Stat 6-dependent induction of myeloid derived suppressor cells after physical injury regulates nitric oxide response to endotoxin.

Objective: To delineate the role of T-helper 2 (Th2) cytokines in the induction of trauma induced myeloid suppressor cells (TIMSC) and the regulation of nitric oxide production.
Background: Trauma induces myeloid cells that express CD11b+/Gr1+ and arginase 1 and exhibit an immune suppressing activity. This article explores the mechanisms that induce TIMSC and the effects on nitric oxide production in response to endotoxin.
Methods: TIMSC were studied in response to Th2 cytokines and a subsequent challenge to endotoxin. The role of Th2 cytokines was studied in STAT6-/- mice. Accumulation of TIMSC in spleens was studied using flow cytometry and immunhistochemistry. Plasma was recovered to measure accumulation of nitric oxide metabolites.
Results: TIMSC accumulated in the spleen of injured mice and were particularly sensitive to IL-4 and IL-13 with large inductions of arginase activity. Significant blunting in both the accumulation of TIMSC in the spleen and induction of arginase 1 was observed in STAT6-/- mice after physical injury. Accumulation of nitric oxide metabolites to endotoxin was observed in STAT6-/- mice.
Conclusion: This study shows that induction of CD11b+/Gr1+ cells after physical injury play an essential role in the regulation of nitric oxide production after a septic challenge. The accumulation and induction of arginase 1 in TIMSC is Th2 cytokine dependent. To our knowledge, the role of TIMSC in the regulation of nitric oxide is a novel finding. This observation adds to the possibility that TIMSC could play an important role in immunosuppression observed after physical injury.