Research Article: Dynamic Innate Immune Responses of Human Bronchial Epithelial Cells to Severe Acute Respiratory Syndrome-Associated Coronavirus Infection

Date Published: January 15, 2010

Publisher: Public Library of Science

Author(s): Tomoki Yoshikawa, Terence E. Hill, Naoko Yoshikawa, Vsevolod L. Popov, Cristi L. Galindo, Harold R. Garner, C. J. Peters, Chien-Te (Kent) Tseng, Rory Edward Morty.

Abstract: Human lung epithelial cells are likely among the first targets to encounter invading severe acute respiratory syndrome-associated coronavirus (SARS-CoV). Not only can these cells support the growth of SARS-CoV infection, but they are also capable of secreting inflammatory cytokines to initiate and, eventually, aggravate host innate inflammatory responses, causing detrimental immune-mediated pathology within the lungs. Thus, a comprehensive evaluation of the complex epithelial signaling to SARS-CoV is crucial for paving the way to better understand SARS pathogenesis. Based on microarray-based functional genomics, we report here the global gene response of 2B4 cells, a cloned bronchial epithelial cell line derived from Calu-3 cells. Specifically, we found a temporal and spatial activation of nuclear factor (NF)κB, activator protein (AP)-1, and interferon regulatory factor (IRF)-3/7 in infected 2B4 cells at 12-, 24-, and 48-hrs post infection (p.i.), resulting in the activation of many antiviral genes, including interferon (IFN)-β, -λs, inflammatory mediators, and many IFN-stimulated genes (ISGs). We also showed, for the first time, that IFN-β and IFN-λs were capable of exerting previously unrecognized, non-redundant, and complementary abilities to limit SARS-CoV replication, even though their expression could not be detected in infected 2B4 bronchial epithelial cells until 48 hrs p.i. Collectively, our results highlight the mechanics of the sequential events of antiviral signaling pathway/s triggered by SARS-CoV in bronchial epithelial cells and identify novel cellular targets for future studies, aiming at advancing strategies against SARS.

Partial Text: Severe acute respiratory syndrome (SARS), caused by a novel human coronavirus (CoV), has established itself as a fatal human respiratory disease [1], [2], [3], [4]. SARS-CoV is transmitted through virus-laden droplets, and likely also via either the aerosol or fecal-oral routes, with the lungs as its main pathological target. While the exact mechanism of SARS pathogenesis remains unknown, pathological examination of lung biopsies and autopsy specimens from SARS patients revealed “diffuse alveolar damage” of varying stages and severities, with extensive disruption of epithelial cells and accumulation of reactive macrophages (MΦs), accompanied by the presence of hemophagocytic syndrome in patients who succumbed to the disease [5], [6], [7], [8]. Strikingly, pulmonary manifestations of SARS patients usually occurred after the clearance of viremia and often in the absence of other opportunistic infections. Taken together, these observations have led to the hypothesis that SARS pathogenesis might stem from ill-regulated and often excessive inflammatory responses within the lungs [5]. The likelihood of SARS being an immune-mediated disease was further supported by reports, within the circulation and the lungs of patients affected by SARS, of highly elevated expressions of various inflammatory mediators, including interleukin (IL)-1, -6, -8; CXCL-10/Interferon-inducible Protein (IP)-10; CCL2/Monocyte Chemoattractant Protein (MCP)-1; CCL5/Regulated on Activation, Normal T Expressed and Secreted (RANTES); and CXCL9/Monokine Induced by interferon-Gamma (MIG) [9], [10], [11], [12], [13], [14]. Such an exacerbated cytokine response was subsequently demonstrated in experimentally infected mice, especially those transgenically expressing human angiotensin-converting enzyme 2 (hACE2) viral receptor [15], [16], [17].

Intense acute inflammatory responses, concomitant with extremely unremarkable IFN-α/β secretion, are the hallmark of SARS-CoV infection. However, the molecular mechanisms attributed to such highly dysregulated innate antiviral responses in SARS-CoV-infected cells, especially those of pathologically relevant lung epithelial cells, remain elusive. We have reported that whereas human bronchial Calu-3 cells secreted high levels of biologically active IL-6, IL-8, and IP-10 in response to SARS-CoV infection, they failed to mount any detectable IFN-α/β response [38]. Because Calu-3 cells are highly heterogeneous with regard to the expression of the ACE2 viral receptor [41], we strived and succeeded to establish various clones of Calu-3 cells, which would be valuable for studying the host response to SARS-CoV infection.



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