The inhibition of vaccination by maternal antibodies is a widely observed phenomenon in human and veterinary medicine. Maternal antibodies are known to suppress the B-cell response. This is similar to antibody feedback mechanism studies where passively transferred antibody inhibits the B-cell response against particulate antigens because of epitope masking. In the absence of experimental data addressing the mechanism underlying inhibition by maternal antibodies, it has been suggested that epitope masking explains the inhibition by maternal antibodies, too. Here we report that in the cotton rat model of measles virus ( IntroductionMaternal antibodies of the immunoglobulin G (IgG) antibody class are transferred from mother to child and protect children against infectious diseases. Over time, passively transferred maternal antibody titers decline and are not protective any longer but interfere with successful vaccination. A well-documented example of this is measles vaccination. 1 Inoculation of seronegative children with a live-attenuated vaccine measles virus (MV) leads first to the development of antibodies specific for the nucleocapsid (MV-N) protein (which is released by infected cells) and subsequently to protective neutralizing antibodies specific for the hemagglutinin (MV-H) and fusion (MV-F) proteins. 2 Neutralizing antibodies recognize at least 15 nonoverlapping neutralizing epitopes on MV-H and 3 on MV-F. 3 Vaccination in the presence of maternal antibodies, however, does not lead to development of protective neutralizing antibodies, 4 whereas the T-cell response is readily detectable. [5][6][7][8][9][10] These findings indicate a specific inhibition of B-cell responses by maternal antibodies. In the absence of experimental data, inhibition of B cells has been postulated to be the result of physical blockage of epitopes by maternal antibodies (epitope masking 11 ). This model is based on antibody feedback mechanism studies. 11,12 In these studies, passive transfer of IgG suppresses the B-cell response against sheep red blood cells. Epitope masking leads to epitope-specific suppression at lower antibody concentrations, whereas at higher antibody concentrations also nonepitopespecific inhibition was observed and explained by steric hindrance. 13 A proposed alternate mechanism is based on the only inhibitory receptor of the IgG binding Fc receptor family, Fc␥-IIB receptor (Fc␥RIIB). On B cells, cross-linking of Fc␥RIIB to the B-cell receptor (BCR) through antigen/antibody complexes leads to inhibition of activation and antibody secretion. 12,[14][15][16] This mechanism was dismissed for the antibody feedback model because IgG is inhibitory in Fc-receptor knockout mice, 17 an IgG3 isotype antibody that in the mouse does not bind to Fc␥RIIB can be inhibitory, 18,19 and in some studies F(abЈ) 2 fragments can also inhibit B-cell responses. 17,20,21 In summary, these studies provide evidence for epitope masking as the main mechanism of inhibition of antibody responses in the antibody feedback model. Whether the...

Measles virus (MV) vaccine effectively protects seronegative individuals against infection. However, inhibition of vaccine-induced seroconversion by maternal antibodies after vaccination remains a problem, as it leaves infants susceptible to MV infection. In cotton rats, passive transfer of MV-specific IgG mimics maternal antibodies and inhibits vaccine-induced seroconversion. Here, we report that immunization in the presence of passively transferred IgG inhibits the secretion of neutralizing antibodies but not the generation of MV-specific B cells. This finding suggested that MV-specific B cells require an additional stimulus to mature into antibody-secreting plasma cells. In order to provide such a stimulus, we generated a recombinant Newcastle disease virus (NDV) expressing the MV hemagglutinin (NDV-H). In contrast to MV, NDV-H induced high levels of type I interferon in plasmacytoid dendritic cells and in lung tissue. In cotton rats immunized with NDV-H, neutralizing antibodies were also generated in the presence of passively transferred antibodies. In the latter case, however, the level and kinetics of antibody generation were reduced. In vitro, alpha interferon stimulated the activation of MV-specific B cells from MV-immune spleen cells. NDV infection (which induces alpha interferon) had the same effect, and stimulation could be abrogated by antibodies neutralizing alpha interferon, but not interleukin 6 (IL-6). In vivo, coapplication of UV-inactivated MV with NDV led to increased MV-specific antibody production in the presence and absence of passively transferred antibodies. These data indicate that MV-specific B cells are being generated after immunization in the presence of maternal antibodies and that the provision of alpha interferon as an additional signal leads to antibody secretion.

Maternal antibodies inhibit seroconversion and the generation of measles virus (MeV)-specific antibodies (both neutralizing and non-neutralizing antibodies) after vaccination whereas T cell responses are usually unaffected. The lack of seroconversion leaves individuals susceptible to vaccine-preventable infections. Inhibition of antibody secretion is due to the inhibition of B cells through a cross-link of the B cell receptor with the inhibitory FcγIIB receptor (CD32) by maternal antibody/vaccine complexes. Here, we demonstrate that a combination of TLR-3 and TLR-9 agonists induces synergistically higher levels of type I interferon in vitro and in vivo than either agonist alone. The synergistic action of TLR-3 and TLR-9 agonists is based on a feedback loop through the interferon receptor. Finally, we have identified CD21 as a potential receptor for interferon α on B cells which contributes to interferon α-mediated activation of B cells in the presence of maternal antibodies. The combination leads to complete restoration of B cell and antibody responses after immunization in the presence of inhibitory MeV-specific IgG. The strong stimulatory action of type I interferon is due to the fact that type I interferon uses not only the interferon receptor but also CD21 as a functional receptor for B cell activation.

Many RNA and DNA viruses activate serine-threonine kinase AKT to increase virus replication. In contrast, measles virus (MV) infection leads to downregulation of AKT. This is thought to be beneficial for the virus because it correlates with immune suppression. To determine whether this is a sacrifice for the virus, we used a recombinant virus and transfected cells expressing constitutively active AKT and evaluated its effect on virus replication. In vitro, overexpression of AKT did not influence virus replication but did affect (cell-type dependent) virus release. In vivo, the recombinant virus did not abrogate inhibition of proliferation of spleen cells from MV-infected cotton rats.

Immunization of neonates is problematic because of the immaturity of their immune system and the presence of maternal antibodies, both of which affect B cell responses. We tested the effects of co-administration of measles vaccine with a combination of TLR-3 (pI:C) and TLR-9 (ODN2216, optimized for human TLR-9) agonists on the ability to induce an effective immune response in neonatal cotton rats. TLR-9 expression in cotton rat lymphocytes was at the same low level as in human lymphocytes, which is in contrast to mice that express higher levels. TLR-3 expression levels were comparable between cotton rats, mice and humans. A combination of TLR-3 and TLR-9 agonists synergistically induced high levels of type I interferon in neonatal spleen cells and higher levels of IL-10 as compared to adult spleen cells. Previously, it was shown that type I interferon stimulates B cell generation and antibody secretion in vitro and in vivo, and that IL-10 has immunomodulatory effects. The simultaneous induction of both type I interferon and IL-10 indicated that this immunization regimen could be both effective and safe. Neonatal cotton rats did not generate neutralizing antibodies after measles vaccination in the first week of life (although a T cell response was detectable). However, co-administration of the TLR-3 and TLR-9 agonist combination with measles vaccine in neonatal cotton rats induced neutralizing antibody responses comparable to those after adult immunization. This immunization regimen was also effective in neonatal cotton rats in the presence of natural maternal antibodies, although antibody titers were lower than after immunization in the absence of maternal antibodies.