Inadequate binding of immune regulator factor H is associated with sensitivity of Borrelia lusitaniae to human complement

Outer surface protein E (OspE) mediates Borrelia burgdorferi sensu stricto strain-specific complement evasion in the eastern fence lizard, Sceloporus undulatus

In North America, Lyme disease is primarily caused by the spirochetal bacterium Borrelia burgdorferi sensu stricto (Bb), which is transmitted between multiple vertebrate hosts and ixodid ticks, and is a model commonly used to study host-pathogen interactions. While Bb is consistently observed in its mammalian and avian reservoirs, the bacterium is rarely isolated from North American reptiles. Two closely related lizard species, the eastern fence lizard (Sceloporus undulatus) and the western fence lizard (Sceloporus occidentalis), are examples of reptiles parasitized by Ixodes ticks. Vertebrates are known to generate complement as an innate defense mechanism, which can be activated before Bb disseminate to distal tissues. Complement from western fence lizards has proven lethal against one Bb strain, implying the role of complement in making those lizards unable to serve as hosts to Bb. However, Bb DNA is occasionally identified in distal tissues of field-collected eastern fence lizards, suggesting some Bb strains may overcome complement-mediated clearance in these lizards. These findings raise questions regarding the role of complement and its impact on Bb interactions with North American lizards. In this study, we found Bb seropositivity in a small population of wild-caught eastern fence lizards and observed Bb strain-specific survivability in lizard sera. We also found that a Bb outer surface protein, OspE, from Bb strains viable in sera, promotes lizard serum survivability and binds to a complement inhibitor, factor H, from eastern fence lizards. Our data thus identify bacterial and host determinants of eastern fence lizard complement evasion, providing insights into the role of complement influencing Bb interactions with North American lizards.

The gut anti-complement activity of Aedes aegypti: Investigating new ways to control the major human arboviruses vector in the Americas

Aedes aegypti is the main urban vector of dengue virus, chikungunya virus and Zika virus due to its great dispersal capacity and virus susceptibility. A. aegypti feed on plant-derived sugars but females need a blood meal for egg maturation. Haematophagous arthropods need to overcome host haemostasis and local immune reactions in order to take a blood meal. In this context, molecules present in the saliva and/or intestinal contents of these arthropods must contain inhibitors of the complement system (CS). CS salivary and/or intestinal inhibitors are crucial to protect gut cells of haematophagous arthropods against complement attack. The present work aimed to investigate the anti-complement activity of A. aegypti intestinal contents on the alternative, classical and lectin pathways of the human complement system. Here we show that A. aegypti gut contents inhibited the human classical and the lectin pathways but not the alternative pathway. The A. aegypti gut content has a serine protease able to specifically cleave and inactivate human C4, which is a novel mechanism for human complement inactivation in haematophagous arthropods. The gut of female A. aegypti was capable of capturing human serum factor H (a negative complement modulator), unlike males. C3 molecules in recently blood-fed female A. aegypti remain in their original state, being inactivated to iC3b soon after a blood feed. A transmission-blocking vaccine using these complement inhibitory proteins as antigens has the potential to interfere with the insect's survival, reproductive fitness and block their infection by the arboviruses they transmit to humans.

Further Insights Into the Interaction of Human and Animal Complement Regulator Factor H With Viable Lyme Disease Spirochetes

Spirochetes belonging to the Borrelia (B.) burgdorferi sensu lato (s.l.) complex differ in their ability to establish infection and to survive in diverse vertebrate hosts. Association with and adaption to various hosts most likely correlates with the spirochetes' ability to acquire complement regulator factor H (FH) to overcome the host's innate immune response. Here we assessed binding of serum FH from human and various animals including bovine, cat, chicken, dog, horse, mouse, rabbit, and rat to viable B. burgdorferi sensu stricto (s.s.), B. afzelii, B. garinii, B. spielmanii, B. valaisiana, and B. lusitaniae. Spirochetes ectopically producing CspA orthologs of B. burgdorferi s.s., B. afzelii, and B. spielmanii, CspZ, ErpC, and ErpP, respectively, were also investigated. Our comparative analysis using viable bacterial cells revealed a striking heterogeneity among Lyme disease spirochetes regarding their FH-binding patterns that almost mirrors the serum susceptibility of the respective borrelial genospecies. Moreover, native CspA from B. burgdorferi s.s., B. afzelii, and B. spielmanii as well as CspZ were identified as key ligands of FH from human, horse, and rat origin while ErpP appears to bind dog and mouse FH and to a lesser extent human FH. By contrast, ErpC did not bind FH from human as well as from animal origin. These findings indicate a strong restriction of distinct borrelial proteins toward binding of polymorphic FH of various vertebrate hosts.

