Localized antibody responses in influenza virus-infected mice

Abstract

Background: Traditionally, vaccine-mediated protective responses were quantified by measuring the level of increase of influenza virus–specific antibodies circulating in blood. However, virus-specific antibodies in serum do not necessarily correlate with protection in vaccinees receiving intranasally administrated live attenuated influenza vaccines (LAIVs). Local mucosal and cellular immune responses are believed to be the protective mechanism induced by LAIVs. Recently, antibody secreting cell (ASC) responses derived from peripheral blood mononuclear cells (Cherukuri A, et al. Vaccine. 2012;356:685-696) of ferrets and antibody obtained via human nasal washes, but not the systemic serum (Barria MI, et al. J Infect Dis. 2013;207:115-124), were found to better correlate with B-cell responses induced by LAIV. ASCs are found in the upper and lower respiratory tract in influenza infections and play an important role in combating influenza infections. Analyses of antigenspecific B-cell receptors on these ASCs were limited by cell-based assays such as ELISPOT or FACS probe by hapten or B-cell tetramers. Although the localized mucosal and systematic ASC responses of Influenza A virus–infected mice are different (Joo HM, et al. Vaccine. 2010;28:2186-2194), direct comparisons of antibodies secreted by ASCs at these locations are lacking. Here, we isolate antibodies secreted by ACSs at multiple anatomical sites and characterize the epitope specificity and other properties of these antibodies systematically. Materials and Methods: Mice intranasally infected with influenza virus (A/HK/68) were used as a model. Lymphocytes from different nodes (eg, cervical lymph nodes [CLNs], which drain the upper respiratory tract, and mediastinal lymph nodes [MLNs], which drain the lower respiratory tract) and from the spleen of infected mice were harvested for cell cultures at days 3, 7, and 28 post-infection. In addition, supernatants of nasal washes, bronchoalveolar lavage fluid, and serum of the mice were harvested. Antibodies secreted by the cultured cells and antibody presented in the harvested body fluids were characterized by influenza A– specific isotyping ELISA, micro-neutralization assay, as well as fine epitope mapping assay using a yeast surface display library for H3 hemagglutinin. Results: Antibody in lymphocyte supernatants (ALS) from cultured cells of MLNs and spleen and antibody in serum were found to be positive for influenza virus–specific IgM at day 3 post-infection. Nasal washes, bronchoalveolar lavage fluid, and ALS from CLNs and MLNs were found to be IgA-positive at day 7 post-infection. High IgG1 and IgG2a responses were detected in ALS from MLNs at day 7 post-infection. The control ALS from cells derived from iliac lymph nodes, which drain the mouse tail but not the respiratory tract, was negative for influenza-specific IgA, IgG1, IgG2a, and IgM throughout the study. Bronchoalveolar lavage fluid collected at day 28 post-infection and ALS from MLNs collected at day 7 post-infection demonstrated of ALS from MLNs collected at day 7 and of serum collected at day 28 post-infection was also performed. The antibody repertoire mappings were comparable and both identified a major immunodominant antigenic site in HA1 and a weaker antigenic site located in HA2. However, two additional antigenic sites were identified in the mapping of ALS from MLNs collected at day 7 when compared with the mapping of the serum obtained at day 28 post-infection. Conclusions: This study illustrated the feasibility of recovering ASC specificity at different localizations after influenza A challenge. With the use of cell-free supernatant, the properties of the ASC-secreted antibodies can be further characterized by various methods traditionally used for serum. The method described will provide information about influenza A–induced antibody responses early post-infection, ie, at the time when the virus was cleared.