Virus-specific enhancement of the expression of CTLA-4 in T cells from target organs during early and late stages of PRRSV-1 infection with strains of different virulence

The pathogenesis of the immune dysregulation in porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) infection is not fully understood yet. In this context, immune checkpoints have been identified as pivotal regulators of inflammatory and immune responses. This study evaluated the expression of CTLA-4 at early and late stages of PRRSV-1 infection in piglets with two strains of different virulence (3249 and Rosalía). Animals infected with the highly virulent Rosalía strain showed more severe clinical signs and a significantly reduced average daily gain. In addition, they presented more severe lung lesions in the early stages of the infection, characterised by interstitial pneumonia, with marked thymus atrophy and enlargement of the tracheobronchial lymph nodes. Although seroconversion was similar in both groups, Rosalía induced earlier and higher viraemia, as well as higher viral loads in the thymus, where the infection persisted for up to 42 days. Gene expression analysis revealed up-regulation of CTLA4 and TIM3 in the lung, tracheobronchial lymph node, and thymus during the early stages of infection, particularly in Rosalía-infected animals, decreasing until the advanced stage in the lung. This was associated with a higher frequency of CD4⁺CTLA-4⁺ and CD8β⁺CTLA-4⁺ T cells in peripheral blood and lung, which persisted, although to a lesser extent, until 42 dpi in animals infected with Rosalía strain. Our findings support the high virulence of Rosalía strain and point out to a role of CTLA-4, as well as other immune checkpoints, such as TIM-3, in modulating the host's immune response.