The Complement System PDF

Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 Sbobina nº 1 The Complement System Ludovica Cecchino & Sara Carbognin Today’s topic is the complement system w...

Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 Sbobina nº 1 The Complement System Ludovica Cecchino & Sara Carbognin Today’s topic is the complement system which is a factor mechanism of the immune response. Innate immunity The immune system has two main components: the innate and adaptive immune responses. The innate response acts as the body’s first line of defence, activating quickly and triggering the adaptive response when needed. The adaptive immune response, in turn, helps enhance and support the innate response and relies on its effector mechanisms. Some believe the innate immune response has an initial non-induced phase consisting of the protection provided by physical barriers like skin, mucosal layers, low pH, and enzymes in saliva. They can be classified as defences since they create an inhospitable environment for microbes, however, they’re always present and don’t require activation, hence they aren’t typically recognized as active immune responses. When a pathogen breaches these barriers, an induced innate immune response is triggered. This involves immune cells like phagocytes, granulocytes, antigen-presenting cells, cytokines and chemokines, as well as the complement system composed of soluble molecules present in our serum. Discovery of the complement system In the late 18th century, Jules Bordet and other scientists, including Paul Ehrlich, conducted experiments with bacteria to investigate their lysis. They already suspected that certain molecules in the body could target and destroy specific pathogens, although these molecules, later known as antibodies, had not yet been fully identified or named. Researchers had already observed that when an individual was immunized with a pathogen, their serum contained substances that specifically acted against that pathogen. This theory was confirmed in Bardot’s experiments: he initially mixed serum from an immunized individual with bacteria and the latter were lysed. Then he used serum from a non- immunized individual, the bacteria remained intact. Thus, proving that immunization produced specific, bacteria-targeting molecules in the serum. Bordet then conducted another test where he heated the immune serum to 57 degrees Celsius before exposing it to bacteria. This heat treatment prevented bacterial lysis, suggesting that whatever was responsible for lysis was sensitive to heat and thus, likely a Ludovica Cecchino, Sara Carbognin 1  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 protein. He then combined heat-inactivated immune serum with non-immune serum, and interestingly, this mixture restored the ability to lyse the bacteria. Through these experiments, Bordet concluded that two components were needed for bacterial lysis: 1. Antibodies: which remained stable at 57 degrees Celsius (obtained from the heat-inactivated immune serum) 2. A heat-sensitive element he called “complement” (obtained from the non-immune serum) → The complement system is a group of proteins found in the serum that supports the immune functions of antibodies. The complement worked with the antibodies to enable effective bacterial killing. An important difference to note is complements are not specific to immunized individuals, they are present in the serum regardless, whereas antibodies are only present among people who had been immunized. Effector functions of antibodies Although a detailed discussion on antibodies will come later, it’s helpful to understand their basic role in pathogen defence: Antibodies are Y-shaped molecules that bind to antigens on the surfaces of pathogens. Contrary to common belief, the mere binding of the antibody to the antigen does not cause lysis, after all, antibodies are just proteins. What does happen is the neutralization of infectious agents. As we know, pathogens (ex: viruses or bacterial toxins) have sites intended to attach to host cells and enter them, however, once antibodies bind to them, they block the pathogen's ability to interact with the cell, preventing replication. This neutralizing activity is especially effective in halting the proliferation of pathogens that rely on host cell entry, as well as toxins that need to bind to specific receptors on target cells. By blocking these interactions, antibodies can effectively neutralize the pathogen or toxin. Another primary function of antibodies is opsonization, during which antibodies bind to the surface of a pathogen (in cases of autoimmunity they can also bind to a self-cell) with their variable regions, coating it entirely. This leaves only the Fc (constant) regions of the antibodies exposed. Immune cells, such as phagocytes and natural killer (NK) cells, have receptors for the Fc regions, meaning they will recognize and bind to the opsonized pathogen or cell. Through this mechanism, phagocytes can engulf and destroy the pathogen, while NK cells can