IMMUNOLOGY - Roit I. - Mir 2000

Chapter 10. Cell-Mediated Immunity Responses

THE ROLE OF MACROPHAGES IN THE IMMUNE RESPONSE

Macrophages participate in the Immune Response at all its stages (Fig. 10.19). First, as already noted, they provide an immediate defensive reaction until the immune response, regulated by antigen-specific T Cells, is amplified. Second, they trigger T Cell activation by Processing and presenting Antigens to them (see Chapter 9). And finally, once activated in turn by T cells, they perform essential Functions in the effector MECHANISMS OF CELL-mediated Immunity, inducing inflammation and destroying microorganisms as well as tumor cells (Fig. 10.20).

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Fig. 10.19. Macrophages carry out the body's defense reaction in the early stage of response to infection, prior to the engagement of specific T- and B-cell-dependent immune mechanisms. Later, macrophage function is reduced to ANTIGEN PROCESSING AND presentation. Finally, in the effector stage of the immune response, antigen-recognizing T cells release cytokines that activate macrophages.

Fig. 10.20. Macrophages and their products are of major importance in the inductive phase of inflammation, as well as in tissue reorganization and post-inflammatory repair (left side of the diagram). The EFFECTOR FUNCTIONS OF macrophages are listed on the right side of the diagram. Their execution can result in tissue damage, such as in delayed-type hypersensitivity reactions.

Cytokines enhance certain macrophage functions

Circulating monocytes are capable of destroying certain microorganisms (see Chapter 17). Upon in vitro cultivation, they largely lose this activity, but under METABOLISM/18.html">The Influence of added cytokines, particularly IFNγ, this activity is restored, accompanied by the activation of additional antimicrobial mechanisms that are not normally expressed by monocytes.

Such cytokine-mediated "activation" is required for macrophages in vitro to destroy many intracellular parasites and certain tumor cells (Fig. 10.21). A classic experiment demonstrating this phenomenon was performed on animals immunized with BCG (from bacillus Calmette-Guérin, a preparation of avirulent bovine-type tuberculosis mycobacteria). Administering purified tuberculin Proteins, i.e., a mixture of Mycobacterium tuberculosis antigens that stimulate T cells, induces resistance to another pathogenic microorganism, Listeria monocytogenes, In addition to stimulating antituberculosis immunity. Analysis of this effect revealed that macrophage stimulation occurs via an antigen-specific mechanism, but leads to an enhancement of their nonspecific bactericidal activity. As further studies showed, lymphocytes from BCG-immunized mice, when cultured in vitro in the presence of a relevant antigen (e.g., purified tuberculin), release into the medium cytokines that enhance the capacity of macrophages to inhibit the proliferation of or destroy both mycobacteria and other microbes.

Fig. 10.21. Destruction of Leishmania by activated macrophages. Cytokines can enhance the destruction of Leishmania enriettii cells within C57 mouse macrophages. Addition of cytokine-containing culture supernatant from a lymphocyte culture to macrophage cultures causes complete destruction of the phagocytized parasites within 48 h (left). In control macrophage cultures lacking cytokines, unrestricted proliferation of Leishmania is observed (right). Giemsa stain. ×800. (Photographs kindly provided by Dr. J. Manuel.)

Macrophages exhibit great functional diversity

Macrophage activity is a complex phenomenon. Activated phagocytic cells acquire an enhanced capacity to destroy certain microorganisms while leaving others unaffected. For example, purified IFNγ stimulates the bactericidal activity of human monocytes against Legionella, yet enhances the growth of Mycobacterium tuberculosis. Such a dual Nature of the effect is due to several reasons:

✵ the multiplicity of effector functions performed by activated macrophages (Fig. 10.20) (antimicrobial activity is discussed in more detail in Chapter 17);

✵ the wide diversity of monocytes and macrophages in their properties; depending on the Tissue and organ, they differ in the expression of MHC class II molecules and Fc receptors, cytokine secretion profiles, and peroxidase production. Nevertheless, most researchers believe that all macrophages belong to a single cell Lineage, and the observed differences are due to successive stages of their maturation and the Influence of the tissue microenvironment;

✵ furthermore, the activation of particular functions may depend not only on The Nature of the macrophages, but also on the specific "spectrum" of cytokines and other proinflammatory stimuli. Macrophage activation is thought to occur in several stages under the influence of sequential stimuli, which can include cytokines, endotoxin, various mediators, and inflammatory regulatory factors. At each stage of activation, macrophages are capable of executing different effector functions and possess characteristic morphological and physiological features (Fig. 10.22).

