As reviewed in [
2], impairment in tumor antigen expression or its processing and presentation by both tumor cells and antigen presenting cells (APC), release of immunouppressive cytokines and prostaglandins as well as pro-apoptotic mechanisms may directly and/or indirectly impair T cell function while favoring tumor cell growth. Finally, tumor cells may promote development and recruitment of regulatory T cells (Treg) and myeloid derived suppressor cells (MDSC). CD4
+CD25
+ Treg in particular, represent an essential mechanism of peripheral tolerance to self antigens [
3]. They selectively express Foxp3, a forkhead/winged helix transcription factor that controls master genes in Treg development/function [
3]. Several neoplasms associate with CD4
+CD25
+ Treg accumulation in the blood and/or in tumors, and this may inversely correlate with patients' survival [
4].
MDSC are a heterogeneous population of cells of myeloid origin [
5], and include immature macrophages, granulocytes, dendritic cells (DC) and other myeloid cells [
2,
5‐
7]. Whereas in the spleen of normal mice they account for less than 5% of the nucleated cells, they rapidly accumulate in secondary lymphoid organs, blood and tissues during inflammation and cancer [
6,
8]. Several soluble factors contribute to alteration of the normal myelopoiesis and recruitment of MDSC to peripheral organs under pathologic conditions, including IL-3, IL-6, IL-10, vascular endothelial growth factor (VEGF), macrophage colony-stimulating factor (M-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) [
6‐
8]. In mice, MDSC are characteristically CD11b
+Gr-1
+, and may also express CD31 [
9], CD124, IL-4 receptor α-chain [
10] and CD80 [
11]. Expression of CD115 on MDSC may correlate with their ability to mediate development of Treg [
12]. In humans MDSC have been described to accumulate in the peripheral blood of patients affected by breast, lung, renal and head and neck carcinomas [
6] and in melanoma [
13], but their phenotype is still poorly defined. MDSC impair T lymphocyte functions through different mechanisms, including immunosuppressive cytokines and perturbation of the arginine metabolism by inducible nitric oxide synthase (iNOS), arginase (Arg), and reactive oxygen species [
14]. More in details, iNOS produces nitric oxide (NO), which interferes with IL-2 receptor signaling [
15], leading to cell cycle arrest. NO is also a key signaling molecule in inflammation-driven diseases, including cancer, where it participates to cancerogenesis, angiogenesis, tumor cell proliferation and invasion [
16]. Furthermore, high Arg activity depletes arginine from the microenvironment, inhibiting T cell activation and proliferation [
17], and favoring T cell apoptosis [
14].
Several Arg and NOS inhibitors have been tested with the purpose to inhibit tumor development and favor antitumor immunity [
18]. As an example, N(G)-monomethyl-L-arginine as been shown to restore anti-tumor immunity
in vitro [
19]. Unfortunately, its use in clinic has been discontinued due to severe toxicity [
18]. N(G) nitro-L-arginine methyl ester (L-NAME) has been reported in several mouse models to inhibit tumor growth [
20‐
22]. Those studies however, did not investigate a direct correlation of its effects on the endogenous tumor-specific immune response. More recently, Serafini et al. [
23] reported that phosphodiesterase-5 inhibitors (sildenafil, tadalafil and vardenafil) down regulate Arg and iNOS expression, thereby impairing the immunosuppressive activity of MDSC. In the mouse models tested, restoration of T cell immunity correlate with substantial delay in tumor progression [
23].
We evaluated whether accumulation of MDSC occurred in BALB/c and C57BL/6 mice bearing the C26GM colon carcinoma and RMA T lymphoma, respectively, and we investigated whether L-NAME and sildenafil restored antitumor immunity and delayed tumor growth.