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

While the development of effective subunit vaccines against African swine fever (ASF) is ongoing, live attenuated vaccines (LAVs) remain the only current strategy capable of inducing robust protective immunity. However, potential biosafety concerns limit their implementation in the field. Thus, further research is required to develop optimized LAVs with better biosafety profiles. Both the ASF virus (ASFV) and derived LAVs suppress innate immune responses of macrophages, thereby limiting their contribution to the induction of protective immune responses. We hypothesized that adjuvants could restore the functionality of LAV-infected macrophages, allowing for a reduction in the vaccine’s effective dose and consequently minimizing the risk of adverse events. To test this hypothesis, we intranasally vaccinated pigs with a suboptimal dose of the LAV BA71ΔCD2, either alone or in combination with two adjuvants derived from the immunostimulatory bacterium Rothia nasimurium . The two immunostimulants enhanced the responsiveness of BA71ΔCD2-infected macrophages, which acquired features of antigen presenting cells. However, both adjuvants reduced the levels of ASFV-specific humoral and cellular responses induced by BA71ΔCD2, consequently decreasing the level of protection against a lethal challenge. Further in vitro analyses demonstrated that adjuvant-activated macrophages acquired an antiviral state, thereby reducing the replication capability of the LAV. Thus, the adjuvant-mediated decline in vaccine efficacy might result from a lower antigen production by infected cells. These results demonstrate that the use of adjuvants combined with ASFV-based LAVs will require a fine-tune manipulation of macrophages, enhancing their functionality while avoiding a significant inhibition of virus replication, reaching the required balance between the levels of viral antigens and innate immune responses to trigger a protective adaptive immunity.