Critical Role of TLR7 Signaling in the Priming of Cross-Protective Cytotoxic T Lymphocyte Responses by a Whole Inactivated Influenza Virus Vaccine

Natalija Budimir,David A Price,Jan Wilschut,Tjarko Meijerhof,Louis Boon,Simke Waijer,Emma Gostick,Anke Huckriede,Aalzen De Haan

doi:10.1371/journal.pone.0063163

Abstract

Current influenza vaccines fail to induce protection against antigenically distinct virus strains. Accordingly, there is a need for the development of cross-protective vaccines. Previously, we and others have shown that vaccination with whole inactivated virus (WIV) induces cross-protective cellular immunity in mice. To probe the mechanistic basis for this finding, we investigated the role of TLR7, a receptor for single-stranded RNA, in induction of cross-protection. Vaccination of TLR7−/− mice with influenza WIV failed to protect against a lethal heterosubtypic challenge; in contrast, wild-type mice were fully protected. The lack of protection in TLR7−/− mice was associated with high viral load and a relative paucity of influenza-specific CD8+ cytotoxic T lymphocyte (CTL) responses. Dendritic cells (DCs) from TLR7−/− mice were unable to cross-present WIV-derived antigen to influenza-specific CTLs in vitro. Similarly, TLR7−/− DCs failed to mature and become activated in response to WIV, as determined by the assessment of surface marker expression and cytokine production. Plasmacytoid DCs (pDCs) derived from wild-type mice responded directly to WIV while purified conventional DCs (cDCs) did not respond to WIV in isolation, but were responsive in mixed pDC/cDC cultures. Depletion of pDCs prior to and during WIV immunization resulted in reduced numbers of influenza-specific CTLs and impaired protection from heterosubtypic challenge. Thus, TLR7 plays a critical role in the induction of cross-protective immunity upon vaccination with WIV. The initial target cells for WIV appear to be pDCs which by direct or indirect mechanisms promote activation of robust CTL responses against conserved influenza epitopes.

Highlights

Influenza continues to represent a major global health burden [1]
We describe the role of the innate pattern recognition receptor Toll-like receptor 7 (TLR7) in whole inactivated virus (WIV)-mediated induction of cytotoxic T lymphocyte (CTL) responses and heterosubtypic cross-protection against influenza infection
In contrast to wt mice, TLR72/2 mice could not be protected from heterosubtypic virus challenge by immunization with WIV, indicating that TLR7 signaling plays a critical role in WIVinduced cross-protective immunity

Summary

Introduction

Influenza continues to represent a major global health burden [1]. vaccination is the cornerstone of control, current seasonal influenza vaccines offer only narrow protection. Vaccine-induced immune responses are largely strain-specific and fail to protect against antigenically-drifted or newly-emerging shifted viruses with pandemic potential. These limitations underscore the need for novel, cross-protective influenza vaccines [4,5]. We and others have shown that vaccination with whole inactivated virus (WIV) induces cross-protection against lethal heterosubtypic infection in mice, in contrast to subunit and split virion vaccines, which did not show any cross-protective capacity [6,7]. This WIV-induced cross-protective effect is mediated principally by CD8+ cytotoxic T lymphocytes (CTLs). WIV-induced cross-protection against heterosubtypic influenza infection in mice depends on the consequent CTL response, which operates to reduce viral load in the lungs after lethal dose challenge

Methods

Results

Conclusion