Virulent African swine fever virus infection of porcine monocytes causes SLA I subversion due to loss of proper ER structure/function

African swine fever virus (ASFV) is a large DNA virus of the Asfarviridae family that causes a fatal hemorrhagic disease in domestic swine and wild boar. Infections with moderately virulent strains predominantly result in a milder clinical course and lower lethality. As target cells of ASFV, monocytes play a crucial role in triggering T-cell-mediated immune defense and ASF pathogenesis. We compared the effect of the highly virulent "Armenia2008" (ASFV-A) virus strain with that of the naturally attenuated "Estonia2014" (ASFV-E) on cellular immune activation in vivo and on primary monocytes ex vivo. Specifically, we asked whether antigen presentation of porcine monocytes is impaired upon ASFV-A infection. ASFV-A-infected monocytes are characterized by lower levels of swine leukocyte antigen (SLA) class I on the cell surface than ASFV-E-infected and uninfected monocytes. Despite stable steady-state SLA I mRNA/protein levels and expression of critical components of the antigen processing machinery, a marked decrease in maturation and reduced surface transport of SLA I were observed in ASFV-A-infected monocytes. The intracellular maturation block of SLA I was accompanied by a loss of functional rough ER structures and a pronounced formation of ER-associated aggresomes. This unsolved cellular stress resulted in a shutdown of overall host cell protein translation, mitochondrial dysfunction, and caspase-3-mediated apoptosis. In contrast, no such cellular subversion phenomenon was found in ASFV-E-infected monocytes. Our findings suggest that in domestic pigs infected with highly virulent ASFV-A, sequential subversion events occur in infected monocytes, likely leading to compromised T-cell activation and impaired downstream responses against ASFV.