Together the OspE homologs were found to constitute an array of proteins that bind factor H via multiple C-terminal domains that are exposed outwards from the Borrelial surface. Charged residue substitutions in the key binding regions account for variations between OspE family members in the optimal binding pH, temperature, and ionic strength. This may help the spirochetes to adapt into various host environments. Our finding that multiple plasmid-encoded OspE proteins act as virulence factors of Borrelia can provide new tools for the prevention and treatment of borreliosis.