Mechanisms of Activation and Evasion of The Complement System by Pathogens

Editors

Lourdes Isaac

University of São Paulo

Tatiana Rodrigues Fraga

University of São Paulo

Angela Silva Barbosa

Butantan Institute

Allan Zajac

University of Alabama at Birmingham

Impact

Review

16 June 2017

Complement Evasion Strategies of Viruses: An Overview

Palak Agrawal

, 3 more and

Arvind Sahu

Activation pathways of the complement system and their targeting by viruses. The complement system is activated primarily by three pathways – CP, LP, and AP. Upper panel: In the CP, antigen-antibody complexes formed on the pathogen surface are recognized by the C1 complex (1) whereas in the LP, specific carbohydrates on the pathogen surface are recognized by MBL/ficolin-MASP complex (2). Both these complexes, upon activation, cleave C4 and C2 that results in the generation of C4bC2a (CP/LP C3 convertase) (3). The CP/LP C3 convertase cleaves C3 into C3b and C3a, where C3b binds and opsonises the pathogen surface and C3a boosts the acquired immune responses (4). When C3b combines with the pre-existing CP/LP C3 convertase, it forms CP/LP C5 convertase (5). In the AP, spontaneous hydrolysis of native C3 by H2O (tick-over process) results in the formation of C3b-like C3 [C3(H2O)] (6), which binds factor B (FB) and upon cleavage by factor D (FD) forms the initial AP C3 convertase (7). The initial AP C3 convertase then cleaves C3 into C3b and C3a (8). The generated C3b molecules bind to the pathogen surface and initiate the formation of surface-bound AP C3 convertase, C3bBb, with the help of FB and FD (9). The surface-bound AP C3 convertase initiates the AP amplification loop (10) resulting in deposition of millions of C3b molecules onto the pathogen surface. Similar to the CP and LP, when C3b combines to the pre-existing AP C3 convertase, it forms the AP C5 convertase (11). Lower panel: The C5 convertases cleave C5 into C5b and C5a (12), where C5b binds to C6 and C7 to form a trimer (C5b-7) (13) that binds to the pathogen surface, while C5a boosts the acquired immune responses. Further binding of C8 to the trimer results in the formation of C5b-8 that penetrates the membrane (14). Finally, C9 binding to C5b-8 and its polymerization completes the MAC formation leading to lysis (15). These activation pathways are regulated at different steps by host complement regulators like factor H (FH), MCP (CD46) complement receptor-1 (CR-1; CD35), DAF (CD55) and C4b-binding protein (C4BP). Viral proteins that target these pathways are: VCP, SPICE, MOPICE, Kaposi’s sarcoma-associated herpesvirus inhibitor of complement activation (KAPOSICA); γ-HV68 RCA, HVS CCPH; non-structural protein 1 of Flaviviruses (NS1); non-structural protein 3/4A of Hepacivirus (NS3/4A); glycoprotein C of HSV-1 (gC-1) and -2 (gC-2), human astrovirus coat protein (CoPt) and M1 protein of INFLV (M1).

Being a major first line of immune defense, the complement system keeps a constant vigil against viruses. Its ability to recognize large panoply of viruses and virus-infected cells, and trigger the effector pathways, results in neutralization of viruses and killing of the infected cells. This selection pressure exerted by complement on viruses has made them evolve a multitude of countermeasures. These include targeting the recognition molecules for the avoidance of detection, targeting key enzymes and complexes of the complement pathways like C3 convertases and C5b-9 formation – either by encoding complement regulators or by recruiting membrane-bound and soluble host complement regulators, cleaving complement proteins by encoding protease, and inhibiting the synthesis of complement proteins. Additionally, viruses also exploit the complement system for their own benefit. For example, they use complement receptors as well as membrane regulators for cellular entry as well as their spread. Here, we provide an overview on the complement subversion mechanisms adopted by the members of various viral families including Poxviridae, Herpesviridae, Adenoviridae, Flaviviridae, Retroviridae, Picornaviridae, Astroviridae, Togaviridae, Orthomyxoviridae and Paramyxoviridae.

35,365 views

151 citations

Major strategies involved in host complement system evasion by T. cruzi. The three complement pathways are inhibited by T. cruzi trypomastigotes. CP inhibition: (A) plasma membrane derived vesicles (PMV), inactivating the C3 convertase; (B,C) TcCRT, binding to C1q, C1r and C1s and inactivates the C1 complex; (D) T-DAF and (E) TcCRP, inactivating the C3 convertase. AP inactivation: (D) T-DAF, (E) TcCRP, (F) FH and (G) gp 58/68, inhibiting the C3 convertase formation. LP inhibition: (A) PMV, inactivating the C3 convertase; (B) TcCRT, binding to MBL and L-Ficolin and (H) CRIT, binding and inactivating C2.