There Is a Method to the Madness: Strategies to Study Host Complement Evasion by Lyme Disease and Relapsing Fever Spirochetes

Lyme disease and relapsing fever are caused by various Borrelia species. Lyme disease borreliae, the most common vector-borne pathogens in both the U.S. and Europe, are transmitted by Ixodes ticks and disseminate from the site of tick bites to tissues leading to erythema migrans skin rash, arthritis, carditis, and neuroborreliosis. Relapsing fever borreliae, carried by ticks and lice, trigger reoccurring fever episodes. Following transmission, spirochetes survive in the blood to induce bacteremia at the early stages of infection, which is thought to promote evasion of the host complement system. The complement system acts as an important innate immune defense mechanism in humans and vertebrates. Upon activation, the cleaved complement components form complexes on the pathogen surface to eventually promote bacteriolysis. The complement system is negatively modulated by a number of functionally diverse regulators to avoid tissue damage. To evade and inhibit the complement system, spirochetes are capable of binding complement components and regulators. Complement inhibition results in bacterial survival in serum (serum resistance) and is thought to promote bloodstream survival, which facilitates spirochete dissemination and disease manifestations. In this review, we discuss current methodologies to elucidate the mechanisms of Borrelia spp. that promote serum resistance and bloodstream survival, as well as novel methods to study factors responsible for bloodstream survival of Lyme disease borreliae that can be applied to relapsing fever borreliae. Understanding the mechanisms these pathogens utilize to evade the complement system will ultimately aid in the development of novel therapeutic strategies and disease prevention to improve human health.

Hide and Seek: How Lyme Disease Spirochetes Overcome Complement Attack

Overcoming the first line of the innate immune system is a general hallmark of pathogenic microbes to avoid recognition and to enter the human host. In particular, spirochetes belonging to the Borrelia burgdorferi sensu lato complex have developed various means to counter the immune response and to successfully survive in diverse host environments for a prolonged period of time. In regard to complement resistance, Borrelia utilize a plethora of immune evasion strategies involves capturing of host-derived complement regulators, terminating complement activation as well as shedding of cell-destroying complement complexes to manipulate and to expeditiously inhibit human complement. Owing to their mode of action, the interacting surface-exposed proteins identified among B. burgdorferi sensu stricto (s.s.), Borrelia afzelii, Borrelia spielmanii, and Borrelia bavariensis can be classified into at least two major categories, namely, molecules that directly interfere with distinct complement components including BBK32, CspA, BGA66, BGA71, and a CD59-like protein or molecules, which indirectly counteract complement activation by binding various complement regulators such as Factor H, Factor H-like protein 1 (FHL-1), Factor H-related proteins FHR-1, FHR-2, or C4Bp. The latter group of genetically and structurally unrelated proteins has been collectively referred to as “complement regulator-acquiring surface proteins” and consists of CspA, CspZ, ErpA, ErpC, ErpP, and the as yet unidentified protein p43. This review focuses on the current knowledge of immune evasion mechanisms exhibited by Lyme disease spirochetes and highlights the role of complement-interfering, infection-associated molecules playing an important part in these processes. Deciphering the immune evasion strategies may provide novel avenues for improved diagnostic approaches and therapeutic interventions.

Travelling between Two Worlds: Complement as a Gatekeeper for an Expanded Host Range of Lyme Disease Spirochetes

Evading innate immunity is a prerequisite for pathogenic microorganisms in order to survive in their respective hosts. Concerning Lyme disease spirochetes belonging to the Borrelia (B.) burgdorferi sensu lato group, a broad range of diverse vertebrates serve as reservoir or even as incidental hosts, including humans. The capability to infect multiple hosts implies that spirochetes have developed sophisticated means to counter the destructive effects of complement of humans and various animals. While the means by which spirochetes overcome the hosts immune defense are far from being completely understood, there is a growing body of evidence suggesting that binding of the key regulator of the alternative pathway, Factor H, plays a pivotal role for immune evasion and that Factor H is an important determinant of host specificity. This review covers (i) the contribution of complement in host-specificity and transmissibility of Lyme disease spirochetes; (ii) the involvement of borrelial-derived determinants to host specificity; (iii) the interplay of human and animal Factor H with complement-acquiring surface proteins of diverse borrelial species; and (iv) the potential role of additional animal complement proteins in the immune evasion of spirochetes.

