Resposta Imune Inata Parte II; Células da Imunidade Inata; Neutrófilos; Macrófagos; Células NK

Understanding the Innate Immune Response

Overview of the Innate Immune Response

• The previous video discussed innate immune responses, focusing on molecular aspects and structures of microorganisms that activate these responses.

• Today's discussion will delve into the functions of key cells involved in the innate immune response, building upon last week's content about physical barriers.

Physical and Chemical Barriers

• Physical barriers include skin and mucous membranes, which prevent microorganism entry into deeper tissues.

• Chemical barriers involve factors like pH levels (acidic or alkaline), which inhibit microbial growth; body temperature also plays a role as it is typically around 37°C.

• These natural barriers are present from birth, with specific characteristics such as skin pH influenced by genetics.

Cellular Components of Innate Immunity

• Upon encountering microorganisms (e.g., through skin cuts), certain cells recognize them and initiate an immune response.

• Key cells in the innate immune response include neutrophils, macrophages, and natural killer (NK) cells. Their roles will be explored further in upcoming slides.

Neutrophils: Key Players in Immune Response

• Neutrophils constitute 50-70% of circulating leukocytes; they are produced at a rate of approximately 50 to 100 billion daily during infections.

• The lifespan of neutrophils is short—about 6 to 8 hours under normal conditions but can extend up to 48 hours during inflammation.

Production and Functionality of Neutrophils

• Neutrophil production occurs in bone marrow, stimulated by granulocyte colony-stimulating factor (G-CSF).

• They are often the first responders during inflammatory phases, playing crucial roles in initial defense mechanisms.

Functions Beyond Phagocytosis

• Contrary to past beliefs that neutrophils primarily perform phagocytosis, they also engage in degradation processes without engulfing pathogens directly.

Understanding Neutrophils and Macrophages in Innate Immunity

The Role of Neutrophils

• Neutrophils are crucial for capturing microorganisms and increasing blood flow; they utilize a process called phagocytosis to recognize pathogens through molecular patterns associated with them.

• These cells possess surface receptors that allow them to identify and respond to microbial patterns, facilitating the phagocytic process.

• Inside neutrophils, granules contain proteases, reactive oxygen species, defensins, lactoferrin, and lysozymes that degrade engulfed microorganisms to aid in pathogen elimination.

• Neutrophils can either ingest pathogens via phagocytosis or release granule contents into the extracellular space to kill microbes directly.

• They also produce neutrophil extracellular traps (NETs), which are DNA structures designed to capture and kill microorganisms outside the cell.

Functions of Macrophages

• Unlike neutrophils found in the bloodstream, macrophages originate from monocytes that migrate into tissues where they mature into resident macrophages.

• Resident macrophages play vital roles in tissue homeostasis by performing phagocytosis on dying cells and promoting inflammation as well as tissue repair processes.

• Macrophages have various names based on their location: Kupffer cells in the liver, microglia in the brain, osteoclasts in bones, and alveolar macrophages in lungs.

• In lymphoid tissues like lymph nodes, macrophages are primarily located at sites of cellular circulation where they interact with dendritic cells and other immune components.

Understanding Macrophage Activation and Function

Macrophage Activation Mechanisms

• The discussion begins with the role of macrophages in blood circulation, highlighting their activation when encountering microbial products.

• Activated macrophages, referred to as M1 macrophages, produce pro-inflammatory cytokines such as IL-1, IL-6, and IL-12 that recruit leukocytes to the site of infection.

• These cytokines are crucial for inflammation and aid in eliminating microorganisms through processes like phagocytosis.

Transition to M2 Macrophages

• Monocytes can differentiate into M2 macrophages under the influence of specific cytokines (e.g., IL-10 and TGF-beta), promoting tissue healing and remodeling.

• M2 macrophages facilitate fibrous tissue formation by secreting growth factors that stimulate fibroblast proliferation.

Role of Natural Killer Cells

Characteristics of NK Cells

• Natural Killer (NK) cells constitute 5% to 15% of blood cells and are more prevalent in organs like the liver and uterus during pregnancy.

• Their presence is significant during pregnancy due to the paternal phenotype being present in the mother's body.

Functions of NK Cells

• NK cells play a critical role in responding to viral infections by targeting infected cells for destruction; they cannot remove viruses from within host cells but induce cell death instead.

• They also target tumor cells, contributing to immune responses against cancer.