Review

01 September 2017

Trypanosoma cruzi Evades the Complement System as an Efficient Strategy to Survive in the Mammalian Host: The Specific Roles of Host/Parasite Molecules and Trypanosoma cruzi Calreticulin

Galia Ramírez-Toloza

and

Arturo Ferreira

6,808 views

42 citations

Mini Review

18 May 2017

Factor H Family Proteins in Complement Evasion of Microorganisms

Mihály Józsi

Human-pathogenic microbes possess various means to avoid destruction by our immune system. These include interactions with the host complement system that may facilitate pathogen entry into cells and tissues, expression of molecules that defuse the effector complement components and complexes, and acquisition of host complement inhibitors to downregulate complement activity on the surface of the pathogen. A growing number of pathogenic microorganisms have acquired the ability to bind the complement inhibitor factor H (FH) from body fluids and thus hijack its host protecting function. In addition to FH, binding of FH-related (FHR) proteins was also demonstrated for several microbes. Initial studies assumed that these proteins are complement inhibitors similar to FH. However, recent evidence suggests that FHR proteins may rather enhance complement activation both directly and also by competing with the inhibitor FH for binding to certain ligands and surfaces. This mini review focuses on the role of the main alternative pathway regulator FH in host–pathogen interactions, as well as on the emerging role of the FHR proteins as enhancers of complement activation.

20,796 views

60 citations

Supernatants of pathogenic (SPL) proteases cleave purified component C5 and C5b from the MAC. (A) Different concentrations (1) 0.6 μg; (2); 0.12 μg; (3) 0.06 μg of SPL from Leptospira interrogans serovar Kennewicki strain Fromm or (4) 0.06 μg SNPL from L. interrogans serovar Patoc strain Patoc I were incubated with 3 μg of purified C5 (left) for 1 or 4 h at 37°C. (B) Alternatively, SPL was incubated with 10% normal human serum (NHS) for 1 h at 37°C (right). The cleavage products were separated after SDS-PAGE and analyzed by Western blot using polyclonal anti-human C5. (C) SPL (Fromm) and SNPL (Patoc) were incubated with 5 μg of purified SC5b-9 complex for 4 h at 37°C. The cleavage product was analyzed by under non-reducing conditions by Western blot using polyclonal anti-C5 antibody.

Original Research

30 May 2017

Pathogenic Leptospira Secreted Proteases Target the Membrane Attack Complex: A Potential Role for Thermolysin in Complement Inhibition

Thais A. Amamura

, 3 more and

Lourdes Isaac

3,226 views

33 citations

Mini Review

12 May 2017

Evasion Mechanisms Used by Pathogens to Escape the Lectin Complement Pathway

Anne Rosbjerg

, 2 more and

Peter Garred

Activation and evasion of lectin pathway (LP) of complement. (A) The LP is activated when the pattern recognition molecules (PRMs) Mannose-Binding Lectin (MBL), ficolins or collectin-10/-11 recognize pathogen associated molecular patterns (PAMPs) on microbial surfaces. Pathogen recognition activates the PRM-associated serine proteases, MASPs, that catalyze C4 and C2 cleavage, leading to C3 convertase (C4b2a) formation. The C3 convertase cleaves C3 into the opsonin C3b and the anaphylatoxin C3a. Activation of C3 also leads to downstream formation of the C5 convertase (C4b2a3b) which cleaves C5 into the anaphylatoxin C5a and the fragment C5b. Attachment of C5b to the pathogen surface initiates formation of the lytic terminal membrane attack complex (C5b-9). The functions of these generated cleavage products include opsonization and lysis of the pathogen as well as generation of an inflammatory response. Complement inhibitory proteins like C1-INH and C4BP prevent excessive complement activation on host cells. (B) Microorganisms have developed multiple ways to evade complement actions and the mechanisms known to interfere with LP activation are: (1) masking of the PAMPs and thus avoiding being recognized by PRMs, (2) surface expression or secretion of proteins that bind and inhibit LP PRMs by disruption of the PRM:MASP complex and/or by impairment of PRM ligand-binding, (3) secretion of proteases that cleave and destruct LP components, (4) recruitment of the host’s complement inhibitory proteins; C1-INH that inhibits the MASP activity and C4BP that inactivates C4b, (5) utilization of LP components for voluntary opsonization by intracellular pathogens, (6) prevention of C3 convertase assembly by hijacking C2 via a surface expressed protein or by blocking the C2 binding-site on C4b via a secreted protein.

