LAMININ CHAINS IN THE BASEMENT-MEMBRANES OF HUMAN THYMUS

In recent studies, the alpha 2 chain of laminin (Ln) has been suggeste d to be the only laminin alpha chain expressed in mouse and human thym us. We have now used chain-specific monoclonal antibodies and indirect immunofluorescence microscopy to study the expression of laminin chai ns in samples of foetal and 6-year-old human thymus. The subepithelial basement membrane of the capsule of foetal 16- to 18-week thymus pres ented a bright immunoreactivity for Ln alpha 1, alpha 3, beta 1, beta 3 and gamma 1 chains but not for alpha 2 chain, suggesting the express ion of laminins-1 and -5. Most cortical and medullary epithelial cells , including Hassall's corpuscles, however, lacked laminin immunoreacti vity. Immunoreactivity for Ln beta 2 chain was only seen in basal lami nae of larger blood vessels. Ln thymic specimens from 6-year-old child ren, immunoreactivity for the laminin alpha 1, alpha 3, beta 1, beta 3 and gamma 1 chains was invariably found in subepithelial basement mem brane of the capsule and that for laminin alpha 2 chain was now also d istinct but more heterogeneous. Furthermore, the thymic subepithelial basement membrane of the capsule at all stages showed immunoreactivity for collagen type VII, forming the anchoring fibres in epithelial bas ement membranes. The subcapsular thymic epithelium also showed immunor eactivity for the BP 230 antigen and beta(4) integrin subunit, both co mponents of hemidesmosomes. The present results show that the thymic s ubepithelial basement membrane of the capsule presents properties whic h are commonly seen in stratified and combined epithelia, and are comp atible with suggestions of the antigenic similarity of thymic epitheli al cells and keratinocytes.