Abstract

ABSTRACT OF THE DISSERTATION Mechanisms of Antibody-Mediated Protection in Severe Acute Respiratory Syndrome Coronavirus 2 Infection by Samantha Rae Mackin Doctor of Philosophy in Biology and Biomedical Sciences Immunology Washington University in St. Louis, 2025 Professor Michael S. Diamond, Chair Coronaviruses (CoVs) are enveloped, non-segmented, positive-sense RNA viruses belonging to the Coronaviridae family. The subfamily Coronavirinae includes four genera: alpha-, beta-, gamma-, and deltacoronaviruses. While gamma- and deltacoronaviruses primarily circulate in wild or domestic birds and have shown the potential for cross-species transmission, alpha- and betacoronaviruses mainly infect mammals, resulting in respiratory illnesses that can be mild or severe. Over the past 25 years, two epidemic CoVs have emerged in humans: severe acute respiratory syndrome (SARS)-CoV in 2002 and Middle East respiratory syndrome (MERS)-CoV in 2012, respectively. SARS-CoV, first detected in Guangdong Province, China, triggered an outbreak of viral pneumonia that quickly spread to 29 countries, infecting over 8,000 individuals and resulting in a 10% fatality rate. The virus was believed to have originated in bats and was transmitted to humans via wild game. MERS-CoV was first identified in Saudi Arabia, responsible for another outbreak of viral pneumonia that spread to 27 countries. Approximately 2,500 people were infected with a fatality rate of 35%. Transmission to humans primarily occurred through contact with dromedary camels. The zoonotic origins of SARS-CoV and MERS-CoV demonstrate the potential threat of coronaviruses for subsequent spillover infections to humans with epidemic or pandemic potential. At the end of 2019, a novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), emerged in Wuhan, China, likely originating from a spillover event involving animals and humans in a market setting. Infection with SARS-CoV-2 causes coronavirus disease 2019 (COVID-19), which has led to approximately 778 million recorded infections and 6.8 million deaths globally. In response, vaccines were rapidly developed and deployed, significantly reducing symptomatic infections, hospitalizations, and mortality. SARS-CoV-2 vaccines all have targeted the viral spike protein derived from strains that circulated in early 2020. However, the continuing evolution of SARS-CoV-2 has jeopardized the immunity generated by these vaccines and the control of virus infection and transmission. Emerging severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants with antigenic changes in the spike protein are neutralized less efficiently by serum antibodies elicited by legacy vaccines against the ancestral Wuhan-1 virus. Nonetheless, these vaccines, including mRNA-1273 and BNT162b2, retained their ability to protect against severe disease and death, suggesting that other aspects of immunity control infection in the lung. Vaccine-elicited antibodies can bind Fc gamma receptors (FcγRs) and mediate effector functions against SARS-CoV-2 variants, and this property correlates with improved clinical coronavirus disease 2019 outcome. However, a causal relationship between Fc effector functions and vaccine-mediated protection against infection has not been established. In our studies, using passive and active immunization approaches in wild-type and FcγR-knockout mice, we determined the requirement for Fc effector functions to control SARS-CoV-2 infection. The antiviral activity of passively transferred immune serum was lost against multiple SARS-CoV-2 strains in mice lacking expression of activating FcγRs, especially murine FcγR III (CD16), or depleted of alveolar macrophages. After immunization with the pre-clinical mRNA-1273 vaccine, control of Omicron BA.5 infection in the respiratory tract also was lost in mice lacking FcγR III. Our passive and active immunization studies in mice suggest that Fc–FcγR engagement and alveolar macrophages are required for vaccine-induced antibody-mediated protection against infection by antigenically changed SARS-CoV-2 variants, including Omicron strains. Over the course of the SARS-CoV-2 pandemic, many monoclonal antibodies (mAbs) that neutralized infection against early strains also experienced reduced neutralization potency against Omicron variants due to the large number of mutations in the receptor binding domain of the spike protein. Nonetheless, some antibodies, including S309, the parent antibody of Sotrovimab, retained protective capacity in animals due to Fc effector function activity. In our experiments, using humanized Fc receptor transgenic mice and genetically modified antibodies, we identify an Fc variant of human S309 that confers optimal protection against SARS-CoV-2. We demonstrate that introducing the G236A (GA) mutation and afucosylation (AFUC) to S309 increases binding to Fc receptors IIA and IIIA, respectively, resulting in enhanced virologic protection in the airways of transgenic mice challenged with SARS-CoV-2 B.1.351. We also identify CCR2+-expressing cells as important mediators of this protection. Furthermore, using MA30, a mouse-adapted strain of SARS-CoV-2, we demonstrate that S309-GA-AFUC improved clinical outcomes, reducing lung pathology and restoring pulmonary function more effectively than the parental S309 antibody. Overall, this work contributes to the characterization and optimization of vaccines and antibody-mediated therapies in murine models of virus pathogenesis. These findings support the critical role of Fc effector functions in antibody-mediated protection against SARS-CoV-2, providing mechanistic insight for the design of future vaccines and therapeutic antibodies.

Committee Chair

Michael Diamond

Committee Members

Adrianus Boon; Deborah Lenschow; Gwendalyn Randolph; James Brien; Jennifer Alexander-Brett

Degree

Doctor of Philosophy (PhD)

Author's Department

Biology & Biomedical Sciences (Immunology)

Author's School

Graduate School of Arts and Sciences

Document Type

Dissertation

Date of Award

11-6-2025

Language

English (en)

Author's ORCID

0009-0009-0160-9858

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