The AP may represent one of the earliest forms of innate immunity. Unlike the CP or LP pathway, the AP can be fully activated in the absence of specific pathogen binding by a “recognition” equivalent to C1q or MBL. In fact, the AP is always “on” at a low level. In addition, the AP forms and uses the distinct C3 convertase C3bBb.

Complement C3 is a disulfide-linked two-chain protein, denoted as α and β , and having a combined apparent molecular weight of approximately 200 kDa. The crystal structure of native C3, shown as a domain-colored ribbon model in Fig. 1A , identified 13 distinct domains, including the thioester domain (TED), which contained the covalent binding site. In the native molecule, the intramolecular thioester bond, formed between the side chains of cysteine and glutamine residues within the sequence CGEQ, is buried within a hydro phobic interface formed between the TED and MG8 domains, which is nevertheless close to the protein’s surface. The subsequent determination of the atomic structure of the activated form of C3 (i.e., C3b) demonstrated a dramatic shift in the location of the TED. Proteolytic cleavage releases the C3a anaphylatoxin peptide, and the TED becomes fully exposed to engage potential targets (see structure based depiction of C3b in Fig. 1B ). Thus the dramatic shift in structure also exposes potential binding sites for factor B of the AP and competing sites for regulators of C3b, such as factor H (FH), membrane cofactor protein (MCP/CD46), complement receptor type 1 (CR1/CD35), and decay accelerating factor (DAF/CD55; all described later in this section). At a low so-called “tick-over” level, the thioester bond undergoes spontaneous hydrolysis, forming C3(H 2O). This conformationally altered C3b-like form of C3 (see Fig. 1B ) allows for binding to factor B, a plasma protein. Factor B is a serine protease that is approximately 30% identical to C2. The binding of factor B by C3(H 2 O) allows factor D, another protease, to cleave factor B to form Ba and Bb. Bb remains associated with C3(H2O) to form the C3(H2O)Bb complex. Factor D appears to function as a serine protease in its native state but can cleave factor B only when bound to C3. Recently, there has been an interesting connection found between factor D and a component of the LP. MASP-1 and MASP-3 are alternative splice products of the same gene with the difference being in the exons encoding the serine protease domain. It was found that a MASP-1/MASP-3 knockout mouse completely lacked AP functionality. Upon further investigation, it was determined that the secreted factor D in this mouse possessed a five-residue propeptide at its amino terminus, suggesting that either MASP-1 or MASP-3 mediated its removal. Subsequently, it was determined that under physiologic conditions, it was solely MASP-3 that was responsible for removing the propeptide from zymogen FD and further, that unlike MASP-1 or MASP-2, MASP-3 circulated predominantly in a proteolytically active form.

Fig1. THE STRUCTURE OF NATIVE C3, ITS CONFORMATIONAL INTERMEDIATES, AND ITS CLEAVAGE FRAGMENTS. (A) Ribbon diagram representation of the X-ray crystal structure of native C3 indicating the 13 domains (bold lettering, color-coded the same as the domain) of which it is composed. (B) Structure-based cartoon representation of the conformational states of intact C3, as well as its cleavage fragments. Where these cartoons are derived from X-ray structures, those structures are depicted as ribbon diagrams adjacent to the cartoon. The remaining cartoons are based on electron micrograph images, as well as established biochemical data. In all cases, the domain colors in the cartoons correspond to those in the ribbon diagrams. Proteolytic activation of C3 to C3b results in an approximate 90-Å downward movement of the thioester domain (TED), a significant repositioning of the CUB (complement C1r/C1s, urchin EGF, bone morphogenic protein 1) domain, and a flipping of the positions macroglobulin 7 (MG7) and MG8 domains. The reorientation of these domains creates binding sites for ligands of C3b that were not present in the native molecule. The reactive thioester produced during this conformational transition is capable of binding a portion of the C3b molecules covalently to a target surface (gray-shaded boxes). Subsequent cleavage of C3b by factor I releases a small C3f fragment and results in a reorientation of the C3c portion of the molecule relative to C3d/TED within iC3b, a molecule that remains bound to the target. This reorientation relative to C3b relieves the steric blockage by MG1 of a portion of the binding site for CR2/CD21, as iC3b is an equivalent ligand to C3dg and C3d with respect to CR2 binding. C3dg and C3c are the products of an additional cleavage by factor I within the CUB domain. A noncomplement protease removes an N-terminal segment from C3dg, yielding the still target-associated C3d fragment. The remaining “squiggle” on C3d represents 16 residues at its C-terminus that are sufficiently flexible that they were not visible in the x-ray crystal structure of C3d. Although the thioester in native C3 is protected from the solvent, native C3 is in conformational equilibrium with a stable conformational intermediate, C3(H 2 O)*, in which the thioester become susceptible to hydrolysis. Although the equilibrium strongly favors the native state, if hydrolysis of the thioester in C3(H 2 O)* occurs, it cannot reform, and the molecule undergoes a unidirectional conformational change to the C3(H2O) stage, which adopts both a C3b-like conformation and functional profile. This conformational transition of intact C3 is the basis of the “tick-over mechanism” for alternative pathway initiation. (Modified from P. Gros, Utrecht University; contains elements previously published in Gros P, Milder FJ, Janssen BJ. Complement driven by conformational changes. Nat Rev Immunol. 2008;8:48.)

