Streptococcus pneumoniae (the pneumococus) ranks among the five leading causes of infectious death worldwide. This single species accounts for a large proportion of respiratory tract (pneumonia, otitis media) and invasive (sepsis, meningitis) bacterial diseases. The development of efficacious conjugate vaccines for children has been based upon protection against the few capsule types that commonly cause disease. However, this strategy is now being eroded by the selection for and replacement by non-vaccine types. The initial step in the interaction of the pneumococcus with its host is colonization of the nasopharynx. Experience with conjugate vaccines has demonstrated that interrupting colonization results in herd immunity that amplifies prevention of disease in the population. To better understand the biology of colonization, during the prior funding period, we utilized a murine model to characterize bacterial and host factors that allow for pneumococcal persistence and its eventual clearance from the mucosal surface. These studies show that carriage, much like infection in normally sterile sites, induces acute inflammation. However, opsonophagocytic killing by this neutrophil-dominated response is not completely effective in clearing colonizing organisms. Complete clearance of carriage requires cellular immunity, and is mediated by the gradual Th17-dependent influx of tissue macrophages into the nasal lumen. The key to the success of the pneumococcus in colonization (and disease), therefore, is its ability to evade the initial inflammatory response it elicits. In specific aim#1, we will identify and characterize the complete set of pneumococcal genes and gene products contributing to evasion of opsonization and phagocytic killing by neutrophils in vitro and in vivo. The key to resolution of pneumococcal carriage appears to be recognition and uptake by macrophages. It remains unclear how the unique population of upper respiratory tract macrophages recognizes colonizing pneumococci. In specific aim#2, we will identify the macrophage receptor(s) (including scavenger receptors and C-type lectins) required for non-opsonic clearance during colonization. Finally, in specific aim#3, we will examine the role of capsule type in colonization and whether type-specific differences are explained by i) evasion of opsonophagocytic clearance by neutrophils and ii) recognition and uptake by upper respiratory tract macrophages. Together these studies will provide mechanistic insight into the three main features of pneumococcal carriage; why it is common, why it is transient, and why it varies greatly among isolates by capsule type.