Chapter 16 : Cell-Mediated Immunity

A synopsis of the elements of CMI. (Left) TH cells can provide cytokines to assist in the activation of macrophages. (Right) CTLs can kill altered self cells such as tumor cells or virally infected cells.

FcRs (in particular, FcγRIII or CD16) expressed on the surface of NK cells allow the NK cell to recognize antibody-coated target cells. Target cells recognized by this mechanism will be killed by the NK cell by ADCC.

Although phagocytosis and complement activation are considered part of the innate (or nonspecific) immune response, products of the specific immune response can enhance the efficiency of these innate immune mechanisms. Antibodies produced by B lymphocytes can mediate classical pathway complement activation and opsonize targets for enhanced phagocytosis. T lymphocytes, particularly TH1 cells, can enhance phagocytic killing by macrophages through their elaboration of cytokines such as IFN-γ, which activates the macrophage.

Activation of a macrophage not only enhances killing of newly phagocytosed bacteria but also can help a macrophage previously infected with intracellular pathogens (such as listeriae) destroy the pathogen. Upon activation, the macrophage fuses its lysosomes with bacteria- laden vesicles.

Activation of the macrophage by a TH1 cell involves an initial priming step and a subsequent cytokine-mediated step. Priming is essential for making the macrophage responsive to the cytokine IFN-γ and can occur either by binding of pathogen-associated molecular pattern (PAMP) to a Toll-like receptor (TLR) (1) or by engagement of macrophage CD40 protein by CD154 on TH cells (2).

Both CTLs and NK cells are capable of cell-mediated cytotoxicity. However, of these two cell types, only the CTL can carry out cytotoxicity in an antigen-specific manner. NK cells have a much more general mechanism of target-cell recognition. NK cells usually engage their target cell by the killer activational receptor (KAR), which binds carbohydrates on the target cell surface. Killing is inhibited if the killer inhibitory receptor (KIR) binds MHC class I protein on the target cell. Thus, by this mechanism NK cells kill targets with abnormal expression of MHC class I.

Through the secretion of cytokines such as IFN-γ, TNF-β/LT, and IL-2, TH1 cells orchestrate CMI by enhancing the activity of macrophages, neutrophils, and CTLs. Further, TH1 cells can enhance CMI by causing B cells to produce antibody isotypes capable of mediating ADCC.

Differentiation to the TH1 phenotype begins when a noncommitted TH cell (TH0) binds IFN-γ, triggering a signal transduction cascade that depends on the transcription factor T-bet. This event leads to some TH1 functions (the secretion of IFN-γ) and to acquisition of responsiveness to IL-12 by expression of the IL-12 receptor (IL-12R) on the T cell. Subsequent binding of IL-12 (also produced by APCs) completes the differentiation process by a mechanism that requires the signal transducer protein STAT4. Upon IL-12 binding, STAT4 is phosphorylated, causing dimerization of STAT4 and its translocation to the T cell's nucleus.

The DTH reaction is initiated when macrophages phagocytose antigen and present it to TH1 cells (A), the latter of which then secrete cytokines that recruit more macrophages and TH1 cells to the area. The cytokines also activate local macrophages (B). If there is a chronic source of antigen, this may ultimately lead to a large mass of activated macrophages and TH1 cells, called a granuloma. (C) In live animals, a DTH reaction is observed only after induction of T-cell memory by prior exposure (sensitization) to the antigen. In animals (left) this can be accomplished by injection of the antigen, whereas in humans (right) this usually occurs by infection with a microorganism such as a mycobacterium. Secondary exposure of the skin to the antigen results in a detectable, red, inflamed, and indurated area. Intentional introduction of the antigen on the skin can be used as an easy and inexpensive test for primary exposure and is the basis of one currently used test for tuberculosis.

For a naive T cell to be activated it must receive two signals from an APC. Signal 1 is the binding of the TCR to MHC-peptide complexes, while signal 2 is the costimulatory signal, which is usually delivered by binding of the CD28 protein of the T cell by the CD80 or CD86 protein of the APC. Delivery of signal 2 alone has no effect on the T cell, whereas delivery of signal 1 alone causes the T cell to enter a state of antigen-specific inactivity called anergy.

Inflammation requires the extravasation of leukocytes from the blood to the interstitial tissue. Extravasation proceeds in several steps, beginning with “rolling adhesion” (top) of the leukocyte by selectin-carbohydrate interactions. Most of these early interactions between the leukocyte and endothelial cell are low affinity. During inflammation, interendothelial cell tight junctions dissociate, inflammatory signals lead to the expression of additional adhesion molecules (such as ICAM-1) on the endothelium, and endothelial cells produce chemokines such as IL-8 (green circles) that act on the leukocyte to activate integrin proteins to their high-affinity form (inset). This activation event is initiated when the chemokine binds its receptor (orange) on the leukocyte membrane and triggers a G-protein-dependent signaling cascade. Following high-affinity binding, the leukocyte arrests on the endothelial cell surface and leaves the vasculature by diapedesis.

(A) Neutrophils (PMNs) are the first cells to extravasate to the interstitial tissue. (B) This correlates with the early expression of E-selectin on vascular endothelium, which binds to sialyl Lewisx carbohydrate moieties on the surface of the PMN.

