Neutrophils, Eosinophils, and Basophils
Granulocytes are present in nearly all forms of inflammation and are amplifiers and effectors of innate immune responses (Fig. 1-3). Unchecked accumulation and activation of granulocytes can lead to host tissue damage, as seen in neutrophil- and eosinophil-mediated systemic necrotizing vasculitis. Granulocytes are derived from stem cells in bone marrow. Each type of granulocyte (neutrophil, eosinophil, or basophil) is derived from a different subclass of progenitor cell, which is stimulated to proliferate by colony-stimulating factors (Table 1-6). During terminal maturation of granulocytes, class-specific nuclear morphology and cytoplasmic granules appear that allow for histologic identification of granulocyte type.
Neutrophils express Fc receptors for IgG (CD16) and receptors for activated complement components (C3b or CD35). Upon interaction of neutrophils with opsonized bacteria or immune complexes, azurophilic granules (containing myeloperoxidase, lysozyme, elastase, and other enzymes) and specific granules (containing lactoferrin, lysozyme, collagenase, and other enzymes) are released, and microbicidal superoxide radicals (OiT) are generated at the neutrophil surface. The generation of superoxide leads to inflammation by direct injury to tissue and by alteration of macromolecules such as collagen and DNA.
Eosinophils express Fc receptors for IgG (CD32) and are potent cytotoxic effector cells for various parasitic cytotoxicity.
FIGURE 1-4
Receptors and ligands involved in human NK cell-mediated NK cell activation is the final result of the engagement of a number of receptors that have opposite functions. A simplified model of the surface receptors and their ligands involved in NK cell activation (green) or inactivation (red) is shown. KIRS are killer immunoglobulin-like receptors. In the absence of inhibitory signals, activating NK cell receptor ligation with molecules on the target cell results in NK cell triggering and target cell lysis. This event occurs in MHC class I HLA-defective cells, such as tumors or virus-infected cells. In the case of normal cells that express MHC class I, the interaction between inhibitory receptors and MHC class I delivers signals that overcome NK cell triggering, thus preventing target cell lysis. Although the cellular natural cytotoxic receptor (NCR) ligands have not yet been identified, the ligands for NG2D are represented by stress-inducible MICA, MICB, and ULBPs. The ligand for 2B4 is CD48, which is expressed by hematopoietic cells, whereas the ligand for NTB-A is itself on target cells. The + and – symbols denote activating or inhibitory signals, respectively.
In Nippostrongylus brasiliensis helminth infection, eosinophils are key cytotoxic effector cells in removal of these parasites. Key to regulation of eosinophil cytotoxicity to N. brasiliensis worms are antigen-specific T helper cells that produce IL-4, thus providing an example of regulation of innate immune responses by adaptive immunity antigen-specific T cells. Intracytoplasmic contents of eosinophils, such as major basic protein, eosinophil cationic protein, and eosinophil-derived neurotoxin, are capable of directly damaging tissues and may be responsible in part for the organ system dysfunction in the hypereosinophilic syndromes. Since the eosinophil granule contains anti-inflammatory types of enzymes (histami-nase, arylsulfatase, phospholipase D), eosinophils may homeostatically downregulate or terminate ongoing inflammatory responses.
Basophils and tissue mast cells are potent reservoirs of cytokines such as IL-4 and can respond to bacteria and viruses with antipathogen cytokine production through multiple TLRs expressed on their surface. Mast cells and basophils can also mediate immunity through the binding of antipathogen antibodies. This is a particularly important host defense mechanism against parasitic diseases. Basophils express high-affinity surface receptors for IgE (FcRI) and, upon cross-linking of basophil-bound IgE by antigen, can release histamine, eosinophil chemo-tactic factor of anaphylaxis, and neutral protease—all mediators of allergic immediate (anaphylaxis) hypersensitivity responses (Table 1-10). In addition, basophils express surface receptors for activated complement components (C3a, C5a), through which mediator release can be directly effected. Thus, basophils, like most cells of the immune system, can be activated in the service of host defense against pathogens, or they can be activated for mediation release and cause pathogenic responses in allergic and inflammatory diseases.
