Microbial infection often begins with colonization. Success for an organism requires persistence during colonization for a sufficient period to allow for host-to-host transmission. For the leading bacterial pathogen Streptococcus pneumoniae (Spn), colonization occurs along the mucosal surfaces of the upper respiratory tract. Extensive experience with vaccination has shown that interruption of colonization is the key to reducing the burden of all Spn disease. Our ongoing project examines the biology of Spn colonization, which occurs in recurrent episodes that are prolonged (lasting weeks to months), variable in duration by serotype, and more common during early life. We have established models of infection in adult and infant mice that recapitulate each of these characteristics of Spn colonization in the natural host. Our preliminary data using these models establish the importance of two bacterial factors, the pore-forming toxin pneumolysin (Ply) and capsule, the determinant of serotype. Additionally, we have shown critical roles for three host factors, the alarmin IL-1α, the cytokine IL-17, and the macrophage scavenger receptor MARCO. This proposal examines the relationship between these factors. In Aim#1, we test the hypothesis that IL-1α is released in a Ply-dependent manner following Spn uptake by neutrophils and triggers the differentiation/proliferation of CD4+ T cells to express IL-17. A further aspect of this aim is that attenuated neutrophil responses during infancy delay the IL-17 response resulting in more persistent colonization. Elevated IL-17 levels are required to drive an increase in numbers of macrophages derived from the pool of inflammatory monocytes. In Aim#2, we test the contribution of the subset of macrophages expressing MARCO as the effectors of clearance and whether MARCO interactions with the Spn capsule dictate clearance dynamics. In this aim, we generate a MARCO humanized mouse to determine if this interaction is responsible for differences in host- specific patterns of colonization among serotypes. In Aim#3, we explore how inflammation induced by Spn colonization promotes exit of the organism from its host (shedding) at levels permissive for transmission in our animal model. Specifically, we test the hypothesis that Ply- dependent stimulation of Type I interferons increase the flow of nasal secretions and Spn shedding. We also test the hypothesis that Spn takes advantage of the recruitment of neutrophils and their ability to bind to the organism to `hitch a ride' out of the host with purulent secretions. The overall goal of this project is an understanding of these determinants of colonization with a long-term view towards a more comprehensive approach to address the continuing public health problem of the pneumococcus.