Immunolocalization of peripheral lymph node addressins in normal and neoplastic human thymuses

Peripheral lymph node addressin is a specific L-selectin ligand of the high endothelial venules that plays an important role in lymphocyte homing to lymph nodes. Tissue selective migration of lymphocytes through this pathway to the thymus has also been proposed. In this work, peripheral lymph node addressin expression was investigated immunohistochemically with a monoclonal antibody, clone MECA-79, in formaldehyde-fixed, paraffin-embedded tissue sections of 5 normal neonatal thymuses, 25 thymomas, 3 thymic carcinomas, and 2 thymic lymphoid hyperplasias. In normal thymuses, peripheral lymph node addressin expression was found in the endothelium of corticomedullary and medullary vessels surrounded by perivascular space. In type B thymomas and thymic lymphoid hyperplasias, peripheral lymph node addressin was detected in the vessels with perivascular spaces, at the medullary differentiation areas, and in paralymphoid follicles, respectively. However, in type A thymomas and thymic carcinomas, MECA-79-positive vessels were restricted to the remnants of pre-existing thymic tissue, and they were absent from the neoplastic areas. These findings suggest that in normal and most neoplastic thymuses, peripheral lymph node addressin is expressed by regions of vascular endothelium corresponding to postcapillary venules that may serve as a pathway for homing of recirculating lymphocytes to the thymus.