Table 1-10
|
EXAMPLES OF MEDIATORS RELEASED FROM HUMAN CELLS AND BASOPHILS |
|
|
MEDIATOR |
ACTIONS |
|
Histamine |
Smooth-muscle contraction, increased vascular permeability |
|
Slow reacting substance of anaphylaxis (SRSA) (leukotriene C4, D4, E4) |
Smooth-muscle contraction |
|
Eosinophil chemotactic factor of anaphylaxis (ECF-A) |
Chemotactic attraction of eosinophils |
|
Platelet-activating factor |
Activates platelets to secrete serotonin and other mediators: smooth-muscle contraction; induces vascular permeability |
|
Neutrophil chemotactic factor (NCF) |
Chemotactic attraction of neutrophils |
|
Leukotactic activity (leukotriene B4) |
Chemotactic attraction of neutrophils |
|
Heparin |
Anticoagulant |
|
Basophil kallikrein of anaphylaxis (BK-A) |
Cleaves kininogen to form bradykinin |
The Complement System
The complement system, an important soluble component of the innate immune system, is a series of plasma enzymes, regulatory proteins, and proteins that are activated in a cascading fashion, resulting in cell lysis. There are four pathways of the complement system: the classic activation pathway activated by antigen/antibody immune complexes, the MBL (a serum collectin;Table 1-3) activation pathway activated by microbes with terminal mannose groups, the alternative activation pathway activated by microbes or tumor cells, and the terminal pathway that is common to the first three pathways and leads to the membrane attack complex that lyses cells (Fig. 1-5).The series of enzymes of the complement system are serine proteases.
Activation of the classic complement pathway via immune complex binding to C1q links the innate and adaptive immune systems via specific antibody in the immune complex. The alternative complement activation pathway is antibody-independent and is activated by binding of C3 directly to pathogens and “altered self” such as tumor cells.
FIGURE 1-5
The four pathways and the effector mechanisms of the complement system. Dashed arrows indicate the functions of pathway components.
In the renal glomerular inflammatory disease IgA nephropathy, IgA activates the alternative complement pathway and causes glomerular damage and decreased renal function. Activation of the classic complement pathway via C1, C4, and C2 and activation of the alternative pathway via factor D, C3, and factor B both lead to cleavage and activation of C3. C3 activation fragments, when bound to target surfaces such as bacteria and other foreign antigens, are critical for opsonization (coating by antibody and complement) in preparation for phagocytosis. The MBL pathway substitutes MBL-associated serine proteases (MASPs) 1 and 2 for C1q, C1r, and C1s to activate C4.The MBL activation pathway is activated by mannose on the surface of bacteria and viruses.
The three pathways of complement activation all converge on the final common terminal pathway. C3 cleavage by each pathway results in activation of C5, C6, C7, C8, and C9, resulting in the membrane attack complex that physically inserts into the membranes of target cells or bacteria and lyses them.
Thus, complement activation is a critical component of innate immunity for responding to microbial infection. The functional consequences of complement activation by the three initiating pathways and the terminal pathway are shown in Fig. 1-5. In general the cleavage products of complement components facilitate microbe or damaged cell clearance (C1q, C4, C3), promote activation and enhancement of inflammation (anaphylatox-ins, C3a, C5a), and promote microbe or opsonized cell lysis (membrane attack complex).
Cytokines
Cytokines are soluble proteins produced by a wide variety of hematopoietic and nonhematopoietic cell types (Tables 1-6 to 1-9). They are critical for both normal innate and adaptive immune responses, and their expression may be perturbed in most immune, inflammatory, and infectious disease states.
Cytokines are involved in the regulation of the growth, development, and activation of immune system cells and in the mediation of the inflammatory response. In general, cytokines are characterized by considerable redundancy; different cytokines have similar functions. In addition, many cytokines are pleiotropic in that they are capable of acting on many different cell types. This pleiotropism results from the expression on multiple cell types of receptors for the same cytokine (see below), leading to the formation of “cytokine networks.” The action of cytokines may be (1) autocrine when the target cell is the same cell that secretes the cytokine, (2) paracrine when the target cell is nearby, and (3) endocrine when the cytokine is secreted into the circulation and acts distal to the source.
Cytokines have been named based on presumed targets or based on presumed functions. Those cytokines that are thought to primarily target leukocytes have been named interleukins (IL-1, -2, -3, etc.). Many cytokines that were originally described as having a certain function have retained those names (granulocyte colony-stimulating factor or G-CSF, etc.). Cytokines belong in general to three major structural families: the hemopoietin family; the TNF, IL-1, platelet-derived growth factor (PDGF), and transforming growth factor (TGF) β families; and the CXC and c-c chemokine families (Table 1-8). Chemokines are cytokines that regulate cell movement and trafficking; they act through G protein-coupled receptors and have a distinctive threedimensional structure. IL-8 is the only chemokine that early on was named an interleukin (Table 1-6).
In general, cytokines exert their effects by influencing gene activation that results in cellular activation, growth, differentiation, functional cell-surface molecule expression, and cellular effector function. In this regard, cytokines can have dramatic effects on the regulation of immune responses and the pathogenesis of a variety of diseases. Indeed, T cells have been categorized on the basis of the pattern of cytokines that they secrete, which results in either humoral immune response (TH2) or cell-mediated immune response (TH1) (Fig. 1-3).
