Activation of extrinsic- and Daxx-mediated pathways in lymphoid tissue of PRRSV-infected pigs

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is a major infectious pathogen in pigs leading to huge economical losses worldwide. PRRSV is able to escape from host immunity and causes transient infections. In the present study, expression of different apoptotic markers and its connection with PRRSV were assessed in tonsil and mediastinal lymph node from PRRSV-infected pigs. Cleaved caspase (CCasp)8, CCasp9, Fas, Daxx, CCasp3 and PRRSV expression were analyzed by immunohistochemistry. An up-regulation of CCasp8, Fas and CCasp3 expression in lymphocytes and macrophages from both organs was found during PRRSV infection, indicating the activation of the extrinsic-mediated pathway of apoptosis. Moreover, Daxx expression was also enhanced in macrophages of both organs, suggesting a simultaneous caspase-independent pathway of apoptosis. A correlation between the expression of the different apoptotic markers and IL-10, IL-6 and TGF-β but not with PRRSV antigen was found in our study, which supports the hypothesis of an indirect mechanism in PRRSV-induced apoptosis. © 2014 Elsevier B.V

Thymic depletion of lymphocytes is associated with the virulence of PRRSV-1 strains

Porcine reproductive and respiratory syndrome virus (PRRSV) exists as two distinct viruses, type 1 (PRRSV-1) and type 2 (PRRSV-2). Atrophy of the thymus in PRRSV-2 infected piglets has been associated with a loss of thymocytes. The present study aimed to evaluate the impact of PRRSV-1 strains of differing virulence on the thymus of infected piglets by analysing the histomorphometry, the presence of apoptotic cells and cells producing cytokines. Thymic samples were taken from animals experimentally infected (with LV, SU1-bel, and 215-06 strains) or mock inoculated animals at 3, 7 and 35 days post-infection (dpi) and processed for histopathological and immunohistochemical analyses. PRRSV antigen was detected in the thymus from 3dpi until the end of the study in all virus-infected animals with the highest numbers of infected cells detected in SU1-bel group. The histomorphometry analysis and counts of CD3+ thymocytes in the thymic cortex displayed significant differences between strains at different time-points (p ≤ 0.011), with SU1-bel group showing the most severe changes at 7dpi. Cell death displayed statistically significant increase in the cortex of all infected animals, with SU1-bel group showing the highest rate at 3 and 7dpi. The number of cells immunostained against IL-1α, TNF-α and IL-10 were predominantly detected in the medulla (p ≤ 0.01). An increase in the number of TNF-α and IL-10 positive cells was observed in LV and SU-1bel groups. Our results demonstrate that different PRRSV-1 strains induced depletion of the thymic cortex due to apoptosis of thymocytes and that the most severe depletion was associated with the highly virulent SU1-bel strain

Porcine reproductive and respiratory syndrome type 1 viruses induce hypoplasia of erythroid cells and myeloid cell hyperplasia in the bone marrow of experimentally infected piglets independently of the viral load and virulence

Porcine reproductive and respiratory syndrome viruses (PRRSV) present a wide phenotypic and genetic diversity. Experimental infections have demonstrated viral replication, including highly pathogenic strains (HP-PRRSV), in primary lymphoid organs such as the thymus. However, studies of the bone marrow are scarce but necessary to help elucidate the immunobiology of PRRSV strains of differing virulence. In this study, whereas viral RNA was detected within the bone marrow of animals experimentally infected with both low virulent Lelystad (LV) and 215-06 PRRSV-1 strains and with the highly virulent SU1-bel strain, PRRSV positive cells were only occasionally detected in one SU1-bel infected animal. PRRSV RNA levels were associated to circulating virus with the highest levels detected in LV-infected pigs. At 3 dpi, a decrease in the proportion of haematopoietic tissue and number of erythroid cells in all infected groups was associated with an increase in TUNEL or cleaved caspase 3 labelling and higher counts of myeloid cells compared to control. The expression of IL-1α and IL-6 was elevated at the beginning of the infection in all infected animals. The expression of TNF-α was increased at the end of the study in all infected groups with respect to control. Different PRRSV-1 strains induced, presummably by indirect mechanisms and independently of viral load and strain virulence, moderate and sustained hypoplasia of erythroid cells and myeloid cell hyperplasia at early stages of infection. These changes were paralleled by a peak in the local expression of IL-1α, IL-6 and TNF-α in all infected groups