Down-Regulated Circulating PMN Function after Injury Despite Enhanced p38 MAPK Activity

Abstract

Background

: Neutrophils (PMN) are initially primed by injury. However, remaining PMN found in the circulation postinjury demonstrate a down-regulated phenotype with inhibited apoptosis, CXCR2 expression, and endotoxin (LPS) responsiveness that may contribute to infectious complications and organ dysfunction. The p38 mitogen activated protein kinase (MAPK) signal transduction pathway has been implicated in regulating each of these PMN functions. We, therefore, hypothesize that p38 signaling is similarly down-regulated in postinjury circulating PMN.

Materials and Methods

: PMN were isolated from trauma patients (ISS > 20, postinjury day 3) and concurrently from healthy volunteers (control). PMN were cultured with LPS (100 ng/mL) and p38 activity assessed by Western blotting of cell lysates using a dual phosphospecific antibody for phosphorylated, active p38. In separate experiments, PMN from healthy volunteers were cultured in plasma from either healthy or injured subjects ± LPS and p38 activity assessed by a cell-free in vitro kinase assay using the transcription factor, ATF-2, as a substrate. Apoptosis and IL-1β secretion of the cultured PMN from each experimental condition were also quantified.

Results

: Circulating PMN from trauma patients have an upregulated LPS signaled p38 MAPK response (threefold) compared to PMN from healthy volunteers (p < 0.05). Furthermore, circulating factors present in trauma plasma can transfer this response to normal PMN. There was no significant alteration in apoptosis following LPS treatment between control and trauma patient's PMN (control: 62 ± 3% vs. trauma: 55 ± 5%), but there was decreased secretion of IL-1β (control: 100 ± 28 pg/mL vs. trauma: 12 ± 5 pg/mL, p < 0.05).

Conclusion

: We conclude that injury down-regulates selective PMN function despite enhanced p38 activity. These results suggest a shift in the role of PMN p38 signal transduction following injury with additional critical regulation of LPS responses downstream to p38 MAPK activity.