The GP2a 91/97/98 amino acid substitutions play critical roles in determining PRRSV tropism and infectivity but do not affect immune responses.

Unlabelled: Porcine reproductive and respiratory syndrome virus (PRRSV) isolates share a restricted cellular tropism. Marc-145 cells derived from African green monkey are one of the few cell lines supporting PRRSV propagation and are commonly used for PRRS vaccine development. However, currently prevalent PRRSV isolates display different Marc-145 cell tropism while the exact determinant is not clarified yet. In this study, we identified for the first time that the 91/97/98 amino acid (aa) substitutions in GP2a of PRRSV play critical roles in determining Marc-145 adaptation. Specifically, multiple series of chimeric viruses were constructed based on four PRRSV infectious clones including Marc-145 adaptive HP-PRRSV-2 strain and Marc-145 non-adaptive NADC34-like PRRSV-2, NADC30-like PRRSV-2, and PRRSV-1 strains. The GP2a 91/97/98 aa substitutions are a sufficient and necessary determinant in NADC34-like and NADC30-like PRRSV-2, a sufficient but not necessary determinant in HP-PRRSV-2, a necessary but not sufficient determinant in PRRSV-1, respectively. In addition, the GP2a substitutions also influenced PRRSV infectivity in PAMs and piglets. Noticeably, the GP2a substitutions did not significantly affect the levels of neutralizing antibodies, porcine T follicular helper (Tfh) cells, and PRRSV-specific IFNγ secreting cells. Overall, our results not only provide new insights into PRRSV tropism and infectivity but also will facilitate PRRS vaccine development.

Importance: Prevalent PRRSV isolates present different cell tropisms . Clarifying the exact determinant of PRRSV tropism is crucial for PRRSV isolation and vaccine development. By constructing chimeric viruses based on four representative PRRSV infectious clones, we identified for the first time that the 91/97/98 amino acid substitutions in GP2a play critical but distinct roles in determining Marc-145 cell tropism for different PRRSV strains. The GP2a 91/97/98 amino acid substitutions also affect PRRSV infectivity in PAMs and piglets but do not influence immune responses. This study not only deciphers an exact determinant of PRRSV tropism and infectivity but also has guiding significance for PRRS vaccine development.