**7. Myeloid-derived suppressor cells (MDSCs)**

An immune cell population responsible for suppression of the immune system in tumors is myeloid-derived suppressor cells (MDSCs). These cells are formed by populations of monocytes and immature granulocytes arising from the bone marrow during pathological conditions [25]. Numerous studies have indicated an increase in the number of cells of this type in cancer in humans and mice [26]; but, its role in dogs is unclear [27].

In humans, MDSCs are responsible for immunosuppression of the tumor site, allowing a more invasive and metastatic characteristic through the production of metalloproteinases [28, 29]. A study [25] on the immunosuppressive action of MDSCs indicated that MDSCs can suppress natural killer cells, dendritic cells, and T lymphocytes, in addition to potentiating the effects of Tregs by producing reactive oxygen species and inducible nitric oxide synthase (iNOs). MDSCs also produce immunoregulatory cytokines, such as TGF-β and IL-10 and decrease the expression of IL-12, which is responsible for the activation of T cells [30].
