Lesions In Thymus Of Piglets Infected With Highly Pathogenic PRRSV

Porcine reproductive and respiratory syndrome (PRRS) was first recognized in the United States in1987and1990respectively, which is characterized by reproductive failure in snows and respiratorydisease in pigs of all ages and caused significant economic losses to the world pig industry. Porcinereproductive and respiratory syndrome virus (PRRSV) is an enveloped, positive single-stranded RNAvirus, belonging to the family Arteriviridae. PRRSV is one of important pathogens causing pig immunedisorderOur previous study has demonstrated that piglets infected with highly pathogenic PRRSV(HP-PRRSV) HuN4strain induced significant thymus atrophy with up to90%thymus reduction in size.The aim of this study was to further investigate the lesions in thymus of piglets infected withHP-PRRSV HuN4strain (Group1) or PRRSV CH-1a strain (Group2) and the changes of thymic T cellpopulations. The lesions were evaluated for the thymus/body weight ratios, pathological changes andvirus load in thymus, apoptosis and ultrastructure of thymocytes. The lesions in thymus of Group1piglets were easily identified. The thymus of pigs in Group1was reduced in size in comparison with thenormal thymus on7day post inoculation (DPI) with the thymus weight/body weight ratios of0.433±0.067(p<0.01). On14DPI, Group1piglets developed severe thymus atrophy with the thymusweight/body weight ratios of0.144±0.039(p<0.01), while the lesions in thymus of Group2piglets werenot observed and the thymus of Group3piglets were normal. For Group1, the viral loads in thymuswere107.3copies/g on3DPI, and up to108.5copies/g virus were detected in a dead piglet on6DPI. Thevirus amounts peaked on10DPI with109copies/g, which subsequently reduced to107.3copies/g on14DPI. For Group2, PRRSV was detected from thymic tissues of approximately58%of piglets with a lowvirus titer level in thymus with a virus load of10^3.4copies/g on3DPI, and the highest virus load was104.3copies/g. For Group1, atrophy of the cortex became more severe with the size of thymic lobuledecreasing and boundaries between thymic cortex and medulla blurring as the days passed. The thymusof one dead piglet on6DPI has atrophic thymic lobule accompanied by thrombus in small vessels. ForGroup2, the main lesions of thymus were characterized with the low-degree depletion of corticalthymocytes, but atrophy in the cortex was not observed. The apoptotic cells in thymus were detectedwith the TUNEL technique. For Group1, numerous TUNEL-positive cells were observed in sections ofthymus on3DPI. On7DPI and10DPI, the numbers of apoptotic cells in the thymus increasedsignificantly, and the number of thymocytes has decreased markedly and almost no apoptotic cells wereobserved on14DPI For Group2and Group3, a small quantity of apoptotic cells were observed inthymus and the numbers of apoptotic cells had no significant increasing during the whole experiment.For Group1, ultrastructurally, numbers of apoptotic cells in the thymus and apoptotic bodies undergoingphagocytosis were observed on3DPI, subsequently, no apoptotic cells were observed on14DPI, butlysis and deletion of thymocytes were observed obviously. For Group1, the proportion of CD4^-CD8⁺subpopulations in the thymus significantly decreased (p<0.01), however, the proportion of CD4⁺CD8^- lymphocytes in circulation significantly decreased and CD4^-CD8⁺lymphocytes in circulationsignificantly increased. For Group2, the proportion of CD4⁺CD8⁺thymocytes was slightly reducedduring the experiment, CD4^-CD8⁺lymphocytes in circulation significantly increased, and CD4^-CD8⁺lymphocytes in circulation significantly increased on14DPI. For Group1, the virus load and numbersof apoptotic cells in lung and peripheral immune organs were higher than that of Group2piglets.