The lectin self of complement factor H

Roles of Siglecs in neurodegenerative diseases

Microglia are resident myeloid cells in the central nervous system (CNS) with a unique developmental origin, playing essential roles in developing and maintaining the CNS environment. Recent studies have revealed the involvement of microglia in neurodegenerative diseases, such as Alzheimer's disease, through the modulation of neuroinflammation. Several members of the Siglec family of sialic acid recognition proteins are expressed on microglia. Since the discovery of the genetic association between a polymorphism in the CD33 gene and late-onset Alzheimer's disease, significant efforts have been made to elucidate the molecular mechanism underlying the association between the polymorphism and Alzheimer's disease. Furthermore, recent studies have revealed additional potential associations between Siglecs and Alzheimer's disease, implying that the reduced signal from inhibitory Siglec may have an overall protective effect in lowering the disease risk. Evidences suggesting the involvement of Siglecs in other neurodegenerative diseases are also emerging. These findings could help us predict the roles of Siglecs in other neurodegenerative diseases. However, little is known about the functionally relevant Siglec ligands in the brain, which represents a new frontier. Understanding how microglial Siglecs and their ligands in CNS contribute to the regulation of CNS homeostasis and pathogenesis of neurodegenerative diseases may provide us with a new avenue for disease prevention and intervention.

Structural studies offer a framework for understanding the role of properdin in the alternative pathway and beyond

Structures of alternative pathway proteins have offered a comprehensive structural basis for understanding the molecular mechanisms governing activation and regulation of the amplification pathway of the complement cascade. Although properdin (FP) is required in vivo to sustain a functional alternative pathway, structural studies have been lagging behind due to the extended structure and polydisperse nature of FP. We review recent progress with respect to structure determination of FP and its proconvertase/convertase complexes. These structures identify in detail regions in C3b, factor B and FP involved in their mutual interactions. Structures of FP oligomers obtained by integrative studies have shed light on how FP activity depends on its oligomerization state. The accumulated structural knowledge allows us to rationalize the effect of point mutations causing FP deficiency. The structural basis for FP inhibition by the tick CirpA proteins is reviewed and the potential of alphafold2 predictions for understanding the interaction of FP with other tick proteins and the NKp46 receptor on host immune cells is discussed. The accumulated structural knowledge forms a comprehensive basis for understanding molecular interactions involving FP, pathological conditions arising from low levels of FP, and the molecular strategies used by ticks to suppress the alternative pathway.

The susceptibility of the kidney to alternative pathway activation-A hypothesis

The glomerulus is often the prime target of dysregulated alternative pathway (AP) activation. In particular, AP activation is the key driver of two severe kidney diseases: atypical hemolytic uremic syndrome and C3 glomerulopathy. Both conditions are associated with a variety of predisposing molecular defects in AP regulation, such as genetic variants in complement regulators, autoantibodies targeting AP proteins, or autoantibodies that stabilize the AP convertases (C3- and C5-activating enzymes). It is noteworthy that these are systemic AP defects, yet in both diseases pathologic complement activation primarily affects the kidneys. In particular, AP activation is often limited to the glomerular capillaries. This tropism of AP-mediated inflammation for the glomerulus points to a unique interaction between AP proteins in plasma and this particular anatomic structure. In this review, we discuss the pre-clinical and clinical data linking the molecular causes of aberrant control of the AP with activation in the glomerulus, and the possible causes of this tropism. Based on these data, we propose a model for why the kidney is so uniquely and frequently targeted in patients with AP defects. Finally, we discuss possible strategies for preventing pathologic AP activation in the kidney.

Genetic variability shapes the alternative pathway complement activity and predisposition to complement-related diseases

The implementation of next-generation sequencing technologies has provided a sharp picture of the genetic variability in the components and regulators of the alternative pathway (AP) of the complement system and has revealed the association of many AP variants with different rare and common diseases. An important finding that has emerged from these analyses is that each of these complement-related diseases associate with genetic variants altering specific aspects of the activation and regulation of the AP. These genotype-phenotype correlations have provided valuable insights into their pathogenic mechanisms with important diagnostic and therapeutic implications. While genetic variants in coding regions and structural variants are reasonably well characterized and occasionally have been instrumental to uncover unknown features of the complement proteins, data about complement expressed quantitative trait loci are still very limited. A crucial task for future studies will be to identify these quantitative variations and to determine their impact in the overall activity of the AP. This is fundamental as it is now clear that the consequences of genetic variants in the AP are additive and that susceptibility or resistance to disease is the result of specific combinations of genetic variants in different complement components and regulators ("complotypes").

