Lysosomal membrane-associated glycoproteins are differentially expressed in acute and chronic human thymic involution.

The lysosomal membrane-associated glycoproteins LAMP-1 and LAMP-2 are the major constituents of lysosomal membranes with still undefined biological functions. As autophagy is an alternative model of programmed cell death in which lysosomes play a crucial role, we hypothesize that LAMPs might participate in this phenomenon in the involuting thymus. Thymic glands from cases with acute (infection induced) and chronic (senile) involution were examined immunohistochemically for the expression of LAMPs. In acute involution LAMP-1 was localized mainly in medullary epithelial cells, in single macrophages and lymphocytes. Hassall's corpuscules were stained less intensely as compared to control specimens. The quantitative analysis showed a significantly elevated LAMP-2 expression compared to LAMP-1. LAMPs were detected with very slight reactivity in the senile thymus. The enhanced expression of LAMPs, and mainly of LAMP-2, in epithelial cells of incidentally involuted thymus might be an indicator of acute cell injury requiring autophagic degradation of damaged structures. The diminished expression of LAMPs in age-involuted thymus could be a sign of the morphological reorganization and the functional disregulation of the gland. In conclusion, we present novel evidence for differential expression of LAMP-1 and LAMP-2 in thymic involution suggesting their possible involvement in the process of accidental involution of the thymic gland.