Cytokine receptors can be grouped into five general families based on similarities in their extracellular amino acid sequences and conserved structural domains. The immunoglobulin (Ig) superfamily represents a large number of cell-surface and secreted proteins. The IL-1 receptors (type 1, type 2) are examples of cytokine receptors with extracellular Ig domains.
The hallmark of the hematopoietic growth factor (type 1) receptor family is that the extracellular regions of each receptor contain two conserved motifs. One motif, located at the N terminus, is rich in cysteine residues. The other motif is located at the C terminus proximal to the transmembrane region and comprises five amino acid residues, tryptophan-serine-X-tryptophan-serine (WSXWS).This family can be grouped on the basis of the number of receptor subunits they have and on the utilization of shared subunits. A number of cytokine receptors, i.e., IL-6, IL-11, IL-12, and leukemia inhibitory factor, are paired with gp130.There is also a common 150-kDa subunit shared by IL-3, IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors. The gamma chain (yc) of the IL-2 receptor is common to the IL-2, IL-4, IL-7, IL-9, and IL-15 receptors. Thus, the specific cytokine receptor is responsible for ligand-specific binding, while the subunits such as gp130, the 150-kDa subunit, and yc are important in signal transduction. The yc gene is on the X chromosome, and mutations in the yc protein result in the X-linked form of severe combined immune deficiency syndrome (X-SCID).
The members of the interferon (type II) receptor family include the receptors for IFN-γ and -ß, which share a similar 210-amino-acid binding domain with conserved cysteine pairs at both the amino and carboxy termini. The members of the TNF (type III) receptorfamily share a common binding domain composed of repeated cysteine-rich regions. Members of this family include the p55 and p75 receptors for TNF (TNFR1 and TNFR2, respectively); CD40 antigen, which is an important B cell-surface marker involved in immunoglobulin isotype switching; fas/Apo-1, whose triggering induces apopto-sis; CD27 and CD30, which are found on activated T cells and B cells; and nerve growth factor receptor.
The common motif for the seven transmembrane helix family was originally found in receptors linked to GTP-binding proteins. This family includes receptors for chemokines (Table 1-7), ß-adrenergic receptors, and retinal rhodopsin. It is important to note that two members of the chemokine receptor family, CXC chemokine receptor type 4 (CXCR4) and ß chemokine receptor type 5 (CCR5), have been found to serve as the two major coreceptors for binding and entry of HIV into CD4-expressing host cells.
Significant advances have been made in defining the signaling pathways through which cytokines exert their effects intracellularly. The Janus family of protein tyrosine kinases (JAK) is a critical element involved in signaling via the hematopoietin receptors. Four JAK kinases,JAK1,JAK2,JAK3, and Tyk2, preferentially bind different cytokine receptor subunits. Cytokine binding to its receptor brings the cytokine receptor subunits into apposition and allows a pair of JAKs to transphosphory-late and activate one another. The JAKs then phospho-rylate the receptor on the tyrosine residues and allow signaling molecules to bind to the receptor, where these molecules become phosphorylated. Signaling molecules bind the receptor because they have domains (SH2, or src homology 2 domains) that can bind phosphorylated tyrosine residues. There are a number of these important signaling molecules that bind the receptor, such as the adapter molecule SHC, which can couple the receptor to the activation of the mitogen-activated protein kinase pathway. In addition, an important class of substrate of the JAKs is the signal transducers and activators of transcription (STAT) family of transcription factors. STATs have SH2 domains that enable them to bind to phos-phorylated receptors, where they are then phosphory-lated by the JAKs. It appears that different STATs have specificity for different receptor subunits. The STATs then dissociate from the receptor and translocate to the nucleus, bind to DNA motifs that they recognize, and regulate gene expression.The STATs preferentially bind DNA motifs that are slightly different from one another and thereby control transcription of specific genes. The importance of this pathway is particularly relevant to lymphoid development. Mutations of JAK3 itself also result in a disorder identical to X-SCID; however, since JAK3 is found on chromosome 19 and not on the X chromosome, JAK3 deficiency occurs in boys and girls.
The Adaptive Immune System
Adaptive immunity is characterized by antigen-specific responses to a foreign antigen or pathogen. A key feature of adaptive immunity is that following the initial contact with antigen (immunologic priming), subsequent antigen exposure leads to more rapid and vigorous immune responses (immunologic memory).The adaptive immune system consists of dual limbs of cellular and humoral immunity. The principal effectors of cellular immunity are T lymphocytes, while the principal effectors of humoral immunity are B lymphocytes. Both B and T lymphocytes derive from a common stem cell (Fig. 1-6).
