The thymus in "bare lymphocyte" syndrome: significance of expression of major histocompatibility complex antigens on thymic epithelial cells in intrathymic T-cell maturation.

Thymic biopsies from two patients with combined immunodeficiency and defective expression of HLA class I and class II antigens on blood mononuclear cells ("bare lymphocyte" syndrome) were investigated. This made possible an evaluation of the significance of HLA antigen expression in a detailed (immuno)histologic study. Both thymuses showed a normal lobular architecture with distinct cortex-medulla areas, well-differentiated epithelium, including ultrastructurally defined subtypes, and Hassall's corpuscles. Normal numbers of lymphoid cells were present and normal T-cell phenotype was found. Using anti-HLA-A,B,C antisera, confluent staining of the medulla (stroma and lymphocytes) was observed. One of the thymuses was found to be negative for HLA class II antigen expression: the other revealed only HLA-DR positivity of nonlymphoid cells in the medulla. These cells were not of epithelial nature as judged from double staining with anti-keratin antibody. There was no expression of HLA-DC/DS. These observations differ from findings in the normal thymus, wherein epithelial cells in the cortex carry HLA class I and class II antigens, and epithelial cells in the medulla express HLA class I, and for a minor part class II antigens. The results indicate a normal sequential acquisition of T-cell differentiation antigens in the thymus of both cases. It is suggested that the expression of HLA class I and class II antigens on epithelial cells in the normal thymus cortex does not play a significant role in the sequential acquisition of differentiation antigens on T lymphocytes.