initiate cytotoxic actions against the targeted cell. Ludovica Cecchino, Sara Carbognin 2  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 The Fc regions of antibodies can also activate the complement system. Complement activation enhances inflammation, promotes direct lysis of microbes (as demonstrated in Jules Bordet’s experiment), and further facilitates phagocytosis by macrophages and neutrophils. Thus, complement activation is a key effector function of antibodies. However, as we will discuss shortly, complement can also be activated independently of antibodies by binding directly to pathogen surfaces. This system can be triggered in multiple ways to achieve the same three outcomes: inflammation, microbial lysis, and enhanced phagocytosis. The complement system The complement system consists of approximately 30 serum proteins (excluding their receptors), most of which possess enzymatic activity but remain inactive under steady-state conditions. These proteins are labelled with numbers, such as C2, C3, C4, C5, and so forth (C stands for complement). Upon activation, they are cleaved, producing fragments denoted by a lowercase letter (e.g., C3a, C3b, C5a, C5b). Complement activation can be initiated through three distinct pathways, with some proteins shared among these pathways. The process involves the sequential proteolysis of proteins to generate enzyme complexes with proteolytic functions: each activated protein cleaves and activates the next. Once activated, complement proteins bind covalently to: microbial surfaces antibodies attached to microbes other antigens apoptotic bodies (since the complement system also plays a role in clearing apoptotic cells) Ludovica Cecchino, Sara Carbognin 3  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 Complement activation is tightly regulated by specific regulatory proteins present on host cells but absent on microbes. This regulation prevents continuous activation and inflammation, despite the constant presence of complement proteins in serum. Furthermore, complement won’t activate against our own cells and destroy them, but only works against pathogens. This stays true in all cases except in immune dysregulations and autoimmunity. So, let’s see how the compliment system is activated, and which are the three pathways: The classical pathway The classical pathway of the complement system earned this name because it was the first to be discovered. However, it evolved later than other pathways because it depends on antibodies. As we know, antibodies (and by extension B-cells and T-cells) only appear with the development of adaptive immunity in certain vertebrates, thus, while Jules Bordet’s experiments identified the classical pathway first, it likely emerged after other complement pathways. It is a significant effector mechanism of adaptive immunity, demonstrating how adaptive immunity can rely on components of innate immunity. This pathway begins with the C1 complex, a multimeric protein composed of C1q and two subunits, C1r and C1s. The C1q component has six regions that bind to the Fc portions of IgG or IgM antibodies. These must already be bound to a pathogen to initiate the pathway because typically, multiple IgG molecules are required for sufficient binding. Once C1q binds to these antibodies, the associated serine proteases, C1r and C1s, become active and initiate a cascade by cleaving additional complement proteins. Ludovica Cecchino, Sara Carbognin 4  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 The next protein in the sequence is C4, it is recruited and cleaved into C4b, which attaches to the pathogen's surface, and C4a, which is released to promote inflammation. Following this, C2 is cleaved into C2a, which binds to C4b, and C2b, which is released. The C4b and C2a complex forms an enzyme called C3 convertase, which cleaves C3, a key component of the complement system involved in all three complement pathways. When C3 is cleaved, it produces C3b, which attaches to the pathogen’s surface or binds to C3 convertase to form a further enzyme complex. In Summary: the classical pathway is initiated when antibodies bind to a pathogen, allowing the C1 complex to recognize and attach to the exposed Fc regions. This attachment triggers a series of cleavages involving C4 and C2, forming C3 convertase, which then cleaves C3 and amplifies the immune response. Q: How does the complement pathway understand that antibodies are bound and not soluble? A: Because when antibodies are soluble it is rare to find them close together, which is instead the case when they have already bonded. The lectin pathway The lectin pathway is initiated by plasma lectins, such as mannose- binding lectin (MBL) and ficolins, which recognize specific carbohydrate patterns on pathogen surfaces. MBL, a member of the collectin family, has a hexameric structure similar to C1q, with six arms that bind to mannose residues present on pathogens. Once MBL binds to a pathogen, it associates with MASP-1 and MASP-2 (MBL-associated serine proteases), which