ALTERATIONS OF THYMUS CORTICAL EPITHELIUM AND INTERDIGITATING DENDRITIC CELLS BUT NO INCREASE OF THYMOCYTE CELL-DEATH IN THE EARLY COURSE OF SIMIAN IMMUNODEFICIENCY VIRUS-INFECTION

The role of the thymus in the pathogenesis of simian acquired immunode ficiency syndrome was investigated in 18 juvenile rhesus monkeys (Maca ca mulatta). The thymus was infected from the first week post-SIVmac i noculation, but the amount of virus-positive cells was very low (< 1 i n 10(4) T cells) as demonstrated by polymerase chain reaction and in s itu hybridization. First morphological alteration was a narrowing of t he cortex at 12 and 24 wpi. Morphometry revealed no increase of pyknot ic T cells but a decrease of the proliferation rate and flow cytometry showed a reduction of the immature CD4+/CD8+ double-positive T cells. Ultrastructural analysis revealed vacuolization shrinkage, and finall y cytolysis of the cortical epithelial cells and the interdigitating d endritic cells. Immunofluorescence staining exhibited a widespread los s of cortical epithelial cells. This damage to the thymic microenviron ment could explain the breakdown of the intrathymic T cell proliferati on. It preceded fully developed simian acquired immunodeficiency syndr ome and is therefore considered to play a major role in its pathogenes is.