Immunology 12-16 PDF

The microbial substances that stimulate innate immunity are often shared by classes of microbes and are called pathogen-associated molecular patterns (PAMPs). Different types of microbes (e.g., viruses, gram-negative bacteria, gram-positive bacteria, fungi) express different PAMPs. These structures...

The microbial substances that stimulate innate immunity are often shared by classes of microbes and are called pathogen-associated molecular patterns (PAMPs). Different types of microbes (e.g., viruses, gram-negative bacteria, gram-positive bacteria, fungi) express different PAMPs. These structures include nucleic acids that are unique to or more abundant in microbes than in host cells, such as double-stranded RNA found in replicating viruses and unmethylated CpG DNA sequences found in bacteria, lipopolysaccharide (LPS) in gram-negative bacteria. The innate immune system recognizes microbial products that are often essential for survival of the microbes.. The recognition of PAMPs by innate immune cells allows to distinguish between self and non -self. The innate immune system also recognizes endogenous molecules that are produced by or released from damaged and dying cells. These substances are called damage associated molecular patterns (DAMPs). DAMPs may be produced as a result of cell damage caused by infections, chemical toxins, burns, trauma, or loss of blood supply. DAMPs are generally not released from cells dying by apoptosis. In some cases, endogenous molecules that are produced by healthy cells are released when the cells are damaged, and they then stimulate innate responses. These molecules are a subset of DAMPs and are often called alarmins because their presence outside cells alarms the immune system that something is causing cell death. The cellular receptors for pathogen and damage-associated molecular patterns are called pattern recognition receptors. They are expressed on the surface, in phagocytic vesicles, and in the cytosol of various cell types. When these cell-associated pattern recognition receptors bind to PAMPs and DAMPs, they activate signal transduction pathways that promote the antimicrobial and proinflammatory functions of the cells in which they are expressed. The receptors of the innate immune system are encoded by inherited (germline) genes, whereas the genes encod ing receptors of adaptive immunity are generated by somatic recombination of gene segments in the precursors of mature lymphocytes. There are different cellular pattern recognition receptors, such as: ⚫ Toll-like receptors (TLRs) are an evolutionarily conserved family of pattern recognition receptors expressed on many cell types that recognize products of a wide variety of microbes, as well as molecules expressed or released by stressed and dying cells. ○ TLRs characterized by a Leucine rich extracellular domain (LRR) and intracellular domain (TIR) responsible for the initiation of signaling cascade. ○ In humans there are 10 different functional TLRs. ○ Mammalian TLRs are involved in responses to a wide variety of molecules that are expressed by microbes but not by healthy mammalian cells. ○ TLRs are also involved in responses to endogenous molecules whose expression or location indicates cell damage. ○ The signaling pathways are initiated by ligand binding to the TLR at the cell surface leading to dimerization of the TLR proteins. Ligand induced TLR dimerization is predicted to bring the TIR domains of the cytoplasmic tails of each protein close to one another. This is followed by immunology Pagina 12  indicates cell damage. ○ The signaling pathways are initiated by ligand binding to the TLR at the cell surface leading to dimerization of the TLR proteins. Ligand induced TLR dimerization is predicted to bring the TIR domains of the cytoplasmic tails of each protein close to one another. This is followed by recruitment adaptor proteins, which facilitate the recruitment and activation of various protein kinases, leading to the activation of different transcription factors. The major transcription factors that are nuclear factor κ B (NFκ B), activation protein 1 (AP-1), interferon response factor 3 (IRF3), and IRF7. NFκ B and AP-1 stimulate the expression of genes encoding molecules required for inflammatory responses, including inflammatory cytokines, chemokines, and endothelial adhesion molecules. IRF3 and IRF7 promote production of type I interferons (IFNα and IFNβ ), which are important for antiviral innate immune responses. ○ When T cells are activated, they undergo a transformation into effector cells, which are crucial for the immune response. This process is heavily influenced by cytokines. TLR activation leads to the enhancement of several antimicrobial mechanisms: The respiratory burst produces toxic reactive oxygen species to kill pathogens. Increase Phagosome-lysosome fusion ensures that the engulfed pathogen is exposed to