Sistema Complemento (aula completa)

Introduction to the Complement System

Overview of the Lesson

• The instructor, Roney, introduces the topic of the complement system and mentions previous lessons available on the channel.

• He notes that this lesson is a reformulation of an earlier lecture recorded during the COVID-19 pandemic in 2020, which was incomplete.

• Today's discussion will cover not only activation processes but also regulation and diseases related to complement deficiencies.

Key Concepts of the Complement System

• The complement system is crucial for both innate and adaptive immune responses, with a focus on its role in humoral immunity.

• Roney emphasizes that he prefers to focus on content rather than appearing on camera during lectures.

Activation and Regulation of the Complement System

Mechanisms of Action

• The complement system consists of over 30 plasma proteins produced by the liver, circulating in inactive forms until needed.

• Activation occurs through various pathways; these proteins must be tightly regulated to prevent damage to host cells.

Historical Context

• Roney discusses Jules Bordet's discovery of the complement system in 1890, highlighting his experiments with serum containing antibodies against bacterial agents.

Bordet's Experiments and Findings

Experimental Observations

• Bordet demonstrated that fresh serum could lyse bacteria at physiological temperatures (37°C).

• However, heating serum above 56°C resulted in loss of lytic function, raising questions about what components were affected by heat.

Implications for Understanding Complement Function

• Bordet's findings led him to hypothesize that there are temperature-sensitive components within serum responsible for lysis.

Complement System Overview

Introduction to the Complement System

• The complement system assists antibodies by enhancing their lytic function, leading to its designation as "complement."

• It consists of serum proteins that interact in a highly regulated manner to produce components that eliminate microorganisms.

Activation Mechanism

• Activation involves sequential cleavage of various proteins, resulting in enzymatic complexes with proteolytic activity.

• The nomenclature for these proteins was initially based on the order of discovery rather than their activation sequence.

Protein Nomenclature and Discovery

• Key complement proteins include C2, C3, C4, C5, C6, C7, C8, and C9; they were named according to their discovery order.

• The classical pathway is introduced as the first discovered route for complement activation.

Fragmentation and Functionality

• Proteins are cleaved into two fragments (A and B), with each fragment having distinct roles; A typically promotes inflammation while B is involved in opsonization.

• Opsonization occurs when complement proteins bind to microorganisms' surfaces, marking them for destruction by immune cells.

Exceptions in Fragmentation Rules

• Not all complement proteins undergo fragmentation; notable exceptions include C6, C7, C8, and C9 which do not get cleaved.

• An exception exists with protein C2 where both fragments (C2a and C2b) are generated but have different functionalities compared to other complements.

Pathways of Complement Activation

Understanding the Classical Pathway of Complement Activation

Overview of Complement Pathways

• The discussion begins with an introduction to the three complement activation pathways: classical, alternative, and lectin pathways. Each pathway has distinct initiation mechanisms but shares common steps in their later stages.

• The classical pathway is initiated by a protein known as C1, which binds to antibodies. This sets off a cascade of reactions leading to complement activation.

Role of Antibodies in Activation

• Only specific immunoglobulin isotypes (IgG and IgM) can activate the classical pathway; other types do not initiate this process.

• The structure of C1 is discussed, highlighting its components and how it interacts with antibodies on pathogens.

Mechanism of Action

• Upon binding to antibodies that are attached to microorganisms, C1 activates through a series of proteolytic cleavages.

• The presence of microorganisms triggers the binding of antibodies, which then allows C1 to engage and activate further components in the pathway.

Protease Activation

• When C1 binds to an antibody, it activates another component called C1r. This leads to a chain reaction where proteases cleave other proteins involved in the complement system.

• Activated C1r cleaves C4 into two fragments (C4a and C4b), each playing different roles in inflammation and opsonization processes.

Formation of Key Structures

• Following the cleavage events, additional components like C2 interact with activated forms from previous steps.

• The interaction between activated fragments leads to the formation of crucial structures such as convertases that enhance immune responses against pathogens.

Conclusion on Convertase Formation

Complement System Activation and Function

Understanding C3 Convertase

• The discussion begins with the role of C3 convertase in the complement system, emphasizing its function in converting C3 into fragments.

• It is explained that C3 convertases cleave C3 into two fragments: C3a, which promotes inflammation, and C3b, which facilitates opsonization.

• The amplification sequence is highlighted where multiple C3 molecules bind around microorganisms to enhance opsonization.

Formation of Other Convertases

• A question arises about whether the combination of proteins (C4b and C2a) can form a different structure with significant activity; the answer is affirmative.

