PROJECT SUMMARY/ABSTRACT Aspergillus species are ubiquitous molds that cause clinical disease in hosts with impaired immunity or abnormal mucosal defenses. In particular, individuals with neutropenia or neutrophil defects are particularly susceptible to invasive pulmonary aspergillosis (IPA), a life-threatening respiratory infection with limited treatment options. Neutrophils play a key role in host defenses against this pathogen but little is known about the contribution of these cells beyond their direct microbicidal role. This project examines how neutrophils influence the development of adaptive immunity to Aspergillus in order to better understand mechanisms of host immunity to Aspergillus during neutropenia. In the preliminary data for this proposal, we report that neutrophil- depleted mice have an exaggerated IL-17A response compared to neutrophil-sufficient mice after Aspergillus. We show that following challenge with Aspergillus, IL-17A-producing CD4+ tissue resident memory (TRM) cells persist in the lung up to three weeks after challenge in neutrophil-depleted mice, and that mice that were neutrophil-depleted during a primary Aspergillus challenge are protected from a secondary challenge with Aspergillus when compared to mice that were neutrophil-sufficient during primary challenge. We therefore seek to test the central hypothesis that, in the absence of neutrophils, Aspergillus induces an exuberant IL-17 response which leads to the persistence of IL-17A-producing CD4+ tissue resident memory cells, and that these cells confer protection against subsequent infection with Aspergillus. To test this hypothesis, we plan to pursue the following specific aims: Aim 1: Determine if CD4+ tissue resident memory lymphocytes protect against Aspergillus infection during neutropenia and Aim 2: Identify pathways involved in neutrophil-independent IL- 17A-mediated protection during rechallenge with Aspergillus. The project brings an innovative approach to the study of this infection through the use of a dual challenge model of IPA which mimics repeated exposure to this fungal pathogen in human hosts and the characterization of a novel and previously undescribed protective role for TRM in the setting of neutrophil-depletion. The proposed studies are relevant to public health by defining a new mechanism of host defense against an important human pathogen that should allow for future development of novel therapeutics or preventative strategies.