Macrophages produce nitric oxide at allograft sites

The current study was designed to determine which cytokines produced during an alloimmune response stimulate macrophage nitric oxide (.N = O) production at allograft sites.

Previous work has demonstrated that rat sponge matrix allograft infiltrating cells produce more .N = O on stimulation with alloantigen than syngeneic graft-infiltrating cells. Addition of NG-monomethyl-L-arginine (NMA), an inhibitor of .N = O synthesis, promotes allospecific cytolytic T-lymphocyte effector function.

Polyurethane sponges were implanted subcutaneously in recipient Lewis rats and injected with 10 x 10(6) ACl splenocytes. On various days after grafting, graft-infiltrating cells were harvested for in vitro study. Adherent macrophages from the graft infiltrating cell population were obtained by a 2- to 3-hour incubation to plastic dishes with subsequent washing to remove nonadherent cells.

Stimulation of unseparated graft-infiltrating cell populations with lipopolysaccharide or interferon-tau resulted in enhanced .N = O synthesis by allograft infiltrating cells compared with syngeneic graft-infiltrating cells, early after grafting. Macrophages recovered from an allograft site spontaneously produce more .N = O than macrophages recovered from syngeneic grafts (p < 0.001). Significantly enhanced levels of .N = O were produced by allograft macrophages compared with syngeneic graft macrophages on stimulation with lipopolysaccharide or interferon-tau (p < or = 0.025).

Nitric oxide appears to be produced in response to the local cytokines secreted by an ongoing rejection reaction. Nitric oxide serves under these circumstances to modulate the alloimmune response.