Abstract
The progressive growth of tumors in patients and experimental animals frequently results in reduced immune competence and an increased appearance of suppressor cells. The cell population responses were identified as T lymphocytes and suppressor leukocytes belonging to the monocyte-macrophage lineage [1–3]. In animal tumor models it has been shown that tumor growth coincided with the appearance of bone marrow-derived suppressor cells, which have characteristics of natural suppressor (NS) cells [4,5]. The role played by these cells in the host-tumor relationship is unclear, although it has been demonstrated that these cells can inhibit NK activity, the generation of cytolytic T cells and block IL-2 production and activity [5,6]. Studies of the mechanism by which the NS cells of tumor-bearers inhibit immune functions suggest that suppressive activity is mediated by TGF-β and nitric oxide [3].
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