Variation in CFHR3 determines susceptibility to meningococcal disease by controlling factor H concentrations

Neisseria meningitidis protects itself from complement-mediated killing by binding complement factor H (FH). Previous studies associated susceptibility to meningococcal disease (MD) with variation in CFH, but the causal variants and underlying mechanism remained unknown. Here we attempted to define the association more accurately by sequencing the CFH-CFHR locus and imputing missing genotypes in previously obtained GWAS datasets of MD-affected individuals of European ancestry and matched controls. We identified a CFHR3 SNP that provides protection from MD (rs75703017, p value = 1.1 × 10^-16) by decreasing the concentration of FH in the blood (p value = 1.4 × 10^-11). We subsequently used dual-luciferase studies and CRISPR gene editing to establish that deletion of rs75703017 increased FH expression in hepatocyte by preventing promotor inhibition. Our data suggest that reduced concentrations of FH in the blood confer protection from MD; with reduced access to FH, N. meningitidis is less able to shield itself from complement-mediated killing.

Sialic acid diversity in the human gut: Molecular impacts and tools for future discovery

Sialic acids are a family of structurally related sugars that are prevalent in mucosal surfaces, including the human intestine. In the gut, sialic acids have diverse biological roles at the interface of the host epithelium and the microbiota. N-acetylneuraminic acid (Neu5Ac), the best studied sialic acid, is a nutrient source for bacteria and, when displayed on the cell surface, a binding site for host immune factors, viruses, and bacterial toxins. Neu5Ac is extensively modified by host and microbial enzymes, and the impacts of Neu5Ac derivatives on host-microbe interactions, and generally on human and microbial biology, remain underexplored. In this mini-review, we highlight recent reports describing how host and microbial proteins differentiate Neu5Ac and its derivatives, draw attention to gaps in knowledge related to sialic acid biology, and suggest cutting-edge methodologies that may expand our appreciation and understanding of Neu5Ac in health and disease.

Functional characterization of 105 Factor H variants associated with atypical HUS: lessons for variant classification

Atypical hemolytic uremic syndrome (aHUS) is a life-threatening thrombotic microangiopathy that can progress, when untreated, to end-stage renal disease. Most frequently, aHUS is caused by complement dysregulation due to pathogenic variants in genes that encode complement components and regulators. Amongst these genes, the Factor H (FH) gene, CFH, presents with the highest frequency (15-20%) of variants and is associated with the poorest prognosis. Correct classification of CFH variants as pathogenic or benign is essential to clinical care but remains challenging owing to the dearth of functional studies. As a result, significant numbers of variants are reported as variants of uncertain significance. To address this knowledge gap, we expressed and functionally characterized 105 aHUS-associated FH variants. All FH variants were categorized as pathogenic or benign, and for each, we fully documented the nature of the pathogenicity. Twenty-six previously characterized FH variants were used as controls to validate and confirm the robustness of the functional assays used. Of the remaining 79 uncharacterized variants, only 29 (36.7%) alter FH in vitro expression or function and are therefore proposed to be pathogenic. We show that rarity in control databases is not informative for variant classification, and we identify important limitations in applying prediction algorithms to FH variants. Based on structural and functional data, we suggest ways to circumvent these difficulties and thereby improve variant classification. Our work highlights the need for functional assays to interpret FH variants accurately if clinical care of patients with aHUS is to be individualized and optimized.

Molecular bases for the association of FHR-1 with atypical hemolytic uremic syndrome and other diseases

aHUS-associated FHR-1 mutants are pathogenic because they acquire capacity to bind sialic acids, which allows C3b-binding competition with FH.

The mechanism by which surface-bound FHR-1 promotes complement activation is the binding and attraction of native C3 to the cell surface.

