Pulmonary dendritic cells and CD8 T cells facilitate protection following influenza A virus vaccination and infection

The severe disease associated with seasonal epidemics of influenza A virus (IAV), as well as pandemic outbreaks, have highlighted the necessity for novel, broadly cross-reactive vaccination and therapeutic strategies against IAV. Our studies have focused on the contribution of IAV-specific CD8 T cells to mediating protection following IAV vaccination and infection as IAV-specific CD8 T cells are required for clearance of IAV. Further, IAV-specific CD8 T cells are typically cross-protective as they are generally directed at highly conserved areas of IAV. Recently, influenza virus-like particles (VLPs) have been developed from recombinant baculoviruses containing influenza proteins hemagglutinin (HA) and/or neuraminidase (NA) on the surface and matrix (M1) in the VLP core. Influenza VLPs induce potent antibody responses and have been shown to provide protection from morbidity and mortality during lethal homo- and hetero-subtypic IAV challenge. This suggests that conserved, VLP-induced CD8 T cell responses may also contribute to the overall protective ability of VLPs. However, whether influenza VLPs can induce influenza-specific CD8 T cell responses and if these T cells are protective during IAV challenge remains unknown.

Here, I demonstrate that a single, intranasal vaccination with VLPs containing HA and M1 leads to a significant increase in HA₅₃₃-specific CD8 T cells in the lungs and lung-draining lymph nodes. Our results also indicate that HA₅₃₃-specific CD8 T cells primed by influenza VLP vaccination are significantly increased in the lungs following lethal IAV challenge. These VLP-induced memory CD8 T cells are critical in providing protection from lethality following subsequent challenge infections, as depletion of CD8 T cells leads to increased mortality, even when total, but not VLP-induced memory, CD8 T cell numbers have been allowed to recover prior to lethal dose IAV challenge. In addition, my studies also importantly demonstrate that these VLP-induced, HA₅₃₃-specific CD8 T cells aid in protection from high-dose, heterosubtypic IAV infections where CD8 T cell epitopes are conserved, but the targets of neutralizing antibodies have been destroyed.

This dissertation further elucidates the requirements for the regulation of the IAV-specific CD8 T cell response in the periphery (i.e. lung) by pDC and CD8α⁺ DC. Our studies have previously demonstrated that pDC or CD8α⁺ DC must present viral antigen in the context of MHC class I along with trans-presentation of IL-15 to effector, IAV-specific CD8 T cells in the lungs to protect the T cells from apoptosis and allow generation of the full magnitude CD8 T cell response needed to clear IAV infection. Herein, I demonstrate that in addition to antigen presentation and IL-15, costimulatory molecules on the surface of pDC and CD8α⁺ DC are also required. However, the specific costimulatory molecules required depends upon both the mouse strain utilized for IAV infection as well as DC subset.

In addition to costimulatory molecules, I also demonstrate that the requirement for pDC and CD8α⁺ DC to be infected differs in order for them to participate in this pulmonary rescue of the IAV-specific CD8 T cell response. While CD8α⁺ DC are able to efficiently cross-present exogenous antigen, pDC must be directly infected and utilize the endogenous, direct antigen presentation pathway to present viral antigen to IAV-specific CD8 T cells in the lungs during IAV infection. These data suggest there are distinct differences between pDC and CD8α⁺ DC in their mechanism of regulating the pulmonary IAV-specific CD8 T cell response, which had not been previously appreciated.

Together, the results presented herein further detail the mechanism of regulation of effector IAV-specific CD8 T cells by DC as well as the contribution of IAV-specific CD8 T cells to a novel, IAV VLP vaccination strategy. These findings highlight the importance of IAV-specific CD8 T cells in mediating protection following IAV vaccination and infection.