Interferon gamma induces cellular protein alteration and increases replication of porcine circovirus type 2 in PK-15 cells

Porcine circovirus type 2 (PCV2) infections may lead to the development of subclinical signs or chronic systemic syndromes, collectively known as "porcine circovirus-associated disease" (PCVAD) in swine. Interferon gamma (IFN-gamma) is known to enhance PCV2 replication in vitro, and immune mediators may act as pivotal factors in triggering PCV2 infection progression toward PCVAD. We determined the effects of IFN-gamma on PCV2 replication in PK-15 cells. PCV2 was cultured in the presence or absence of exogenous swine IFN-gamma (swIFN gamma). Growth curve analysis in PK-15 cells revealed that PCV2 could replicate to a significantly higher titer in swIFN gamma medium. To investigate the host cell response upon PVC2 infection, differential expression of proteins in PCV2-infected PK-15 cells with or without swIFN gamma stimulation was analyzed by proteomics (LC-MS/MS) analysis. A large proportion of the differentially expressed proteins in swIFN gamma-treated PCV2-infected cells were found to be involved in apoptosis, cellular stress responses, cell survival/proliferation pathways, and inflammatory responses. We further confirmed the expression of these differentially expressed proteins at the mRNA levels by qRT-PCR. PCV2 infection in PK-15 cells in the presence of IFN-gamma resulted in upregulation of cellular proteins in responses to stress, cell survival, and cell proliferation (Hsp90, MAP3K7, RAS-GTPase, c-myc, and 14-3-3 epsilon) as well as in an increase in the levels of proteins (CASP9 and TRAF5) related to the apoptosis pathways. Thus, PCV2 exploits several cellular biological processes through IFN activation for enhancing viral replication. This is the first evidence of IFN-gamma promoting PCV2 replication in vitro via a mechanism similar to that used by several human viruses.