Characterisation of lung cell infiltration during the early stage of infection by porcine reproductive and respiratory syndrome virus type 1 strains of different virulence

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Virulent strains of porcine reproductive and respiratory syndrome virus (PRRSV) have been associated with more severe clinical signs and more pronounced interstitial pneumonia, as well as other associated lung lesions, than classical PRRSV strains. However, there is no description regarding the activation of different mechanisms linking the strain virulence with the severity of pulmonary lesions. This study aims to analyse the pulmonary cell infiltration from piglets at the early stage of infection by several PRRSV-1 strains of differing virulence. A retrospective study was carried out using lung tissue samples from piglets infected with classical (215-06, LV), moderately virulent strain (3249) and virulent (Lena, SU1-bel, and Rosalía) PRRSV-1 strains, which were evaluated by gross pathology, histopathology, and immunohistochemical methods. Immunohistochemistry was performed to detect specific cell markers, including CD163, calprotectin, CD14, CD3, CD20, and TTF-1. Piglets infected with highly virulent PRRSV-1 strains, such as Lena and Rosalía, showed more severe lung pathology, including a marked interstitial pneumonia compared to classical PRRSV-1 strains. The higher frequency of cells infiltrating the alveolar septa was made up of CD3 + and TTF-1 + cells, followed by calprotectin + cells, with the higher counts being observed in the lungs of piglets infected with virulent PRRSV-1 strains, especially in those infected by Rosalía strain. The proportion of the studied cell populations was similar between strains, with no differences at the first week post-infection, which indicates that the mechanisms activated after infection with strains of different virulence may be similar but differing in intensity at this stage of infection. • The cellular infiltrate in PRRSV-1 interstitial pneumonia is characterised. • Virulent strains caused the most severe lung lesions compared to classical strains. • Interstitial pneumonia was mainly made up of CD3 + , TTF-1 + and calprotectin + cells. • The type cell ratio was uniform among strains of different virulence. • Mechanisms activated by PRRSV-1 strains seem to be similar but varying in intensity.

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