Distinct patterns of innate immune activation by clinical isolates of respiratory syncytial virus

Abstract

Respiratory syncytial virus (RSV) is a major respiratory pathogen of infants and young children. Multiple strains of both subgroup A and B viruses circulate during each seasonal epidemic. Genetic heterogeneity among RSV genomes, in large part due to the error prone RNA-dependent, RNA polymerase, could mediate variations in pathogenicity. We evaluated clinical strains of RSV for their ability to induce the innate immune response. Subgroup B viruses were used to infect human pulmonary epithelial cells (A549) and primary monocyte-derived human macrophages (MDM) from a variety of donors. Secretions of IL-6 and CCL5 (RANTES) from infected cells were measured following infection. Host and viral transcriptome expression were assessed using RNA-SEQ technology and the genomic sequences of several clinical isolates were determined. There were dramatic differences in the induction of IL-6 and CCL5 in both A549 cells and MDM infected with a variety of clinical isolates of RSV. Transcriptome analyses revealed that the pattern of innate immune activation in MDM was virus-specific and host-specific. Specifically, viruses that induced high levels of secreted IL-6 and CCL5 tended to induce cellular innate immune pathways whereas viruses that induced relatively low level of IL-6 or CCL5 did not induce or suppressed innate immune gene expression. Activation of the host innate immune response mapped to variations in the RSV G gene and the M2-1 gene. Viral transcriptome data indicated that there was a gradient of transcription across the RSV genome though in some strains, RSV G was the expressed in the highest amounts at late times post-infection. Clinical strains of RSV differ in cytokine/chemokine induction and in induction and suppression of host genes expression suggesting that these viruses may have inherent differences in virulence potential. Identification of the genetic elements responsible for these differences may lead to novel approaches to antiviral agents and vaccines.

Genotype-dependent and non-gradient patterns of RSV gene expression

“…The most abundant mRNA they observed was associated with the G gene [40]. Levitz et al reported the G gene to be the most highly expressed gene at later time-points in A549 cells infected with isolates from the RSV/B subgroup [43]. Thus, recent published data indicate that patterns of RSV gene expression vary and do not always follow a gradient.…”

“…By homology with other NNS viruses, it is widely assumed that transcription in RSV follows a gradient, where the extent to which a gene is transcribed falls with its distance from the 3’ promoter [29, 39, 40]. Earlier studies reported data consistent with a gradient [39, 41, 42]; however, recent studies show mRNA abundances that peak at the G gene, which is located in the middle of the genome [40, 43]. We recently reported the G gene to be the most abundant in clinical samples obtained from RSV/A- and RSV/B-infected infants [44].…”

Non-gradient and genotype-dependent patterns of RSV gene expression

“…This is believed to be due to transcriptional attenuation at the intergenic (IG) regions, although the mechanisms underlying the attenuation are unknown [34]. Earlier studies reported results consistent with a gradient [32,35,36]; however, recent studies show RSV mRNA abundances that peak at the G gene, which is the seventh gene downstream of the 3' promoter of the genome [33,37]. We recently reported the G gene to be the most abundant in clinical samples obtained from RSV/A-and RSV/B-infected infants [38].…”

Syncitial virus respiratory infections in children – immunological aspects

“…Although it is a common viral infection, there is still no efficient therapy. The hurdles in obtaining an efficient therapy reside in several issues: the immaturity of the immune system of infants and young children, the different virulence potential of clinical strains that trigger a different cytokine/chemokine response and the involvement of multiple molecular, still unknown pathways (Levitz et al, 2017). The identification of antiviral immune mechanisms and a comprehensive characterization of virus-specific immune responses, using new cellular and animal models would help to define new aspects of the viral -host crosstalk that would modulate the severity of infection.…”