T Lymphocyte Response to a Virus

When a foreign particle, such as a virus, infects the body, it is taken up by macrophages (or dendritic cells) via phagocytosis and partially digested. Within the macrophage, the partially digested virus particles provide foreign antigens that are moved to the surface of the cell membrane. At the membrane, these foreign antigens form a complex with the class-2 MHC molecules. This combination of MHC molecules and foreign antigens is required for interaction with the receptors on the surface of helper T cells. The macrophages thus "present" the antigens to the helper T cells and, in this way, stimulate activation of the T cells (fig. 15.17). It should be remembered that T cells are "blind" to free antigens; they can respond only to antigens presented to them by dendritic cells and macrophages in combination with class-2 MHC molecules.

The first phase of macrophage-T cell interaction then occurs: the macrophage is stimulated to secrete the cytokine known as interleukin-1. As previously discussed, interleukin-1 stimulates cell division and proliferation of T lymphocytes. The activated helper T cells, in turn, secrete macrophage colony-stimulating factor and gamma interferon, which promote the activity of macrophages. In addition, interleukin-2 is secreted by the T lymphocytes and stimulates the macrophages to secrete tumor necrosis factor, which is particularly effective in killing cancer cells.

Killer T cells can destroy infected cells only if those cells display the foreign antigen together with their class-1 MHC molecules (fig. 15.18). Such interaction of killer T cells with the

CD4 coreceptor

Class-2 MHC molecule

Heiper T ceiis

CD4 coreceptor

Class-2 MHC molecule

Heiper T ceiis

Foreign particle

Activated helper T cell

Foreign particle

Activated helper T cell

B cell

■ Figure 15.17 The interaction of antigen-presenting cells, T cells, and B cells. (a) An electron micrograph showing contact between a macrophage (left) and a lymphocyte (right). As illustrated in (b), such contact between a macrophage (or other antigen-presenting cell) and a T cell requires that the helper T cell interact with both the foreign antigen and the class-2 MHC molecule on the surface of the macrophage. In this figure, the helper T cell is now activated and able to interact with a B cell.

From Alan S. Rosenthal, "Current Concepts: Regulation of the Immune Response—Role of the Macrophage" in New England Journal of Medicine, vol. 303:1153, 1980, fig 2. Copyright © 1980 Massachusetts Medical Society. All rights reserved.

Killer T cell

Viral antigen CD8 coreceptor Class-1 MHC molecule

Killer T cell

Viral antigen CD8 coreceptor Class-1 MHC molecule

Tissue cell infected with

Tissue cell infected with

Target cell

This mechanism also helps to maintain certain parts of the body—such as the inner region of the eye and the tubules of the testis—as immunologically privileged sites. These sites harbor molecules that the immune system would mistakenly treat as foreign antigens if the site were not somehow protected. The Sertoli cells of the testicular tubules (chapter 20; see fig. 20.17), for example, protect developing sperm from immune attack through two mechanisms. First, the tight junctions between adjacent Sertoli cells form a barrier that normally prevents exposure of the immune system to the developing sperm. Second, the Sertoli cells produce FAS ligand, which triggers apoptosis of any T lymphocytes that may enter the area.

Some tumor cells, unfortunately, have also been found to produce FAS ligand, which may defend the tumor from immune attack by triggering the apoptosis of lymphocytes. The role of the immune system in the defense against cancer is discussed in a later section.

Destruction of infected cell

■ Figure 15.18 A killer T cell destroys an infected cell. In order for a killer T cell to destroy a cell infected with viruses, the T cell must interact with both the foreign antigen and the class-1 MHC molecule on the surface of the infected cell.

foreign antigen-MHC class-1 complex also stimulates proliferation of those killer T cells. In addition, proliferation of the killer T lymphocytes is stimulated by interleukin-2 secreted by the helper T lymphocytes that were activated by macrophages, as previously described (fig. 15.19).

The network of interactions among the different cell types of the immune system now spreads outward. Helper T cells, activated to an antigen by macrophages or other antigen-presenting cells, can also promote the humoral immune response of B cells. In order to do this, the membrane receptor proteins on the surface of the helper T lymphocytes must interact with molecules on the surface of the B cells. This occurs when the foreign antigen attaches to the immunoglobulin receptors on the B cells, so that the B cells can present this antigen together with its class-2 MHC molecules to the receptors on the helper T cells (fig. 15.20). This interaction stimulates proliferation of the B cells, their conversion to plasma cells, and their secretion of antibodies against the foreign antigens.

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