Abstract
Fast evolution in the field of the replicase nsp2 represents a most prominent feature of porcine reproductive and respiratory syndrome virus (PRRSV). Here, we determined its biological significance in viral pathogenesis by constructing interlineage chimeric mutants between the Chinese highly pathogenic PRRSV (HP-PRRSV) strain JXwn06 (lineage 8) and the low-virulent NADC30-like strain CHsx1401 (lineage 1). Replacement with nsp2 from JXwn06 was surprisingly lethal to the backbone virus CHsx1401, but combined substitution with the structural protein-coding region (SP) gave rise to viable virus CHsx1401-SPnsp2JX. Meanwhile, a derivative carrying only the SP region (CHsx1401-SPJX) served as a control. Subsequent animal experiments revealed that acquisition of SP alone (CHsx1401-SPJX) did not allow CHsx1401 to gain much virulence, but additional swapping of HP-PRRSV nsp2 (CHsx1401-SPnsp2JX) enabled CHsx1401 to acquire some properties of HP-PRRSV, exemplified by prolonged high fever, microscopic lung hemorrhage, and a significant increase in proinflammatory cytokines in the acute stage. Consistent with this was the transcriptomic analysis of persistently infected secondary lymphoid tissues that revealed a much stronger induction of host cellular immune responses in this group and identified several core immune genes (e.g., TLR4, IL-1β, MPO, etc.) regulated by HP-PRRSV nsp2. Interestingly, immune activation status in the individual groups correlated well with the rate of viremia clearance and viral tissue load reduction. Overall, the above results suggest that the Chinese HP-PRRSV nsp2 is a critical virulence regulator and highlight the importance of nsp2 genetic variation in modulating PRRSV virulence and persistence via immune modulation. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) has been a major threat to the world swine industry. In the field, rapid genetic variations (e.g., deletion, mutation, recombination, etc.) within the nsp2 region present an intriguing conundrum to PRRSV biology and pathogenesis. By making chimeric mutants, here, we show that the Chinese highly pathogenic PRRSV (HP-PRRSV) nsp2 is a virulence factor and a much stronger inducer of host immune responses (e.g., inflammation) than its counterpart, currently epidemic, NADC30-like strains. Differences in the ability to modulate host immunity provide insight into the mechanisms of why NADC30-like strains and their derivatives are rising to be the dominant viruses, whereas the Chinese HP-PRRSV strains gradually give away center stage in the field. Our results have important implications in understanding PRRSV evolution, interlineage recombination, and persistence.