Innate Immune Antagonism of Flavivirus Infection and the Impact of Preexisting Anti-Flavivirus Antibodies on Secondary ZIKV Infection in the Human Placenta Open Access

Zimmerman, Matthew (Fall 2020)

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Humoral immunity is an essential component of the protective immune response to flavivirus infection however, the role of immunological cross-reactivity and the consequences to secondary flavivirus infection outcome remain controversial. Since its introduction to Brazil in 2015, Zika virus (ZIKV), has caused an epidemic of fetal congenital malformations within the Americas due to its unique ability to infect the human placenta and invade the fetal compartment. Because ZIKV is a mosquito-borne flavivirus with a high degree of sequence and structural homology to Dengue virus (DENV), the role of immunological cross-reactivity in ZIKV and DENV infections and its effects on vertical transmission of ZIKV from mother to child through the placenta has also been of great concern. In this dissertation, we demonstrate that viral immune complexes generated from cross-reactive DENV antibodies can enhance ZIKV infection in human placental macrophages, Hofbauer cells (HCs). We determined that this enhancement is IgG-subclass specific and likely results from increased Fcγ receptor-mediated binding and entry as well as dampening of antiviral immune responses in HCs. Using second trimester ex vivo human chorionic villous explants, we observed that ZIKV immune complexes can utilize the neonatal Fc receptor (FcRn)-mediated IgG transport system to traffic across the immunologically privileged syncytiotrophoblast layer of the placenta and target HCs within the villous core. Using both DENV- and ZIKV-immune sera, we also observed that ZIKV-specific IgM in early convalescent sera had a substantial impact on IgG-mediated transcytosis of ZIKV across the placental barrier. Finally, using a systems biology based approach, explored the ability of West Nile virus (WNV) and ZIKV to inhibit phosphorylation of STAT5, a previously undescribed target of viral antagonism, and downregulate STAT5-target genes downstream of RIG-I stimulation, type I interferon (IFN), and IL-4, but not GM-CSF, signaling in human monocyte-derived dendritic cells. We also found that blockade of STAT5 phosphorylation was specific to WNV and ZIKV, with no inhibition occurring during DENV1-4 or YFV-17D infection. Altogether, these findings implicate complex mechanisms of ZIKV entry into the placenta and innate immune antagonism that could greatly influence the development of flavivirus antivirals, vaccine design, and strategies for vaccine administration.  

Table of Contents

Table of Contents

Chapter 1: Introduction 12

Introduction to Pathogenic Flaviviruses 13

Innate Immune Responses against Flavivirus Infection 16

Human Antibody Responses to DENV and ZIKV 19

Cross-reactive antibody responses between DENV and ZIKV: Protective vs. Pathogenic 23

DENV and ZIKV disease severity in flavivirus-naïve individuals 26

Cross reactive antibodies in animal models of systemic ZIKV ADE 27

Introduction to the human placenta 28

Hofbauer Cells: Key Regulators within the Placenta 30

Innate Immunity at the Placental Barrier 32

Passing the TORCH: Mechanisms of ZIKV Entry into the Placenta 33

ZIKV Infection of the Human Placenta 34

Chapter 2: Cross-reactive dengue virus antibodies augment Zika virus infection of human placental macrophages 40

Introduction 41

Results 44

Discussion 53

Materials and Methods 77

Chapter 3: Anti-Zika virus IgM Inhibits Translocation and Enhancement of Zika virus Infection within the Human Placenta 87

Introduction 88

Results 93

Discussion 96

Materials and Methods 107

Chapter 4: STAT5: A Target of Antagonism by Neurotropic Flaviviruses 111

Introduction 112

Results 115

Discussion 122

Materials and Methods 135

Chapter 5: Discussion 140

Summary of WNV and ZIKV antagonism of STAT5 findings 141

Viral factors affecting STAT5 activation 142

Potential STAT5 Antagonism in other Target Tissues 143

Summary of Placental ZIKV Infection Findings 145

New Paradigm: Viral transcytosis using the placental FcRn pathway? 146

The Reciprocity of Immune Enhancement between ZIKV and DENV 149

Vertical Transmission of other Flaviviruses 151

The Many Flavors of ZIKV Vaccines 154

Monoclonal Antibodies as Therapeutics and Vaccine Adjuvants 156

The Spread of ZIKV and Potential for Reemergence 158

Concluding Thoughts and Questions 161

Works Cited 163






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