Activation of macrophage cytostatic effector mechanisms during acute graft-versus-host disease: release of intracellular iron and nitric oxide–mediated cytostasis

Abstract

During acute graft-versus-host disease (GVHD) the activation of macrophages (Mφ) is mediated by 2 signals, interferon (IFN)-γ and bacteria-derived lipopolysaccharide (LPS). A cascade of inflammatory responses that includes the release of mediators of tissue injury follows Mφ activation. Among the tissues characteristically targeted during acute GVHD are epithelial tissues of the skin and gastrointestinal tract that normally undergo continuous proliferation and are therefore sensitive to cytostatic processes. We have investigated whether Mφ can mediate cytostatic mechanisms capable of interrupting cell proliferation during acute GVHD. GVHD was induced in nonirradiated C57BL/6XAF1 (B6AF1) mice by the injection of 60 × 106 (acute GVHD) or 30 × 106 (nonlethal GVHD) C57BL/6 (B6) lymphoid cells. Mφ from animals undergoing acute GVHD could be triggered by normally insignificant quantities of LPS to mediate a cytostatic effect on target cells, resulting in the complete shutdown of cellular proliferation. The same amounts of LPS had no effect on Mφ from normal or syngeneically transplanted animals. Mφ mediated the release of significant quantities of intracellular iron from target cells undergoing cytostasis. Reversal of cytostasis occurred following inhibition of nitric oxide (NO) production by NG-monomethyl-L-arginine (NMMA). Production of NO by LPS-triggered Mφ reflected the severity of GVHD. NO release increased significantly during acute GVHD but was only transiently increased during nonlethal GVHD. The results provide evidence that, as a result of activation during acute GVHD, Mφ produce NO and induce the release of iron from target cells, resulting in a potent cytostatic effect that inhibits cellular proliferation.

The mechanism of nitrogen monoxide (NO)‐mediated iron mobilization from cells

“…Previous studies have clearly demonstrated that NO has a marked effect on cellular Fe metabolism [25][26][27]32]. Indeed, NO-mediated Fe depletion of tumour target cells by activated macrophages could play an important role in immune surveillance [3][4][5]15,16,45]. Our previous studies have shown that NO-mediated Fe mobilization is potentiated by incubating cells with D-glucose due to the subsequent generation of GSH [32].…”

Pathophysiology of Acute Graft-versus-Host Disease

“…130,131 As a result of activation during GVHD, macrophages produce NO and induce the release of iron from target cells, resulting in an inhibition of the recovery of injured target tissues by inhibiting proliferation of epithelial stem cells in the gut and skin. 132 More recently a central role of inflammatory cytokines in acute GVHD was confirmed in a murine study by using bone marrow chimeras wherein mortality and target organ injury was prevented by the neutralization of TNF-and IL-1 particularly for CD4-mediated acute GVHD but also in part, for CD8-mediated disease. 23 These inflammatory effectors may synergize with the lytic component provided by CTLs in a complex milieu of chemotactic signals and cytokine cascades resulting in the amplification of local tissue injury and further promotion of an inflammatory response, which ultimately leads to the observed target tissue destruction in the transplant recipient.…”