Section F – The T cell response – cell-mediated immunity
F5 C LONAL EXPANSION AND
are expanded to carry out their effector function and develop into memory cells.
Helper T cells There are two kinds of CD4+helper T cells (Th1 and Th2) with different func- tions. Each develops from uncommitted Th0 cells following initial contact with a microbe. Th1 cells are predominately involved in mediating inflammatory immune responses (through the activation of macrophages), while Th2 cells are primarily involved in the induction of humoral immunity (via the activation of B cells). In this regard, Th0 to Th1 cell development is encouraged when a Th0 cell recognizes a microbial peptide presented by an infected macrophage that is producing IL-12 (Fig. 2). In contrast, Th0 cells are encouraged to become Th2 cells under the influence of IL-4 released by B cells and other cells (e.g. mast cells). Thus, following activation by specific peptide antigen, Th1 cells derived from Th0 cells produce cytokines such as IFNγand TNFαthat primarily act on macrophages. Cytokines produced by Th2 cells (IL-4, IL-5, IL-6, and IL-13) are involved mainly in B cell differentiation and maturation. IL-10 is also produced F5 – Clonal expansion and development of effector function 133
Th Th
Th CD40L CD40
IL-1 IL-2
IL-12
Fig. 1. Initial priming of helper T cells through dendritic cells.
Macrophages/
dendritic
cells Th1
Th2 Th0
Microbe IFNγ
IL-4
IFNγ TNFα
IL-4,5,10,13
Activates MΦ, Induces B cells to class switch Ig to IgG1 or IgG3, suppresses Th2 cells
Activates B cells, induces Ig class Switch to IgG2, IgA or IgE, suppresses Th1 cells
IL-12
Produces Functions
APC
Fig. 2. Two types of helper T cells. Following uptake of a microbe, APCs produce IL-12, and present microbial-derived peptides to specific Th0 cells. In the presence of IL-12 and IFNg, Th0 cells differentiate into Th1 cells whereas in the presence of IL-4 and other Th2 cytokines the Th0 cells differentiate into Th2 cells.
by Th2 cells and regulates the activity of Th1 cells (Topic G5). It is important to remember that in most cases, an immune response elicits both Th1 and Th2 activities, although there are some instances where one or the other is more effective in mediating protection (Topic H2).
Th2 cells
Participation of T cells is required for B cell responses to most antigens (Topic E2). T cells most effective in inducing the production of antibody from B cells, especially of the IgA and IgE isotype, are the helper CD4 Th2 cells. Th2 cells induce B cells to produce Ig, switch the isotype of Ig being produced, and induce affinity maturation of the Ig. This involves not only cytokines but direct engagement of surface molecules on the T and B cells (cognate interactions) which trigger their activation.
More specifically, Th2 cells recognize antigenic peptides in MHC class II molecules on the surface of antigen-specific B cells and through interaction with other surface molecules are activated (Topics F2 and F4). The interaction of CD40L on T cells with CD40 on B cells induces B cell proliferation and class switching to IgE- and IgA-producing cells (Topic D3, Fig. 4). Cytokines produced by Th2 cells, including IL-4, IL-5, and IL-6) act as growth and differ- entiation factors for B cells.
Th1 cells
Role of Th1 cells in macrophage recruitment and activation. The response to a variety of intracellular parasites is dependent upon functionally intact Th1 cells. For example, the immune responses to Leishmania and mycobacteria are severely diminished if the host cannot produce IFNγand TNFα. This is because, in the absence of these mediators, infected macrophages cannot become activated to kill the pathogen. Although other cytokines can augment macrophage activities, both IFNγand TNFαare critical for effective macrophage activation.
Th1 cells when activated also produce chemokines that assist in the recruit- ment of monocytes, and colony-stimulating factor (GM-CSF) that induces their differentiation into macrophages at the site of infection. In addition, IL-3 increases the production and release of monocytes from the bone marrow. Also, TNFαfrom Th1 cells alters the surface properties of endothelial cells to promote the adhesion of monocytes at the site of infection (Topic B4). The coordinated production of these mediators allows the infiltration of T cells and monocytes to the site of inflammation where their interaction leads to macrophage differentia- tion, activation and the elimination of the pathogen (Fig. 3).
Role of Th1 cells in isotype switching and affinity maturation. Th1 cells may also induce B cells to produce Ig, switch the isotype of Ig produced, and undergo affinity maturation of the Ig (Topic D3, Fig. 4). As with Th2 cells, the interaction of CD40L on Th1 cells with CD40 on B cells induces B cell prolifera- tion and class switching. Cytokines are also important, but in this case IFNγand TNFαare involved, resulting in signals and help for the development of B cells that produce primarily IgG antibodies.
