Investigation of Neutrophils’ Role During Influenza A Virus Infection

Submission note: A thesis submitted in total fulfillment of the requirements for the degree of Doctor of Philosophy to the Department of Biochemistry and Genetics, School of Molecular Science, College of Science, Health and Engineering, La Trobe University, Victoria, Australia.

Influenza A virus (IAV) infection is highly contagious and can cause severe respiratory disease with significant morbidity and mortality. Neutrophils are rapidly deployed innate immune cells serving as an important immune barrier and bridging innate and adaptive immunity. Although a causal relationship exists between excessive neutrophil recruitment and IAV-induced lung injury, little is known about influenza lethality-associated neutrophil effector proteins following IAV infection. Here we first used semi-quantitative proteomics to map comparative changes in the proteomes of bone marrow, blood and lung neutrophils at homeostasis and following a lethal IAV infection. Type I interferon (IFN)-induced proteins are already elevated in the bone marrow neutrophils and further upregulated during migration to the infection site following IAV infection in C57BL/6J (B6) mice. Further, the absence of type I IFN signalling in Ifnar1 deficient mice impairs upregulation of these proteins and induces a significant increase in the number of lung infiltrating neutrophils compared with those in B6 WT mice following IAV infection. Interestingly, the excessive neutrophilia is suppressed by merely preserving type I IFN-responding lung alveolar macrophages in a thoracic shielding chimera model, indicative of the necessity of type I IFNs in regulating neutrophil recruitment. As neutrophils upregulate MHC molecules and express IAV proteins during IAV infection, their potential in antigen presentation is subsequently assessed. In agreement with previous studies, neutrophils are able to activate CD8+ T cells in an MHC I- and TAP-dependent manner. In addition, neutrophils can present antigens to CD4+ T cells at a low but reproducible level in vitro and ex vivo. Further investigation is required to clarify the impact of in vivo MHC II-dependent antigen presentation by neutrophils on the overall cellular and humoral immunity to IAV. Altogether, neutrophils appear to play a diverse and prominent role in shaping up both innate and adaptive immunity than previously anticipated.