Project Summary Recent years have seen a re-emergence of pertussis as a major public health concern, despite successful mass vaccination programs. Critical pertussis disease causes over 150,000 deaths annually. Antibiotics are only effective if given before the onset of the characteristic cough. For these reasons, there is an urgent need for the development of new treatments. The long-term goal of our group is to develop anti-pertussis therapeutics. Our central hypothesis is that targeting tissue damage and inflammation induced by bacterial virulence factors will have a greater benefit than traditional bactericidal therapies. We have demonstrated the potential of sphingosine-1-phosphate receptor (S1PR) agonists in reducing pulmonary inflammation in mice and baboons. The objective of this work is to understand the beneficial elements of the S1PR agonist-mediated response to guide the next generation of anti- microbials. We identified two peptidoglycan recognition proteins (PGLYRP), -1, and -4 an antimicrobial protein, as differentially regulated by S1PR agonism. PGLYRPs elicit bactericidal activity through interactions with peptidoglycan (PGN). PGYLRP1 but not PGLYRP4 can stimulate inflammatory responses following PGN binding. We hypothesize that PGLYRP4 competes with PGLYRP1 to mediate inflammatory responses. Further, we propose that this is mediated by inhibition of host triggering receptor on myeloid cells 1 (TREM1). TREM-1 is an amplifier of inflammatory responses. It is upregulated following pattern recognition receptor activation and its activity amplifies inflammatory cytokine expression. TREM-1 ligands include PGLYRP1-PGN complexes. TREM-1 expression is reduced by S1PR agonism and inhibitor treated mice show reduced inflammatory pathology following B. pertussis infection. We hypothesize that TREM-1 acts as a checkpoint between beneficial bacterial control and detrimental inflammatory pathology.