Factor H (FH)–related proteins are a group of partly characterized complement proteins thought to promote complement activation by competing with FH in binding to surface-bound C3b. Among them, FH-related protein 1 (FHR-1) is remarkable because of its association with atypical hemolytic uremic syndrome (aHUS) and other important diseases. Using a combination of biochemical, immunological, nuclear magnetic resonance, and computational approaches, we characterized a series of FHR-1 mutants (including 2 associated with aHUS) and unraveled the molecular bases of the so-called deregulation activity of FHR-1. In contrast with FH, FHR-1 lacks the capacity to bind sialic acids, which prevents C3b-binding competition between FH and FHR-1 in host-cell surfaces. aHUS-associated FHR-1 mutants are pathogenic because they have acquired the capacity to bind sialic acids, which increases FHR-1 avidity for surface-bound C3-activated fragments and results in C3b-binding competition with FH. FHR-1 binds to native C3, in addition to C3b, iC3b, and C3dg. This unexpected finding suggests that the mechanism by which surface-bound FHR-1 promotes complement activation is the attraction of native C3 to the cell surface. Although C3b-binding competition with FH is limited to aHUS-associated mutants, all surface-bound FHR-1 promotes complement activation, which is delimited by the FHR-1/FH activity ratio. Our data indicate that FHR-1 deregulation activity is important to sustain complement activation and C3 deposition at complement-activating surfaces. They also support that abnormally elevated FHR-1/FH activity ratios would perpetuate pathological complement dysregulation at complement-activating surfaces, which may explain the association of FHR-1 quantitative variations with diseases.

A Protective and Pathogenic Role for Complement During Acute Toxoplasma gondii Infection

The infection competence of the protozoan pathogen Toxoplasma gondii is critically dependent on the parasite’s ability to inactivate the host complement system. Toxoplasma actively resists complement-mediated killing in non-immune serum by recruiting host-derived complement regulatory proteins C4BP and Factor H (FH) to the parasite surface to inactivate surface-bound C3 and limit formation of the C5b-9 membrane attack complex (MAC). While decreased complement activation on the parasite surface certainly protects Toxoplasma from immediate lysis, the biological effector functions of C3 split products C3b and C3a are maintained, which includes opsonization of the parasite for phagocytosis and potent immunomodulatory effects that promote pro-inflammatory responses and alters mucosal defenses during infection, respectively. In this review, we discuss how complement regulation by Toxoplasma controls parasite burden systemically but drives exacerbated immune responses locally in the gut of genetically susceptible C57BL/6J mice. In effect, Toxoplasma has evolved to strike a balance with the complement system, by inactivating complement to protect the parasite from immediate serum killing, it generates sufficient C3 catabolites that signal through their cognate receptors to stimulate protective immunity. This regulation ultimately controls tachyzoite proliferation and promotes host survival, parasite persistence, and transmissibility to new hosts.

Effect of red seaweed sulfated galactans on initial steps of complement activation in vitro

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Red algal galactans can participate in cell surface biology involving complement system.

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Polysaccharides inhibit C3 binding to LPS with direct dependence on degree of sulfation.

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Degree of sulfation mattered in carrageenans capacity to reduce C4 binding to mannan.

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C4 binding to antibodies was activated in the presence of carrageenans.

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No relevant structural characteristics were observed in ameliorating C5 cleavage by plasmin.

The research described here presents data on the effect of galactans of red algae, carrageenans (λ/μ/ν-, κ-, κ/β-, and ι/κ-types), and agar on complement system activation in normal human serum. The experiments were based on well surfaces coated with triggering agents for binding initiating complement components —C3 and C4. The sulfated galactans inhibited C3 binding to lipopolysaccharide with direct dependence on the sulfation degree of polysaccharides. Sulfation degree was also important in carrageenans’ capacity to reduce C4 binding to mannan. However, C4 binding to antibodies was considerably activated by carrageenans, especially with 3,6-anhydrogalactose. The gelling carrageenans were able to block antigen binding centers of total serum IgM and with more intensity than non-gelling. No structural characteristics mattered in ameliorating C5 cleavage by plasmin in extrinsic protease complement activation, but λ/μ/ν- and κ/β-carrageenans almost completely inhibited C5 cleavage. Thus, galactans participated in cell surface biology by imitating surface glycans in inhibition of C3 binding and mannose binding lectin, but as to the tthe heclassical pathway these substances stimulated complement, probably due to their structure based on carrabiose.

