Impairment Of The Function Of Polymorphonuclear Leukocytes After PRRSV Infection

Porcine reproductive and respiratory syndrome (PRRS) was identified as one ofthe most important etiological agents in multi-factorial respiratory disease of swineand can predispose pigs to secondary infections by many kinds of pathogens, usuallybacteria. Many researchers in recent years have focused on the study of the increasedsusceptibility to secondary bacterial infection after PRRSV infection. Associationswere calculated between PRRSV and the other etiological agents, the results provedthat pigs were predisposed to infection by bacteria.Recent studies suggested several possible explanations for an increasedsusceptibility to secondary bacterial infection following infection by PRRSV.Decreased functioning of macrophages from PRRSV-infected pigs has been found.Thus at7days following PRRSV infection, alveolar macrophages had a decreaseduptake of opsonized H. parasuis and decreased superoxide anion production; at9daysthere was increased intracellular survival of H. parasuis along with decreasedsuperoxide anion production.Plymorphonuclear leukocytes (PMNs) also play a crucial role in the primaryimmunological defense against infectious agents by clearing bacteria through hostinnate receptors. They have several well-established functions including thephagocytosis of opsonized particles and the production of reactive oxygen andnitrogen species, utilized in the killing of foreign target cells. PMNs interact withopsonized immune complexes through Fcγ receptors (FcγRs) which bind the Fcdomain of immunoglobulin G (IgG). Fc receptors fall into two categories: theactivating and the inhibitory, which respectively transmit their signals via ITAM orITIM terminal sequences.Regulatory T (Treg) cells have been confirmed to be crucial for establishingself-tolerance, controlling inflammatory activities, down-regulation of variousimmune responses and maintaining host homeostasis. They are a subset of circulatingCD4+CD25+T cells, with expression of the transcription factor forkhead box P3(FoxP3) as a useful Treg selection marker. Porcine FoxP3gene was first identified in 2009. It was demonstrated in2010that exposure to classical PRRSV, both in vitroand in vivo, could increase the numbers of viral-specific CD4+CD25+FoxP3+Tregs,which suggested a novel immunomodulatory mechanism induced by classical PRRSV.Yet whether the negative immunomodulation of host immune response by H-PRRSValso involves Tregs, and the difference in Treg modulation between H-PRRSV andclassical PRRSV remain unclear.In order to understand the mechanism for an increased susceptibility tosecondary bacterial infection, we investigated the antibody-dependent phagocytosisbehaviour of PMNs after infection by PRRSV strains BJ-4and HN07-1. The numbersand activities of CD4+CD25+FoxP3+Tregs after H-PRRSV or classical PRRSVinfection were compared to study the immunomodulatory mechanism of H-PRRSV.In our study the PMNs from piglets inoculated with either PRRSV strainsshowed a sharp down-regulation in IC binding and IC-mediated phagocytosis. Theexpression of the inhibitory receptor FcγRIIb was elevated, whereas that of theactivating receptor FcγRIII was suppressed in both PRRSV HN07-1and BJ-4infections, contributing to the decreased phagocytosis of PMNs and possibly to thesuppression of immune responses. This is in consistent with previous investigationsthat suggest the decrease of FcγRIII on the surface of neutrophils is the major cause ofthe reduction in phagocytosis. Pro-inflammatory cytokines such as TNF have beenshown to depress the levels of FcγR, particularly of FcγRIII, in PMNs. We haveshown this effect to be enhanced in those animals infected with PRRSV. Since wefound that the serum levels of TNF-α and IL-1β rise following PRRSV infection thismay be a contributory factor in the observed fall in PMN’s level of Ig-dependentphagocytosis following infection.We found that following infection the oxygen burst function of the PMNs wasdepressed. The production of oxygen and N free radicals is dependent on pinocytosisand phagocytosis but their production also further enhances phagocytic activity. Sucha reduction could therefore be an additional factor in the observed diminution ofphagocytic activity. The interdependence of phagocytosis and free radical productionis important in the killing of engulfed microorganisms. A reduction in the oxygenburst activity during PRRSV infection could explain the secondary infections in these animals.After H-PRRSV (HN07-1) or classical PRRSV (BJ-4) infection, the numbers ofCD4+CD25+FoxP3+Tregs and the amount of FoxP3expression in porcine PBLsshowed a sharp increase, and these were greater in HN07-1. Proliferations ofPHA-stimulated PBLs were noticeably decreased after co-culturing withPRRSV-induced CD4+CD25+Tregs, but no significant difference was observedbetween the two strains. The number of CD3+CD4+cells in PRRSV HN07-1infectiondecreased stronger than in BJ-4infection, which led to a remarkably greater decreasein ratios of CD4+/CD8+cell contents in PRRSV HN07-1infection.Finally, in all respects, the consequences of infection by the more highlypathogenic strain HN07-1were greater on all of the parameters measured in this study.Whatever traits the PRRSV carries which make the host more susceptible tosecondary infection, they appear likely to be more prominently expressed in PRRSVHN07-1. These observations may help to understand the mechanism of the increasedsusceptibility to secondary bacterial infection following PRRSV infection.