Interaction Between NK Cells and Macrophages

Cytokine Production

• NK cells destroy infected or transformed cells while activating macrophages through cytokine signaling.

• Upon receiving signals like IL-12 from activated macrophages, NK cells produce interferon-gamma (IFN-gamma), enhancing macrophage activity against pathogens.

Mechanisms of Macrophage and NK Cell Interaction

Role of Macrophages in Immune Response

• The macrophage destroys pathogens through a series of events, producing cytokines that enhance its activity.

• Activated macrophages become more potent, improving their ability to eliminate phagocytosed microorganisms.

Activation and Inhibition of NK Cells

• NK cells utilize two critical receptors: an activation receptor and an inhibition receptor to determine the fate of target cells.

• When both receptors engage simultaneously with a normal cell, the NK cell receives an inhibitory signal, preventing activation.

Decision-Making Process in NK Cells

• The simultaneous binding of activation and inhibition receptors leads to the inactivation of NK cells when interacting with healthy cells.

• If only the activation receptor is engaged without inhibition signals from MHC class I molecules, the NK cell will be activated.

Recognition of Infected Cells

• An infected cell may lose MHC class I expression, making it susceptible to detection by NK cells.

• The absence of MHC class I allows for the engagement of activation receptors on NK cells, leading to their activation.

Mechanism of Action Upon Activation

• Once activated, NK cells release granules containing perforin and granzymes that induce apoptosis in infected cells.

• Perforin creates pores in target cell membranes while granzymes enter through these pores to trigger programmed cell death (apoptosis).

Intracellular Signaling Pathways

• The interaction between activating and inhibiting receptors initiates intracellular signaling pathways within NK cells.

Understanding NK Cell Activation and Inhibition

Mechanisms of NK Cell Regulation

• The activation of Natural Killer (NK) cells involves signals from activating receptors, while inhibitory receptors can inhibit this process. The presence of both types of receptors determines the cell's response.

• Inhibitory receptors activate protein tyrosine phosphatases that remove phosphate groups, preventing the transmission of activating signals and leading to NK cell inhibition.

• If an inhibitory receptor is occupied, it prevents activation; however, if it is unoccupied, activating signals can proceed, allowing for NK cell activation.

• The balance between activating and inhibitory signals on the surface of NK cells is crucial for their function in immune responses against infections.

Role of Dendritic Cells in Immune Response

• Dendritic cells play a significant role in innate immunity by detecting pathogens and relaying information to adaptive immunity. They bridge both immune responses.

• Recent discoveries have identified new lymphoid cells involved in innate immunity that work alongside dendritic cells and mast cells to enhance immune defense mechanisms.

Types and Functions of Dendritic Cells

• Dendritic cells are essential for initiating adaptive immune responses by presenting antigens to T-cells after encountering pathogens.

• There are different subsets of dendritic cells: plasmacytoid dendritic cells produce interferons, while conventional dendritic cells reside in tissues and help activate T-cells.

Transcription Factors in Dendritic Cell Development

• Each subset of dendritic cells develops under specific transcription factors that regulate their differentiation and function within the immune system.

• Transcription factors are proteins that bind to DNA regions to promote RNA synthesis necessary for producing specific proteins related to immune responses.

Hematopoietic Stem Cells as Precursors

• All leukocytes originate from hematopoietic stem cells located primarily in bone marrow, which differentiate into various blood cell types including macrophages and dendritic cells.

Understanding Hematopoietic Stem Cell Differentiation

Differentiation of Hematopoietic Stem Cells

• The initial stage involves hematopoietic stem cells differentiating into monocyte and dendritic cell precursors within the bone marrow.

• The term "differentiation" is emphasized over "transformation," which is reserved for instances where normal cells become cancerous.

• Monoblasts further differentiate into monocytes as they enter the bloodstream, which eventually develop into macrophages.

Types of Dendritic Cells

• In addition to monocytes, conventional dendritic precursor cells are also found in the bloodstream.

• Macrophages can differentiate upon reaching tissues, responding to microorganisms by becoming activated phagocytic cells.

• Conventional dendritic cells perform phagocytosis and migrate to lymph nodes to present antigens to T lymphocytes.

Role of Dendritic Cells in Immune Response

• Dendritic cells provide a crucial interface for detecting pathogens alongside resident macrophages, activating adaptive immunity by stimulating T lymphocytes.

• Plasmacytoid dendritic cells (pDCs), distinct from conventional ones, play a role in producing type I interferons essential for antiviral responses.