are functionally similar to C1r and C1s in the classical pathway. This binding activates MASP-1 and MASP-2, triggering a proteolytic cascade identical to the classical pathway. The lectin pathway begins with the cleavage of C4 into C4a and C4b. C4b attaches to the pathogen surface, followed by the cleavage of C2 into C2a and C2b. The C4b and C2a fragments then combine to form the C3 convertase enzyme, which subsequently cleaves C3, amplifying the complement response. Ludovica Cecchino, Sara Carbognin 5  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 The alternative pathway The third pathway, known as the alternative pathway, was discovered later than the classical and lectin pathways, but it is likely evolutionarily older, surely older than the classical pathway at least. This pathway differs in its activation mechanism which is centred on the role of the complement protein C3, which circulates in significant quantities. Adequate levels of C3 are critical for effective pathogen response, and deficiencies can lead to immunodeficiency or immune-mediated diseases. If C3 levels fall below a certain threshold, it may indicate either a genetic deficiency in C3 production or accelerated C3 degradation, often indicating there is dysregulated complement activation. This is typically due to either deficiencies in complement inhibitors or diseases involving rapid immunocomplex formation, which depletes C3. Under normal conditions, a small portion of circulating C3 undergoes spontaneous, low-level degradation in a process known as "C3 tick-over", producing fragments C3a and C3b. If C3b does not encounter a pathogen or a surface to bind to, it rapidly becomes inactive. However, if C3b does attach to a pathogen surface, it initiates the alternative pathway, enabling the complement system to maintain constant low-level surveillance for pathogens. This low-grade activation ensures the immune system can quickly respond to the presence of an infection. When a microbe is present, C3b can directly bind to its surface, recruiting another complement protein, factor B. Factor B is then cleaved into two fragments: Bb and Ba. Bb binds to C3b, forming the alternative pathway’s version of C3 convertase, designated as C3bBb. This C3 convertase performs the same function as the others from the classical and lectin pathways but is structurally distinct. The C3 convertase significantly amplifies complement activation by cleaving large quantities of C3 into C3b, unlike the low-level C3b generation occurring in the "tick-over" process. This amplification is essential for robust complement activation, enabling an effective immune response. Here is a comparison of the three complement activation pathways: In the alternative pathway, the C3 convertase is designated as C3bBb, while in the classical and lectin pathways, it is labelled as C4bC2a. Ludovica Cecchino, Sara Carbognin 6  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 All C3 convertases have the same function: they cleave C3 into C3b and C3a at higher levels. When additional C3b binds to these C3 convertases, they become C5 convertases, which are denominated C3bBbC3b in the alternative pathway and C4bC2aC3b in the classical and lectin pathways. These C5 convertases are essential for further complement activation. Late steps of complement formation The enzyme C5 convertase initiates the cleavage of C5 into two fragments, C5b and C5a. After which the complement proteins C6, C7, and C8 are recruited in sequence (these proteins are not cleaved). The cascade culminates with C9, a polymeric protein that integrates into the target cell membrane, forming a pore. This process resembles the function of perforins used by immune cells (natural killer cells and cytotoxic T lymphocytes) to create membrane pores in target cells, facilitating apoptosis by releasing proteases. In this complement pathway, however, the pore formation by C9 leads to osmotic lysis rather than apoptosis. These components, once assembled on the cell surface, form the membrane attack complex (MAC), which disrupts the target cell membrane, causing it to burst. This is the mechanism discovered by Jules Bordet which demonstrates how target cell lysis can occur without direct involvement of immune cells. In the classical pathway, all that is required is antibody binding to initiate this lethal sequence, no assistance from cytotoxic immune cells needed. Functions of the complement There are several functions the complement system can complete: Ludovica Cecchino, Sara Carbognin 7  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 It developed to form the MAC and induce cell lysis. However, bacteria countered this by developing capsules to shield themselves from complement-mediated killing. As a result, complement-mediated lysis is not effective against all bacteria. It is particularly crucial for a subset of pathogens, such as those in the Neisseria genus, which have thin cell walls and are vulnerable to MAC insertion. Other bacteria, which can resist MAC formation, are more susceptible