• The formation of C5 convertase from these components is introduced, noting its role in further activating the complement system.

Fragmentation Process

• The process by which C5 convertase cleaves itself into two fragments (C5a and C5b) is described, with each fragment having distinct functions: inflammation for C5a and opsonization for C5b.

• It’s emphasized that most complement proteins are cleaved into two fragments by proteases, aiding in their respective roles within immune responses.

Differences Among Complement Pathways

• An overview of the three activation pathways—classical, alternative, and lectin—is provided. Each pathway initiates differently but converges at a similar point in the activation cascade.

Classical Pathway Activation Without Antibodies

• A question regarding whether classical pathway activation can occur without antibodies leads to a discussion on innate immunity's rapid response before adaptive immunity develops.

• It’s noted that antibodies take time to form post-infection; however, other structures like pentraxins can initiate classical pathway activation even in their absence.

Role of Pentraxins

• Pentraxins such as serum amyloid P component (SAP), CRP (C-reactive protein), and others are mentioned as substitutes for antibodies during early immune responses.

• These pentraxins bind to microbial surfaces (e.g., phosphorylcholine), facilitating classical pathway initiation through interactions with complement components like C4 and C2.

Understanding the Alternative Pathway of Complement Activation

Introduction to Complement System

• The alternative pathway of the complement system is introduced, emphasizing that it operates without a prior trigger, unlike other pathways.

• The discussion begins with spontaneous cleavage of C3 protein, which is crucial for understanding how the complement system functions at a basal physiological level.

Spontaneous Cleavage and Its Implications

• At low physiological levels, C3 undergoes spontaneous cleavage but does not bind to healthy cells; instead, it interacts with water molecules in absence of pathogens.

• In the presence of microorganisms, spontaneous cleavage becomes significant as C3b binds to microbial surfaces, initiating opsonization.

Mechanism of Action in Alternative Pathway

• The process involves further cleavage of C3 into fragments (C3a and C3b), where C3b attaches to pathogen surfaces.

• Factor B is recruited to bind alongside C3b on the pathogen surface, setting off a cascade reaction.

Role of Factor D and Fragmentation

• Factor D acts as a protease that cleaves factor B into two fragments (Ba and Bb), contributing to inflammation and opsonization processes.

• Understanding these fragmentations clarifies how the complement system activates without being overly complex.

Formation of Convertases

• The combination of C3b and Bb forms an alternative convertase (C3bBb), which amplifies the response by converting more C3 into its active form.

• This amplification is critical since relying solely on spontaneous cleavage would be insufficient for effective immune response.

Stabilization by Properdin

• Properdin stabilizes the alternative convertase complex (C3bBb), enhancing its activity against pathogens.

• This stabilization allows for sequential cleavages leading to increased opsonization and inflammatory responses against invading microorganisms.

Amplification Process Overview

• A sequence occurs where multiple C3 proteins are cleaved into their active forms (C3a and C3b), significantly boosting opsonization efforts against pathogens.

• The interaction between various components leads to enhanced inflammation through additional fragmentations aiding in immune response.

Importance of Structural Changes

• Questions arise about whether combinations like C3bBb lead to new structures with important activities; indeed they do contribute significantly to immune function.

Inflammation and Complement Pathways

Overview of Inflammation and Opsonization

• The discussion begins with the concept of inflammation, highlighting its role in opsonization, where fragment B facilitates binding to microorganisms.

• The lecturer introduces the lectin pathway, noting its similarity to the classical pathway, emphasizing that both pathways share common processes after initial activation.

Lectin Pathway Activation

• The protein involved in this process is identified as MBL (mannose-binding lectin), which binds specifically to mannose on microorganisms.

• MBL's function is clarified: it initiates the lectin pathway by attaching to mannose-containing microbes, marking the start of this immune response.

Protease Activation in Lectin Pathway

• The interaction between MBL and pathogens activates proteases MASP-1 and MASP-2, crucial for subsequent steps in complement activation.

• Upon binding, MASP-1 activates MASP-2, which then cleaves C4 and C2 proteins necessary for forming convertases.

Formation of Convertases

• Following MBL binding to mannose, C4 and C2 are recruited; their cleavage leads to the formation of C3 convertase specific to the lectin pathway.

• This process results in smaller fragments that contribute to inflammation while larger fragments assist in opsonization.

Comparison with Classical Pathway

• The lecturer emphasizes that despite differences in initiation (lectin vs. classical), both pathways converge at similar stages leading to C3 convertase formation.

• Fragmentation continues with C3 being cleaved into two parts—one promoting inflammation and another facilitating opsonization.