Intercellular adhesion molecules can be classified into several categories on the basis of their structure and function. Molecules of the integrin family bind to Ig superfamily receptors with either low or high affinity, depending on the integrin's activation state. Selectin molecules bind to carbohydrate moieties of mucins via the calcium-dependent (C-type) lectin domain. The mucin MAdCAM (mucosal addressin cell adhesion molecule) is unusual in that it can bind to both integrins (via its Ig-like domains) and selectins (via its carbohydrate moieties).

Some of the products of activated macrophages. The effects of each product are indicated with green arrows. O2•, superoxide; H2O2, hydrogen peroxide; HOCl, hydrogen hypochlorite.

The importance of CMI in protection against intracellular pathogens is illustrated by infection of IFN-γ-deficient mice with Mycobacterium bovis (attenuated strain BCG). Normal mice (blue line) are able to kill BCG following infection due to an effective TH1- mediated DTH response. However, IFN-γ-deficient mice (red line) are unable to kill the microorganisms and die of overwhelming infection within weeks. Reproduced from D. K. Dalton et al., Science 259:1739–1742, 1993, with permission.

The occurrence of two different forms of leprosy correlates with the TH cell (TH1 or TH2) polarity of the immune response. A properly contained (tuberculoid) infection is associated with a TH1 response, while an ineffective immune response, leading to widely disseminated (lepromatous) infection, is associated with a TH2 cytokine profile. The figure shows dot blots in which mRNA samples from the lesions of four patients with the tuberculoid form of leprosy are compared to those of four patients with the lepromatous form, following hybridization with probes specific for the indicated cytokines. Reproduced from P. A. Sieling and R. L. Modlin, Immunobiology 191:378– 387, 1994, with permission.

The TH1 phenotype correlates with the less severe form of tuberculosis because TH1 cytokines recruit and activate macrophages (A), resulting in the formation of a granuloma (B) that contains the infection. Specific Th1 T cells are activated by peptide-MHC complexes to secrete cytokines that are chemotactic for cells, including monocytes/macrophages. Other Th1 cytokines, especially IFN-γ, cause the activation of the macrophages in the tissues (A). In the chronic form of delayed-type hypersensitivity, an organized arrangement of cells is formed, with the specific T cells on the periphery and activated macrophages in the interior causing tissue damage (B). Some macrophages fuse into multinucleated “giant” or “epithelioid” cells.

Direct activation of a naive CD8+ CTL precursor by a virus-infected DC. (Top) The CTL receives signal 1 (TCR-MHC-peptide) and signal 2 (CD28-CD80/86) from the DC. (Bottom) This causes the CTL to produce both IL-2 and its receptor (IL-2R), stimulating CTL activation in an autocrine manner.

TH cells may assist the activation of naive CTLs by two different mechanisms. (A) The CTL, TH cell, and DC may form a tripartite complex, in which the DC simultaneously presents antigen to TH cells (1) and CTLs (2). Activation of the helper cell results in production of IL-2 (3), while presentation of antigen to the CTL results in synthesis of the IL-2R (4). Helper cell-produced IL-2 can then bind the CTL IL-2R in a paracrine manner (5), completing activation of the CTL. (B) Alternatively, the TH cell may form a bipartite complex with only a DC (1), activating the DC to increase its expression of the CD80 or CD86 costimulatory molecules (2). This activated DC may then be able to stimulate production of both IL-2 (3) and the IL-2R (4) on the CTL, allowing autocrine IL-2 stimulation (5). In either model, but particularly in mechanism B, up-regulated dendritic cell expression of CD80 or CD86 is a key requirement for costimulation via interaction with CD28 on the T cell. An efficient way of up-regulating CD80 or CD86 on dendritic cells is signaling via CD40 after its ligation by CD154 expressed by activated CD4 T helper cells.

Stages of CTL-mediated cytotoxicity. Initial recognition of an infected target cell occurs via the TCR-peptide-MHC class I complex, which induces reorganization of the CTL membranes and organelles toward the area of plasma membrane contact with the target cell. The membrane-bound secretory granules then fuse with the plasma membrane, and perforin, granzymes, and other potentially cytotoxic factors are released directionally toward the target cell. The CTL then detaches, and the target cell is eventually killed by apoptosis. Some CTL factors such as granulysin are also toxic to the microbes infecting the target cell.

Cytotoxicity by CTLs can be caused by the introduction of perforin and granzymes to the target cells. (A) One view is that perforin forms stable multimers in the target cell plasma membrane, forming transmembrane pores. Other data suggest that pores are not needed for delivery of mediators of apoptosis and perforin serves merely as a translocator complexed to serglycan, a chondroitan sulfate proteoglycan. Granzyme B enters cells also complexed to serglycan and activates pro-caspase-3, which in turn becomes caspase-3, a key mediator of apoptosis. (B) Structure of granzyme B. The three-dimensional structure of human granzyme B compared to caspase-3, key mediators of cell death with cleavage specificity for aspartic acid in P1. From J. Rotonda et al., Chem. Biol. 8:357–368, 2001, with permission.

Cell signaling pathway following Fas (CD95) ligation. Fas ligation leads to trimerization of Fas, causing recruitment of the adaptor protein FADD (Fas-associated protein with death domain) and pro-caspase-8. FADD and pro-caspase-8 include death effector domains (DED), which are found in proteins with both pro- and antiapoptotic activity. Pro-caspase-8 is cleaved to its active form (caspase-8), which initiates a cascade of caspase activation events. The terminal caspase (caspase-6) acts on numerous cellular substrates to bring about cell death.

A synopsis of several FcRs, which are represented schematically with the CD designations, affinities for antibody, and tissue distributions of the receptors listed underneath.