BGA66 and BGA71 facilitate complement resistance of Borrelia bavariensis by inhibiting assembly of the membrane attack complex

Borrelia (B.) bavariensis exhibits a marked tropism for nervous tissues and frequently causes neurological manifestations in humans. The molecular mechanism by which B. bavariensis overcomes innate immunity, in particular, complement remains elusive. In contrast to other serum-resistant spirochetes, none of the B. bavariensis isolates investigated bound complement regulators of the alternative (AP) and classical pathway (CP) or proteolytically inactivated complement components. Focusing on outer surface proteins BGA66 and BGA71, we demonstrated that both molecules either inhibit AP, CP and terminal pathway (TP) activation, or block activation of the CP and TP respectively. Both molecules bind complement components C7, C8 and C9, and thereby prevent assembly of the terminal complement complex. This inhibitory activity was confirmed by the introduction of the BGA66 and BGA71 encoding genes into a serum-sensitive B. garinii strain. Transformed spirochetes producing either BGA66 or BGA71 overcome complement-mediated killing, thus indicating that both proteins independently facilitate serum resistance of B. bavariensis. The generation of C-terminally truncated proteins as well as a chimeric BGA71 protein lead to the localization of the complement-interacting binding site within the N-terminus. Collectively, our data reveal a novel immune evasion strategy of B. bavariensis that is directed against the activation of the TP.

Identification of Borrelia species after creation of an in-house MALDI-TOF MS database

Lyme borreliosis (LB) is a multisystemic disease caused by Borrelia burgdorferi sensu lato (sl) complex transmitted to humans by Ixodes ticks. B. burgdorferi sl complex, currently comprising at least 19 genospecies, includes the main pathogenic species responsible for human disease in Europe: B. burgdorferi sensu stricto (ss), B. afzelii, and B. garinii. In this study, for the first time, MALDI-TOF MS was applied to Borrelia spp., supplementing the existing database, limited to the species B. burgdorferi ss, B.spielmanii and B. garinii, with the species B. afzelii, in order to enable the identification of all the species potentially implicated in LB in Europe. Moreover, we supplemented the database also with B. hermsii, which is the primary cause of tick-borne relapsing fever in western North America, B. japonica, circulating in Asia, and another reference strain of B. burgdorferi ss (B31 strain). The dendrogram obtained by analyzing the protein profiles of the different Borrelia species reflected Borrelia taxonomy, showing that all the species included in the Borrelia sl complex clustered in a unique branch, while Borrelia hermsii clustered separately. In conclusion, in this study MALDI-TOF MS proved a useful tool suitable for identification of Borrelia spp. both for diagnostic purpose and epidemiological surveillance.

Factor H-related proteins

Factor H-related proteins (CFHRs) are plasma glycoproteins related in structure and antigenicity to each other and to the complement inhibitory protein factor H. Such proteins are found in most mammals but their number and domain composition vary. This chapter summarizes our current knowledge on the human factor H-related proteins. In contrast to factor H, they have no strong complement inhibitory activity, although for some of them regulatory or complement modulatory activity has been reported. A common feature of CFHRs is that they bind to the C3b component of complement. Novel links between CFHRs and various diseases (C3 glomerulopathies, atypical hemolytic uremic syndrome and age-related macular degeneration) have been revealed in recent years, but we are still far from understanding their biological function.

Borrelia valaisiana resist complement-mediated killing independently of the recruitment of immune regulators and inactivation of complement components

