Vaccination is currently the best method for preventing influenza virus infection and for reducing disease severity. Unfortunately, current vaccine regimens suffer from several major drawbacks and efficacy can vary dramatically year to year. Improved vaccine strategies are desperately needed. Preclinical testing is critical for evaluating potential safety and immunogenicity of next generation influenza virus vaccine candidates. Mice are an ideal first model organism because there are a wealth of reagents and genetic tools allow refined experimental approaches and the capacity to do rigorous mechanistic studies, longitudinal kinetic analyses, sampling of mucosal tissue, and assessment of protection through lethal challenge, all at low cost. While mice are not natural hosts for influenza virus infection they can still recapitulate many aspects of human disease. Additionally, some strains are directly pathogenic while others can be readily mouse adapted. Unfortunately, many therapeutic interventions that were successful in mice have failed to translate to humans. This could be due to several factors including species genetic differences and/or environmental factors. We have previously demonstrated that exposing SPF laboratory mice to diverse pathogens from pet store mice recapitulates many of the human cellular and molecular immune signatures absent in standard mouse models. Preliminary studies demonstrate that heterologous protection and influenza-specific serum antibody isotypes are dramatically altered in ‘dirty’ cohoused (CoH) compared to SPF mice, which more closely resemble what has been observed in humans. We hypothesize that CoH mice will better predict immune responses to influenza infection and vaccination compared to standard mouse models. We will test this hypothesis in three aims. Aim 1 will compare the immune response to a panel of adjuvants used in humans between SPF and CoH mice. Where available, we will compare transcriptional profiles to human data. Aim 2 will determine how a diverse infection history impacts the generation, function, and durability of influenza-specific memory T cells, including extensive analyses of lung immune surveillance.to influenza virus infection and vaccination. Aim 3 will determine the immune response to live attenuated, seasonal split and adjuvanted split vaccinations in SPF and CoH mice. This aim will also evaluate the immune response to less immunogenic targets of universal influenza vaccines. Collectively, this proposal will rigorously evaluate the immune response to influenza virus infection and vaccination in mice with diverse infection histories and immune signatures that more closely align with humans. We propose that the intrinsic advantages of the CoH mouse model will significantly complement ferret or other large animal models to improve the pipeline for preclinical testing and enhance translation potential of next generation influenza virus vaccines.