Cyclophosphamide induces the development of early myeloid cells suppressing tumor cell growth by a nitric oxide-dependent mechanism

Adoptive immunotherapy with cyclophosphamide (Cy) increases the host resist ance against tumor growth. The precise mechanism(s) by which this therapy e nhances tumor suppression is unclear. Cy induces the development of early m yeloid cells that may be strongly antiproliferative through NO production. These cells are similar to the natural suppressor cells found in normal bon e marrow with a potential antitumor effect. Here we have addressed whether the development of NO-producing cells may be involved in this tumor resista nce in Cy-treated mice. The results show a synergism between Cy treatment a nd tumor-specific lymphocytes transferred systemically (i.v.) or locally (W inn's assay) that results in a strong tumor suppression. Inhibition of NO p roduction by N(G-)monomethyl-(L)-arginine at the site of tumor inoculation results in a loss of the protection achieved by the combined therapy. Cy-tr eated mice develop splenic early myeloid (CD11b, Gr-1, CD31 (ER-MP12), ER-M P20, ER-MP54) cells producing large amounts of NO upon T cell-derived signa ls (IFN-gamma plus CD40 ligation) able to inhibit tumor cell growth in vitr o. Early myeloid cells (ER-MP54(+)) and cells expressing inducible NO synth ase are increased at the site of tumor challenge in mice treated with the c ombined therapy, but not in those treated with Cy or immune cell transfer a lone. Thus, Cy Induces the expansion of early myeloid cells, inhibiting tum or cell growth by a mechanism involving NO. Both the recruitment and the ac tivation of these myeloid cells at the site of tumor challenge appear to be dependent on the presence of tumor-specific lymphocytes.