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The thymus has two important roles in controlling the specificity of T lymphocytes. First, T cells differentiating in the thymus are rendered tolerant of ‘self’ antigens, particularly antigens encoded by the major histocompatibility complex, the H–2 complex in mice¹. Second, the thymus imbues T cells with the property of H–2-restricted recognition of antigen, that is, the capacity of T cells to react with foreign antigens presented in association with self H–2 gene products^2,3. Until recently it has generally been assumed that self-tolerance and H–2-restricted specificity both reflect early T-cell contact with self H–2 determinants expressed on thymic epithelial cells. Recent evidence suggests, however, that intrathymic cells of the macrophage/dendritic cell (MØ/DC) lineage also have a role in shaping T-cell specificity^4–7. In particular, it has been found that the tolerance to graft-type H–2 determinants which normally ensues …