The non-structural protein Nsp2TF of porcine reproductive and respiratory syndrome virus down-regulates the expression of Swine Leukocyte Antigen class I

Abstract

Porcine reproductive and respiratory syndrome virus (PRRSV) is arguably the most economically-important global swine pathogen. Here we demonstrated that PRRSV down-regulates Swine Leukocyte Antigen class I (SLA-I) expression in porcine alveolar macrophages, PK15-CD163 cells and monocyte-derived dendritic cells. To identify the viral protein(s) involved in SLA-I down-regulation, we tested all 22 PRRSV structural and non-structural proteins and identified that Nsp1α and Nsp2TF, and GP3 significantly down-regulated SLA-I expression with Nsp2TF showing the greatest effect. We further generated a panel of mutant viruses in which the Nsp2TF protein synthesis was abolished, and found that the two mutants with disrupted -2 ribosomal frameshifting elements and additional stop codons in the TF domain were unable to down-regulate SLA-I expression. Additionally we demonstrated that the last 68 amino acids of TF domain in Nsp2TF are critical for this function. Collectively, the results indicate a novel function of Nsp2TF in negative modulation of SLA-I expression.

Nonstructural Protein 4 of Porcine Reproductive and Respiratory Syndrome Virus Modulates Cell Surface Swine Leukocyte Antigen Class I Expression by Downregulating β2-Microglobulin Transcription

“…1B). These observations indicate that the HP-PRRSV strain used in this study was able to downregulate SLA-I expression on the cell surface, in agreement with previous observations (11)(12)(13)(14).…”

“…Disruption of the SLA-I antigen presentation pathway is thought to be employed by PRRSV as a strategy for modulating the host immune response (11)(12)(13)(14), although the molecular basis of this process is unclear. PRRSV was shown previously to downregulate SLA-I expression on the cell surface via viral Nsp1␣-mediated proteasomal degradation of the SLA-I HC (11).…”

“…For example, mouse cytomegalovirus utilizes viral gp34, gp40, and gp48 to block SLA-I at the cell surface, cause its retention in the ER, and target SLA-I to lysosomes, respectively (26)(27)(28). We therefore speculated that multiple PRRSV proteins might be involved in its modulation of the SLA-I antigen presentation pathway, and Nsp1␣, Nsp2TF, and GP3 have been shown to downregulate SLA-I expression on the cell surface (11,12). In the current study, we identified Nsp4 as being involved in the downregulation of SLA-I expression on the cell surface, though Cao et al (12) previously reported that Nsp4 had no significant effect on SLA-I expression on the cell surface.…”

“…We therefore speculated that multiple PRRSV proteins might be involved in its modulation of the SLA-I antigen presentation pathway, and Nsp1␣, Nsp2TF, and GP3 have been shown to downregulate SLA-I expression on the cell surface (11,12). In the current study, we identified Nsp4 as being involved in the downregulation of SLA-I expression on the cell surface, though Cao et al (12) previously reported that Nsp4 had no significant effect on SLA-I expression on the cell surface. Rapid molecular evolution is known to result in diverse PRRSV isolates with multifarious pathogenicities (29), and the apparent discrepancy between our and Cao et al's results may be attributable to differences in the roles of Nsp4 between the PRRSV strains used.…”

“…We transiently expressed Flag-Nsp4 in PK-15 cells (a cell line used to analyze the effect of PRRSV on SLA-I expression [11,12]) and determined its effect on SLA-I expression on the cell surface by flow cytometry. Flag-N was expressed as a negative control.…”