Sweet turning bitter: Carbohydrate sensing of complement in host defence and disease

The complement system plays a major role in threat recognition and in orchestrating responses to microbial intruders and accumulating debris. This immune surveillance is largely driven by lectins that sense carbohydrate signatures on foreign, diseased and healthy host cells and act as complement activators, regulators or receptors to shape appropriate immune responses. While carbohydrate sensing protects our bodies, misguided or impaired recognition can contribute to disease. Moreover, pathogenic microbes have evolved to evade complement by mimicking host signatures. While complement is recognized as a disease factor, we only slowly start to appreciate the role of carbohydrate interactions in the underlying processes. A better understanding of complement's sweet side will contribute to a better description of disease mechanisms and enhanced diagnostic and therapeutic options. This review introduces the key components in complement-mediated carbohydrate sensing, discusses their role in health and disease, and touches on the potential effects of carbohydrate-related disease intervention. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

Characterization of Binding Properties of Individual Functional Sites of Human Complement Factor H

Factor H exists as a 155,000 dalton, extended protein composed of twenty small domains which is flexible enough that it folds back on itself. Factor H regulates complement activation through its interactions with C3b and polyanions. Three binding sites for C3b and multiple polyanion binding sites have been identified on Factor H. In intact Factor H these sites appear to act synergistically making their individual contributions difficult to distinguish. Recombinantly expressed fragments of human Factor H were examined using surface plasmon resonance (SPR) for interactions with C3, C3b, iC3b, C3c, and C3d. Eleven recombinant proteins of lengths from one to twenty domains were used to show that the three C3b-binding sites exhibit 100-fold different affinities for C3b. The N-terminal site [complement control protein (CCP) domains 1-6] bound C3b with a K_d of 0.08 μM and this interaction was not influenced by the presence or absence of domains 7 and 8. Full length Factor H similarly exhibited a K_d for C3b of 0.1 μM. Unexpectedly, the N-terminal site (CCP 1-6) bound native C3 with a K_d of 0.4 μM. The C-terminal domains (CCP 19-20) exhibited a K_d of 1.7 μM for C3b. We localized a weak third C3b binding site in the CCP 13-15 region with a K_d estimated to be ~15 μM. The C-terminal site (CCP 19-20) bound C3b, iC3b, and C3d equally well with a K_d of 1 to 2 μM. In order to identify and compare regions of Factor H that interact with polyanions a family of 18 overlapping three domain recombinant proteins spanning the entire length of Factor H were expressed and purified. Immobilized heparin was used as a model polyanion and SPR confirmed the presence of heparin binding sites in CCP 6-8 (K_d 1.2 μM) and in CCP 19-20 (4.9 μM) and suggested the existence of a weak third polyanion binding site in the center of Factor H (CCP 11-13). Our results unveil the relative contributions of different regions of Factor H to its regulation of complement, and may contribute to the understanding of how defects in certain Factor H domains lead to disease.

Siglecs that Associate with DAP12

Siglecs are a family of transmembrane receptor-like glycan-recognition proteins expressed primarily on leukocytes. Majority of Siglecs have an intracellular sequence motif called immunoreceptor tyrosine-based inhibitory motif (ITIM) and associate with Src homology region 2 domain-containing tyrosine phosphatase-1 (SHP-1), and negatively regulate tyrosine phosphorylation-mediated intracellular signaling events. On the other hand, some Siglecs have a positively charged amino acid residue in the transmembrane domain and associate with DNAX activation protein of 12 kDa (DAP12), which in turn recruits spleen tyrosine kinase (Syk). These DAP12-associated Siglecs play diverse functions. For example, Siglec-15 is conserved throughout vertebrate evolution and plays a role in bone homeostasis by regulating osteoclast development and function. Human Siglec-14 and -16 have inhibitory counterparts (Siglec-5 and -11, respectively), which show extremely high sequence similarity with them at the extracellular domain but interact with SHP-1. The DAP12-associated Siglec in such "paired receptor" configuration counteracts the pathogens that exploit the inhibitory counterpart. Polymorphisms (mutations) that render DAP12-associated inactive Siglecs are found in humans, and some of these appear to be associated with sensitivity or resistance of human hosts to bacterially induced conditions. Studies of mouse Siglec-H have revealed complex and intriguing functions it plays in regulating adaptive immunity. Many questions remain unanswered, and further molecular and genetic studies of DAP12-associated Siglecs will yield valuable insights with translational relevance.