Spirochetes belonging to the Borrelia (B.) burgdorferi sensu lato complex differ in their resistance to complement-mediated killing, particularly in regard to human serum. In the present study, we elucidate the serum and complement susceptibility of B. valaisiana, a genospecies with the potential to cause Lyme disease in Europe as well as in Asia. Among the investigated isolates, growth of ZWU3 Ny3 was not affected while growth of VS116 and Bv9 was strongly inhibited in the presence of 50% human serum. Analyzing complement activation, complement components C3, C4 and C6 were deposited on the surface of isolates VS116 and Bv9, and similarly the membrane attack complex was formed on their surface. In contrast, no surface-deposited components and no aberrations in cell morphology were detected for serum-resistant ZWU3 Ny3. While further investigating the protective role of bound complement regulators in mediating complement resistance, we discovered that none of the B. valaisiana isolates analyzed bound complement regulators Factor H, Factor H-like protein 1, C4b binding protein or C1 esterase inhibitor. In addition, B. valaisiana also lacked intrinsic proteolytic activity to degrade complement components C3, C3b, C4, C4b, and C5. Taken together, these findings suggest that certain B. valaisiana isolates differ in their capability to resist complement-mediating killing by human serum. The molecular mechanism utilized by B. valaisiana to inhibit bacteriolysis appears not to involve binding of the key host complement regulators of the alternative, classical, and lectin pathways as already known for serum-resistant Lyme disease or relapsing fever borreliae.

TRIF mediates Toll-like receptor 2-dependent inflammatory responses to Borrelia burgdorferi

TRIF is an adaptor molecule important in transducing signals from intracellularly signaling Toll-like receptor 3 (TLR3) and TLR4. Recently, TLR2 was found to signal from intracellular compartments. Using a synthetic ligand for TLR2/1 heterodimers, as well as Borrelia burgdorferi, which is a strong activator of TLR2/1, we found that TLR2 signaling can utilize TRIF. Unlike TRIF signaling by other TLRs, TLR2-mediated TRIF signaling is dependent on the presence of another adaptor molecule, MyD88. However, unlike MyD88 deficiency, TRIF deficiency does not result in diminished control of infection with B. burgdorferi in a murine model of disease. This appears to be due to the effects of MyD88 on phagocytosis via scavenger receptors, such as MARCO, which are not affected by the loss of TRIF. In mice, TRIF deficiency did have an effect on the production of inflammatory cytokines, suggesting that regulation of inflammatory cytokines and control of bacterial growth may be uncoupled, in part through transduction of TLR2 signaling through TRIF.

Complement regulator-acquiring surface proteins of Borrelia burgdorferi: Structure, function and regulation of gene expression

Borrelia burgdorferi, the etiological agent of Lyme disease, exploits an array of strategies to establish infection and to overcome host innate and adaptive immune responses. One key borrelial immune escape mechanism involves the inactivation of host complement attack through acquisition of human immune regulators factor H (CFH), factor H-like protein 1 (FHL1), factor H-related protein 1 (CFHR1), CFHR2, and/or CFHR5. Binding of these host proteins is primarily mediated by bacterial surface-exposed proteins that have been collectively referred to as complement regulator-acquiring surface proteins, or CRASPs. Different strains of B. burgdorferi produce as many as 5 different CRASP molecules that comprise 3 distinct, genetically unrelated groups. Depending on bacterial genetic composition, different combinations of these proteins can be found on the borrelial outer surface. The 3 groups differ in their gene location, gene regulatory mechanisms, expression patterns during the tick-mammal infection cycle, protein sequence and structure as well as binding affinity for complement regulators and other serum proteins. These attributes influence the proteins' abilities to contribute to complement resistance of this emerging human pathogen. In this review, we focus on the current knowledge on structure, function, and gene regulation of these B. burgdorferi infection-associated proteins.

Contribution of the infection-associated complement regulator-acquiring surface protein 4 (ErpC) to complement resistance of Borrelia burgdorferi

Borrelia burgdorferi evades complement-mediated killing by interacting with complement regulators through distinct complement regulator-acquiring surface proteins (CRASPs). Here, we extend our analyses to the contribution of CRASP-4 in mediating complement resistance of B. burgdorferi and its interaction with human complement regulators. CRASP-4 (also known as ErpC) was immobilized onto magnetic beads and used to capture proteins from human serum. Following Western blotting, factor H (CFH), CFH-related protein 1 (CFHR1), CFHR2, and CFHR5 were identified as ligands of CRASP-4. To analyze the impact of native CRASP-4 on mediating survival of serum-sensitive cells in human serum, a B. garinii strain was generated that ectopically expresses CRASP-4. CRASP-4-producing bacteria bound CFHR1, CFHR2, and CFHR5 but not CFH. In addition, transformed spirochetes deposited significant amounts of lethal complement components on their surface and were susceptible to human serum, thus indicating that CRASP-4 plays a subordinate role in complement resistance of B. burgdorferi.