Mechanisms of host protection against pathogen-associated proteases in acute lung injury

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Project Summary/Abstract: The broad, long term objective is to identify host-protective mechanisms that counter pathogen-initiated lung inflammation and injury. Many pathogens secrete proteases to cause direct damage to the lung, but bacteria, fungi and viruses can also co-opt host proteases to increase pathogenicity and promote lung injury. We have studied Pseudomonas aeruginosa (PA) as a model of infection-induced injury to probe how a pathogen-encoded protease called Pseudomonas elastase LasB, a metalloprotease released into the extracellular milieu by the PA type II secretion system, induces lung tissue damage and secondary trigger of host-derived serine proteases such as neutrophil elastase (NE). We previously identified thrombospondin-1 (TSP-1), a matricellular protein secreted by a variety of cells following injury that disarms both pathogen-encoded LasB and host protease NE, to limit lung injury and inflammation. Key questions that have arisen from this work is how an unregulated proteolytic environment drives excessive inflammatory response and dysregulated repair following injury, and what are the host factors that calibrate this response in the lung. Our preliminary findings suggest that a feed-forward neutrophilic inflammatory response occurs in the proteolytic environment of PA infection through N-terminal processing of IL-36γ that is exaggerated in the absence of TSP-1. Moreover, platelet TSP-1 appears protective against PA-induced lung injury, but the precise mechanism related to TSP-1's role at the alveolar-capillary interface remains unknown. Furthermore, we show that PA elastase activity in clinical strains confer excessive inflammation and injury in mice and is associated with worse clinical outcomes when compared with non-elastase producers. Based upon these findings, we propose the following aims utilizing genetically deficient mice, cell-specific conditional knockout systems, and PA clinical respiratory isolates obtained from the ICU to (1) elucidate the mechanisms by which TSP-1 counters the hyperinflammatory response mediated by proteolytic processing of the pro-inflammatory cytokine IL-36γ; (2) examine the contribution of TSP-1 and platelet TSP-1 in protection against alveolar barrier disruption and stabilization of the early provisional matrix following lung injury; (3) determine whether PA ICU respiratory isolates with elastolytic properties drive unwarranted inflammation and persistent tissue injury in the susceptible host. A better understanding of host biology during severe respiratory infection could prove useful in the rational design of targeted therapeutics against pathogen-derived proteases and deregulated host inflammation as adjuncts to current antimicrobial agents and supportive pulmonary and critical care management.