The importance of antiviral CD8+ T cell recognition of alternative reading frame (ARF)–derived peptides is uncertain. In this study, we describe an epitope (NS1-ARF21–8) present in a predicted 14-residue peptide encoded by the +1 register of NS1 mRNA in the influenza A virus (IAV). NS1-ARF21–8 elicits a robust, highly functional CD8+ T cell response in IAV-infected BALB/c mice. NS1-ARF21–8 is presented from unspliced NS mRNA, likely from downstream initiation on a Met residue that comprises the P1 position of NS1-ARF21–8. Derived from a 14-residue peptide with no apparent biological function and negligible impacts on IAV infection, infectivity, and pathogenicity, NS1-ARF21–8 provides a clear demonstration of how immunosurveillance exploits natural errors in protein translation to provide antiviral immunity. We further show that IAV infection enhances a model cellular ARF translation, which potentially has important implications for virus-induced autoimmunity.
Plant immunity relies on the perception of Microbe-Associate Molecular Patterns (MAMPs) from invading microbes to induce defense responses that suppress attempted infections. It has been proposed that MAMP-triggered immunity (MTI) suppresses bacterial infections by suppressing the onset of bacterial virulence. However, the mechanisms by which plants exert this action are poorly understood. Here, we showed that MAMP perception in Arabidopsis (Arabidopsis thaliana) induces the accumulation of free amino acids in a salicylic acid (SA)-dependent manner. When co-infiltrated with Glutamine and Serine, two of the MAMP-induced highly accumulating amino acids, Pseudomonas syringae pv. tomato DC3000 expressed low levels of virulence genes and failed to produce robust infections in otherwise susceptible plants. When applied exogenously, Glutamine and Serine directly suppressed bacterial virulence and growth, bypassing MAMP perception and SA-signaling. In addition, an increased level of endogenous Glutamine in the leaf apoplast of a gain-of-function mutant of Glutamine Dumper-1 rescued the partially compromised bacterial virulence- and growth-suppressing phenotype of the SA-induced deficient-2 (sid2) mutant. Our data suggest that MTI suppresses bacterial infections by delaying the onset of virulence with an excess of amino acids at the early stages of infection.
Plants live under the constant challenge of microbes that probe the environment in search of potential hosts. Plant cells perceive microbe-associated molecular patterns (MAMPs) from incoming microbes and activate defense responses that suppress attempted infections. Despite the substantial progress made in understanding MAMP-triggered signaling pathways, the downstream mechanisms that suppress bacterial growth and disease remain poorly understood. Here, we uncover how MAMP perception in Arabidopsis (Arabidopsis thaliana) elicits dynamic changes in extracellular concentrations of free L-amino acids (AAs). Within the first 3 hours of MAMP perception, a fast and transient inhibition of AA uptake produces a transient increase in extracellular AA concentrations. Within 4 and 12 hours of MAMP perception, a sustained enhanced uptake activity decreases the extracellular concentrations of AAs. Gene expression analysis showed that salicylic acid (SA)-mediated signaling contributes to inducing the expression of AA/H+ symporters responsible for the MAMP-induced enhanced uptake. A screening of loss-of-function mutants identified the AA/H+ symporter Lysin/Histidine Transporter-1 (LHT1) as an important contributor to MAMP-induced enhanced uptake of AA. Infection assays in lht1-1 seedlings revealed that high concentrations of extracellular AA promote bacterial growth in the absence of induced defense elicitation but contribute to suppressing bacterial growth upon MAMP perception. Overall, the data presented in this study reveal a mechanistic connection between MAMP-induced plant defense and suppression of bacterial growth through the modulation of AA transport activity.
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