Thymic lymphocytes and thymic epithelial cells in 4 cases of congenital immunodeficiency

Congenital defects of predominantly cell-mediated immunity without other major defects or recognizable enzyme deficiency are classified "severe combined immunodeficiency" (SCID). Affected thymuses are severely reduced in size, microscopically they show only few lymphocytic cells and a cortico-medullary boundary is missing as well as Hassal's corpuscules. The primary defect is not yet clear. In this study 4 such cases were examined. The intrathymic epithelial cells showed expression of different cytokeratins and the typical strong positivity for the major histocompatibility complex. Lymphoid cells were phenotyped immunohistochemically and appeared as a very small population of cortical thymocytes, which was especially clear in a prenatally diagnosed and aborted fetus. His thymus, not altered by stress of infection or bone marrow transplantation, showed those few lymphocytes clustered around epithelial cells. The finding suggests an arrest in maturation and cell amplification rather than the absence of stem cells in the pathogenesis of SCID. Alterations in the thymic microenvironment could not be revealed by our methods.