Recombination of Porcine Reproductive and Respiratory Syndrome Virus: Features, Possible Mechanisms, and Future Directions

Abstract

Recombination is a pervasive phenomenon in RNA viruses and an important strategy for accelerating the evolution of RNA virus populations. Recombination in the porcine reproductive and respiratory syndrome virus (PRRSV) was first reported in 1999, and many case reports have been published in recent years. In this review, all the existing reports on PRRSV recombination events were collected, and the genotypes, parental strains, and locations of the recombination breakpoints have been summarized and analyzed. The results showed that the recombination pattern constantly changes; whether inter- or intra-lineage recombination, the recombination hotspots vary in different recombination patterns. The virulence of recombinant PRRSVs was higher than that of the parental strains, and the emergence of virulence reversion was caused by recombination after using MLV vaccines. This could be attributed to the enhanced adaptability of recombinant PRRSV for entry and replication, facilitating their rapid propagation. The aim of this paper was to identify common features of recombinant PRRSV strains, reduce the recombination risk, and provide a foundation for future research into the mechanism of PRRSV recombination.

Keywords: PRRSV; recombination; vaccine safety; viral evolution.

Figure 1

The recombination patterns of all the reported PRRSV-2 recombinant cases. The diagram of the full-length genome structure includes the major ORFs’ positions and boundaries, referring to VR-2332. (A) The inter-lineage and (B) intra-lineage recombination model diagram. Each junction of different colors means the breakpoints and indicates that the recombination occurs in this position. The different colors on the right indicate the parental lineages. The yellow rectangle represents the major parent, while the green rectangle box shows the minor parent. The recombination frequency of each coding or non-coding region in the (C) inter-lineage and (D) intra-lineage recombinant PRRSVs. The x-axis shows the position of the PRRSV-2 genome, and the y-axis represents the percentage of recombination events that occurred in the specific composition of the recombinant genome. (E) The MLV-related recombination patterns of PRRSV-2. The different colors of the box depict the different PRRS MLV vaccines shown on the right, and orange indicates all the other parents who are unrelated to the MLV vaccine. The major and minor parents are shown on the right, and the identified lineages are labeled.

Figure 2

Schematic diagram illustrating the recombinant mechanism of PRRSV. During PRRSV infection, PRRSV enters the cell via the CD163 receptor. Different PRRSVs co-infect the same cell and can switch viral templates in replication. RdRp can switch from donor to receptor template to generate recombinant PRRSVs. The recombinant virion is finally released after transcription, translation, and assembly. The two different PRRSV genomes are represented by yellow and blue lines, respectively.