Increase of CD163 but not sialoadhesin on cultured peripheral blood monocytes is coordinated with enhanced susceptibility to porcine reproductive and respiratory syndrome virus infection

Porcine reproductive and respiratory syndrome virus (PRRSV) has a restricted tropism mainly for porcine alveolar macrophages (PAMs), but not for peripheral blood monocytes (BMo) in vivo. Previous research showed that only a few BMo became susceptible to PRRSV infection after 1 day culture. Porcine sialoadhesin (PoSn) and CD163 are identified to be the two main PRRSV receptors for binding and internalization. Both receptors are not expressed on BMo, or only expressed at low levels, which may explain why PRRSV cannot infect them. The relationship of BMo differentiation/aging, PRRSV receptor level, and susceptibility to PRRS virus infection has not been thoroughly investigated. In this study, BMo were successfully cultured with pig serum plus L929 cell culture supernatant. Our results showed that both the mRNA and protein expression levels of PoSn were significantly increased after 5-day culture. The mRNA level of CD163 was enhanced more than 20-fold after 1-day culture; CD163-positive BMo increased dramatically from about 2% after 2 h- culture to about 50% after 96-h culture. Furthermore, cultured BMo became much more permissive to PRRSV infection, and the percentage of PRRSV-infected BMo was at least the same as PAMs, if not higher, when infected with CH-1a, the first PRRSV strain isolated in China, or HV, a highly virulent strain. Three other PRRSV strains including VR2332, and two classical Chinese isolates could also infect cultured BMo as well. Most importantly, PRRS virus was successfully isolated from 14 of 15 antibody-positive serum samples using cultured BMo. These results suggest that the enhanced susceptibility of cultured BMo to PRRS virus is coordinated with increased CD163 expression, but less related to the delayed (day 5) increased expression of PoSn. Thus, cultured BMo could be an alternative choice for PRRS virus isolation and identification.