C3(H2O)Bb is an enzymatic complex capable of cleaving native C3. This complex is a fluid-phase C3 convertase. Although it is formed only in small amounts, it can cleave many molecules of C3. Much of the C3b produced in this process is inactivated by hydrolysis, but some attaches covalently to the surface of host cells or pathogens. C3b bound in this way is able to bind factor B, allowing its cleavage by factor D to yield Ba and Bb. The result is the formation of C3bBb, a C3 convertase akin to C4b2a found in the classical and MBL pathways, with the capability of initiating an amplification cascade (see Fig. 2).

Fig2. SCHEMATIC OVERVIEW OF THE COMPLEMENT CASCADE. The initiation events of the classical, lectin, and alternative pathways are at the left. The C1 complex of the CP recognizes an array of antibody Fc regions via its C1q pattern recognition entity. The induced conformational distortion of the C1q arms leads to the autoactivation of the associated C1r 2 C1s 2 enzymatic complex. MBL of the lectin pathway’s MBL-MASP complex is similarly a pattern recognition molecule for pathogen-specific carbohydrate arrays, whose binding leads to the activation of the associated MASP1 or MASP2 enzymatic complexes, albeit via a distinct mechanism from C1. Both activated C1s and activated MASP-2 can cleave C4, with some of the resulting nascent C4b covalently binding to the target surface (covalent bond denoted by thick black vertical stroke). The binding of C2 to the target-bound C4b, followed by C2’s cleavage by either activated forms of C1s, MASP-2, or MASP-1 yields the CP C3 convertase, C4b2a. Some nascently activated C3b molecules deposit on the target as opsonins, where they are ligands for immune cell-associated complement receptors either as C3b, or as its subsequent degradation products iC3b, C3dg, and C3d. If nascent C3b binds to the CP C3 convertase, the latter becomes capable of cleaving C5. The AP initiates spontaneously in the fluid phase when factor B is recruited to the thioester-hydrolyzed C3b-like entity, C3(H2O). Cleavage by factor D yields a fluid-phase initial AP C3 convertase C3(H2O)Bb. Some of the C3b generated by this entity may covalently bind to the target surface, whereupon a much more efficient surface-bound, and properdin-stabilized, AP C3 convertase, C3bBbP, assembles. A surface-bound AP C3 convertase can also assemble using C3b deposited via the CP as a nidus. Via the depicted amplification loop, the AP C3 convertase deposits many opsonic C3b molecules on the target surface. As for the CP, the association of a second C3b molecule with the AP C3 convertase enables the latter to cleave C5. The generation of C5b by either the CP or AP C5 convertase initiates the self-assembly of the C5b678(9) n membrane attack complex (MAC). Of the liberated fluid-phase complement split products, namely C4a, Ba, C2b, C3a, and C5a, the latter two have well established roles as inflammatory mediators. (Adapted from Fig. 1 of Chen, JY, Cortes, C, and Ferreira, V. Properdin: a multifaceted molecule involved in inflammation and disease. Mol Immunol. 2018;102:588–572.Copyright © 2018 Elsevier Ltd. All rights reserved.)

In light of the nonspecific nature of C3b binding in the AP, it is not surprising that a number of complement regulators exist both in the plasma and on host cell membranes to prevent complement activation on self-tissues. Some of these regulatory components are mentioned now for the sake of clarity; more detailed attention is provided later in this chapter ( Table 1). CR1 (CD35) and DAF (CD55) compete with factor B for binding to C3b on the cell sur face and can displace Bb from a convertase that has already formed. Factor I (FI), a serum protease, in concert with CR1 or MCP (CD46) can prevent convertase formation by converting C3b into its inactive derivative, iC3b. CR1 is unique among the FI cofactors in facilitating an additional proteolytic cleavage of iC3b to yield C3c and C3dg (see Fig. 1B ). Trimming of the latter by noncomplement proteases yields the proteolytic limit fragment C3d, which structurally corresponds to the TED domain (see Fig. 1B ). Another complement regulatory protein found in the plasma is FH. FH binds C3b and is able to compete with factor B and displace Bb from the convertase. FH also acts as a cofactor for FI to convert C3b to iC3b. In addition to interaction sites for C3b, FH possesses two distinct binding sites for polyanionic molecules, particularly various sulfated glycosaminoglycans (e.g., heparan sulfate) or arrays of sialic acid (e.g., from membrane surface glycoproteins) found on host surfaces in contact with blood plasma. Although these polyanion binding sites are not required for FH to regulate fluid phase AP C3 convertase, they are required for its activity on surface-bound C3bBb. In fact, this is the basis for FH being able to discriminate between AP C3 convertase adventitiously deposited on host tissue versus that deposited on a microbial surface because the latter do not possess either the sulfated glycosaminoglycans or the sialic acid arrays.

