Pathogenesis and immune response to respiratory coronaviruses in their natural porcine host

Porcine respiratory coronavirus (PRCV) is a naturally occurring pneumotropic coronavirus in the pig, providing a valuable large animal model to study acute respiratory disease. PRCV pathogenesis and the resulting immune response was investigated in pigs, the natural large animal host. We compared two strains, ISU-1 and 135, which induced differing levels of pathology in the respiratory tract to elucidate the mechanisms leading to mild or severe disease. The 135 strain induced greater pathology which was associated with higher viral load and stronger spike-specific antibody and T cell responses. In contrast, the ISU-1 strain triggered mild pathology with a more balanced immune response and greater abundance of T regulatory cells. A higher frequency of putative T follicular helper cells was observed in animals infected with strain 135 at 11 days post-infection. Single-cell RNA-sequencing of bronchoalveolar lavage revealed differential gene expression in B and T cells between animals infected with 135 and ISU-1 at 1 day post infection. These genes were associated with cell adhesion, migration, and immune regulation. Along with increased IL-6 and IL-12 production, these data suggest that heightened inflammatory responses to the 135 strain may contribute to pronounced pneumonia. Among BAL immune cell populations, B cells and plasma cells exhibited the most gene expression divergence between pigs infected with different PRCV strains, highlighting their potential role in maintaining immune homeostasis in the respiratory tract. These findings indicate the potential of the PRCV model for studying coronavirus induced respiratory disease and identifying mechanisms that determine infection outcomes. Author summaryUnderstanding how our immune system reacts to respiratory viruses, like SARS-CoV-2, is crucial to developing better treatments. While most COVID-19 infections are mild, some cases lead to severe lung damage, but we do not fully understand why. To study this, we used pigs, which respond more like humans compared to small animals, to explore how the immune system deals with respiratory coronaviruses. We tested two porcine respiratory coronavirus strains that caused different levels of lung damage. The more severe strain triggered a strong immune response and high inflammation, leading to lung pathology similar to that seen in severe COVID-19 cases. By contrast, the milder strain caused a balanced immune response, including more regulatory T cells that help control inflammation. We also found changes in genes related to antibody-producing cells, which may be important for controlling respiratory pathology. Interestingly, changes in immune responses and gene expression lasted long after the virus was cleared, potentially making individuals more vulnerable to future infections - similar to the "long COVID" symptoms seen in people. We propose that this pig model could help us study coronavirus-induced lung damage and test new therapies to prevent severe disease.