The IFN-γ–producing Th1 subset is induced by microbes that are ingested by and have evolved to survive and replicate within phagocytes; it is the major effector T-cell population in phagocyte-mediated host defense.
Differentiation of Th1 Cells
Th1 differentiation is driven mainly by the cytokines IL-12 and IFN-γ and occurs in response to microbes that activate DCs, macrophages, and NK cells (Fig. 1). The differentiation of antigen activated CD4+ T cells to Th1 effectors is stimulated by many intracellular bacteria, such as Listeria and mycobacteria, and by some parasites, such as Leishmania, all of which infect DCs and macrophages. Th1 differentiation is also stimulated by viruses and by protein antigens administered with strong adjuvants. A common feature of these infections and immunization conditions is that they elicit innate immune reactions that are associated with the production of certain cytokines, including IL-12, IL-18, and type I IFNs. All of these cytokines promote Th1 development; of these, the most potent is IL-12, produced by DCs and macrophages activated by microbes. Many microbes stimulate NK cells to produce IFN-γ, which is itself a Th1-inducing cytokine and also acts on DCs and macrophages to induce more IL-12 secretion. After their differentiation, Th1 cells secrete IFN-γ, which results in more Th1 differentiation and thus amplifies the reaction. In addition, IFN-γ inhibits the differentiation of naive CD4+ T cells to the Th2 and Th17 subsets, thus promoting the polarization of the immune response in one direction. T cells may further enhance Th1 development by virtue of CD40L on activated T cells engaging CD40 on the DCs and macrophages and stimulating IL-12 secretion.
Fig1. Differentiation of Th1 cells. Interleukin-12 (IL-12) produced by dendritic cells and macrophages in response to microbes, including intracellular microbes, and interferon-γ (IFN-γ) produced by NK cells (all part of the early innate immune response to the microbes) activate the transcription factors T-BET, STAT1, and STAT4, which stimulate the differentiation of naive CD4+ T cells to the Th1 subset. IFN-γ produced by the Th1 cells amplifies this response and inhibits the development of Th2 and Th17 cells. Other cytokines, including type I IFNs and IL-18, also promote Th1 differentiation but are not shown. TCR, T-cell receptor.
IFN-γ and IL-12 stimulate Th1 differentiation by inducing and activating the transcription factors T-BET, STAT1, and STAT4 (see Fig. 1). T-BET, a member of the T-box family of transcription factors, is induced in naive CD4+ T cells in response to antigen and IFN-γ. IFN-γ also activates STAT1 (signal transducer and activator of transcription 1), which in turn stimulates expression of T-BET. T-BET then promotes IFN-γ production through a combination of direct transcriptional activation of the IFNG gene and by inducing chromatin remodeling of the IFNG promoter region. The ability of IFN-γ to stimulate T-BET expression and the ability of T-BET to enhance IFN-γ transcription set up a positive amplification loop that drives differentiation of T cells toward the Th1 phenotype. IL-12 con tributes to Th1 commitment by binding to receptors on antigen stimulated CD4+ T cells and activating the transcription factor STAT4, which further enhances IFN-γ production.
Functions of Th1 Cells
The principal function of Th1 cells is to activate macrophages to ingest and destroy microbes (Fig. 2). The same reaction of Th1-mediated macrophage activation is involved in injurious DTH, which is a component of many inflammatory diseases. Some of these conditions are characterized by granulomatous inflammation, including infectious diseases (such as tuberculosis) and chronic inflammatory diseases (such as sarcoidosis and Crohn’s disease).
Fig2. Functions of Th1 cells. Th1 cells secrete interferon-γ (IFN-γ), which acts on macrophages to increase phagocytosis and killing of microbes in phagolysosomes. Th1 cells also produce TNF (tumor necro sis factor), which activates neutrophils and macrophages and promotes inflammation (not shown). APC, Antigen-presenting cell.
Before discussing the activation of macrophages and how they destroy microbes, we will describe the properties of IFN-γ, the cytokine responsible for most of the specialized functions of Th1 cells.
Interferon-γ IFN-γ
is the principal macrophage-activating cytokine. IFN-γ is also called immune or type II IFN. Although its name interferon is shared with the antiviral type I IFNs, it is not a potent antiviral cytokine, and it functions mainly as an activator of effector cells of the immune system.
IFN-γ is a homodimeric protein belonging to the type II cytokine family. In addition to CD4+ Th1 cells, ILC1s, NK cells, and CD8+ T cells also produce IFN-γ. NK cells secrete IFN-γ in response to activating ligands on the surface of infected or stressed host cells or in response to IL-12; in this setting, IFN-γ functions as a mediator of innate immunity. In adaptive immunity, Th1 cells produce IFN-γ in response to antigen recognition, and production is enhanced by IL-12 and IL-18. The receptor for IFN-γ is composed of two structurally homologous polypeptides belonging to the type II cytokine receptor family, called IFNγR1 and IFNγR2. IFN γ binds to and induces the dimerization of the two receptor chains. This leads to activation of the associated Janus kinases JAK1 and JAK2 and ultimately to phosphorylation and dimerization of STAT1, which stimulates transcription of several genes.
IFN-γ–induced genes encode many different molecules that mediate the biologic activities of this cytokine, described next.
• IFN-γ activates macrophages to kill phagocytosed microbes. IFN-γ works with other signals to induce the type of macro phage activation that is called classical activation, discussed in more detail later.
• IFN-γ maintains lineage commitment of CD4+ T cells to the Th1 subset and inhibits the development of Th2 and Th17 cells. These effects on T-cell subset differentiation occur because IFN-γ itself induces the expression of transcription factors needed for Th1 development and maintenance and inhibits transcription factors involved in Th2 and Th17 differentiation.
