Porcine reproductive and respiratory syndrome virus (PRRSV) is a major swine pathogen that causes considerable economic losses globally. Previous studies have demonstrated that the L1 branch PRRSV SD-R vaccine candidate offers effective clinical protection against multiple PRRSV sublineages, including L1.8 (NADC30-like), L1.5 (NADC34-like), and L8.7 (HP-PRRSV and HP-like PRRSV). However, the mechanisms underlying cross-protection in SD-R remain to be fully elucidated. Based on three different PRRSV-MLV vaccines (CH-1R, HuN4-F112, and SD-R), we systematically evaluated the humoral and cellular immune responses in piglets following immunization with these vaccines against various PRRSV strains. The neutralization assay results showed that sera obtained from piglets immunized with SD-R or HuN4-F112 at 28 days post-vaccination (dpv) exhibited neutralizing activity against both homologous and heterologous viral strains, although the titers were relatively low. Further neutralization tests revealed that sera from SD-R-immunized piglets at 28dpv exhibited variable neutralization potency against different circulating strains in China. The T cell proliferation assay demonstrated that both SD-R and HuN4-F112, particularly SD-R, exhibited enhanced capacity to induce the proliferation of antigen-specific T cells, including Th cells and cytotoxic T lymphocytes (CTLs), as well as IFN-γ-secreting cells. Moreover, SD-R immunization in piglets can induce broad-spectrum specific T-cell responses against the major prevalent strains circulating in China. Therefore, these findings indicate that cellular immunity is closely associated with the cross-protective effect observed in early SD-R immunity, with humoral immunity playing a secondary role.

Keywords: Cellular immunity; Cross-protection; Humoral immunity; SD-R.