destructive enzymes and chemicals. Increase Phagosome acidification creates an optimal environment for enzyme activity that degrades the pathogen. β-Defensins are produced to directly kill microbes and modulate the immune response. TLRs recognize not only PAMPs but also DAMPs The body prevents immune activation by PAMPs from commensal microbes: The mucous layer of the intestines acts as a physical barrier that separates commensal bacteria from the epithelial cells lining the gut. This prevents direct contact between the Pathogen-Associated Molecular Patterns (PAMPs) of commensal microbes and the Toll- like receptors (TLRs) on the intestinal epithelial cells. The mucus is rich in antimicrobial peptides and immunoglobulin A (IgA), which further helps to neutralize microbes and prevent unnecessary immune responses. Intestinal epithelial cells express TLRs, but these receptors are often found on the basolateral membrane (the side facing the underlying tissue) rather than the apical membrane (the side facing the gut lumen where commensal microbes reside). This strategic localization ensures that TLRs are exposed only to microbes or their PAMPs when there is a breach in the intestinal barrier (e.g., during an infection or injury). Even if the immune system is occasionally activated by commensal microbes, the body has mechanisms to dampen this response. Regulatory T cells (Treg lymphocytes) play a critical role in suppressing excessive immune responses. Commensal microbes are known to induce the expansion of Tregs, which secrete anti-inflammatory cytokines like IL-10 and TGF-β. These cytokines help to suppress immune responses and maintain immune tolerance in the gut. By doing this, Tregs prevent undesired inflammation and immune reactions against commensal bacteria while allowing the immune system to stay alert to harmful pathogens. ⚫ NOD-like receptors (NLRs): In addition to TLRs, which sense pathogens outside cells or in endosomes. Also these receptors are associated to signal transduction pathways that promote inflammation or type I IFN responses (antiviral). These receptors induce the formation of enzyme complexes called inflammasomes. They can sense both PAMPs and DAMPs. The family includes more than 20 different cytosolic proteins. Expressed by monocytes, macrophages, dendritic cells, mast cells, epithelial cells. NLR proteins contain a C-terminal LRR (leucine-rich repeats) that senses the presence of ligands, a central NOD (nucleotide oligomerization domain also called NACHT) required for NLR proteins to bind one another and form oligomers, and an effector N-terminal domain which recruits adaptor proteins to form signaling complexes. In response to stimuli like PAMPs, DAMPs, Endogenous Molecules/Ions, NOD proteins undergo self-oligomerization, where identical NOD proteins bind to each other, forming a structure called an inflammasome complex. Each NOD protein in the inflammasome binds to another protein with a CARD (Caspase Activation and Recruitment Domain). The CARD domain is crucial for recruiting the inactive precursor of caspase-1, known as pro-caspase-1. Once pro-caspase-1 is recruited to the inflammasome complex, it undergoes autoproteolytic cleavage, meaning it cleaves itself to generate the active form of caspase-1. Active caspase-1 has specific substrates, the pro-inflammatory cytokines IL-1β and IL-18, which are initially produced as inactive precursors (called pro-IL-1β and pro-IL-18). immunology Pagina 13  are initially produced as inactive precursors (called pro-IL-1β and pro-IL-18). Caspase-1 cleaves these inactive cytokine precursors, transforming them into their active forms: IL-1β and IL-18. Both of these cytokines are potent mediators of inflammation: ○ IL-1β plays a key role in fever, inflammation, and the activation of immune cells. ○ IL-18 enhances the production of interferon-gamma (IFN-γ), an important cytokine for activating immune responses, particularly against intracellular pathogens. Caspase 1 also cleaves the protein gasdermin D which polymerizes to form pores allowing the release of active IL-1b and IL-18; two potent inflammatory cytokines. IL-1β is a potent pro-inflammatory cytokine that plays a central role in the immune response. IL-1β is produced by antigen-presenting cells (APCs) such as dendritic cells and macrophages in response to infection or tissue damage. IL-1β can act in an autocrine manner, meaning that it not only affects nearby cells but also influences the same APCs that secreted it. This autocrine signaling enhances the APCs' ability to present antigens more effectively. IL-1β promotes the upregulation of Major Histocompatibility Complex (MHC) molecules on the surface of APCs. These molecules are critical for antigen presentation. By increasing MHC expression, IL-1β enhances the ability of APCs to display antigens, which is crucial for the activation of T cells and the initiation of an adaptive immune