The proportion and distribution of immunocompetent cells in various tissues reflect cell traffic, homing patterns, and functional capabilities. Bone marrow is the major site of maturation of B cells, monocytes-macrophages, dendritic cells, and granulocytes and contains pluripotent stem cells that, under the influence of various colony-stimulating factors, are capable of giving rise to all hematopoietic cell types. T cell precursors also arise from hematopoietic stem cells and home to the thymus for maturation. Mature T lymphocytes, B lymphocytes, monocytes, and dendritic cells enter the circulation and home to peripheral lymphoid organs (lymph nodes, spleen) and mucosal surface-associated lymphoid tissue (gut, genitourinary, and respiratory tracts) as well as the skin and mucous membranes and await activation by foreign antigen.
T Cells
The pool of effector T cells is established in the thymus early in life and is maintained throughout life both by new T cell production in the thymus and by antigen-driven expansion of virgin peripheral T cells into “memory” T cells that reside in peripheral lymphoid organs. The thymus exports ~2% of the total number of thymocytes per day throughout life, with the total number of daily thymic emigrants decreasing by ~3% per year during the first four decades of life.
Mature T lymphocytes constitute 70-80% of normal peripheral blood lymphocytes (only 2% of the total-body lymphocytes are contained in peripheral blood), 90% of thoracic duct lymphocytes, 30-40% of lymph node cells, and 20-30% of spleen lymphoid cells. In lymph nodes, T cells occupy deep paracortical areas around B cell germinal centers, and in the spleen, they are located in periarteriolar areas of white pulp. T cells are the primary effectors of cell-mediated immunity, with subsets of T cells maturing into CD8+ cytotoxic T cells capable of lysis of virus-infected or foreign cells (short-lived effector T cells). Two populations of long-lived memory T cells are triggered by infections: effector memory and central memory T cells. Effector memory T cells reside in nonlymphoid organs and respond rapidly to repeated pathogenic infections with cytokine production and cytotoxic functions to kill virus-infected cells. Central memory T cells home to lymphoid organs where they replenish long- and short-lived and effector memory T cells as needed.
FIGURE 1-6
Development stages of T and B cells. Elements of the developing T and B cell receptor for antigen are shown schematically. The classification into the various stages of B cell development is primarily defined by rearrangement of the immunoglobulin (Ig), heavy (H), and light (L) chain genes and by the absence or presence of specific surface markers.
In general, CD4+ T cells are also the primary regulatory cells of T and B lymphocyte and monocyte function by the production of cytokines and by direct cell contact (Fig. 1-2). In addition,T cells regulate erythroid cell maturation in bone marrow, and through cell contact (CD40 ligand) have an important role in activation of B cells and induction of Ig isotype switching.
Human T cells express cell-surface proteins that mark stages of intrathymic T cell maturation or identify specific functional subpopulations of mature T cells. Many of these molecules mediate or participate in important T cell functions (Table 1-1, Fig. 1-6).
The earliest identifiable T cell precursors in bone marrow are CD34+ pro-T cells (i.e., cells in which TCR genes are neither rearranged nor expressed). In the thymus, CD34+ T cell precursors begin cytoplasmic (c) synthesis of components of the CD3 complex of TCR-associated molecules (Fig. 1-6).Within T cell precursors, TCR for antigen gene rearrangement yields two T cell lineages, expressing either TCRaß chains or TCRγδ chains. T cells expressing the TCRaß chains constitute the majority of peripheral T cells in blood, lymph node, and spleen and terminally differentiate into either CD4+ or CD8+ cells. Cells expressing TCRy5 chains circulate as a minor population in blood; their functions, although not fully understood, have been postulated to be those of immune surveillance at epithelial surfaces and cellular defenses against mycobacterial organisms and other intracellular bacteria through recognition of bacterial lipids.
In the thymus, the recognition of self-peptides on thymic epithelial cells, thymic macrophages, and dendritic cells plays an important role in shaping the T cell repertoire to recognize foreign antigen (positive selection) and in eliminating highly autoreactive T cells (negative selection). As immature cortical thymocytes begin to express surface TCR for antigen, autoreactive thymocytes are destroyed (negative selection),thymocytes with TCRs capable of interacting with foreign antigen peptides in the context of self-MHC antigens are activated and develop to maturity (positive selection), and thymocytes with TCR that are incapable of binding to self-MHC antigens die of attrition (no selection). Mature thymocytes that are positively selected are either CD4+ helper T cells or MHC class II-restricted cytotoxic (killer) T cells, or they are CD8+ T cells destined to become MHC class I-restricted cytotoxic T cells. MHC class I- or class II-restricted means that T cells recognize antigen peptide fragments only when they are presented in the antigen-recognition site of a class I or class II MHC molecule, respectively (Chap. 2).
After thymocyte maturation and selection, CD4 and CD8 thymocytes leave the thymus and migrate to the peripheral immune system. The thymus continues to be a contributor to the peripheral immune system, well into adult life, both normally and when the peripheral T cell pool is damaged, such as occurs in AIDS and cancer chemotherapy.