The complement system is a crucial defensive network that protects the host against invading pathogens. It is part of the innate immune system and can be initiated via three pathways: the lectin, classical and alternative activation pathway. Overall the network compiles a group of recognition molecules that bind specific patterns on microbial surfaces, a group of associated proteases that initiates the complement cascade, and a group of proteins that interact in proteolytic complexes or the terminal pore-forming complex. In addition, various regulatory proteins are important for controlling the level of activity. The result is a pro-inflammatory response meant to combat foreign microbes. Microbial elimination is, however, not a straight forward procedure; pathogens have adapted to their environment by evolving a collection of evasion mechanisms that circumvent the human complement system. Complement evasion strategies features different ways of exploiting human complement proteins and moreover features different pathogen-derived proteins that interfere with the normal processes. Accumulated, these mechanisms target all three complement activation pathways as well as the final common part of the cascade. This review will cover the currently known lectin pathway evasion mechanisms and give examples of pathogens that operate these to increase their chance of invasion, survival and dissemination.

29,003 views

20 citations

Complement evasion strategies of Escherichia coli. To overcome complement attack, E. coli has evolved diverse immune evasion strategies: (A) capsular polysaccharide may confer resistance to complement-mediated killing. Factor H (FH), the main soluble regulator of the alternative pathway, may interact with polyanions such as sialic acid, present in the K1 capsular polysaccharide of E. coli. Downregulation of the alternative pathway may occur, thereby impairing membrane attack complex (MAC) deposition. (B) E. coli proteases such as Pic and extracellular serine protease P (EspP) may cleave complement components of all three pathways or, in the case of StcE, may recruit C1 inhibitor (C1-INH) to the surface of target cells, locally downregulating complement activation by the classical pathway. (C) Acquisition of the soluble complement regulators FH and C4b-binding protein (C4BP) by outer membrane protein W (OmpW) and by outer membrane protein A (OmpA)/new lipoprotein I (NlpI) may allow downregulation of the alternative and the classical/lectin pathways of complement, respectively. Binding of proteins of the FH family to Stx2 may result in complement activation contributing to tissue damage (renal injury) observed during the course of hemolytic uremic syndrome caused by enterohemorrhagic E. coli.

Mini Review

20 April 2017

How Escherichia coli Circumvent Complement-Mediated Killing

Afonso G. Abreu

and

Angela S. Barbosa

12,151 views

67 citations

Review

20 April 2017

The Complement System: A Prey of Trypanosoma cruzi

Kárita C. F. Lidani

, 2 more and

Iara J. de Messias-Reason

Trypanosoma cruzi is a protozoan parasite known to cause Chagas disease (CD), a neglected sickness that affects around 6–8 million people worldwide. Originally, CD was mainly found in Latin America but more recently, it has been spread to countries in North America, Asia, and Europe due the international migration from endemic areas. Thus, at present CD represents an important concern of global public health. Most of individuals that are infected by T. cruzi may remain in asymptomatic form all lifelong, but up to 40% of them will develop cardiomyopathy, digestive mega syndromes, or both. The interaction between the T. cruzi infective forms and host-related immune factors represents a key point for a better understanding of the physiopathology of CD. In this context, the complement, as one of the first line of host defense against infection was shown to play an important role in recognizing T. cruzi metacyclic trypomastigotes and in controlling parasite invasion. The complement consists of at least 35 or more plasma proteins and cell surface receptors/regulators, which can be activated by three pathways: classical (CP), lectin (LP), and alternative (AP). The CP and LP are mainly initiated by immune complexes or pathogen-associated molecular patterns (PAMPs), respectively, whereas AP is spontaneously activated by hydrolysis of C3. Once activated, several relevant complement functions are generated which include opsonization and phagocytosis of particles or microorganisms and cell lysis. An important step during T. cruzi infection is when intracellular trypomastigotes are release to bloodstream where they may be target by complement. Nevertheless, the parasite uses a sequence of events in order to escape from complement-mediated lysis. In fact, several T. cruzi molecules are known to interfere in the initiation of all three pathways and in the assembly of C3 convertase, a key step in the activation of complement. Moreover, T. cruzi promotes secretion of plasma membrane-derived vesicles from host cells, which prevent the activity of C3 convertase C4b2a and thereby may hinder complement. In this review, we aim to present an overview on the strategies used by T. cruzi in order to circumvent the activation of complement and, consequently, its biological effects.

