Experiments Revealed the Essential Elements of Positive and Negative Selection

Direct evidence that binding of thymocytes to class I or class II MHC molecules is required for positive selection in the thymus came from experimental studies with knockout mice incapable of producing functional class I or class II MHC molecules (Table 10-1). Class I-deficient mice were found to have a normal distribution of double-negative, double-positive, and CD4+ thymocytes, but failed to produce CD8+ thymocytes. Class II-deficient mice had double-negative, double-positive, and CD8+ thymocytes but lacked CD4+ thymocytes. Not surprisingly, the lymph nodes of these class II-deficient mice lacked CD4+ T cells. Thus, the absence of class I or II MHC molecules prevents positive selection of CD8+ or CD4+ T cells, respectively.

Further experiments with transgenic mice provided additional evidence that interaction with MHC molecules plays a role in positive selection. In these experiments, rearranged ap-TCR genes derived from a CD8+ T-cell clone specific for influenza antigen plus H-2^ class I MHC molecules were injected into fertilized eggs from two different mouse strains,

TABLE 10-1

Effect of class I or II MHC deficiency on thymocyte populations*


Cell type

Control mice

Class I deficient

Class II deficient

*Plus sign indicates normal distribution of indicated cell types in thymus. Minus sign indicates absence of cell type.

one with the H-2k haplotype and one with the H-2d haplo-type (Figure 10-6). Since the receptor transgenes were already rearranged, other TCR-gene rearrangements were suppressed in the transgenic mice; therefore, a high percentage of the thymocytes in the transgenic mice expressed the T-cell receptor encoded by the transgene. Thymocytes expressing the TCR transgene were found to mature into CD8+ T cells only in the transgenic mice with the H-2k class I MHC haplotype (i.e., the haplotype for which the transgene receptor was restricted). In transgenic mice with a different MHC haplotype (H-2d), immature, double-positive thymocytes expressing the transgene were present, but these thymocytes failed to mature into CD8+ T cells. These findings confirmed that interaction between T-cell receptors on immature thymocytes and self-MHC molecules is required for positive selection. In the absence of self-MHC molecules, as in the H-2d transgenic mice, positive selection and subsequent maturation do not occur.

Evidence for deletion of thymocytes reactive with self-antigen plus MHC molecules comes from a number of experimental systems. In one system, thymocyte maturation was analyzed in transgenic mice bearing an ap TCR transgene specific for the class I Dh MHC molecule plus H-Y antigen, a small protein that is encoded on the Y chromosome and is therefore a self-molecule only in male mice. In this experiment, the MHC haplotype of the transgenic mice was H-2h, the same as the MHC restriction of the transgene-encoded receptor. Therefore any differences in the selection of thymocytes in male and female transgenics would be related to the presence or absence of H-Y antigen.

Analysis of thymocytes in the transgenic mice revealed that female mice contained thymocytes expressing the H-Y-specific TCR transgene, but male mice did not (Figure 10-7). In other words, H-Y-reactive thymocytes were self-reactive in the male mice and were eliminated. However, in the female transgenics, which did not express the H-Y antigen, these cells were not self-reactive and thus were not eliminated. When thymocytes from these male transgenic mice were cul tured in vitro with antigen-presenting cells expressing the H-Y antigen, the thymocytes were observed to undergo apoptosis, providing a striking example of negative selection.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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