to different functions of the complement system. Another crucial function of the complement system is promoting inflammation. The proteolytic fragments C5a, C4a, and C3a induce acute inflammation by activating mast cells, neutrophils, and endothelial cells. C5a and C3a are also known as anaphylatoxins because they trigger mast cell degranulation, leading to a response similar to anaphylaxis, which is a hypersensitivity reaction. Even though anaphylaxis is triggered by allergens and not pathogens, the symptoms can be similar because when mast cells degranulate, they release histamines and other granule contents that contribute to inflammation. The complement system also mediates the solubilization of immune complexes. These complexes can form physiologically, particularly when a large amount of antigen is introduced in a short period of time. This commonly occurs during a recall immunization, when a high dose of antigen is administered to an individual who already has antibodies from a previous exposure. The antibodies bind to the new antigen, forming immune complexes composed of multiple antigen-antibody interactions. These complexes can be large structures, and if they deposit in small blood vessels, they may cause inflammation and endothelial damage. Therefore, it is crucial for immune complexes to be properly cleared. Complement plays a key role in this process by recognizing the Fc regions of antibodies in the immune complexes. This activation allows phagocytic cells, which have receptors for complement components, to capture and transport Ludovica Cecchino, Sara Carbognin 8  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 the immune complexes to the liver and spleen, where they are eliminated by macrophages and other phagocytes. The last function we’re going to discuss is opsonization, which enhances phagocytosis. Opsonization occurs when antibodies cover the surface of a pathogen, however, it can also be mediated by complement components, such as C3b, which bind to the surface of a pathogen like antibodies. This allows the pathogen to be fully opsonized and subsequently recognized by phagocytes through specific receptors that bind to complement components. Phagocytes have specific receptors for complement components. Opsonization refers to the process of coating a foreign cell with molecules—such as antibodies or complement proteins—that make the cell more recognizable and susceptible to phagocytosis. Phagocytosis is the process by which phagocytes internalize foreign material. This material is engulfed into phagolysosomes, where it is degraded by reactive oxygen species (ROS), nitric oxide, and various proteases. How do phagocytes identify which cells to engulf? 1. "Eat me" signals: Cells often display specific surface markers that signal they should be phagocytosed. These signals can be a result of damage, infection, or apoptosis. 2. Opsonization by antibodies: Foreign cells can be opsonized by antibodies, making them easier for phagocytes to recognize. Phagocytes have Fc gamma receptors (FcγRs) on their surface, which specifically recognize the Fc region of IgG antibodies that coat the target cells. This helps phagocytes identify and engulf antibody-opsonizedpathogens or immune complexes (ICs). 3. Binding to complement: Phagocytes also have complement receptors on their surface, allowing them to recognize and bind to complement-coated targets, such as opsonized pathogens. This binding enhances phagocytosis, further promoting the immune response. Ludovica Cecchino, Sara Carbognin 9  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 COMPLEMENT RECEPTORS: The most important action of complement receptors is to facilitate the uptake and destruction of pathogens by phagocytic cells. This occurs by the specific recognition of bound complement components by complement receptors (CRs) on phagocytes. Example: complement receptor 1 can recognize the C3B fragments that opsonize the pathogens. The activation of the complement receptor or the FCgamma receptor can bring to the internalization of the opsonized pathogen. Sometimes, when they are both triggered, the phagocytosis process is even more efficient. Type 1 complement receptor: CR1, also called CD35. One of the most common complement receptors that is present on phagocytes, but it is also present on mononuclear phagocytes, which includes macrophages, monocytes, some immature dendritic cells, neutrophils, B and T cells, erythrocytes, eosinophils, FDCs. FDCs are follicular dendritic cells. They are cells of stromal origin that are found in the B-cell follicles of the secondary lymphoid organs, where they play a very important role both in homeostasis and the immune response. During homeostasis, FDCs are responsible for the formation of the B-cell follicles, because they produce CXCL13. The reason why B-cells are attracted into these B-cell follicles is because they sense a chemokine that is produced by FDCs = FDCs create a gradient in the B-cell follicles so that B-cells are attracted there. Follicular dendritic cells