Live Attenuated Influenza Vaccine in Children Induces B-Cell Responses in Tonsils

Influenza continues to be an important infectious disease, with annual epidemics claiming up to half a million lives and causing a significant economic burden [1]. Annual seasonal immunization with inactivated trivalent influenza vaccine(TIV) is the most widely used and cost-effective measure for limiting the impact of influenza. An alternative vaccination strategy is to use live attenuated influenza vaccine(LAIV), which was licensed in Europe for children (2–17 years old) in 2012 [2]. LAIV is genetically stable and attenuated to have limited replication in the upper respiratory tract. Meta-analysis of LAIV efficacy studies have demonstrated up to 80% efficacy to matched strains in children <6 years old and 40% efficacy in adults [3–5]. However, the immunological mechanisms and correlates of protection of LAIV are not yet clearly understood. Serum antibody levels are known to underestimate the protection achieved by LAIV [6]. Other immunological mechanisms are thought to be involved in conferring protection after intranasal immunization, and mucosal responses warrant further investigation. Tonsilsare local lymph nodes serving the upper respiratory tract and are a collection of mucosa-associated lymphoid tissues. They consist of a pharyngeal ( adenoid) and lingual tonsiland 2 tubal and palatine tonsils(referred to as tonsils). Tonsilsplay a key role in eliciting mucosal immune responses against respiratory pathogens [7], but their role in eliciting immune response against antigens delivered by intranasal vaccination is not widely reported. Delivery of LAIV via the intranasal route is perhaps the most efficient way of boosting mucosal immunity at the site of viral entryand induces a weaker systemic response as compared to that of TIV [8]. The tonsil's location at the site of entry into the upper respiratory tract suggests a major role in anti-influenza immunity. The tonsillar epithelium is composed of deep crypts to maximize the surface area exposed to antigens, with Langerhans and M cells transporting luminal antigens into the tonsillar tissue [9, 10]. Evidence suggests that tonsilshave functional T cells and can provide B cells for mucosal effector sites, including upper airway mucosa and lacrimal and salivary glands [11, 12]. In this unique study, we have vaccinated young children with LAIV at specific time points prior to elective tonsillectomy. We aimed to characterize the early local immune responses after LAIV vaccination, using the blood, saliva, and tonsilsobtained from these children. We have previously reported that the systemic B- and T-cell responses persisted for 1 year after LAIV vaccination in some children [13]. Here we show that the LAIV induces early salivary antibody and B-cell responses in the tonsils, which may play a significant role in mediating protection against influenza.