Toxoplasma gondii Recruits Factor H and C4b-Binding Protein to Mediate Resistance to Serum Killing and Promote Parasite Persistence in vivo

Regulating complement is an important step in the establishment of infection by microbial pathogens. Toxoplasma gondii actively resists complement-mediated killing in non-immune human serum (NHS) by inactivating C3b, however the precise molecular basis is unknown. Here, a flow cytometry-based C3b binding assay demonstrated that Type II strains had significantly higher levels of surface-bound C3b than Type I strains. However, both strains efficiently inactivated C3b and were equally resistant to serum killing, suggesting that resistance is not strain-dependent. Toxoplasma activated both the lectin (LP) and alternative (AP) pathways, and the deposition of C3b was both strain and lectin-dependent. A flow cytometry-based lectin binding assay identified strain-specific differences in the level and heterogeneity of surface glycans detected. Specifically, increased lectin-binding by Type II strains correlated with higher levels of the LP recognition receptor mannose binding lectin (MBL). Western blot analyses demonstrated that Toxoplasma recruits both classical pathway (CP) and LP regulator C4b-binding proteins (C4BP) and AP regulator Factor H (FH) to the parasite surface to inactivate bound C3b-iC3b and C3dg and limit formation of the C5b-9 attack complex. Blocking FH and C4BP contributed to increased C5b-9 formation in vitro. However, parasite susceptibility in vitro was only impacted when FH was blocked, indicating that down regulation of the alternative pathway by FH may be more critical for parasite resistance. Infection of C3 deficient mice led to uncontrolled parasite growth, acute mortality, and reduced antibody production, indicating that both the presence of C3, and the ability of the parasite to inactivate C3, was protective. Taken together, our results establish that Toxoplasma regulation of the complement system renders mice resistant to acute infection by limiting parasite proliferation in vivo, but susceptible to chronic infection, with all mice developing transmissible cysts to maintain its life cycle.

Structural Basis for Properdin Oligomerization and Convertase Stimulation in the Human Complement System

Properdin (FP) is a positive regulator of the immune system stimulating the activity of the proteolytically active C3 convertase C3bBb in the alternative pathway of the complement system. Here we present two crystal structures of FP and two structures of convertase bound FP. A structural core formed by three thrombospondin repeats (TSRs) and a TB domain harbors the convertase binding site in FP that mainly interacts with C3b. Stabilization of the interaction between the C3b C-terminus and the MIDAS bound Mg2+ in the Bb protease by FP TSR5 is proposed to underlie FP convertase stabilization. Intermolecular contacts between FP and the convertase subunits suggested by the structure were confirmed by binding experiments. FP is shown to inhibit C3b degradation by FI due to a direct competition for a common binding site on C3b. FP oligomers are held together by two sets of intermolecular contacts, where the first is formed by the TB domain from one FP molecule and TSR4 from another. The second and largest interface is formed by TSR1 and TSR6 from the same two FP molecules. Flexibility at four hinges between thrombospondin repeats is suggested to enable the oligomeric, polydisperse, and extended architecture of FP. Our structures rationalize the effects of mutations associated with FP deficiencies and provide a structural basis for the analysis of FP function in convertases and its possible role in pattern recognition.

Biophysical analysis of sialic acid recognition by the complement regulator Factor H

Complement factor H (FH), an elongated and substantially glycosylated 20-domain protein, is a soluble regulator of the complement alternative pathway (AP). It contains several glycan binding sites which mediate recognition of α2-3-linked sialic acid (FH domain 20) and glycosaminoglycans (domains 6–8 and 19–20). FH also binds the complement C3-activation product C3b, a powerful opsonin and focal point for the formation of C3-convertases of the AP feedback loop. In freely circulating FH the C3b binding site in domains 19–20 is occluded, a phenomenon that is not fully understood and could be mediated by an intramolecular interaction between FH’s intrinsic sialylated glycosylation and its own sialic acid binding site. In order to assess this possibility, we characterized FH’s sialylation with respect to glycosidic linkage type and searched for further potential, not yet characterized sialic acid binding sites in FH and its seven-domain spanning splice variant and fellow complement regulator FH like-1 (FHL-1). We also probed FH binding to the sialic acid variant Neu5Gc which is not expressed in humans but on heterologous erythrocytes that restrict the human AP and in FH transgenic mice. We find that FH contains mostly α2-6-linked sialic acid, making an intramolecular interaction with its α2-3-sialic acid specific binding site and an associated self-lock mechanism unlikely, substantiate that there is only a single sialic acid binding site in FH and none in FHL-1, and demonstrate direct binding of FH to the nonhuman sialic acid Neu5Gc, supporting the use of FH transgenic mouse models for studies of complement-related diseases.

