African swine fever (ASF) is an acute and hemorrhagic infectious disease caused by African swine fever virus (ASFV), which is listed as an animal epidemic disease that must be reported by The World Organization for Animal Health and that causes serious economic losses to China and even the whole world. Currently, the entry mechanism of ASFV is not fully understood. Especially in the early stages of virus entry, the host factors required for ASFV entry have not yet been identified and characterized. In this study, we demonstrated that ASFV externalized phosphatidylserine (PS) on the envelope functioned as viral apoptotic mimicry, which interacts with AXL, a tyrosine kinase receptor, to mediate ASFV entry into porcine alveolar macrophages (PAMs). We found that AXL was the most pronounced phosphatidylserine receptor (PSR) affecting ASFV entry in PAMs by RNA interference screening. Knockout AXL gene expression remarkably decreased ASFV internalization and replication in MA104 cells. Furthermore, the antibody against AXL extracellular domains effectively inhibited the ASFV entry. Consistent with these results, the deletion of the intracellular kinase domain of AXL and the treatment of the AXL inhibitor, R428, significantly inhibited the internalization of ASFV. Mechanistically, AXL facilitated the internalization of ASFV virions via macropinocytosis. Collectively, we provide evidence that AXL is a coreceptor for ASFV entry into PAMs, which expands our knowledge of ASFV entry and provides a theoretical basis for identifying new antiviral targets. IMPORTANCE African swine fever (ASF) is a highly contagious infectious disease caused by the ASF virus (ASFV), with a mortality rate of up to 100%. ASFV has caused huge economic losses to pig farming worldwide. Specific cellular surface receptors are considered crucial determinants of ASFV tropism. However, the host factors required for ASFV entry have not yet been identified, and the molecular mechanism of its entry remains unclear. Here, we found that ASFV utilized phosphatidylserine (PS) on the surface of virions to masquerade as apoptotic mimicry and facilitated virus entry by interacting with host factor AXL. We found that knockout of AXL remarkably decreased ASFV internalization and replication. The antibody against AXL extracellular domains and AXL inhibitor R428 significantly inhibited the internalization of ASFV via macropinocytosis. The current work deepens our understanding of ASFV entry and provides clues for the development of antiviral drugs to control ASFV infection.
Keywords: AXL; African swine fever virus; apoptotic mimicry; macropinocytosis; receptor.