Table1. Control Proteins of the Classical and Alternative Pathways

Pathogen surfaces are normally not afforded the protection offered by these regulators. Persistence of the C3bBb convertase on microbial surfaces may additionally be favored by the positive regulator properdin (P). The positive modulation of the AP by properdin has traditionally been thought to be attributable to its ability to prolong the lifetime of the AP C3 convertase by forming a C3bBbP complex in which properdin contacts segments in both C3b and Bb (see Fig. 2 ). This mechanism may indeed be the dominant one exhibited by properdin, however, there is also evidence for properdin displaying pattern recognition functionality for some, but not all, AP targets. For example, native properdin, which circulates predominantly as a trimer, binds to zymosan (yeast cell walls), Chlamydia pneumoniae, and necrotic, or late apoptotic, mammalian cells, but not to Neisseria meningitidis or Neisseria gonorrhoeae. Because of properdin’s trimeric nature, even if it uses two of its subunits to bind to the target surface, one is still left that can recruit C3b, or C3(H2O), from the fluid phase to the target surface. The properdin-bound C3b/C3(H2O) may then act as a plat form for recruiting factors B and D, thereby forming a surface-bound AP C3 convertase. Although this mechanism can be demonstrated to function in vitro, its physiologic relevance has yet to be established.

After forming, the C3bBb convertase rapidly cleaves more C3 to C3b, which can participate in the formation of more molecules of C3bBb convertase. The AP thereby activates an amplification loop that can proceed on the surface of a pathogen but not on a host cell (see Fig. 2). An additional point regarding amplification by the AP is that C3b deposited on a target as a result of activation of either the CP or the LP can act as a nidus for the formation of an AP C3 convertase. It has been argued that this AP augmentation mechanism is responsible for upwards of 80% of the downstream complement effector mechanisms initiated via the classical or LPs.

Although specific antibody is not required for AP activation, many classes of immunoglobulin can facilitate AP activation. The mechanism by which this occurs remains elusive, although some evidence indicates that C3b covalently bound to IgG displays a reduced rate of inactivation to iC3b by factors H and I. However, in contrast to CP activation, which requires Fc, AP activation can occur with F(ab)′2 fragments.

An instructive demonstration for the role of antibody in continuing the AP cascade, with possible ramifications for human disease, comes from a murine model of rheumatoid arthritis. Mice do not spontaneously develop rheumatoid arthritis. However, a murine model has been developed in which expression of antibodies specific for the ubiquitously expressed cytoplasmic protein glucose-6-phosphate can cause joint destruction reminiscent of human rheumatoid arthritis. Interestingly, the disease state, through complement-mediated joint destruction, can occur even if the specific antibodies are of isotypes incapable of fixing complement through the CP. The response may be localized to the joints because of the absence of complement cascade regulators on cartilage.

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قسم التطوير يقيم دورة عن أخلاقيات المهنة للمنتسبين الجدد

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قسم الشؤون الفكرية يطلق فعاليات الأسبوع الثالث من برنامج البذرة الصالحة

المزيد

الآخبار الصحية

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لتعزيز الطاقة والتغلب على التعب.. عليك بـ7 طرق بسيطة

لتحسين الهضم والتحكم في الشهية.. ما أفضل وقت لتناول التمر؟

خبراء يكشفون تأثير الاستحمام في الظلام على جودة النوم !

المزيد

الآخبار التكنلوجية

إزالة ثاني أكسيد الكربون من الغلاف الجوي.. ما دور المحيطات والبحار؟

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المزيد

مواضيع ذات صلة

B-Lymphocyte Coreceptors

The Complement System : Lectin Pathway

The Complement System: Classical Pathway

Secondary Lymphoid Tissue

The Hematopoietic Microenvironment

The Pro-B- and Pre-B-Cell Checkpoints

Immunoglobulin Gene Rearrangement : Heavy Chain Gene Rearrangement

Transcriptional Regulation of B-Cell Development

Stages of B-Cell development

Genetic Engineering of Natural Killer Cells

The role of Natural Killer Cells in Human Disease

Natural Killer Cells and Hematopoietic Cell Transplantation