Continuation of Complement Activation

• After reaching C5 activation through similar mechanisms as seen in classical pathways, further processes lead towards effective immune responses against pathogens.

Alternative Initiation Mechanisms

• A question arises regarding alternative methods for initiating the lectin pathway without MBL or other components; potential similarities with ficolins are discussed.

Ficolins' Role

• Ficolins are introduced as another type of collectin that can activate complement systems similarly by binding different microbial structures compared to MBL.

Summary of Differences Between Collectins

Activation of the Complement System

Overview of Lectin Pathway

• The discussion introduces a protein similar to MBL (mannose-binding lectin), which also initiates the lectin pathway, highlighting that there are multiple proteins involved in this process.

Steps Following C5 Activation

• The focus shifts to the late stages of complement system activation after C5 has been cleaved, leading to the formation of MAC (membrane attack complex).

Formation and Function of MAC

• The MAC is described as a structure that attacks the plasma membrane of microorganisms. It involves several components including C6, C7, and others leading up to C9.

• The process includes polymerization of C9 on the surface of microorganisms, forming pores that disrupt their integrity.

Mechanism of Action

• The final goal is to create holes in microbial membranes through polymerized C9, resulting in destabilization and potential death of bacteria due to loss of cytoplasmic content.

• Multiple reactions can occur simultaneously on different points along a microorganism's membrane, leading to various pore formations.

Visual Representation

• A visual representation shows how C9 polymers surround a microorganism's membrane, illustrating the concept of MAC effectively creating holes or "marks" on it.

Importance of Regulation in Complement Activation

• Discussion transitions into why regulation is crucial for complement activation; uncontrolled activation could damage host cells alongside targeting pathogens.

Reasons for Regulation Necessity

• Two main reasons for regulating complement activation are outlined:

• Continuous low-level spontaneous activation can harm normal tissues if unchecked.

• Even when activated against pathogens, control is necessary to prevent collateral damage from degradation products affecting adjacent healthy cells.

Regulatory Proteins Involved

• Emphasis on regulatory proteins that manage complement activity; these include both circulating proteins and those embedded in cell membranes.

Types and Functions of Regulators

• Introduction to specific regulators like C1 inhibitor (C1NH), which plays a role in inhibiting early steps in complement activation.

Inhibition of C1 and the Role of Regulators in Complement Activation

Mechanism of NH as a C1 Inhibitor

• The NH acts as an inhibitor of C1, specifically mimicking normal substrates for C1r, which is crucial for understanding its function.

• When NH binds to antibodies (e.g., IgM or IgG), it activates C1r, leading to the cleavage and activation of C1s, transforming it into a protease.

• The activation process involves the binding of NH to antibodies, which subsequently activates the classical pathway by cleaving components.

Regulation of Classical Pathway Activation

• The presence of regulatory inhibitors like NH allows for dissociation from active complexes (C1r and C1rs), preventing further enzymatic activity.

• This regulation is essential when resolving immune processes during infections; complement activity must cease to avoid damage to host cells.

Overview of Complement Regulators

• A group of regulators known as inhibitors of C3b and factor 4p includes membrane proteins such as MCP (CD46), CR1, and DAF.

• These membrane-bound proteins are critical in protecting host cells from complement-mediated lysis by inhibiting pathways that could lead to cell damage.

Interaction with Host Cells

• Factors like Factor H and plasma proteins play roles in regulating complement activity on host cell surfaces, preventing unwanted activation.

• For instance, when C3b binds to a host cell surface without proper regulation, it can lead to the formation of harmful convertases that activate the classical pathway.

Mechanisms Preventing Cell Lysis

• To prevent lysis on normal host cells due to complement activation, regulatory proteins bind to components like C3b and inhibit their functions.

• Proteins such as DAF prevent further progression along both classical and alternative pathways by blocking necessary interactions between factors involved in these pathways.

Role of Factor I in Degradation

• Factor I plays a role in degrading activated components (C3b and C4b), but requires cofactors like MCP or Factor H for effective action.

Understanding Complement System Components

Inactive Forms and Fragments of C3

• The inactive form of C3 is referred to as C3b, which is crucial for the complement system's function.

• Fragment C3f is derived from the cleavage of C3b, indicating its role in the complement activation process.

• Phagocytes like macrophages and neutrophils possess receptors for C3dg, facilitating the uptake of microorganisms marked by this fragment.

Regulators of Membrane Attack Complex (MAC)

• CD59 acts as a regulator that inhibits MAC formation on normal mammalian cells, preventing damage to host tissues.

• CD59 prevents pore formation by interfering with the assembly of MAC components on cell membranes.

