Trauma-induced heme release increases susceptibility to bacterial infection

Infection is a common complication of major trauma that causes significantly increased morbidity and mortality. The mechanisms however, linking tissue injury to increased susceptibility to infection remain poorly understood. To study this relationship, we present a novel murine model where a major liver crush injury is followed by bacterial inoculation into the lung. We find that such tissue trauma both impaired bacterial clearance and was associated with significant elevations in plasma heme levels. While neutrophil (PMN) recruitment to the lung in response to Staphylococcus aureus was unchanged after trauma, PMN cleared bacteria poorly. Moreover, PMN show >50% less expression of TLR2, which is responsible, in part, for bacterial recognition. Administration of heme effectively substituted for trauma. Last, day 1 trauma patients (n=9) showed similar elevations in free heme to that seen after murine liver injury and circulating PMN showed similar TLR2 reduction compared to volunteers (n=6). These findings correlate to high infection rates.