Mechanism of antibody-dependent enhancement in severe acute respiratory syndrome coronavirus infection

Severe lymphopenia is a clinical feature of Severe Acute Respiratory Syndrome (SARS) patients. However, lymphocytes do not express receptor for SARS-CoV, neither the widely accepted viral receptor angiotensin converting enzyme 2 (ACE2) nor the putative receptors Dendritic Cell- and Liver/lymph-Specific Intercellular adhesion molecule-3-Grabbing Non-integrin (DC-SIGN and L-SIGN). Our group previously showed in vitro that, SARS-CoV Spike pseudotyped particles (SARSCoVpp) could infect human B cells only when inoculated in presence of anti-SARSCoV Spike immune serum. Such observations raised concerns about the possible occurrence of antibody-dependent enhancement (ADE) of infection, a phenomenon during which a virus bounded by antibodies could gain entry into cells through mechanisms involving complement receptors or Fc receptors. Recently, we have demonstrated the participation of the human Fc gamma receptor II (hFcγRII) molecules in granting SARS-CoV an opportunity to infect human immune cells. The aim of this study was to decipher the molecular mechanism leading to antibodymediated, FcγRII-dependent infection of immune cells by SARS-CoV. By using transduction experiment, I highlighted that different members of the hFcγRII family (namely hFcγRIIA, hFcγRIIB1 and hFcγRIIB2) could confer susceptibility to ADE of SARS-CoVpp infection. I further demonstrated that purified anti-viral immunoglobulin G, but not other soluble factor(s) from heat-inactivated immune serum, was the determinant for occurrence of ADE infection. Additionally, with the development of a cell-cell fusion assay, I illustrated that in contrast to the ACE2- dependent pathway, ADE infection did not occur at the plasma membrane, but rather require internalization of virus/antibodies immune complexes by the target cells. In line with this hypothesis, my results using a panel of FcγRII-expressing mutants demonstrated that binding of immune complexes to cell surface FcγRII was a prerequisite but was not sufficient to trigger ADE infection. In these experiments, only FcγRII signaling-competent constructions conferred susceptibility to ADE of SARS-CoVpp infection. Altogether my results point toward a role of the anti-SARS-CoV Spike IgG in vitro in granting SARS-CoV an opportunity to infect cells bearing signaling-competent FcγRII receptors. If further confirmed, such observations could have implications for understanding SARS-CoV tropism and SARS pathogenesis, as well as warrant for careful design of SARS vaccines and immunotherapy based on anti-viral antibodies.published_or_final_versionMicrobiologyMasterMaster of Philosoph

Transmission potential of influenza in respiratory droplets

Influenza virus is a respiratory infection of public health importance, because it is associated with a substantial burden of morbidity and mortality in Hong Kong and elsewhere. The traditional paradigm suggests that influenza virus transmits between humans primarily by respiratory droplets and secondarily by contact, while there is controversy over the role of aerosol transmission. Interventions to control influenza epidemics often require identification of infected persons, which is challenging without laboratory testing, while asymptomatic infections may have a role in community spread. I conducted a series of studies on the potential contribution of aerosols to the transmission of influenza, the efficacy of surgical face masks against aerosols, the potential for a new rapid test to be used for the identification of cases of influenza virus infection, and the potential importance of asymptomatic infections in the spread of influenza epidemics. In one study, I used a Gesundheit-II (G-II) device to collect exhaled breath particles from outpatients in two size fractions, ≥5μm (coarse) and <5μm (fine) and maintained viability of any viruses collected in the fine fraction. In a second study, I used a cyclone NIOSH air sampler to detect influenza virus in fine particles in patient rooms. In the third study, I used data from three separate studies of outpatients to characterize the clinical diagnostic performance of the Sofia Influenza A+B Fluorescent Immunoassay. Finally, I conducted a systematic review and meta-analysis of published estimates of the proportion of influenza virus infections that are asymptomatic. In the first study, I showed that influenza A virus, including A(H1N1)pdm09 and A(H3N2), were found in both fine and coarse fractions of exhaled breath particles. I also showed that wearing a surgical face mask led to a significant and substantial reduction in influenza A virus shedding in the coarse fraction of exhaled breath. In the second study I detected influenza virus RNA in aerosols at low concentrations half of the time in patient rooms with patients with confirmed influenza virus infection. I estimated that the sensitivity and specificity of the Sofia rapid test for detection of influenza A and B were 77% (95% CI 71-82%) and 91% (95% CI 89-92%) respectively. In the meta-analysis I found that estimates of the asymptomatic fraction were affected by study design, with estimates from outbreak investigations having a pooled mean of 16% (95% CI: 13%, 19%), while estimates from cohort studies adjusting for incidence of non-influenza illnesses fell in the range 65%-85%. Influenza virus could be detected in fine particles in the exhaled breath of outpatients with influenza, and in the rooms of inpatients with confirmed influenza, supporting a role of aerosol transmission in the spread of influenza in the community. The Sofia rapid test had higher sensitivity than other point-of-care tests, but poorer specificity leading to a low positive predictive value outside of peaks in influenza activity. Uncertainty remains about the frequency of asymptomatic infections in the community which depends on how infections are identified, and their importance in transmission.published_or_final_versionPublic HealthDoctoralDoctor of Philosoph