19,395 views

67 citations

Structural comparison of AvGAPDH and human liver GAPDH. Ribbon representations of the AvGAPDH and human liver GAPDH (PDB ID 1ZNQ) holoenzymes in gray. The molecular surfaces of two subunits are shown in wheat color. The NAD+ cofactor bound to the two subunits represented as ribbons are shown as sphere models in CPK color. Regions where structural differences are more prominent are shown in teal, boxed, and magnified as panels (A–C). (A) S loop. (B) Structurally distinct loop regions on the outer surface of AvGAPDH. (C) Adenine ring subpocket of the active site. Several pharmacologically relevant compounds have been discovered that target the adenine ring subpocket of GAPDH from trypanosomatids. The superposition, molecular surface, and (A–C) were produced with PyMOL (Schrodinger LLC). (D) Sequence alignment including the protein sequences of AvGAPDH and human liver GAPDH.

Original Research

10 April 2017

Crystal Structure of Glyceraldehyde-3-Phosphate Dehydrogenase from the Gram-Positive Bacterial Pathogen A. vaginae, an Immunoevasive Factor that Interacts with the Human C5a Anaphylatoxin

Javier Querol-García

, 10 more and

M. Cristina Vega

6,386 views

24 citations

Review

20 February 2017

Role of Streptococcus pneumoniae Proteins in Evasion of Complement-Mediated Immunity

Greiciely O. Andre

, 5 more and

Michelle Darrieux

Role of pneumococcal virulence factors in evasion from the Complement System. The polysaccharide capsule prevents binding of IgG, IgM, and CRP to the bacterial surface, as well as C3b and iC3b, thereby impairing complement activation by classical (CP) and alternative pathways (AP). The proteins have been grouped according to their interactions with complement. T indicates direct inhibition by pneumococcal antigens. PspA affects C3 convertase formation by interfering with FB and prevents CRP binding to phophocholine on the bacterial wall, thus inhibiting activation of the classical pathway. PspC binds to FH and C4BP promoting the inhibition of C3 convertase formation and accelerating C3 convertase decay. PspC also cleaves C3 molecules generating products that cannot activate complement. Binding of PspC to Vitronectin reduces MAC formation. Phts bind to FH and cleave C3. LytA binding to FH and C4BP reduces C3 convertase formation and promotes its dissociation. It also inhibits the interaction between CRP and C1q. In addition, LytA is able to split up C3b and iC3b. PLY released from the bacterium activates the classical pathway through interactions with IgG, C1q, and L-ficolin, and depletes complement. The exoglycosidases NanA, BgaA, and StrH remove sialic acid (SA) from complement components; also, SA favors FH activity in regulation of C3 convertase. PepO binds to C1q and activates complement when released from the pneumococcal surface, leading to depletion of complement components. PepO binding to C4BP results in down regulation of classical pathway activation. Eno decreases C3 convertase formation by binding to C4BP. GAPDH on its free form or attached to the bacterial surface binds to C1q, likely promoting complement activation. PGK impairs MAC assembly by binding to C5, C7, and C9. Tuf binds to FH and FHL-1 inhibiting formation and accelerating dissociation of C3 convertase. Tuf can also bind to CFHR-1, but the implications of this interaction are not completely elucidated. The main result of such interactions is an impaired bacterial phagocytosis. Also, pneumococcal proteins use complement molecules as bridges to interact with host receptors and favor bacterial adherence and invasion. PspC exploits FH to adhere and invade host cells. PepO uses C1q to increase bacterial adherence. RrgA improves colonization by binding to complement receptor 3 (CR3). Finally, the ability to interact with PLG – as demonstrated for PepO, Eno, and GAPDH with PLG was shown to degrade the host extracellular matrix components (ECM) and to improve invasion, while PepO, PGK and Tuf bound to PLG inactivate C3 and C3b.

The complement system plays a central role in immune defense against Streptococcus pneumoniae. In order to evade complement attack, pneumococci have evolved a number of mechanisms that limit complement mediated opsonization and subsequent phagocytosis. This review focuses on the strategies employed by pneumococci to circumvent complement mediated immunity, both in vitro and in vivo. At last, since many of the proteins involved in interactions with complement components are vaccine candidates in different stages of validation, we explore the use of these antigens alone or in combination, as potential vaccine approaches that aim at elimination or drastic reduction in the ability of this bacterium to evade complement.

17,541 views

78 citations