Prior infection and passive transfer of neutralizing antibody prevent replication of severe acute respiratory syndrome coronavirus in the respiratory tract of mice

Abstract

Following intranasal administration, the severe acute respiratory syndrome (SARS) coronavirus replicated to high titers in the respiratory tracts of BALB/c mice. Peak replication was seen in the absence of disease on day 1 or 2, depending on the dose administered, and the virus was cleared within a week. Viral antigen and nucleic acid were detected in bronchiolar epithelial cells during peak viral replication. Mice developed a neutralizing antibody response and were protected from reinfection 28 days following primary infection. Passive transfer of immune serum to naïve mice prevented virus replication in the lower respiratory tract following intranasal challenge. Thus, antibodies, acting alone, can prevent replication of the SARS coronavirus in the lung, a promising observation for the development of vaccines, immunotherapy, and immunoprophylaxis regimens.

FIG. 1.

Kinetics and dose-response of SARS-CoV replication in the respiratory tract of mice. The graphs show the mean titers of virus detected on the indicated days in the lower respiratory tract (A) and upper respiratory tract (B) of four BALB/c mice per group following intranasal administration of the indicated doses of SARS-CoV. Error bars associated with each data point indicate standard errors, and the dotted line indicates the lower limit of detection of virus in 10% (wt/vol) (lungs) and 5% (wt/vol) (nasal turbinates) suspensions.

FIG. 2.

Histopathology, immunohistochemistry, and in situ hybridization of mouse lung tissues harvested on day 2 following infection. (A) Focal and mild peribronchiolar mononuclear inflammatory infiltrate. Hematoxylin and eosin stain; magnification, ×158. (B) SARS-CoV antigens in multiple bronchiolar epithelial cells. Immunoalkaline phosphatase staining, naphthol-fast red substrate with light hematoxylin counterstain; original magnification, ×158. (C and D) SARS-CoV nucleic acids in multiple bronchiolar epithelial cells. Immunoalkaline phosphatase staining, naphthol-fast red substrate with light hematoxylin counterstain. Original magnification: C, ×100; D, ×250.

FIG. 3.

Detection of viral nucleic acid in lung homogenates by RT-PCR. RNA extracted from serial 10-fold dilutions of lung homogenates obtained from mice that received passive transfers of immune or nonimmune serum was subjected to RT-PCR. For each dilution, the cycle number at which amplicons were detected is indicated. SARS-CoV nucleic acid was not detected at 45 cycles (dotted line) in lung homogenates from three mice (×, ▵, and □) that received passive transfers of immune serum but was detected in the virus stock (•) and also when virus was added exogenously to lung homogenates from mice that had received immune serum (⧫) or nonimmune serum (▪).