MMP9 and infection: Furthermore, Mmp9+ neutrophils were more efficiently recruited to sites of bacterial infection (67.3% Mmp9+ vs. 32.7% Mmp9- of all LysC+ neutrophils at infection site; n = 33; paired t test; P = 0.002) (Fig. 2g) and the overall frequency of Mmp9+ cells was increased during infection (mean = 56.4% Mmp9+ in infected and 39.2% Mmp9+ in PBS treated, n = 5; paired t test; P = 0.009) (Supplementary Fig. 2d, e).