The Killing Activity of Cytotoxic T Cells Is Antigen Specific and MHC Restricted
Clearly MHC molecules did not evolve to prevent the exchange of surgical grafts. MHC molecules play an essential role in the recognition of virus-infected cells by cytotoxic T cells, which are also called cytolytic T lymphocytes (CTLs). In virus-infected cells, MHC molecules interact with protein fragments derived from the virus and display these fragments on the cell surface, where CTLs, charged with eliminating the infection, can recognize them. How such fragments of antigen are generated and displayed will be described below. CTLs that have receptors capable of recognizing a particular peptide-MHC complex unleash a payload of lethal molecules onto the infected target cells, destroying the target-cell membranes. The destruction of these target cells can be readily measured by the release of their cytoplasmic contents when they physically disintegrate. Thus CTL killing of infected host cells requires (1) MHC presentation of antigenic peptides from the pathogen on the host cell surface, (2) CTLs expressing antigen-MHC–specific T-cell receptors on their surface that can recognize the MHC-antigen complex, and (3) the activation of the CTL killing machinery once the T-cell receptors have bound to the MHC-antigen complex.
Mice that have recovered from a particular viral infection are a ready source of CTLs that can recognize and kill target cells infected with the same virus. If CTLs are obtained from a mouse that has successfully cleared an infection with influenza virus, cytotoxic activity is observed against influenza-infected target cells, but not against uninfected controls (Figure 23-22). Moreover, the influenza-specific CTLs will not kill target cells infected with a different virus, such as vesicular stomatitis virus. CTLs can even discriminate between closely related strains of influenza virus, and can do so with pinpoint precision: differences of a single amino acid in the viral antigen may suffice to prevent recognition and killing by CTLs. These experiments show that CTLs are truly antigen specific and do not simply recognize some attribute that is shared by all virus-infected cells, regardless of the identity of the virus.
EXPERIMENTAL FIGURE 23-22 Chromium (51Cr) release assay allows the direct demonstration of the cytotoxicity and specificity of cytotoxic T cells in a heterogeneous population of cells. (a) A suspension of spleen cells containing cytotoxic (killer) T cells is prepared from mice that have been exposed to a particular virus (e.g., influenza virus) and have cleared the infection. Target cells obtained from mice of the same strain are infected with the identical virus or left uninfected. After infection, cellular proteins are labeled nonspecifically by incubation of the target-cell suspension with 51Cr. When the radiolabeled target cells are incubated with the suspension of cytotoxic T cells, the killing of infected target cells results in release of the 51Cr-labeled proteins. Uninfected target cells are not killed and retain their radioactive contents. Lysis of cells by cytotoxic T cells can therefore be readily detected and quantitated by measuring the radioactivity released into the supernatant. (b) Cytotoxic T cells (CTLs) harvested from mice that have been infected with virus X can be tested against various target cells to determine the specificity of CTL-mediated killing. CTLs capable of lysing virus X–infected target cells 1 cannot kill uninfected cells 2 or cells infected with a different virus, Y 3. When these CTLs are tested on virus X–infected targets from a strain of mice that carries an altogether different MHC type (strain b), again no killing is observed 4. Cytotoxic T-cell activity is thus virus specific and restricted by the MHC.
In this example, it is assumed that the CTLs harvested from an influenza-immune mouse are assayed on influenza-infected target cells derived from the same strain of mouse (strain a). However, if target cells from a completely unrelated strain of mouse (strain b) are infected with the same strain of influenza and used as targets, the CTLs from the strain a mouse are unable to kill the infected strain b target cells (see Figure 23-22b, 1 vs. 4). It is therefore not sufficient that the antigen (an influenza-derived protein) is present; recognition of the antigen by CTLs is restricted by mouse strain–specific elements. Genetic mapping has shown that these restricting elements are encoded by genes in the MHC. Thus CTLs from one mouse strain that is immune to influenza will kill influenza-infected target cells from another mouse strain only if the two strains match at the MHC loci for the relevant MHC molecules. This phenomenon is therefore known as MHC restriction, and the MHC molecules involved are called restriction elements.