African swine fever (ASF) virus has been reported to infect cells of the monocyte family, probably macrophage-like cells, but there is variation in the apparent susceptibility of these cells. We have demonstrated that the phenotype and activity of porcine monocytic cells varies between different isolations and also upon culture. The variation during culture is dependent upon the phenotype of the cells at the time of isolation. As for the susceptibility of porcine monocytes/macrophages to infection by ASF virus, it was seen that this could be related to the variation in cell phenotype and activity. The susceptibility was determined by the relative density of particular subpopulations of cells present. Whilst inflammatory macrophages did not have an apparent role to play, phagocytic activity was influential. Furthermore, the expression of CD44 and the DH59 myeloid cell marker was important, whereas the relevance of MHC Class II expression was variable. Overall, it was concluded that susceptibility to infection required that a culture be dominated by CD44-positive cells which were non-inflammatory, of low phagocytic activity, and characterizable as being of the myeloid (DH59-positive) lineage.
