Impact of flavivirus vaccine-induced immunity on primary Zika virus antibody response in humans.

BACKGROUND: Zika virus has recently spread to South- and Central America, causing congenital birth defects and neurological complications. Many people at risk are flavivirus pre-immune due to prior infections with other flaviviruses (e.g. dengue virus) or flavivirus vaccinations. Since pre-existing cross-reactive immunity can potentially modulate antibody responses to Zika virus infection and may affect the outcome of disease, we analyzed fine-specificity as well as virus-neutralizing and infection-enhancing activities of antibodies induced by a primary Zika virus infection in flavivirus-naïve as well as yellow fever- and/or tick-borne encephalitis-vaccinated individuals. METHODOLOGY: Antibodies in sera from convalescent Zika patients with and without vaccine-induced immunity were assessed by ELISA with respect to Zika virus-specificity and flavivirus cross-reactivity. Functional analyses included virus neutralization and infection-enhancement. The contribution of IgM and cross-reactive antibodies to these properties was determined by depletion experiments. PRINCIPAL FINDINGS: Pre-existing flavivirus immunity had a strong influence on the antibody response in primary Zika virus infections, resulting in higher titers of broadly flavivirus cross-reactive antibodies and slightly lower levels of Zika virus-specific IgM. Antibody-dependent enhancement (ADE) of Zika virus was mediated by sub-neutralizing concentrations of specific IgG but not by cross-reactive antibodies. This effect was potently counteracted by the presence of neutralizing IgM. Broadly cross-reactive antibodies were able to both neutralize and enhance infection of dengue virus but not Zika virus, indicating a different exposure of conserved sequence elements in the two viruses. CONCLUSIONS: Our data point to an important role of flavivirus-specific IgM during the transient early stages of infection, by contributing substantially to neutralization and by counteracting ADE. In addition, our results highlight structural differences between strains of Zika and dengue viruses that are used for analyzing infection-enhancement by cross-reactive antibodies. These findings underscore the possible impact of specific antibody patterns on flavivirus disease and vaccination efficacy.

CD4 T Cell Determinants in West Nile Virus Disease and Asymptomatic Infection.

West Nile (WN) virus infection of humans is frequently asymptomatic, but can also lead to WN fever or neuroinvasive disease. CD4 T cells and B cells are critical in the defense against WN virus, and neutralizing antibodies, which are directed against the viral glycoprotein E, are an accepted correlate of protection. For the efficient production of these antibodies, B cells interact directly with CD4 helper T cells that recognize peptides from E or the two other structural proteins (capsid-C and membrane-prM/M) of the virus. However, the specific protein sites yielding such helper epitopes remain unknown. Here, we explored the CD4 T cell response in humans after WN virus infection using a comprehensive library of overlapping peptides covering all three structural proteins. By measuring T cell responses in 29 individuals with either WN virus disease or asymptomatic infection, we showed that CD4 T cells focus on peptides in specific structural elements of C and at the exposed surface of the pre- and postfusion forms of the E protein. Our data indicate that these immunodominant epitopes are recognized in the context of multiple different HLA molecules. Furthermore, we observed that immunodominant antigen regions are structurally conserved and similarly targeted in other mosquito-borne flaviviruses, including dengue, yellow fever, and Zika viruses. Together, these findings indicate a strong impact of virion protein structure on epitope selection and antigenicity, which is an important issue to consider in future vaccine design.

Structural Influence on the Dominance of Virus-Specific CD4 T Cell Epitopes in Zika Virus Infection.

Zika virus (ZIKV) has recently caused explosive outbreaks in Pacific islands, South- and Central America. Like with other flaviviruses, protective immunity is strongly dependent on potently neutralizing antibodies (Abs) directed against the viral envelope protein E. Such Ab formation is promoted by CD4 T cells through direct interaction with B cells that present epitopes derived from E or other structural proteins of the virus. Here, we examined the extent and epitope dominance of CD4 T cell responses to capsid (C) and envelope proteins in Zika patients. All patients developed ZIKV-specific CD4 T cell responses, with substantial contributions of C and E. In both proteins, immunodominant epitopes clustered at sites that are structurally conserved among flaviviruses but have highly variable sequences, suggesting a strong impact of protein structural features on immunodominant CD4 T cell responses. Our data are particularly relevant for designing flavivirus vaccines and their evaluation in T cell assays and provide insights into the importance of viral protein structure for epitope selection and antigenicity.

Multiple Modes of Action of a Monoclonal Antibody against Multidrug-Resistant Escherichia coli Sequence Type 131-H30.

The multidrug-resistant H30 subclone of extraintestinal pathogenic Escherichia coli sequence type 131 (ST131-H30) has spread worldwide. This clone expresses a conserved lipopolysaccharide (LPS) O antigen, O25b. Previously, we described monoclonal antibodies (MAbs) specific to the O25b antigen and characterized them as diagnostic and therapeutic tools. In this study, evidence is provided that besides the previously shown complement-mediated bactericidal effect, an O25b-specific humanized MAb, A1124, also enhances opsonophagocytic uptake by the murine macrophage cell line RAW 264.7. Both phagocyte-dependent killing and phagocyte-independent killing, triggered by A1124, were confirmed in human whole blood. Furthermore, A1124 was shown to neutralize endotoxin activity of purified LPS of clinical isolates. This activity was demonstrated in vitro using both RAW 264.7 cells and a human Toll-like receptor 4 (TLR4) reporter cell line, as well as in a murine model of endotoxemia using purified LPS for challenge. Significant protective efficacy of A1124 at low doses (<1 mg/kg of body weight) was shown in murine and rat models of bacteremia. The contribution of the bactericidal and anti-inflammatory effects was dissected in the mouse bacteremia model through depletion of complement with cobra venom factor (CVF). Protective efficacy was lost in complement-depleted mice, suggesting the essential role of complement-mediated activities for protection in this model. These data suggest that A1124 exhibits different mechanisms of action, namely, direct complement-mediated and opsonophagocytic killing as well as endotoxin neutralization in various challenge models. Which of these activities are the most relevant in a clinical setting will need to be addressed by future translational studies.