PspA binds necroptotic cells to cause disease and transmit

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Influenza A virus (IAV) profoundly enhances the susceptibility of lung epithelial cells for pneumolysin-mediated necroptosis. Briefly, pneumolysin is the pore-forming toxin produced by Streptococcus pneumoniae (Spn), whereas necroptosis is a caspase-independent form of programmed cell death that results in cell lysis. Herein, we will determine the molecular basis and full biological consequence of a new key observation: Spn binds to necroptotic respiratory epithelial cells via Pneumococcal surface protein A (PspA). Briefly, our preliminary results show that PspA binds to host-derived (h)GAPDH on dying cells and this property directly contributes to IAV/Spn disease severity. Furthermore, Spn/sloughed epithelial cell aggregates formed in the nasopharynx likely promote Spn transmission to a naive host. Herein we test the hypothesis that during IAV/Spn superinfection a high level of epithelial cell necroptosis occurs that promotes PspA-mediated binding to cells. This property directly enhances Spn outgrowth and promotes Spn transmission. AIM 1: Determine the molecular basis for PspA-mediated adhesion to necroptotic lung epithelial cells (LEC). Spn adhesion to LEC is PspA-dependent, enhanced when cells undergo necroptosis, and mediated by PspA binding to hGAPDH found on the surface of dying cells. We will identify the domain of PspA responsible for hGAPDH binding, how conserved this domain is across sequenced strains of Spn, and the affinity of representative PspA variants to hGAPDH. We will create and test the ability of isogenic mutants in the PspA hGAPDH binding motif to bind dying LEC. We will identify the region of hGAPDH that is bound by PspA. AIM 2: Determine the biological impact of PspA-mediated adhesion on IAV/Spn pneumonia severity. PspA is required for the enhanced disease severity that occurs during IAV superinfection. We will determine if hGAPDH binding alters the canonical role of PspA, which is inhibiting lactoferricin-mediated killing. We will determine how PspA-binding influences the localization of Spn within the airway and how this is impacted by co-infection with IAV, neutralization of pneumolysin, or blocking of necroptosis. We will determine if PspA- mediated binding of Spn to dying LEC promotes their outgrowth in otherwise nutrient restricted conditions. We will determine how antibody against the hGAPDH-binding motif of PspA alters overall disease progression. AIM 3: Determine the requirement of PspA mediated adhesion to colonization and transmission. Spn binds to dying mucosal epithelial cells during colonization and they are together expelled in nasal secretions. Sloughed Spn/host cell aggregates are infectious and thought to promote Spn survival on fomites. We will determine the requirement for PspA on the formation of Spn/host cell aggregates, moreover, how pneumolysin, IAV superinfection, and necroptosis inhibition influences their number in secretions. We will determine the requirement of PspA hGAPDH binding for transmission to a naive host. We will determine if antibody against PspA and/or pneumolysin reduces transmission rates and shortens the length of colonization.