Cytokine (IL-8, IL-6, TNF-alpha) and soluble TNF receptor-I release from human peripheral blood mononuclear cells after respiratory syncytial virus infection

Abstract

During the initial phase of respiratory syncytial virus (RSV) infection, when a low virus-cell ratio is most probable, signs of inflammation are detectable in the infected respiratory tissue. Therefore we analysed the release of the proinflammatory cytokines interleukin-6 (IL-6), IL-8, tumour necrosis factor-alpha (TNF-alpha), and the soluble form of the TNF receptor-I (sTNFR-I), from peripheral blood mononuclear cells (PBMC) after exposure to low infectious RSV doses (multiplicity of infection, MOI, 0.001-1) and incubation times of up to 24 hr. The PBMC secreted IL-8 in a time- and virus dose-dependent fashion. As was verified by Northern blot analysis, the increased IL-8 secretion rate was accompanied by an enhanced IL-8 mRNA steady-state level. The infection of the PBMC after 4 hr post-RSV exposure was verified by detection of RSVSH genomic RNA and mRNA after reverse transcription and polymerase chain reaction (PCR) amplification. In addition, after 24 hr post-infection we determined the percentage of infected cells by specific immunofluorescence using monoclonal antibodies directed against the F- and G-proteins. After exposure of PBMC to inactivated RSV, we observed only RSVSH genomic RNA and a reduced IL-8 release. Thus, even the binding and/or phagocytosis of RSV by PBMC induced an IL-8 synthesis to some extent. Following an incubation time of 24 hr, PBMC exposed to small RSV doses synthesized and released high amounts of IL-6 into the cell supernatant. In contrast, only low amounts of TNF-alpha were released from PBMC. In addition to the release of the proinflammatory cytokines, an enhanced level of the sTNFR-I was measured in the cell supernatants at a MOI of 0.1. However, there was no correlation between TNFR-I membrane expression and cell supernatant concentration. Co-culture experiments performed with PBMC and human epithelial cells (A549) revealed that the enhanced IL-8 secretion profile observed in the coculture was partially dependent on the cytokines TNF-alpha, IL-1 beta and TNF-beta/lymphotoxin released by the cells themselves.