MPO and osteomyelitis: To begin to determine the mechanisms underlying the substance P-mediated increase in local MPO levels in S. aureus infected bone tissue, we assessed the early localized protein expression of the leukocyte-attracting chemokines, CCL2, CCL4, CCL7, CXCL1, CXCL2, and CXCL5, in the absence and presence of aprepitant treatment in our murine model of osteomyelitis.