Modeling the Early Events of Severe Acute Respiratory Syndrome Coronavirus Infection In Vitro

Abstract

The clinical picture of severe acute respiratory syndrome (SARS) is characterized by pulmonary inflammation and respiratory failure, resembling that of acute respiratory distress syndrome. However, the events that lead to the recruitment of leukocytes are poorly understood. To study the cellular response in the acute phase of SARS coronavirus (SARS-CoV)-host cell interaction, we investigated the induction of chemokines, adhesion molecules, and DC-SIGN (dendritic cell-specific ICAM-3-grabbing nonintegrin) by SARS-CoV. Immunohistochemistry revealed neutrophil, macrophage, and CD8 T-cell infiltration in the lung autopsy of a SARS patient who died during the acute phase of illness. Additionally, pneumocytes and macrophages in the patient's lung expressed P-selectin and DC-SIGN. In in vitro study, we showed that the A549 and THP-1 cell lines were susceptible to SARS-CoV. monocytes, and activated T cells in a chemotaxis assay. We also demonstrated that DC-SIGN was inducible in THP-1 as well as A549 cells after SARS-CoV infection. Our in vitro experiments modeling infection in humans together with the study of a lung biopsy of a patient who died during the early phase of infection demonstrated that SARS-CoV, through a dynamic interaction with lung epithelial cells and monocytic cells, creates an environment conducive for immune cell migration and accumulation that eventually leads to lung injury.Severe acute respiratory syndrome (SARS) in adults causes new pulmonary infiltration, lymphopenia, thrombocytopenia, and high levels of proinflammatory cytokines and chemokines (30) and C-reactive protein (28) in the sera. The clinical picture is characterized by a cascade of immunological events leading to pulmonary inflammation and respiratory failure (9, 17), resembling adult acute respiratory distress syndrome (ARDS) (8). High levels of chemokines and cytokines, triggered by the host immune response to SARS coronavirus (SARS-CoV), are believed to contribute to the progressive pulmonary infiltration of macrophages (16), polymorphonuclear leukocytes (2), and T cells (11) and to eventual diffuse alveolar damage and fibrosis (12). However, it remains to be determined how the cellular response in the early stage of virus-host cell interaction results in the sequence of events that leads to the severe clinical outcome.In situ hybridization and immunohistochemistry revealed that both SARS-CoV nucleic acids and antigens are present within type II pneumocytes (26). Alveolar macrophages are also reported to harbor SARS-CoV (23). Hence, it is important to investigate how the interaction between SARS-CoV and pneumocytes and macrophages influences the subsequent events in the lung.DC-SIGN (dendritic cell-specific ICAM-3-grabbing nonintegrin) is a type II C-type lectin that is naturally expressed in human dendritic cells. It has been reported that DC-SIGN binds SARS-CoV and mediates its entry into myeloid dendritic cells by binding to the spike protein (31). However, the inducibility of DC-SIGN in cells encountering the virus and its si...

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