Effect of macrophage-specific colony-stimulating factor (CSF-1) on swine monocyte/macrophage susceptibility to in vitro infection by African swine fever virus

Abstract

Swine cells of the monocyte/macrophage lineage (MM) proliferate and survive for several weeks in vitro in medium supplemented with the murine macrophage-specific hematopoietic growth factor, colony-stimulating factor 1 (CSF-1). The extent to which MM, cultured in CSF-1, supported African swine fever virus (ASFV) growth in vitro was investigated. MM, cultured in medium with CSF-1, were sensitive to infection and viral-induced cytopathogenic damage by both natural field isolates of ASFV and fibroblast-adapted ASFV strains, as were primary MM (P-MM). Without CSF-1, blood mononuclear leukocytes (MNL), containing lymphocytes and MM, and P-MM could be reliably used in microculture for ASFV titration when inoculated at times limited to no more than 3 to 5 days after culture inception; inclusion of CSF-1 in the media stimulated continued MM survival and growth, and allowed for the use of MNL and P-MM for ASFV titration when inoculated as long as 2 to 3 weeks after microculture inception. MM that were propagated beyond 1 week in secondary culture in medium with CSF-1 (MM-CSF) were useful in microcultures for infective-ASFV titration, only when the cells were kept in medium with CSF-1 and inoculated no later than 3 days of culture inception. In vitro studies of ASFV infection in P-MM and in MM-CSF showed comparable kinetics in ASFV-induced hemadsorption (HAd), cytopathogenic effect (CPE), cytoplasmic viral antigens and nucleic acid material. Compared to P-MM in culture without CSF-1, relatively minor delays in CPE onset induced by some ASFV strains were noticed in MM-CSF and in P-MM that were placed in media with CSF-1. The effects of ASFV on DNA synthesis in the virus-susceptible MM, cultured with or without CSF-1, were also examined at different times of infection by measurement of 3H-thymidine (3H-TdR) incorporation into total precipitable culture material. ASFV-infection of P-MM, placed in culture medium with CSF-1, caused a pronounced transient increase in total 3H-TdR incorporation at the early onset of CPE and HAd. When compared to uninfected P-MM that were stimulated by CSF-1 to synthesize DNA, infected P-MM failed to incorporate 3H-TdR after CPE was fully evident. For P-MM that were cultured without CSF-1 and for MM-CSF, that were kept in culture with CSF-1, transient increases in 3H-TdR incorporation at the onset of CPE and HAd by ASFV-infection were evident, but were much less pronounced.