are cells of stromal origin that are called dendritic cells because they have dendrites, but they have nothing to do with the dendritic cells of the hematopoietic system. B-cells and T-cells are typically localized in distinct regions within the lymph nodes. B-cells reside in structures known as B-cell follicles, while T-cells are found in the paracortex. This spatial segregation is due to the presence of different chemokine gradients in these areas. B-cells express the chemokine receptor CXCR5, which directs them to the chemokine CXCL13, produced by follicular dendritic cells (FDCs) within the B-cell follicles. In contrast, Ludovica Cecchino, Sara Carbognin 10  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 T-cells lack this receptor and therefore localize in the paracortex, where a different chemokine gradient is present. During an immune response, FDCs play a crucial role by serving as a physical scaffold where antigens can be captured and retained. These antigens bind to complement receptors on the surface of FDCs, allowing B-cells to recognize them. B-cells are most efficiently activated when antigens are presented on the surface of cells, as opposed to in a soluble form. Soluble antigens are harder for B-cells to recognize because the B-cell receptor (BCR) needs to engage multiple epitopes to become fully activated. When antigens are presented on the surface of cells or in multiple copies, they cross-link the BCR, which enhances the activation of B-cells. This is where FDCs are particularly important, as they have complement receptors on their surface that facilitate antigen capture. FDCs do not phagocytose the antigens, unlike macrophages, but rather they bind the antigens and retain them for extended periods. This allows B-cells ample time to mature their BCRs and increase their specificity for the antigen. FDCs express complement receptor 1 (CR1), which can also be found on other hematopoietic cells. CR1 recognizes C3b with high affinity, and to a lesser extent, C4b and I- C3b (inactive C3b). CR1 activation typically leads to phagocytosis by phagocytes and the clearance of immune complexes. Both phagocytes and erythrocytes express CR1. When immune complexes form, erythrocytes can bind them via CR1 and transport them to the liver or spleen, where they are cleared by phagocytes. Type 2 complement receptor, CR2 or C21: CD21 is expressed by B-lymphocytes, follicular dendritic cells (FDCs), and certain epithelial cells. One of its primary roles is as a coreceptor for B-cell activation. It also plays a key part in antigen trapping within germinal centers, which are specialized structures that form within B-cell follicles as B-cells mature their B-cell receptors (BCRs). Within these germinal centers, FDCs capture and retain antigens, allowing B-cells to recognize them for further activation and differentiation. In addition to its immune functions, CD21 also serves as a receptor for Epstein-Barr virus (EBV). However, this is not a normal physiological role of the immune response. Instead, it represents how EBV exploits the immune system to gain entry into B-cells, where it can persist in a latent form for years. During this latent phase, the virus remains dormant unless it reactivates and begins replicating. In this way, EBV hijacks the B-cell machinery for its own survival. Ludovica Cecchino, Sara Carbognin 11  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 Type 3 complement receptor, also called C3: Immunologists know it mostly as CD11B because this is a marker for all macrophages, phagocytes, neutrophils, NK cells, monocytes, and all myeloid cells that have some function in phagocytosis. Type 4 complement receptor, also known as CD11C: Expressed by the same kind of cells as type 3 complement receptor, but more expressed in dendritic cells, with respect to macrophages. The function is very similar. They all bind complement members and subunits that lead them to an increased phagocytosis. Why did complement evolve physiologically? Complement evolved primarily to protect the body from pathogens, activating in response to infectious microorganisms. However, it also plays a role in protecting host cells. This dual function is possible because complement regulators are present on host cells, preventing inappropriate activation of the complement system against the body's own tissues. Some of these complement regulators are involved in the removal of apoptotic or modified host cells. Importantly, this process occurs without causing inflammation, ensuring that the immune system clears dead or damaged cells without triggering an immune response or unnecessary inflammation. Pathological conditions In conditions such as immunodeficiencies or other immune dysregulations, the complement system can be inappropriately activated, leading to pathology. For example: Defective complement regulators can allow the complement to activate on host cells, leading to tissue damage. Ludovica Cecchino, Sara Carbognin 12  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 Inefficient