Cellular receptors play a critical role in viral infection. At least seven cellular molecules have been identified as putative viral entry mediators for porcine reproductive and respiratory syndrome virus (PRRSV). Accumulating data indicate that among these candidates, CD163, a cysteine-rich scavenger receptor on macrophages, is the major receptor for PRRSV. This review discusses the recent advances and understanding of the entry of PRRSV into cells, viral pathogenesis in CD163 gene-edited swine, and CD163 as a potential target of receptor–ligand for the control of PRRS.

The incursion of African swine fever virus (ASFV) into Eurasia presents a threat to the world’s swine industry. Highly sensitive and specific diagnostic assays are urgently needed for rapid detection during an outbreak, post-outbreak investigation, and disease surveillance. In this study, a highly specific and repeatable blocking ELISA (bELISA) was developed using a recombinant p30 protein as the antigen combined with biotinylated mAb against p30 as the detection antibody. Initial test validation included sera from 810 uninfected animals and 106 animals experimentally inoculated with ASFV or recombinant alphavirus/adenovirus expressing p30. Receiver operating characteristic (ROC) analysis of the data calculated an optimal percentage of inhibition (PI) cutoff value of 45.92%, giving a diagnostic sensitivity of 98.11% and diagnostic specificity of 99.42%. The coefficient of variation of an internal quality control serum was 6.81% for between runs, 6.71% for within run, and 6.14% for within plate. A time course study of infected pigs showed that bELISA was able to detect seroconversion as early as 7 days post-inoculation. Taken together, these results demonstrate that bELISA can be used as an alternative serological test for detecting ASFV infection.

Porcine reproductive and respiratory syndrome (PRRSV) is an enveloped single-stranded positive-sense RNA virus and one of the main pathogens that causes the most significant economical losses in the swine-producing countries. PRRSV is currently divided into two distinct species, PRRSV-1 and PRRSV-2. The PRRSV virion envelope is composed of four glycosylated membrane proteins and three non-glycosylated envelope proteins. Previous work has suggested that PRRSV-linked glycans are critical structural components for virus assembly. In addition, it has been proposed that PRRSV glycans are implicated in the interaction with host cells and critical for virus infection. In contrast, recent findings showed that removal of N-glycans from PRRSV does not influence virus infection of permissive cells. Thus, there are not sufficient evidences to indicate compellingly that N-glycans present in the PRRSV envelope play a direct function in viral infection. To gain insights into the role of N-glycosylation in PRRSV infection, we analysed the specific contribution of the envelope protein-linked N-glycans to infection of permissive cells. For this purpose, we used a novel strategy to modify envelope protein-linked N-glycans that consists of production of monoglycosylated PRRSV and viral glycoproteins with different glycan states. Our results showed that removal or alteration of N-glycans from PRRSV affected virus infection. Specifically, we found that complex N-glycans are required for an efficient infection in cell cultures. Furthermore, we found that presence of high mannose type glycans on PRRSV surface is the minimal requirement for a productive viral infection. Our findings also show that PRRSV-1 and PRRSV-2 have different requirements of N-glycan structure for an optimal infection. In addition, we demonstrated that removal of N-glycans from PRRSV does not affect viral attachment, suggesting that these carbohydrates played a major role in regulating viral entry. In agreement with these findings, by performing immunoprecipitation assays and colocalization experiments, we found that N-glycans present in the viral envelope glycoproteins are not required to bind to the essential viral receptor CD163. Finally, we found that the presence of N-glycans in CD163 is not required for PRRSV infection.

Porcine reproductive and respiratory syndrome virus (PRRSV) is the most economically important pathogen of swine, yet the molecular mechanisms governing its entry into host cells remain incompletely understood. The minor envelope glycoprotein GP2, together with GP3 and GP4, forms an essential complex that engages the entry receptor CD163; however, the specific GP2 regions required for receptor association have not been fully defined experimentally. Here, we investigated GP2 processing, intracellular trafficking and interactions with viral glycoproteins and CD163. We demonstrate that the GP2 signal peptide (SP) is cleaved in both transfected and infected cells and is necessary and sufficient for ER localization. Removal of the SP disrupted GP2 maturation, impaired interactions with GP3, GP4 and GP5, and significantly reduced viral infectivity in infectious clone assays. Deletion of the SP also abolished GP2–CD163 association, indicating that proper SP-dependent processing is required for receptor engagement. Using co-immunoprecipitation and colocalization analyses, we identified two highly conserved regions within the GP2 ectodomain that associate with CD163. Deletion of either region completely eliminated PRRSV infection. Together, these findings define the structural determinants within GP2 required for association with CD163 and advance our understanding of the early steps of PRRSV entry.

Siglec-1, also known as sialoadhesin (Sn) or CD169, is a product of the SIGLEC1 gene. A large body of work performed on cell culture systems concludes that Siglec-1 is a primary receptor for porcine reproductive and respiratory syndrome virus (PRRSV) isolates in the genotype 1 or PRRSV-1 group. In this study, primary cultures of PAMs from SIGLEC-1 knockout (KO) pigs showed resistance to infection using both PRRSV-1 and PRRSV-2 isolates, which is consistent with previous observations. However, infection of KO and WT pigs with a PRRSV-1 isolate, SD01-08, resulted in viremia in both groups. Interestingly, virus nucleic acid levels in sera were significantly greater in the KO pigs at 21 and 28 days after infection. Mean virus load, calculated as area under the viremia curve over the infection period, was also significantly greater for the KO group. At the level of the pig, Siglec-1 as a receptor for PRRSV may not be biologically relevant. The increased viremia observed for the KO pigs may reflect a role for Siglec-1 in the regulation of the immune response to PRRSV-1.