Additional Proteins Inhibiting MAC Formation

• Protein S also inhibits MAC formation but operates differently than CD59; it binds to soluble complexes like C5b-C7.

• Protein S's mechanism involves binding near where the complement cascade initiates, effectively blocking MAC assembly.

Summary of Complement Regulation

• Key regulators include:

• C1 inhibitor (inhibits C1r and C1s),

• Membrane-bound proteins (MCP, CR1),

• Factor H and C4bp, which degrade active fragments,

• CD59 and Protein S, both inhibiting MAC formation.

Functions Beyond Membrane Attack

Opsonization and Phagocytosis

• The primary function of the complement system includes opsonization, enhancing phagocytosis by marking pathogens with opsonins like C3b.

Inflammatory Response Stimulation

• Complement fragments stimulate inflammatory responses, contributing to immune defense mechanisms against infections.

Mechanism Overview

Fagocitose e Resposta Inflamatória

Mecanismos de Fagocitose

• A fagocitose é facilitada pela presença de anticorpos, como o IgG, que se ligam a microrganismos, aumentando a eficiência do processo.

• O receptor CR1 não é tão eficiente na indução da fagocitose em comparação com os anticorpos ligados aos microrganismos, que melhoram a ligação e a captura.

• Em ambientes ricos em citocinas, como o interferão gama, os macrófagos tornam-se mais eficazes na fagocitose devido à ativação mediada por receptores específicos.

• Além do CR1, outros receptores como CR3 e CR4 também desempenham papéis importantes no reconhecimento e na captura de patógenos durante a fagocitose.

• A compreensão dos mecanismos de fagocitose é crucial para entender as respostas imunes e inflamatórias no corpo humano.

Processo Inflamatório

• Durante uma resposta inflamatória, neutrófilos rolam nas moléculas de adesão para realizar diapedese e migrar para o local da infecção.

• Macrófagos residentes liberam citocinas pró-inflamatórias que atuam sobre o endotélio vascular, promovendo a expressão de moléculas de adesão essenciais para a migração celular.

• As proteínas do complemento ajudam na remoção de microrganismos ao aumentar a adesão das células sanguíneas ao endotélio vascular durante processos inflamatórios.

• Fragmentos como C5a atuam nas células endoteliais aumentando as moléculas de adesão (selectinas), facilitando ainda mais a migração dos leucócitos para os tecidos afetados.

• Mastócitos presentes nos tecidos liberam mediadores inflamatórios quando ativados por fragmentos do complemento, contribuindo para vasodilatação e aumento da permeabilidade vascular.

Efeitos dos Fragmentos do Complemento

• Os fragmentos C4a e C5a induzem desgranulação em mastócitos, resultando em liberação de histamina e outras substâncias que aumentam a inflamação.

• A desgranulação dos mastócitos leva à vasodilatação e aumento da permeabilidade vascular, permitindo maior influxo celular na área inflamada.

• O C5a reforça a adesão estável dos neutrófilos às células endoteliais através da ativação das integrinas, facilitando sua passagem do vaso sanguíneo para os tecidos inflamados.

Understanding the Role of Complement System in Disease

Overview of Microorganisms and Complement System

• The discussion begins with the formation of microorganisms, emphasizing their structure and function related to length.

• A live microorganism is compared to one that has undergone complement system action, highlighting how cytoplasmic content is expelled leading to bacterial death.

Functionality of the Complement System

• The complement system's role in inflammation and disease processes is introduced, indicating its importance in immune response.

• Research articles on PubMed reveal a wealth of information regarding diseases associated with complement deficiencies, suggesting a broader understanding beyond general knowledge.

Deficiencies in Complement Proteins

• Specific deficiencies like C2 and C4 are discussed; individuals lacking these proteins have a 50% chance of developing lupus or similar conditions due to impaired removal of circulating immune complexes.

• The accumulation of immune complexes can lead to skin lesions or kidney issues such as glomerulonephritis, illustrating the clinical implications of these deficiencies.

Alternative Pathways and Infections

• Individuals with complement deficiencies may still utilize alternative pathways for immune functions, which could mitigate some risks associated with their condition.

• Deficiency in C3 leads to severe bacterial infections; this highlights the critical nature of this protein in maintaining health.

Clinical Implications and Genetic Disorders

• Deficiencies in other proteins (C5-C9) primarily result in susceptibility to specific infections like meningitis and gonorrhea, underscoring serious health risks.

• The hereditary condition known as hereditary angioedema arises from a deficiency in regulatory proteins (C1 inhibitor), causing acute symptoms including abdominal pain and respiratory obstruction.

Conclusion on Importance of Complement Proteins