Modulation of Immune Tolerance via Siglec-Sialic Acid Interactions

One of the key features of the immune system is its extraordinary capacity to discriminate between self and non-self and to respond accordingly. Several molecular interactions allow the induction of acquired immune responses when a foreign antigen is recognized, while others regulate the resolution of inflammation, or the induction of tolerance to self-antigens. Post-translational signatures, such as glycans that are part of proteins (glycoproteins) and lipids (glycolipids) of host cells or pathogens, are increasingly appreciated as key molecules in regulating immunity vs. tolerance. Glycans are sensed by glycan binding receptors expressed on immune cells, such as C-type lectin receptors (CLRs) and Sialic acid binding immunoglobulin type lectins (Siglecs), that respond to specific glycan signatures by triggering tolerogenic or immunogenic signaling pathways. Glycan signatures present on healthy tissue, inflamed and malignant tissue or pathogens provide signals for “self” or “non-self” recognition. In this review we will focus on sialic acids that serve as “self” molecular pattern ligands for Siglecs. We will emphasize on the function of Siglec-expressing mononuclear phagocytes as sensors for sialic acids in tissue homeostasis and describe how the sialic acid-Siglec axis is exploited by tumors and pathogens for the induction of immune tolerance. Furthermore, we highlight how the sialic acid-Siglec axis can be utilized for clinical applications to induce or inhibit immune tolerance.

Evolution of sialic acids: Implications in xenotransplant biology

All living cells are covered with a dense “sugar-coat” of carbohydrate chains (glycans) conjugated to proteins and lipids. The cell surface glycome is determined by a non-template driven process related to the collection of enzymes that assemble glycans in a sequential manner. In mammals, many of these glycans are topped with sialic acids (Sia), a large family of acidic sugars. The “Sialome” is highly diverse owing to various Sia types, linkage to underlying glycans, range of carriers, and complex spatial organization. Presented at the front of cells, Sia play a major role in immunity and recognition of “self” versus “non-self,” largely mediated by the siglecs family of Sia-binding host receptors. Albeit many mammalian pathogens have evolved to hijack this recognition system to avoid host immune attack, presenting a fascinating host-pathogen evolutionary arms race. Similarly, cancer cells exploit Sia for their own survival and propagation. As part of this ongoing fitness, humans lost the ability to synthesize the Sia type N-glycolylneuraminic acid (Neu5Gc), in contrast to other mammals. While this loss had provided an advantage against certain pathogens, humans are continuously exposed to Neu5Gc through mammalian-derived diet (eg, red meat), consequently generating a complex immune response against it. Circulating anti-Neu5Gc antibodies together with Neu5Gc on some human tissues mediate chronic inflammation “xenosialitis” that exacerbate various human diseases (eg, cancer and atherosclerosis). Similarly, Neu5Gc-containing xenografts are exposed to human anti-Neu5Gc antibodies with implications to sustainability. This review aimed to provide a glimpse into the evolution of Sia and their implications to xenotransplantation.

Common and rare genetic variants of complement components in human disease

Genetic variability in the complement system and its association with disease has been known for more than 50 years, but only during the last decade have we begun to understand how this complement genetic variability contributes to the development of diseases. A number of reports have described important genotype-phenotype correlations that associate particular diseases with genetic variants altering specific aspects of the activation and regulation of the complement system. The detailed functional characterization of some of these genetic variants provided key insights into the pathogenic mechanisms underlying these pathologies, which is facilitating the design of specific anti-complement therapies. Importantly, these analyses have sometimes revealed unknown features of the complement proteins. As a whole, these advances have delineated the functional implications of genetic variability in the complement system, which supports the implementation of a precision medicine approach based on the complement genetic makeup of the patients. Here we provide an overview of rare complement variants and common polymorphisms associated with disease and discuss what we have learned from them.

The properdin pathway: an "alternative activation pathway" or a "critical amplification loop" for C3 and C5 activation?

