**3. Tumor‐induced immunosuppression**

It has now been well documented that tumor-induced immunosuppression affects NK cell receptor repertoire and leads to progressive local and systemic inhibition of NK cell function. In tumors, a complex composition of immunosuppressive molecules TGF-β, IL-10, IDO, prostaglandin E2 (PGE2), vascular endothelial growth factor (VEGF), nitric oxide synthase (NOS), and reactive oxygen species (ROS) are produced by regulatory immune cells such as Treg, MDSC, TAM, and by tumor cells themselves (**Figure. 1**). These factors generate a chronic inflammatory immunosuppressive milieu that leads to the suppression of the antitumor effector NK cell function that supports tumor progression [3, 7, 13, 65]. In this sense, considering that the expression of activating NK cell receptors is decreased in most malignancies, and as some studies report that the expression of inhibitory KIR and CD94/ NKG2A receptors on NK cells in different tumors remains unchanged, this suggests that activating receptors are main targets of tumor-mediated suppression [13].

Furthermore, NK cell dysfunction due to decreased activating NK cell receptor expression may be mediated by chronic tumor cell ligand-NK cell receptor engagement that leads to an exhausted NK cell phenotype characterized by upregulated programmed death 1 (PD-1) checkpoint immunoreceptor expression [66]. Conversely, the appearance of soluble NK cell ligands due to proteolytic cleavage from tumor cells also leads to NK cell dysfunction due to chronic NK cell receptor stimulation in the absence of target tumor cells [26].

involved in antibody-dependent cell-mediated cytotoxicity (ADCC) [57], as well as cytokine

CD16, as well as NKp46, associates with two cytoplasmatic domains composed of FcεRIγ or T-cell receptor (TCR) ζ chains that comprise ITAM, which upon ligand-binding become phosphorylated and induce signal transduction by activation of nonreceptor tyrosine kinases

The expression CD16 as a prominent NK cell cytotoxic receptor has been found to be decreased on NK cells in breast cancer and MM patients [18, 58] not only due to postactivational receptor internalization but also following target cell induced activation of MMP, namely ADAM 17, as shown during *in vitro* NK cell cultivation with tumor cells [59, 60]. Moreover, as CD16 defines the two functionally different NK cell subsets, its decreased expression influences the ratio of these subsets, leading to the loss CD16bright cytotoxic subset, a finding that has been

detected in numerous malignancies such as breast cancer, MM, and melanoma [6, 33].

This is a member of the C-type lectin family that is primarily designated as an activating receptor that plays a role in NK cell-mediated cytotoxicity and it has been previously shown that natural killer cell surface protein P1A (NKR-P1A)/CD161 participates in triggering NK cell cytotoxicity against numerous human tumor cell lines [61], while more recently, it has been suggested that its activating mechanism may require interaction with costimulatory receptors [62]. On identification of its lectin-like transcript 1 (LLT1) ligand, its inhibitory potential has recently been described, but its function still remains controversial. In support of the inhibitory function of NKR-P1A, it has been shown that its cytoplasmic tail contains tyrosine residue in an atypical motive (AxYxxL) that may function as a weak ITIM [63, 64]. There are some reports of decreased expression and cytokine-mediated upregulation of NKR-P1A/CD161 in

It has now been well documented that tumor-induced immunosuppression affects NK cell receptor repertoire and leads to progressive local and systemic inhibition of NK cell function. In tumors, a complex composition of immunosuppressive molecules TGF-β, IL-10, IDO, prostaglandin E2 (PGE2), vascular endothelial growth factor (VEGF), nitric oxide synthase (NOS), and reactive oxygen species (ROS) are produced by regulatory immune cells such as Treg, MDSC, TAM, and by tumor cells themselves (**Figure. 1**). These factors generate a chronic inflammatory immunosuppressive milieu that leads to the suppression of the antitumor effector NK cell function that supports tumor progression [3, 7, 13, 65]. In this sense, considering that the expression of activating NK cell receptors is decreased in most malignancies, and as some studies report that the expression of inhibitory KIR and CD94/ NKG2A receptors on NK cells in different tumors remains unchanged, this suggests that

activating receptors are main targets of tumor-mediated suppression [13].

production, proliferation, and postactivational NK cell apoptotic death [56].

Syk and ZAP-70 [57].

76 Natural Killer Cells

**2.4. Natural killer cell surface protein P1A**

metastatic melanoma and MM [29, 33, 58].

**3. Tumor‐induced immunosuppression**

In this sense, it has now been established that during the development of solid tumors, NK cells are frequently rendered functionally impaired as a consequence of cancer immunoediting that induces immune tolerance to tumors owing to impairment of NK cell receptor repertoire and signaling, as well as immunoselection of nonimmunogenic tumor cells.

**Figure 1.** Mechanisms of NK cell receptor dysregulation in tumors. The tumor induces alterations in NK cell activating receptors by producing suppressive tumor-derived mediators including immunosuppressive cytokines (transforming growth factor beta - TGF-β), enzymes (indolamin-2,3-dioxygenase - IDO), factors (L-kynurenine, prostaglandin E2 - PGE2) that together with the presence of hypoxia can suppress NK cell antitumor activity. Chronic engagement of NK cell activating receptors with either tumor cell surface-expressed or shed NK cell ligands (NKG2D ligand - NKG2D-L) leads to progressive inhibition of NK cell antitumor response. Also, tumor-expressed proliferating cell nuclear antigen (PCNA) associated with HLA-I molecule by binding NKp44 activating receptor induces unconventional inhibitory signals in NK cells. Moreover, interferon gamma (IFN-γ) released by NK cells induces increased expression of HLA-I molecules on tumor cells, as well as tumor-produced IDO, that lead to inhibition of NK cell function.
