CD63 and bacterial pneumonia: Children with PARDS also had increased neutrophil surface expression of CD63, a marker of primary granule exocytosis (Fig. 1F), and sphingosine 1-phosphate receptor 3 (S1PR3), a protein that forms a heterodimer with the neutrophil IL8-induced chemotaxis receptor, CXCL1, and is detected with increased abundance in blood neutrophils from adults with bacterial pneumonia (Fig. 1G)16.