Comparison of Primary Virus Isolation in Pulmonary Alveolar Macrophages and Four Different Continuous Cell Lines for Type 1 and Type 2 Porcine Reproductive and Respiratory Syndrome Virus

Abstract

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) has a highly restricted cellular tropism. In vivo, the virus primarily infects tissue-specific macrophages in the nose, lungs, tonsils, and pharyngeal lymphoid tissues. In vitro however, the MARC-145 cell line is one of the few PRRSV susceptible cell lines that are routinely used for in vitro propagation. Previously, several PRRSV non-permissive cell lines were shown to become susceptible to PRRSV infection upon expression of recombinant entry receptors (e.g., PK15^Sn-CD163, PK15^S10-CD163). In the present study, we examined the suitability of different cell lines as a possible replacement of primary pulmonary alveolar macrophages (PAM) cells for isolation and growth of PRRSV. The susceptibility of four different cell lines (PK15^Sn-CD163, PK15^S10-CD163, MARC-145, and MARC-145^Sn) for the primary isolation of PRRSV from PCR positive sera (both PRRSV1 and PRRSV2) was compared with that of PAM. To find possible correlations between the cell tropism and the viral genotype, 54 field samples were sequenced, and amino acid residues potentially associated with the cell tropism were identified. Regarding the virus titers obtained with the five different cell types, PAM gave the highest mean virus titers followed by PK15^Sn-CD163, PK15^S10-CD163, MARC-145^Sn, and MARC-145. The titers in PK15^Sn-CD163 and PK15^S10-CD163 cells were significantly correlated with virus titers in PAM for both PRRSV1 (p < 0.001) and PRRSV2 (p < 0.001) compared with MARC-145^Sn (PRRSV1: p = 0.22 and PRRSV2: p = 0.03) and MARC-145 (PRRSV1: p = 0.04 and PRRSV2: p = 0.12). Further, a possible correlation between cell tropism and viral genotype was assessed using PRRSV whole genome sequences in a Genome-Wide-Association Study (GWAS). The structural protein residues GP2:187L and N:28R within PRRSV2 sequences were associated with their growth in MARC-145. The GP5:78I residue for PRRSV2 and the Nsp11:155F residue for PRRSV1 was linked to a higher replication on PAM. In conclusion, PK15^Sn-CD163 and PK15^S10-CD163 cells are phenotypically closely related to the in vivo target macrophages and are more suitable for virus isolation and titration than MARC-145/MARC-145^Sn cells. The residues of PRRSV proteins that are potentially related with cell tropism will be further investigated in the future.

Keywords: MARC-145; PK15S10-CD163; PK15Sn-CD163; PRRSV; macrophages; virus isolation/production.

Figure 1

Comparison of virus titers obtained with the five different cell types. Virus titers obtained with each cell line were labeled with different colors. Data are given by dot plot including the mean value of the titers obtained from all the tested samples. The detection limit (0.96 log₁₀TCID₅₀) was indicated with the blue dashed line. Differences were considered significantly different at p < 0.05.

Figure 2

Correlation analysis of type 1 and type 2 PRRSV titration results obtained with PAM and one of the four continuous cell lines (PK15^Sn-CD163, PK15^S10-CD163, MARC-145^Sn, or MARC-145). Each bullet represents the virus titer obtained with PAM (X-axis) versus the titer obtained with one of the four continuous cell lines (PK15^Sn-CD163, PK15^S10-CD163 MARC-145^Sn, and MARC-145) (Y-axis). The line of linear regression was generated and presented in the figure with a 95% confidence interval (CI_95%).

Figure 3

Phylogenetic analysis and the GWAS analysis between genotype and virus-cell tropism for PRRSV1 and PRRSV2 field isolates. (A) The phylogenetic tree containing both PRRSV1 and PRRSV2 full-genome sequences with available reference sequences from GenBank, labeled with countries. (B) The distribution of the isolates (PRRSV1 and PRRSV2) from the current study in the full-genome phylogenetic tree. (C) Phylogenetic and phenotypic correlation analysis. The phylogenetic tree was constructed with an IQ-Tree. The tree and the titration data set were combined. The bootstrap values above 70 were indicated with blue circles in each node of the tree. The heatmap in the first column, besides the tree, represents the titration data tested in the five cell lines, as indicated. The second column represents the residues that are potentially linked to MARC-145 permissiveness. The third column shows the residues that are potentially linked to preferences for PAM. Legends for all the symbols are listed in the figure. The asterisk (*) represents a stop codon in the corresponding residue.