Host immune responses to porcine circovirus type 2d infection in C57BL/6 mice: lymphocyte activation, thymic T-cell dynamics, and Treg expansion

Porcine circovirus type 2 (PCV2), a major immunosuppressive pathogen, causes substantial economic losses to the global swine industry. PCV2d is the dominant genotype in multiple regions, but its immunopathological mechanisms, particularly virus‒host immune crosstalk, remain unclear. Here, we evaluated immune responses to a rescued replication-competent PCV2d in C57BL/6 mice, focusing on lymphocyte activation, thymic T-cell apoptosis and dynamics, T-cell immunophenotype shifts, and splenic regulatory T (Treg) cell dynamics. CD69-based assessment of lymphocyte activation revealed time-specific patterns: CD8⁺ T cells were activated throughout infection, with stronger activation at 7–14 days post-infection (dpi); γδ T cells were activated only early; CD4⁺ T cells and natural killer (NK) cells were activated late (14–28 dpi); and B cells were persistently activated. Thymic analysis revealed subset-specific apoptosis; for example, CD8⁺ T cells underwent early apoptosis across all stages, immature CD4⁻CD8⁻ T cells were vulnerable late, and altered dynamics were detected: CD4⁺ and CD8⁺ T cells first increased and then decreased from early to late infection, while CD4⁺CD8⁺ T cells showed the opposite trend. T-cell immunophenotyping with CD44 and CD62L revealed reduced numbers of naive CD4⁺/CD8⁺ T cells, increased numbers of memory T cells, and upregulated the expression of PD-1, a marker of T-cell exhaustion. Splenic Treg cells (CD4⁺CD25⁺Foxp3⁺) expanded, with a peak at 21 dpi. These findings demonstrate the multifaceted immunomodulatory effects of PCV2d, including innate activation, adaptive immune suppression, thymic dysfunction, T-cell exhaustion, and Treg-mediated immunosuppression. This work improves our understanding of PCV2d pathogenesis, aiding the development of targeted control strategies and vaccines for swine PCV2.