removal of apoptotic cells can result in necrosis, which induces significant inflammation. Preventing unwanted inflammation and ensuring the non-inflammatory removal of apoptotic cells by phagocytes is critical for maintaining immune homeostasis. Complement evasion by pathogens Complement regulators are generally not found on microorganisms, but some pathogens can evade complement activation by recruiting host complement regulatory proteins. Certain microbes produce proteins that mimic human complement regulators, allowing them to avoid immune detection and destruction. Complement-mediated inflammation can be inhibited by specific microbial gene products, further enabling pathogens to evade the immune response. In many situations, uncontrolled or dysregulated activation of complement can contribute to immune-mediated pathology, emphasizing the need for tight regulation to balance effective immune defense and the prevention of tissue damage. REGULATORS Complement regulators can be either cell-bound or soluble. For example, CR1, which is expressed on all phagocytes, functions as a complement regulator. This means that phagocytes expressing CR1 are protected from complement attack, as CR1 can bind to cells that have been opsonized by a complement. However, if complement proteins, such as C3b, attempt to bind directly to the phagocyte’s surface, the C3b is quickly inactivated. Other molecules that act as co- receptors for CR1 help in this inactivation process, preventing complement activation on the phagocyte. Soluble Regulators: C1 inhibitor (C1NH) regulates the classical pathway by inhibiting the C1 complex. Factor H is a regulator of the alternative pathway, preventing overactivation of complement. Properdin helps stabilize complement in certain contexts, and the absence of properdin acts as a regulatory mechanism to prevent complement activation on host cells. Factor I works in conjunction with cofactors such as MCP, CR1, or Factor H to degrade C3b into iC3b (inactive C3b). It does the same with C4b, converting it into inactive forms such as C4c or C4d. When Factor I is functioning properly, it regulates the cleavage of C3b, acting only on cells that lack the necessary co-factors, thus preventing complement activation on host cells. Ludovica Cecchino, Sara Carbognin 13  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 When C3b attaches to a cell surface that expresses CR1 (a complement receptor and regulator), Factor I can cleave C3b into its inactive form (iC3b) after the C3b- CR1 complex is formed. This process helps regulate complement activation and prevent excessive immune responses. It has been observed that patients with Factor I deficiency experience similar health issues to those with C3 deficiency. How is this possible? The answer lies in Factor I's role in regulating C3b. Without Factor I, C3b cannot be properly inactivated. As a result, C3b binds to cell surfaces or pathogens, leading to the formation of C3 convertases. This accelerates the breakdown of C3, and as a consequence, C3 levels in the serum drop significantly. This depletion is not due to a problem with the C3 gene itself, but rather because C3 is being consumed at an increased rate due to the lack of regulation by Factor I. Without sufficient C3, the complement system cannot be properly activated when needed, such as during an infection. As a result, these patients are more susceptible to various types of infections, including those caused by bacteria and viruses. Additional Regulation by Complement Receptors Another level of regulation in the complement system is mediated by complement receptors. For example, CR1 on cell surfaces helps disassemble C3 convertases. When a C3 convertase forms, these regulators can break it down into C4b and C2a, preventing further cleavage of C3 and controlling excessive complement activation. There are also other inhibitors that block the formation of the MAC (membrane attack complex), further regulating complement activity. Additionally, some inhibitors cleave anaphylatoxins (such as C3a and C5a) and block their ability to activate receptors on immune cells like neutrophils and mast cells, preventing inappropriate inflammation. Ludovica Cecchino, Sara Carbognin 14  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 Defects in the complement system Inherited deficiencies of components in both the classical and alternative complement pathways have been identified in humans, with the exception of Factor B in the alternative pathway. This indicates that many genetic mutations affecting complement components have already been discovered. Deficiencies in genes involved in the classical pathway—such as C1q, C1r, C1s, C2, or C4—result in immune complex diseases. The classical pathway is essential for the clearance of immune complexes, and when it is impaired, these complexes accumulate. More than 50% of patients with these deficiencies go on to develop autoimmune diseases, particularly lupus. Lupus is a systemic autoimmune disease driven by immune complexes. Pathogenesis