The dynamics of the B cell response during influenza A virus infection

Abstract

Although vaccination remains the most effective method of managing influenza epidemics, there is still much that remains to be characterized about humoral immunity against the varying contexts in which influenza infection can occur. With the continuous subversion of humoral immunity by seasonal influenza through antigenic drift and the potential of zoonotic influenza viruses adapting and spreading through human populations through antigenic shift, improving our understanding of B cell immunity against different types of influenza infection could provide important insights into improving management of epidemics and vaccine formulations. In order to understand B cell responses during influenza infection, the well-characterized C57BL/6 mouse model was used to investigate and compare humoral responses in the context of different influenza infection histories. Markers that identified specific B cell subsets such as germinal centre (GC) B cells and plasmablasts were analysed by flow cytometry paired with influenza virus-specific B cell ELISPOT assays to investigate strain-specific antibody secreting responses within the same experiment. As the surface glycoprotein HA is thought to be the immunodominant response for B cell responses against influenza virus, the prediction is that the greater the antigenic differences between the HA of the first and second infecting strains, the more primary-like the response to the second strain would be. Primary and homologous secondary B cell responses in the mediastinal lymph node (MLN) and spleen were first characterized using this model to establish baseline responses against influenza virus before heterosubtypic infection was studied through infection of mice with H1N1A/Puerto Rico/8/34 (PR8) virus followed by H3N2 A/Udorn/305/72 (Udorn) virus 7 weeks later. Unexpectedly, a secondary-like plasmablast, GC B cell and Udorn-specific antibody secreting cells was observed during heterosubtypic infection, with earlier and higher magnitude B cell responses. These findings suggested a possible role for cross-subtype T cell memory in modulating B cell responses. The effect of antigenic drift on the B cell responses during influenza infection was then analysed with the same model. Mice were infected with H3N2 strains isolated between 1972 and 1979, representing different antigenic distance from a virus isolated in 1982 (Ph82). Seven weeks post infection mice were reinfected with Ph82 and the B cell response over the course of infection examined. It was found that infection of strains up to 10 years apart appeared to induce a secondary-like B cell response in the secondary lymphoid organs when compared to baseline primary and secondary responses against Ph82 virus. Prior infection with any H3N2 strain also resulted in minimal viral replication during the secondary challenge when compared to primary infection groups. However, data from both primary antibody inhibition and HA-specific B cell responses appears to suggest a narrower threshold of recognition, around a maximum of 3 years drift before serum and HA-specific responses cease to bind with other strains. Taken together, secondary-like B cell responses in both heterosubtypic and drift models of infection and in the case of drift responses, irrespective of reactivity of HA-specific B cells, appear to refute the hypothesis that virus-specific B cell responses would reflect antigenic relatedness between the HA of the infecting strains. Overall, data from this study identifies the diversity of the overall B cell response against influenza infection in the context of prior exposure to strains of different antigenic properties. Further study into the reactivity of these B cells against different influenza virus components and the role of memory T cells in the observed responses may provide important insights into the nature of host immunity against the ever-shifting target of influenza virus.