A 28-year-old man presents with a 3-day history of painful swelling over his right shin after a minor abrasion while playing cricket. On examination, the area is warm, erythematous, edematous, and tender. There is no fluctuance or abscess. Gram stain of the exudate shows Gram-positive cocci in clusters. Culture grows Staphylococcus aureus (methicillin-sensitive). A skin biopsy shows acute inflammatory infiltrate with prominent neutrophils, dilated vessels with stasis, fibrin deposition in the dermis, and early fibroblast proliferation at the periphery. The exudate is serous-purulent in character. Which of the following mediators is MOST responsible for the vascular permeability and neutrophil chemotaxis observed in this acute inflammatory response?