of Lupus The underlying cause of lupus is the presence of auto-reactive B and T cells targeting DNA. These auto-reactive cells produce auto-antibodies against circulating DNA, particularly when DNA levels are elevated due to cellular damage. For example, exposure to sunlight can increase DNA release, which is why lupus patients are advised to avoid direct sun exposure. Sunlight-induced DNA damage triggers the formation of auto-antigens, leading to the production of auto-antibodies. In genetically predisposed individuals, these auto-antibodies form immune complexes, which deposit in tissues and blood vessels, causing widespread inflammation. If these patients also have defects in the classical complement pathway, the immune system's ability to clear these immune complexes is compromised, exacerbating the condition. This leads to increased immune complex formation and chronic inflammation throughout the body. This is why autoimmune diseases like lupus are considered multifactorial—a combination of genetic predisposition and environmental factors, such as sunlight exposure, contribute to disease development. Over 50% of patients with mutations in classical complement pathway genes develop lupus. Complement Deficiencies and Infection Risk C3 deficiency is associated with a high incidence of serious pyogenic bacterial infections, some of which can be fatal. This underscores the central role of C3 in opsonization, enhancing phagocytosis, and the destruction of pathogens. In contrast, deficiencies in the terminal complement components (C5, C6, C7, C8, and C9) predispose individuals to disseminated infections caused by Neisseria species, such as Neisseria meningitidis and Neisseria gonorrhoeae. While certain bacteria with thick cell walls are resistant to lysis by the membrane attack complex (MAC), Neisseriaspecies, which have thinner cell walls, are particularly susceptible. Therefore, individuals with defective MAC function(due to deficiencies in the terminal complement components) have difficulty clearing Neisseria infections. This can result in severe, life-threatening infections. Ludovica Cecchino, Sara Carbognin 15  Basic Mechanisms of Disease Mirela Kuka 1 4.11.2024 What happens when complement is activated excessively or improperly? Excessive inflammation: Overactivation of complement can lead to acute inflammatory responses. Anti-endothelial antibodies: For example, in the case of organ transplants, these antibodies can trigger complement activation, leading to immune-mediated damage. Endothelial damage: Complement activation can damage the endothelial cells lining blood vessels. MAC complex formation: The formation of the membrane attack complex (MAC) on endothelial cells can contribute to tissue damage. Systemic vasculitis and immune complex glomerulonephritis: These conditions result from the deposition of antigen-antibody complexes in the walls of blood vessels and kidney glomeruli, leading to inflammation and tissue injury. While these are normal physiological responses of the complement system, problems arise when the complement response is directed against self-antigens or becomes dysregulated in the context of chronic inflammation, where the antigen is not adequately cleared. In such cases, complement can contribute to immune-mediated damage, as seen in various autoimmune diseases. Complement Activation in Cancer The role of complement in cancer is still debated, with both positive and negative aspects. Therapeutic potential: On one hand, activating complement against cancer cells can be beneficial. For example, antibodies that target cancer cells can opsonize these cells, marking them for destruction by complement. If the classical complement pathway is activated in response to these antibodies, cancer cells can be lysed by the complement system, which is desirable in cancer therapy. In fact, many immunotherapy approaches today make use of monoclonal antibodies that activate the complement system, helping to target and destroy cancer cells. Thus, understanding how to harness complement activation effectively for cancer treatment could be highly beneficial. Pro-tumoral effects: On the other hand, excessive complement activation and inflammation in the tumor microenvironment can have pro-tumoral effects. Some myeloid cells in the tumor microenvironment, particularly those exposed to C3a and C5a (anaphylatoxins released during complement activation), can become immunosuppressive. This helps tumors evade immune detection and promotes tumor growth. Thus, while complement activation can target cancer cells, it can also promote immune suppression and tumor progression. Researchers are still working to fully understand the dual role of complement in cancer. The goal is to leverage complement activation in a way that targets cancer cells effectively while avoiding its pro-tumoral effects. Optimizing the use of complement in cancer therapy remains a key area of research to ensure that its benefits are maximized while minimizing potential harm. Ludovica Cecchino, Sara Carbognin 16