Date Published: June 20, 2013
Publisher: Public Library of Science
Author(s): Oksana Vratskikh, Karin Stiasny, Jürgen Zlatkovic, Georgios Tsouchnikas, Johanna Jarmer, Urs Karrer, Michael Roggendorf, Hedwig Roggendorf, Regina Allwinn, Franz X. Heinz, Ted C. Pierson.
The live attenuated yellow fever (YF) vaccine has an excellent record of efficacy and one dose provides long-lasting immunity, which in many cases may last a lifetime. Vaccination stimulates strong innate and adaptive immune responses, and neutralizing antibodies are considered to be the major effectors that correlate with protection from disease. Similar to other flaviviruses, such antibodies are primarily induced by the viral envelope protein E, which consists of three distinct domains (DI, II, and III) and is presented at the surface of mature flavivirions in an icosahedral arrangement. In general, the dominance and individual variation of antibodies to different domains of viral surface proteins and their impact on neutralizing activity are aspects of humoral immunity that are not well understood. To gain insight into these phenomena, we established a platform of immunoassays using recombinant proteins and protein domains that allowed us to dissect and quantify fine specificities of the polyclonal antibody response after YF vaccination in a panel of 51 vaccinees as well as determine their contribution to virus neutralization by serum depletion analyses. Our data revealed a high degree of individual variation in antibody specificities present in post-vaccination sera and differences in the contribution of different antibody subsets to virus neutralization. Irrespective of individual variation, a substantial proportion of neutralizing activity appeared to be due to antibodies directed to complex quaternary epitopes displayed on the virion surface only but not on monomeric E. On the other hand, DIII-specific antibodies (presumed to have the highest neutralizing activity) as well as broadly flavivirus cross-reactive antibodies were absent or present at very low titers. These data provide new information on the fine specificity as well as variability of antibody responses after YF vaccination that are consistent with a strong influence of individual-specific factors on immunodominance in humoral immune responses.
The live-attenuated yellow fever (YF) vaccine based on the 17D virus strain is considered to be one of the most successful vaccines ever produced , . Since its development in the 1930s by Max Theiler, several hundred million doses have been administered and its effectiveness in protecting from disease has been reported to be at least 90% . Recent studies, including systems biology approaches , analyzing innate, cellular and humoral immune responses after YF vaccination indicate that all arms of the immune system are activated, leading to a polyfunctional response that is most likely essential for the long-lasting immunity induced by this vaccine , , . Despite the broad immunological stimulation, there is strong evidence that humoral immunity mediated by virus-neutralizing antibodies is the primary effector mechanism of protection . Such antibodies may persist for more than 45 years and apparently protect against all naturally occurring genotypes of YF virus .
The primary goal of this study was to gain insight into individual variations in fine specificities of the antibodies and their possible impact on virus neutralization after YF vaccination. This was accomplished by quantifying subsets of antibodies directed to distinct domains of the viral envelope protein E and determining their contribution to virus neutralization. Our data provide evidence for extensive differences in the specificities and relative proportions of antibody populations induced by YF vaccination in different individuals. This conclusion is primarily based on the observation that the ratios of reactivities in ELISAs with the monomeric E, DI+II and prM relative to the virion ELISA reactivities varied substantially (compare Figure 5). Substantial variation was found in the ratio between virion ELISA reactivities and neutralization titers, suggesting a strong influence of antibody subset composition on the functional activity of individual sera. Further confirmation of the observed heterogeneities was obtained by the quantitative analysis of antibodies in sera depleted with recombinant antigens. Specifically, depletion with the monomeric sE resulted in a strongly diverging pattern of neutralizing activity removed (ranging from 0 to 79%) which in several instances did not match the reactivity pattern of the virion ELISA (compare the panels in Figure 6 A), and similar differences were also observed in depletions with DI+II. Our data not only demonstrate extensive differences in the fine specificities of antibody subsets in post-vaccination sera, but also that these heterogeneities can strongly affect functional activities such as virus neutralization. It is likely that these findings are related to cooperative and/or competitive interactions between antibody populations directed to the same target antigen but displaying different fine specificities, avidities and concentrations. Such effects have been described in studies with monoclonal antibodies , ,  and were proposed for explaining variations in the efficiency of polyclonal sera to neutralize influenza virus  and HIV . In the case of YF and other flaviviruses, the phenomenon of virus breathing has to be considered as an additional layer of complexity, because it allows binding of antibodies to epitopes that are seemingly inaccessible in a static model of virion structure but become exposed through dynamic motions of the virion shell , , , , , . Antibodies to such sites can thus contribute to virus neutralization and increase the potential individual variation.