Persistent viral infections can be an important medical problem, with persistently infected (PI) cells extending viral shedding, maintaining inflammation, and providing potential sources for new viral variants. Given that PI cells can acquire resistance to some innate immune pathways, we tested the hypothesis that complement (C')-mediated lysis of parainfluenza virus 5 (PIV5)-infected cells would differ between acute-infected and PI cells. Biochemical and real-time cell viability assays showed effective C'-mediated lysis of A549 lung cells acutely infected with PIV5, through pathways that depended on C3 and C5, but largely independent of C6. A PIV5 PI cell line established by long-term culturing of acutely infected A549 cells showed a high-level persistent expression of PIV5 proteins and infectious virus. Under conditions that led to effective lysis of acute PIV5-infected cells, the PI cells were nearly completely resistant to C'-mediated killing. This lack of C' killing was not due to failure to activate C', since C'-treated PIV5 PI cells had extensive C3 and membrane attack complex deposition, as well as production of C3a and C5a. Transcriptomics analysis revealed the C' cascade as the most significantly upregulated pathway in PIV5 PI cells versus acute infection. Biochemical analyses showed that resistance to C' killing correlated with increased expression in PI cells of two major C' inhibitors: complement factor H and Vitronectin. The finding of acquisition of C' resistance after the transition from acute PIV5 infection to PI cells raises the potential to inform therapeutics for PIs based on modulating C' pathways.
Importance: A persistent infection (PI) with RNA viruses can extend virus shedding, prolong inflammation, and be a source of new viral variants. Since profound changes to innate immune pathways can occur in PI cells, it was important to test PI cells for changes in sensitivity to the complement (C') system, powerful innate immune pathways capable of lysing infected cells. Using parainfluenza virus 5 (PIV5) as a model system, we show that PI cells are nearly completely resistant to C'-mediated lysis, in stark contrast to high sensitivity of acute PIV5-infected cells to C' killing. A key finding was the upregulated expression in PI cells of two C' inhibitors: Vitronectin and complement factor H. These are important results with strong potential to inform therapeutics, given that polymorphisms in C' genes can correlate with severity of viral infections, and clinical trials are underway with new drugs that modulate C' responses.
Keywords: Complement; persistent infection.