Adjuvant-induced macrophage activation compromises BA71ΔCD2-mediated protection against African swine fever virus.

While effective subunit vaccines against African swine fever (ASF) are still under development, live attenuated vaccines (LAVs) remain the only approach capable of inducing robust protective immunity, though biosafety concerns limit their field use. Thus, further research is required to improve LAV safety while maintaining its immunogenicity. Because both the ASF virus (ASFV) and derived LAVs suppress macrophage innate responses, we hypothesized that adjuvants could restore functionality of LAV-infected cells, allowing dose reduction and consequently minimizing the risk of adverse events. To test this, we intranasally vaccinated pigs with a suboptimal dose of the LAV BA71ΔCD2, either alone or combined with two immunostimulatory adjuvants derived from Rothia nasimurium. Both immunostimulants enhanced the in vitro responsiveness of BA71ΔCD2-infected macrophages, which acquired antigen-presenting features. However, in vivo, both adjuvants lowered humoral and cellular responses induced by BA71ΔCD2, consequently decreasing protection against lethal challenge. Further in vitro analyses demonstrated that adjuvant-activated macrophages acquired an antiviral state, limiting LAV replication. These results suggest the hypothesis that reduced vaccine efficacy might result from insufficient antigen production. Overall, the study indicates that combining adjuvants with ASFV-based LAVs would require a fine-tune macrophage activation to enhance functionality without excessively inhibiting viral replication.