Characteristics of Plasmacytoid Dendritic Cells

• pDC morphology resembles plasma cells rather than conventional dendritic cells due to their rounded shape and fewer dendrites.

• These pDCs are primarily located in blood and lymphoid organs, serving as key sources of antiviral cytokines like interferon-alpha and beta.

Antiviral Mechanisms Induced by Interferons

• Type I interferons induce an antiviral state in neighboring cells, inhibiting viral protein synthesis and degrading viral RNA.

• This antiviral state prevents new virus formation through multiple mechanisms including inhibition of gene expression related to viruses.

Summary of Immune System Activation

• The innate immune response is initiated by plasmacytoid dendritic cells that help control viral infections before adaptive immunity kicks in.

Mast Cells and Innate Lymphoid Cells in Inflammation

Role of Mast Cells in Inflammation

• Mast cells are located adjacent to blood vessels and release granules that induce changes in these vessels, initiating the inflammatory process.

• Upon activation, mast cells release mediators that recruit more immune cells, exacerbating inflammation through their numerous granules.

• Mast cells also play a role in mounting immune responses against helminths (intestinal worms), indicating their importance beyond just inflammation.

• They release substances like histamines, prostaglandins, and cytokines (e.g., TNF), which contribute to vasodilation and increased capillary permeability during inflammation.

• The discussion transitions to innate lymphoid cells (ILCs), highlighting their significance alongside mast cells in the initial stages of acute inflammation.

Characteristics of Innate Lymphoid Cells

• ILCs possess transcription factors that categorize them as lymphoid-like but lack the diverse receptors found in adaptive immune lymphocytes.

• Unlike B and T cells with high receptor diversity for recognizing various antigens, ILCs have limited variability due to fewer antigen receptors.

• There are three types of ILCs: ILC1, ILC2, and ILC3. Each type is characterized by specific transcription factors guiding their differentiation.

• While B and T cells express diverse antigen receptors (BCR for B cells; TCR for T cells), ILCs do not have such extensive receptor diversity but still play crucial roles in immunity.

• Despite lacking diverse receptors, ILC characteristics stem from transcription factors that define their functions within the immune system.

Functions and Importance of Innate Lymphoid Cells

• ILCs reside within tissues where they engage with both commensal microorganisms and potential pathogens at mucosal barriers.

• These cells are involved in tissue remodeling after damage while also communicating with the nervous system regarding tissue responses to pathogens.

• Recent research has revealed the existence of these innate lymphoid cell types over the past decade, emphasizing their previously underappreciated roles in immunity.

• All innate lymphoid cells express a common marker (ID2), differentiating into subtypes based on specific transcription factors influencing cytokine production profiles.

Immune Response Mechanisms

Cytokines and Immune Cell Interactions

• Discussion on intracellular microorganisms and the role of cytokines IL-5 and IL-13 in allergic inflammation and parasitic responses, particularly with larger parasites like helminths.

• Comparison of cytokine profiles from T cells (TH1, TH2, TH17), suggesting that these profiles reflect functions of previously discovered immune cells.

• Explanation of how TH1 cells are involved in cellular responses, mirroring the roles of other T cell types in immune reactions.

Innate vs. Adaptive Immunity

• Observation that innate immune cells (like LC1, LC2, LC3) appear to be pre-prepared for responding to various pathogens before adaptive immunity (TH1, TH2, TH17) is fully activated.

• Question raised about whether an effective immune response can occur prior to the full activation of adaptive T cells.

Physical and Chemical Barriers in Immune Defense

• Overview of physical barriers such as ciliated epithelial tissue in the respiratory tract that helps expel trapped organisms through synchronized movement.

• Mention of antibodies and phagocytes present in the digestive tract contributing to microbial defense mechanisms.

Role of Microbiota

• Description of how normal microbiota occupy binding sites on mucosal surfaces to prevent pathogenic colonization.

• Discussion on alkaline pH regions within the intestine limiting microorganism growth alongside mechanical movements aiding clearance.

Additional Protective Mechanisms

• Identification of enzymes like lysozyme found in bodily fluids that contribute to bacterial cell wall degradation as a protective measure.

• Examination of skin's acidic pH due to sebaceous secretions which inhibit pathogen growth along with commensal microbiota occupying potential infection sites.

Urinary Tract Defenses

• Summary of urinary tract defenses including urine flow, acidic pH levels, lysozyme presence, and lactic acid contributing to limiting microbial growth.

Conclusion Reflection