response. immunology Pagina 14 THE INFLAMMATORY RESPONSE mercoledì 23 ottobre 2024 16:43 THE INFLAMMATORY RESPONSE ⚫ The principal way by which the innate immune system deals with infections and tissue injury is to stimulate acute inflammation, which is the accumulation of leuko cytes, plasma proteins, and fluid derived from the blood at an extravascular tissue site of infection or injury. ⚫ Typically, the leukocyte that is recruited first from the blood into sites of inflammation is the neutrophil because it is the most abundant leukocyte in the blood and the most rapid responder to chemotactic signals. Blood monocytes, which become macrophages in the tissue, become increasingly prominent over time and may be the dominant population in some reactions. ⚫ The delivery of cells and proteins to the inflammatory site is dependent on reversible changes in blood vessels in the infected or damaged tissue. These changes include increased blood flow into the tissue due to vascular dilation. ⚫ All of these changes are induced by cytokines and small-molecule mediators initially derived from resident sentinel cells in the tissue, such as mast cells, macrophages, DCs, and endothelial cells, in response to PAMP or DAMP stimulation. ⚫ Acute inflammation can develop in minutes to hours and last for days. Chronic inflammation is a process that takes over from acute inflammation if the infection is not eliminated or the tissue injury is prolonged. It usually involves recruitment and activation of monocytes and lymphocytes. Chronic inflammatory sites also often undergo tissue remodeling, with angiogenesis and fibrosis. Cytokines play a central role in acute inflammation by mediating communication between immune cells, initiating and regulating the immune response, and driving the inflammatory process. Role of Cytokines in Acute Inflammation - Cytokines are signaling molecules secreted by immune cells (such as macrophages, dendritic cells, and T cells) in response to pathogens or tissue damage. - They help recruit and activate other immune cells to the site of infection or injury, increase vascular permeability (allowing immune cells to reach the affected tissue), and regulate the inflammatory response. Some key cytokines involved in acute inflammation are: 1. Interleukin-1 (IL-1) Key roles in acute inflammation: ○ Induction of fever: IL-1 acts on the hypothalamus in the brain to raise body temperature, which helps limit the growth of pathogens. ○ Increased vascular permeability: IL-1 increases the permeability of blood vessels, allowing immune cells, proteins, and fluids to move from the bloodstream into the tissues where infection or injury is present. ○ Recruitment of immune cells: It enhances the recruitment of neutrophils and other immune cells to the site of infection. ○ Promotion of T cell activation: IL-1 helps activate T cells, improving their ability to respond to antigens presented by antigen-presenting cells (APCs). ○ Stimulation of other cytokines: IL-1 stimulates the release of other pro-inflammatory cytokines such as IL-6 and TNF-α, enhancing the inflammatory cascade. 2. Tumor Necrosis Factor-Alpha (TNF-α) Key roles in acute inflammation: ○ Vascular changes: TNF-α increases the expression of adhesion molecules on endothelial cells (lining of blood vessels), making it easier for immune cells to adhere to the blood vessel walls and migrate to the site of infection or injury. ○ Activation of immune cells: TNF-α activates and recruits neutrophils, macrophages, and other immune cells to the inflamed tissue. ○ Fever and systemic effects: Like IL-1, TNF-α induces fever by acting on the hypothalamus. ○ Apoptosis and tissue destruction: At high levels, TNF-α can promote cell death (apoptosis) in certain cells, which may contribute to tissue damage in chronic inflammation or severe infections. ○ Shock in sepsis: Excessive production of TNF-α, especially during infections like sepsis, can lead to septic shock, characterized by widespread inflammation, low blood pressure, and organ failure. 3. Interleukin-6 (IL-6) Key roles in acute inflammation: ○ Acute-phase response: IL-6 induces the liver to produce acute-phase proteins, such as C- reactive protein (CRP) and fibrinogen, which are important markers of inflammation. ○ B and T cell activation: IL-6 promotes the differentiation and activation of B cells (for antibody production) and T cells. ○ Enhancement of neutrophil production: IL-6 stimulates the bone marrow to produce more neutrophils, increasing the body's ability to respond to infections. ○ Chronic inflammation link: IL-6 also plays a role in the transition from acute to chronic inflammation. 