This review is not intended to cover in detail all aspects of the discovery and evolution of our understanding of the "alternative pathway" of complement activation, there are many excellent reviews that do this (see Fearon (CRC Crit Rev Immunol 1:1-32, 1979), Pangburn and Müller-Eberhard (Springer Semin Immunopathol 7:163-192, 1984)), but instead to give sufficient background for current concepts to be put in context. The prevailing textbook view, of components having a primary role as an alternative "pathway" for C3 activation, is challenged, with an argument developed for the primary role of the system being that of providing a surface-dependent amplification loop for both C3 and C5 activation. Whatever the mechanism by which the initial C3b molecule is generated, deposition onto a surface has the potential to target that surface for elimination. Elimination or escape from initial targeting is determined by a sophisticated and highly regulated amplification loop for C3 activation. This viewpoint of the system is then briefly developed to provide a context for therapeutic treatment of disease caused, at least in part, by dysregulated amplification of C3 activation, and to highlight some of the challenges that such therapies will face and need to address.

How novel structures inform understanding of complement function

During the last decade, the complement field has experienced outstanding advancements in the mechanistic understanding of how complement activators are recognized, what C3 activation means, how protein complexes like the C3 convertases and the membrane attack complex are assembled, and how positive and negative complement regulators perform their function. All of this has been made possible mostly because of the contributions of structural biology to the study of the complement components. The wealth of novel structural data has frequently provided support to previously held knowledge, but often has added alternative and unexpected insights into complement function. Here, we will review some of these findings focusing in the alternative and terminal complement pathways.

Are humans prone to autoimmunity? Implications from evolutionary changes in hominin sialic acid biology

Given varied intrinsic and extrinsic challenges to the immune system, it is unsurprising that each evolutionary lineage evolves distinctive features of immunoreactivity, and that tolerance mechanisms fail, allowing autoimmunity. Humans appear prone to many autoimmune diseases, with mechanisms both genetic and environmental. Another rapidly evolving biological system involves sialic acids, a family of monosaccharides that are terminal caps on cell surface and secreted molecules of vertebrates, and play multifarious roles in immunity. We have explored multiple genomic changes in sialic acid biology that occurred in human ancestors (hominins), some with implications for enhanced immunoreactivity, and hence for autoimmunity. Human ancestors lost the enzyme synthesizing the common mammalian sialic acid Neu5Gc, with an accumulation of the precursor sialic acid Neu5Ac. Resulting changes include an enhanced reactivity by some immune cells and increased ability of macrophages to kill bacteria, at the cost of increased endotoxin sensitivity. There are also multiple human-specific evolutionary changes in inhibitory and activating Siglecs, immune cell receptors that recognize sialic acids as "self-associated molecular patterns" (SAMPs) to modulate immunity, but can also be hijacked by pathogen molecular mimicry of SAMPs. Altered expression patterns and fixed or polymorphic SIGLEC pseudogenization in humans has modulated both innate and adaptive immunity, sometimes favoring over-reactivity. Meanwhile, dietary intake of Neu5Gc (derived primarily from red meats) allows metabolic incorporation of this non-human molecule into human cells--apparently the first example of "xeno-autoimmunity" involving "xeno-autoantigen" interactions with circulating "xeno-autoantibodies". Taken together, some of these factors may contribute to the apparent human propensity for autoimmunity.

Hiding in plain sight: immune evasion by the staphylococcal protein SdrE

The human immune system is responsible for identification and destruction of invader cells, such as the bacterial pathogen Staphylococcus aureus In response, S. aureus brings to the fight a large number of virulence factors, including several that allow it to evade the host immune response. The staphylococcal surface protein SdrE was recently reported to bind to complement Factor H, an important regulator of complement activation. Factor H attaches to the surface of host cells to inhibit complement activation and amplification, preventing the destruction of the host cell. SdrE binding to Factor H allows S. aureus to mimic a host cell and reduces bacterial killing by granulocytes. In a new study published in Biochemical Journal, Zhang et al. describe crystal structures of SdrE and its complex with the C-terminal portion of Factor H. The structure of SdrE and its interaction with the Factor H peptide closely resemble a family of surface proteins that recognize extracellular matrix components such as fibrinogen. However, unbound SdrE forms a novel 'Closed' conformation with an occluded peptide-binding groove. These structures reveal a fascinating mechanism for immune evasion and provide a potential avenue for the development of novel antimicrobial agents to target SdrE.