Transcript 4th Lecture
PHL 633 Immunotoxicology Fourth Lecture By Abdelkader Ashour, Ph.D. Phone: 4677212 Email: [email protected] Innate Immunity, Overview Innate immune response acts as a first line of defense against anything nonself With respect to infectious agents, the innate immune system eliminates most potential pathogens before significant infection occurs Two general types of cells are involved in innate host resistance NK cells Professional phagocytes Functions of NK: Cytolysis Cytokine production NK cells are located primarily in blood, all lymphoid organs, liver and lungs The primary mediators of cell death are perforin and granzyme These mediators are expelled onto the surface of the target cell, which subsequently undergoes apoptosis Innate Immunity, Overview NKT cells have been described relatively recently. They express cell surface markers characteristic of both NK and T cells NKT cells are considered important for innate immunity because they are activated fairly early in an immune response and they do not possess immunologic memory Much information exists regarding NKT cell surface markers, but most prominent are expression of a TCR and a C-lectin-type NK receptor, to which glycolipids may bind Functionally, NKT cells are similar to NK cells in that they produce cytokines and mediate cytolysis of target cells Innate Immunity, Overview Phagocytic cells include polymorphonuclear cells (PMN; neutrophils) and the monocyte/macrophage The precursors of the macrophage and PMN develop from pluripotent stem cells that have become committed to the myeloid lineage Differentiation into macrophage or PMN is dependent on the interaction with specific colony-stimulating factors (CSFs) such as macrophage-CSF (M-CSF), granulocyte-CSF (G-CSF), granulocyte-macrophage-CSF (GM-CSF), interleukin (IL)-3 and others Within the bone marrow, both cell types undergo several rounds of replication before entering the bloodstream where they circulate for about 10 h and then enter the tissues where they perform effector functions for about 1–2 days PMNs are capable of passing through the cell membrane of blood vessels and thereby represent a primary line of defense against infectious agents They are excellent phagocytic cells and can eliminate most microorganisms through the release of various reactive oxygen species, such as superoxide, singlet oxygen, ozone, hydrogen peroxide, and hydroxyl radicals Their phagocytic activity is greatly enhanced by the presence of complement and antibody deposited on the surface of the foreign target. They are also important in the induction of an inflammatory response Innate Immunity, Overview Macrophages are terminally differentiated monocytes. Upon exiting the bone marrow, monocytes circulate within the bloodstream for about 1 day. At that time, they begin to distribute to the various tissues, where they can then differentiate into macrophages Macrophages can be found in all tissues, most notably in the liver, lung, spleen, kidney, peritoneum, and brain Within different tissues, macrophages have distinct properties and vary in extent of surface receptors, oxidative metabolism, and expression of MHC class II This is likely due to the factors present within the microenvironment in which the monocyte differentiates The liver macrophages, or Kupffer cells, are primarily responsible for particulate and microbial clearance from the blood. They express high levels of MHC class II, are actively phagocytic, and release several soluble mediators. Thus, they are the primary cells responsible for the acute phase response Alveolar (lung) macrophages remove foreign particulate matter from the alveolar space. They are self-renewing and have a particularly long life span. These cells actively secrete proteases and bactericidal enzymes, such as lysozyme Splenic macrophages also phagocytose particulate material and polysaccharides from the blood and tissue Innate Immunity, Overview Mononuclear phagocytes within the CNS are known as microglia They are responsible for antigen presentation in immunologic diseases of the CNS. Microglia have a very slow turnover time, and thus recruitment of monocytes to areas of inflammation within the CNS is also slow In addition to the cellular components of innate immunity, there are several soluble components. These include the complement cascade, acute phase proteins, granzyme and perforin, and various cytokines, chemokines, and interferons The complement cascade is important in innate immunity because of its activation through the lectin pathway. Further, C3a and C5a, which are chemokines generated during the cascade, recruit phagocytic cells to the site of complement activation Innate Immunity, Overview Acute phase proteins, such as serum amyloid A, serum amyloid P, and C-reactive protein, participate in an acute phase response to infection by binding bacteria and facilitating complement activation Granzyme and perforin work in conjunction, with perforin disrupting the target cell membrane, allowing granzyme to enter and mediate cell lysis by several mechanisms Cytokines, chemokines and interferons are also critical soluble components of the innate immune system. There are also several shared characteristics of such mediators that warrant discussion for the purposes of understanding immunotoxicological mechanisms: I. The primary function of cytokines, chemokines and interferons is cell–cell interactions cellular activation, initiation or termination of intracellular signaling events, proliferation, differentiation, migration, trafficking, or effector functions II. Although some of these molecules might be constitutively expressed, most are inducible in response to antigens, cellular stressors, or other cytokines. Thus many cytokines, chemokines, and interferons are not stored in the cell, but rather are tightly regulated, so that they are quickly generated on demand An example of a cytokine important in innate immunity is IL-1, which is rapidly transcribed in response to various stimulants using the critical transcription factor nuclear factor κB (NF-κB) Innate Immunity, Overview III. Many cytokines share common receptor subunits such that should one particular subunit of a receptor be adversely affected by an immunotoxic agent, the functional outcome might be amplified. An example is the common γ chain, which is shared by the IL-2, IL-4, IL-7, IL-9, IL-13, IL-15 and IL-21 receptors Although a compound might “only” affect transcription of the common γ chain, the immunologic consequences of affecting this target could be quite destructive. Specifically for innate immunity, there are several cytokines, chemokine and interferons that play critical roles, such as the proinflammatory cytokines IL-1, IL-6, and TNF-α and the type I interferons Innate Immunity, Cytokines Innate Immunity, Cytokines Innate Immunity, Cytokines Characteristics of Selected Immune Cells