Role of Th1 cells in induction of cytotoxicity by CD8+ CTLs. Antigen- presenting cells (APCs) initially process and present microbial peptides via the exogenous pathway in association with MHC class II molecules (Topic F2). Th1
134 Section F – The T cell response – cell mediated immunity
cells recognize and are activated by interaction with these cells, and in turn influence APC function. They do this by direct cell–cell interaction and signal- ing and through release of cytokines that act on APCs, including IFNγ. As a result of these interactions with specific Th1 cells, APCs become ‘conditioned’
to more efficiently present peptides to CTLs via their MHC class I molecules and to Th cells through MHC class II (Topic F6, Fig. 1).
Cytotoxic T cells Recognition of antigen and activation
Peptides derived from viral proteins are processed via the endogenous route and are presented on the cell surface by MHC class I molecules, marking this cell as infected and as a target for CTL killing. CTLs express cell surface CD8 which binds to the nonpolymorphic region of MHC class I (expressed on all nucleated cells), restricting these killer T cells to recognizing only cells present- ing peptide in MHC class I molecules (Topic F2). This interaction also serves to stabilize the interaction of the T cell receptor with specific peptides bound to the polymorphic part of the MHC class I molecule (Fig. 4). Other surface co-stimulatory and adhesion molecules such as LFA-1 are important for close interaction of the CTL with the infected cell and for activating its cytotoxic F5 – Clonal expansion and development of effector function 135
Macrophage Inflammatory
CD4ⴙ T cell
IL-1 IFN-γ TNF-α IL-3 GMCSF TGF
CD154 (CD40L)
CD40 TNF-α
IFN-γ
Fig. 3. Macrophage activation by CD4+inflammatory T cells. Cytokines released by Th1 cells, as well as signaling through direct contact of cell surface receptors, increase: (a) fusion of lysozomes and phagosomes; (b) production of nitric oxide and oxygen radicals for killing pathogens; and (c) expression of MHC class II molecules and TNF receptors by
macrophages. Note that CD154/CD40 interactions are also important in activation of the macrophage .
Fig. 4. CTL recognize peptides associated with MHC class I molecules. CD8 binds to non- polymorphic MHC class I stabilizing this interaction and enhancing killing. The interaction of LFA-1 and ICAM-1 is also important in killing of the target.
CTL
MHC class I
CD8 LFA-1 ICAM-1
Virus infected target cell
machinery (Topic F4). This activation step also induces the expression of FasL on the CTL, which can interact with Fas expressed on the surface of the virus- infected cell.
Mechanisms of cytotoxicity
Mature CTLs, generated with the help of Th1 cells, contain the cytotoxic machinery required to kill virus-infected cells. In particular, these CTLs are able to induce programed cell death (apoptosis) of the virus-infected cells through two distinct pathways: (a) release of lytic granules containing perforin and granzymes which enter the target cell; (b) interaction of FasL on the CTL with Fas on the target cell.
1. Perforin-induced apoptosis.CTL contain large cytolytic granules and are diffi- cult to distinguish morphologically from NK cells (also called large granular lymphocytes; Topic B1). These intracytoplasmic granules contain proteases, granzyme A and granzyme B, and perforin, a molecule similar to C9 of the complement pathway (Topic D8). On interaction of the CTL with a virus- infected cell, the granules move toward the portion of the membrane close to the point of contact with the target cell. On fusion with the membrane, the granules release perforins which polymerize in the membrane of the infected cell creating pores that allow entry of the proteases (Fig. 5). These enzymes cleave cellular proteins, the products of which initiate induction of programed cell death (apoptosis). CTL then re-synthesize their granular contents in preparation for specific killing of another infected cell.
2. Fas-mediated apoptosis. Nucleated cells of the body infected with some viruses upregulate expression of Fas (CD95). CTL activated to release their granules by their first encounter with antigen presented by MHC class I molecules, are induced to upregulate FasL which then also allows them to kill specific virus-infected cells by an additional mechanism through interac- tion with surface CD95 (Fig. 6).
The importance of apoptosis as a killing mechanism used by the immune system is that targeted cells can be removed rapidly by phagocytes without
136 Section F – The T cell response – cell mediated immunity
CTL
Virus-infected cell
(a) (b)
Lytic granules
CTL
Virus infected target cell
Granzymes Perforins Granule
Fig. 5. Apoptosis induced by release of lytic granules. (a) Lytic granules containing perforin and granzymes accumulate at the point of contact of CTL with virus-infected cell. (b) The granule contents are released and the perforins polymerize in the infected cell membrane allowing entry of granzymes into the target cell which induce apoptosis.
initiating inflammatory responses. Another mechanism of cell death – necrosis – results from tissue trauma or certain kinds of infection and leads to acute inflammation through the production of inflammatory cellular products.
F5 – Clonal expansion and development of effector function 137
CTL Virus-infected cell MHC I peptide FasL (CD178)
Fas (CD95) TCR
Apoptotic signals
Fig. 6. Apoptosis induced by Fas/FasL interactions. CTL have preformed FasL in their granules which is rapidly expressed on their surface when they attach via their TCR to the target cell. Ligation of Fas on the virus-infected cell by FasL on the CTL is an additional mechanism for induction of apoptosis.
Section F – The T cell response – cell-mediated immunity