4. Interleukin-8 (IL-8) Key roles in acute inflammation: ○ Neutrophil recruitment: IL-8 is a powerful chemoattractant for neutrophils, guiding them to the site of infection or injury where they can phagocytose and kill pathogens. ○ Activation of neutrophils: IL-8 also activates neutrophils, increasing their ability to adhere to the endothelium and enhancing their phagocytic capabilities. ○ Amplification of the inflammatory response: By recruiting neutrophils and other immune cells to the area, IL-8 helps amplify the local inflammatory response to eliminate pathogens quickly. immunology Pagina 15  to the area, IL-8 helps amplify the local inflammatory response to eliminate pathogens quickly. 5. Interleukin-12 (IL-12) Key roles in acute inflammation: ○ Th1 differentiation: IL-12 promotes the differentiation of naïve T cells into Th1 cells (a type of helper T cell), which are crucial for fighting intracellular pathogens such as viruses and some bacteria. Th1 cells produce interferon-gamma (IFN-γ), which activates macrophages and enhances their pathogen-killing abilities. ○ Activation of NK cells: IL-12 stimulates natural killer (NK) cells, which play a key role in the early defense against infections by producing IFN-γ and directly killing infected cells. ○ Promotion of cell-mediated immunity: IL-12 drives the immune response towards cell- mediated immunity, which is essential for controlling and eliminating intracellular pathogens. Inflammatory response is characterized by two main events: a) One involving microcirculation (fluids and proteins from circulation to injured or infected tissue) b) One involving leukocytes (cellular component) The goal of inflammatory response is to redirect fluids and cells to tissue damage site. The first to arrive are fluids, whereas for cells arrival more time is required because they have to actively move (exit) from circulation to the tissue (2-12 min). Leukocyte must: 1) Adhere to the endothelium surface (thetering and rolling): Tethering and Rolling: This phase is characterized by low-affinity interactions between adhesion molecules on the endothelial cells and their corresponding ligands on leukocytes. Selectins (e.g., P-selectin and E-selectin) are expressed on the activated endothelial cells in response to inflammatory cytokines like TNF-α and IL-1. These selectins bind to sialylated carbohydrates (such as sialyl Lewis X) present on glycoproteins on the surface of leukocytes. These bonds are weak and transient, allowing the leukocytes to slow down and roll along the endothelial surface rather than adhere firmly. 2) Cross the endothelium (extravasation): Once rolling leukocytes come to a near stop, they undergo firm adhesion and transmigrate through the endothelial layer into the surrounding tissue. This process is known as extravasation or diapedesis. Key Mechanisms: Firm adhesion: ○ Following rolling, chemokines (e.g., CXCL8/IL-8) released from the site of infection or tissue injury bind to chemokine receptors on the leukocytes. This binding triggers the activation of integrins on the leukocyte surface. ○ These integrins switch from a low-affinity to a high-affinity state, allowing them to bind tightly to ICAM-1 and VCAM-1, which are expressed on activated endothelial cells. ○ This strong adhesion halts the rolling leukocyte, ensuring firm attachment to the endothelium. ○ IL-8 is a chemokine responsible for endothelium activation (increased adhesivness) and neutrophil activation(increased integrin expression) and attraction. Transmigration (Diapedesis): ○ Leukocytes, firmly attached to the endothelium, squeeze through the endothelial cell junctions, a process called diapedesis. ○ This process involves the reorganization of the actin cytoskeleton in both endothelial cells and leukocytes, which allows the leukocyte to pass through the gaps between endothelial cells. ○ PECAM-1 (Platelet endothelial cell adhesion molecule-1), expressed on both endothelial cells and leukocytes, facilitates this process by promoting the movement of leukocytes across the endothelial barrier. 3) Arrive to the damaged area following a chemotactic gradient (chemotaxis): Once leukocytes have crossed the endothelial barrier, they must migrate toward the exact location of infection or tissue damage. This movement is guided by chemotactic gradients. Key Mechanisms: Chemotaxis: ○ Leukocytes move in response to chemokines and other chemoattractants (such as C5a, fMLP, or leukotriene B4) that form a concentration gradient from the site of infection or injury. ○ The receptors on leukocytes detect these gradients, triggering intracellular signaling pathways that direct the movement of the cell. ○ Leukocytes migrate by polymerizing actin at their leading edge and contracting actin filaments at their trailing edge, allowing them to crawl toward the source of the chemotactic signal. 4) Eliminate the microbes or the source of injury. Leukocytes, particularly neutrophils and macrophages, use phagocytosis, ROS production, degranulation, and cytokine secretion to destroy pathogens and clear damaged tissue. Efferocytosis is the process by which dying or dead cells are cleared away by phagocytic cells. immunology Pagina 16