Severe Acute Respiratory Syndrome (SARS) is a severe respiratory illness caused by a newly identified virus named SARS coronavirus (SARS-CoV). The disease emerged in late 2002 and spread to several countries in early 2003 and was responsible for 8,098 cases including 774 deaths worldwide. The syndrome is characterized by fever, chills or rigors, headache and non-specific symptoms such as malaise and myalgias, followed by cough, and dyspnea. The severe morbidity and mortality associated with SARS make it imperative that effective means to prevent and treat the disease be developed and evaluated, especially since it is not known whether the virus will reappear or whether it will be independently reintroduced into the human population. We have developed animal models for SARS in order to evaluate immunoprophylaxis and immunotherapy for SARS and to understand the pathogenesis of the disease. Animal models for SARS: We have studied the replication of SARS-CoV in mice, hamsters and non-human primates. Following intranasal administration, SARS-CoV replicated to high titer in the respiratory tract of BALB/c mice and Golden Syrian hamsters. Peak replication was seen in the absence of disease on days 1 or 2, depending on the dose administered. SARS-CoV replicated to a higher titer and for a longer duration in the respiratory tract of hamsters and extrapulmonary spread of SARS-CoV to the liver and spleen was detected in hamsters but not in mice. SARS-CoV antigen was detected in epithelial cells of the respiratory tract of mice and hamsters but was not accompanied by inflammation in mice. In contrast, an inflammatory response was seen in hamster lungs coincident with viral clearance with focal and later more generalized consolidation in pulmonary tissue, and eventual pulmonary tissue repair. Despite high levels of virus replication and associated pathology in the respiratory tract, the hamsters showed no clinical evidence of disease. SARS-CoV was also administered to rhesus, cynomolgus and African Green monkeys via the intranasal and intratracheal routes. Virus replication was seen in the absence of disease; the level of replication in the upper and lower respiratory tract of the monkeys was low in rhesus and cynomolgus monkeys and modest in African Green monkeys (AGM) and the virus was cleared within 10 days. Moderate to high titers of SARS-CoV with associated interstitial pneumonitis were detected in the lungs of AGMs on day 2 and were resolving by day 4 post-infection. All the animal models tested developed a serum neutralizing antibody response and were protected from re-infection 28 days following primary infection. There was no evidence of enhanced disease upon challenge in previously infected mice, hamsters or AGMs. Passive transfer of immune serum protected naive mice and hamsters from virus replication in the respiratory tract following intranasal challenge. SARS-CoV replicates in the respiratory tract of BALB/c mice and hamsters to levels that will permit an evaluation of the efficacy of vaccines, immunotherapeutic and antiviral drug treatment strategies. The nonhuman primate species that we evaluated supported replication of SARS-CoV and will be useful for the evaluation of the immunogenicity of candidate vaccines, but the lack of apparent clinical illness, variability from animal to animal in level of viral replication, and rapid clearance of virus and pneumonitis in African Green monkeys are considerations that preclude this species from being the ideal model for the evaluation of the efficacy of vaccines against viral challenge. Our observations in mice and hamsters, that primary infection provides protection from re-infection and that antibody alone can protect against viral replication, suggest that vaccines that induce neutralizing antibodies and strategies for immunoprophylaxis or, perhaps, immunotherapy are likely to be effective in SARS. Pathogenesis studies: We had established earlier that SARS-CoV replicates in 4 to 6 week old BALB/c mice but is rapidly cleared within 5 to 7 days. Although the infection elicits a humoral immune response, it is unlikely to appear early enough to contribute to viral clearance. In collaborative studies with Drs. Glass and Murphy of the Laboratory of Host Defenses, NIAID, studies of the mechanisms underlying the clearance of SARS-CoV from the lungs of C57BL/6 (B6) mice suggested that NK cells and adaptive cellular immunity do not play a role in clearance Determining which component of innate immunity suppresses viral infection in this model is an important question that could provide insight into the pathogenesis and potential treatment and prevention of SARS in human populations. Pulmonary levels of inflammatory cytokines were not elevated in B6 or BALB/c mice. However, some proinflammatory chemokines (especially CXCR3) were up-regulated in the lungs of young SARS-CoV-infected B6 mice How these chemokines are induced, what role they play in viral clearance and why the chemokines fail to elicit more of an inflammatory response is not clear remains to be determined. Vaccine studies: We have collaborated with scientists at the NIH, at academic institutions and in industry to evaluate the immunogenicity and efficacy of a number of vaccines against SARS-CoV in animal models including an inactivated vaccine, two vectored vaccines, and a DNA vaccine. In collaboration with Dr. Peter Collin?s laboratory in LID, we evaluated a recombinant attenuated parainfluenza virus type 3 expressing the S protein and found that immunization with was protective. The results demonstrated that the S protein alone can mediate the induction of a protective immune response and indicated that the mucosal route of immunization is protective. In collaboration with Dr. Bernard Moss? laboratory (Laboratory of Viral Diseases, NIAID), we demonstrated that the SARS-CoV spike protein expressed by a highly attenuated recombinant vaccinia virus induces protective immunity against subsequent SARS-CoV challenge in mice. In collaboration with Dr. Gary Nabel?s laboratory (Vaccine Research Center, NIAID), we showed that a DNA vaccine encoding the spike glycoprotein of the SARS-CoV induces T cell and neutralizing antibody responses, as well as protective immunity, in a mouse model. Viral replication was reduced by more than six orders of magnitude in the lungs of mice vaccinated with S plasmid DNA expression vectors, and protection was mediated by a humoral but not a T-cell-dependent immune mechanism. Immunoprophylaxis with Monoclonal Antibodies: We extended our observation that antibodies alone were able to protect mice from SARS infection by evaluating the ability of monoclonal antibodies (MAbs) against the spike protein of SARS-CoV to protect mice from infection with SARS-CoV. The MAbs that we evaluated were produced by different methods. Dr. Antonio Lanzavecchia from the Institute for Research in Biomedicine in Switzerland developed an improved method of EBV transformation of human memory B cells and used this method to isolate several neutralizing and non-neutralizing monoclonal antibodies that had been selected in the course of natural infection. One such antibody specific for the SARS-CoV spike protein had potent in vitro neutralizing activity and conferred protection in mice. In collaboration with Dr. Donna Ambrosino at the University of Massachusetts, we evaluated MAbs derived from transgenic mice with human immunoglobulin genes that were immunized with the recombinant spike glycoprotein ectodomain of SARS-CoV. Mice that received antibody were completely protected from virus replication in lungs. In these studies, we have clearly established the efficacy of antibodies for immunoprophylaxis against SARS-CoV in the mouse model.
Showing the most recent 10 out of 27 publications