• IFN-γ stimulates the expression of several different proteins that contribute to enhanced antigen presentation and T-cell activation. These proteins include MHC molecules; many proteins involved in antigen processing, including components of the proteasome; and B7 costimulators on APCs.
• IFN-γ activates macrophages, DCs, and other cells to produce cytokines that amplify the host response. These cytokines include tumor necrosis factor (TNF), IL-1, and chemokines, which recruit more leukocytes and induce inflammation, and IL-12, which provides a positive feedback loop for IFN-γ production and Th1 development.
Other Th1 Cytokines
In addition to IFN-γ, Th1 cells produce TNF, which contributes to the recruitment of leukocytes and enhanced inflammation. Th1 cells are also sources of IL-10, which functions mainly to inhibit DCs and macrophages and thus to suppress Th1 activation. This is an example of a negative feedback loop in T-cell responses.
Th1-Mediated Classical Macrophage Activation and Killing of Phagocytosed Microbes
Th1 cells activate macrophages through contact-mediated signals delivered by CD40L-CD40 interactions and by IFN-γ (Fig. 3). This pathway of macrophage activation is called classical macrophage activation, to distinguish it from alternative macrophage activation induced by Th2 cytokines, described later. Classically activated macrophages are also called M1 macrophages. When Th1 cells are stimulated by antigen, the cells express CD40L on their surface and secrete IFN-γ, which inter act with CD40 and IFN-γ receptors, respectively, on the sur face of macrophages. The actions of IFN-γ synergize with the actions of CD40L, and together they are potent stimuli for macrophage activation. CD40 signals activate the transcription fac tors NF-κB (nuclear factor κB) and AP-1 (activator protein 1), and, as discussed earlier, IFN-γ activates the transcription factor STAT1. These transcription factors together induce the expression of genes encoding several enzymes that localize in phagolysosomes of macrophages, including inducible nitric oxide synthase (iNOS), which stimulates the production of nitric oxide (NO), and lysosomal enzymes. Macrophage activation is also associated with the assembly of the enzyme phagocyte oxidase in the membrane of the phagolysosome, which induces the production of reactive oxygen species (ROS). The induction of CD40L on the T cells requires that the T cells be activated by antigen. This ensures that macrophages that are presenting antigens to the T cells (i.e., the macrophages that are harboring intracellular microbes) are also the macrophages that will be in contact with T cells and thus most efficiently activated by the T cells.
Fig3. Macrophage activation by Th1 cells. (A) Macrophages are activated by CD40L-CD40 interactions and by interferon-γ (IFN-γ) expressed by Th1 cells and perform several functions that kill microbes, stimulate inflammation, and enhance the antigen-presenting capacity of the cells. (B) The principal responses of macrophages activated by the classical activation pathway, and their roles in T cell–mediated host defense, are listed. Macrophages are also activated during innate immune reactions and perform similar functions (see Chapter 4). IL, Interleukin; MHC, major histocompatibility complex; NO, nitric oxide; ROS, reactive oxygen species; TNF, tumor necrosis factor.
Activated macrophages kill phagocytosed microbes mainly by the actions of NO, ROS, and lysosomal enzymes. All of these potent microbicidal agents are produced within the lysosomes of macrophages and kill ingested microbes after phagosomes fuse with lysosomes. These toxic substances may also be released into adjacent tissues, where they kill extracellular microbes and may damage normal tissues.
Inherited immunodeficiencies have established the critical importance of Th1 cells in cell-mediated immunity against intracellular pathogens. Homozygous mutations affecting the IFN-γ receptor, IL-12, IL-12 receptor, STAT1, and other signaling molecules involved in Th1 differentiation cause defects in the development of Th1 cells. Patients with these inherited mutations are susceptible to infection with M. tuberculosis, low-virulence environmental mycobacteria, and the attenuated Mycobacterium bovis strain used in the bacillus Calmette Guérin (BCG) vaccine. As a group, these disorders are called Mendelian susceptibility to mycobacterial disease. The patients are also susceptible to infections with other intracellular bacteria, such as Salmonella, and protozoan parasites, further establishing the critical role of the Th1 response in defense against intracellular microbes. Rare patients make autoantibodies against their own IFN-γ and are also susceptible to mycobacterial and viral infections. Humans with inherited mutations in CD40L (X-linked hyper-IgM syndrome) and mice in which the gene for CD40 or CD40L is knocked out are highly susceptible to infections with intracellular microbes, notably the fungus Pneumocystis jirovecii, whose eradication requires T cell–dependent macrophage activation. These patients and knockout mice also have defects in helper T cell–dependent antibody production because of the critical role of the CD40L-CD40 interaction in B-cell activation.
Macrophages activated by Th1 cells are involved in several other reactions of host defense (see Fig. 3). They stimulate inflammation mainly through the secretion of cytokines, including TNF and IL-1, which recruit more leukocytes, enhancing the host’s ability to destroy infectious pathogens. Activated macrophages may amplify cell-mediated immune responses by becoming more efficient APCs because of increased levels of molecules involved in antigen processing and increased surface expression of MHC class II molecules and costimulators, and by producing cytokines (such as IL-12) that stimulate T-lymphocyte differentiation into effector cells.
Some tissue injury may normally accompany Th1 cell–mediated immune reactions to microbes because the microbicidal products released by activated macrophages and neutrophils are capable of injuring normal tissue and do not discriminate between microbes and host tissue. This tissue injury usually resolves as the infection is cleared. However, excessive Th1 reactions are the cause of tissue injury in many chronic inflammatory diseases.
