Characterization of thymus atrophy in piglets infected with highly pathogenic porcine reproductive and respiratory syndrome virus

Our previous study has demonstrated that piglets infected with highly pathogenic PRRSV (HP-PRRSV) induced significant thymus atrophy. The aim of this study was to further investigate the lesions in thymus of piglets infected with HP-PRRSV or PRRSV and the changes of thymic T cell populations. The lesions were evaluated for the thymus/body weight ratios, pathological changes and virus load in thymus, apoptosis and ultrastructure of thymocytes. The result showed that thymus/body weight ratios of HP-PRRSV-infected piglets were significantly reduced compared to PRRSV-infected or control piglets, and thymic lesions were characterized by severe cortical depletion of thymocytes. The number of thymocytes undergoing apoptosis was increased approximately forty-fold in piglets infected with HP-PRRSV than that of PRRSV on 7 days post-inoculation (DPI). Double-positive thymocytes of piglets infected with HP-PRRSV were suppressed to a greater degree than either single positive subpopulation, but similar results were not observed in piglets infected with PRRSV. These results suggested that HP-PRRSV induced abundant apoptosis might result in severe thymus atrophy and the depletion of thymocytes.