Fig. 10.22. Macrophage activation sometimes requires the combined action of multiple cytokines, as well as microbial factors. 1. For optimal TNFα release, macrophages must be primed by IFNγ and then triggered by microbial products (e.g., endotoxin) that induce cytokine release. The combined effect of IFNγ and endotoxin provides The amount of TNFα sufficient to trigger stage 2 macrophage activation. 2. Activation of nitric oxide production requires IFNγ, but the actual initiation of production is driven by TNFα.

In some cases, stimulating a specific functional activity of macrophages requires multiple signals. For instance, to induce maximal production of nitric oxide NO, which is toxic to Bacteria and tumor cells, mouse macrophages must be stimulated first with IFNγ and then with TNFα (Fig. 10.22). This effect is much more difficult to elicit in human macrophages. In most cases, it requires a series of stimuli, such as exposure to several cytokines combined with simultaneous cross-linking of FcεRII (CD23). Human macrophages isolated from inflammatory sites sometimes express inducible nitric oxide synthase, but they contain low concentrations of tetrahydrobiopterin, the cofactor necessary for its synthesis. Because nitric oxide performs numerous signaling functions unrelated to its toxic action, it can be presumed that the toxicant is not this nitrogen compound itself, but rather predominantly peroxynitrites formed by the interaction of NO with oxygen reduction products. Normally, such an interaction occurs only at sites of inflammation and upon stimulation of macrophage phagocytic activity.

In humans, calcitriol participates in macrophage activation and The regulation of Th1/Th2 balance

Under the influence of IFNγ, human macrophages express 1-α-hydroxylase, which can convert inactive circulating 25-hydroxycholecalciferol into active 1,25-dihydroxycholecalciferol (also known as vitamin D₃ or calcitriol). Calcitriol receptors are present on the macrophage surface, and calcitriol further stimulates these cells (Fig. 10.23). In addition, via a negative feedback mechanism, calcitriol exerts a potent suppressive effect on Th1 lymphocytes. This effect likely serves as one of the pathways for switching the immune response from a Th1 to a Th2 type when the pathogen cannot be cleared from Tissues and inflammation as a cell-mediated immune response becomes chronic. This mechanism is of particular importance in humans, because in sarcoidosis and tuberculosis, calcitriol production can be so substantial that it leaks from the site of macrophage activation into the bloodstream, causing elevated Blood calcium levels.

Fig. 10.23. In humans, IFNγ upregulates the expression of 1-α-hydroxylase in macrophages, enabling them to convert inactive circulating 25-hydroxycholecalciferol into calcitriol. This is an example of an autocrine feedback mechanism that provides additional macrophage activation while simultaneously downregulating Th1 cell activity.

In addition to positive regulation, there is also negative regulation of macrophage effector functions

It has been established that macrophages can not only be activated, but also deactivated. Suppression of their functions can be induced by prostaglandin E and, partially (not across all effector mechanisms), by glucocorticoids. Recently, a macrophage deactivating factor (MDF) has been isolated and purified from tumor cell culture media; this factor is capable of reversing the IFNγ-induced enhancement of highly active oxygen metabolite production and, to some extent, NO generation (Fig. 10.24). A similar effect is exerted by IL-4 and Calcitonin Gene-related peptide (CGRP), as well as the family of TGFβ-like cytokines.

Fig. 10.24. IFNγ action enhances the capacity of macrophages to produce highly active oxygen metabolites and nitric oxide, whereas A number of other humoral factors inhibit this process.