**9. NK cells in alarming situations**

Situations related to cellular stress resulting from the presence of intracellular infectious agents and changes in self-elements (the presence of damaged-self [damage-associated molecular patterns (DAMPs)] or altered-self [tumor-associated molecular patterns (TAMPs)]) induce an increase in the expression of ligands that can "exclusively" stimulate activating receptors (major histocompatibility complex class I-related chain A [MICA], MICB, UL-16-binding proteins [ULBP]1, ULBP2, ULBP3, ULBP4, viral hemagglutinin, Newcastle disease hemagglutinin-neuraminidase, B7-H6, etc.) and consequently activate NK cell cytotoxicity.

## **10. NK cell-based immunotherapy in cancer**

not associated with CD94. Its signaling is achieved by association with two dimers of DAP10, a transmembrane adaptor molecule containing a tyrosine-based signaling motif (YINM), carrying phosphatidylinositol-3 kinase-binding sites in their cytoplasmic tails and recruiting growth fac-

Both NKG2A and NKG2B are alternative spliced products from a single gene. They can dimerize with CD94 to form inhibitory receptors through their cytoplasmic domains, which contain two ITIMs. Conversely, CD94-NKG2C, CD94-NKG2E, and CD94-NKG2H dimers and homodi-

There are at least three activating and two inhibitory MHC class I-independent receptors.

Natural cytotoxicity receptors (NCRs) are composed by a heterogeneous group of molecules belonging to IgSF and include NKp46 (NCR1), NKp44 (NCR2), and NKp30 (NCR3) activating receptors targeting most tumor cell lines. They are characterized by a type I transmembrane domain containing a positively charged amino acid residue and a short cytoplasmic tail. All these transmembrane type I receptors are expressed almost exclusively by NK cells. Binding of one or more of these receptors with a specific ligand leads to the increased NK cell activation and cytotoxicity [40]. It has been reported that these receptors can initiate tumor targeting

C-type lectin receptors include mostly killer cell lectin-like receptor subfamily G member 1 (KLRG1) and KLRB1 (also known as NK1.1, NKR-P1A, or CD161), which inhibit the cytotoxicity of NK cells and therefore prevent tissue damage. NKR-P1A is encoded by the KLRB1 gene and recognizes lectin like transcript-1 (LLT1) as a functional ligand. Its signaling in NK cells has previously been known to involve the activation of acid sphingomyelinase, which represent the catabolic pathway for N-acyl-sphingosine generation as a second messenger for the induction of apoptosis, proliferation, and differentiation [42]. KLRG1 is expressed by

subset of γδ T cells [43]. KLRG1 can bind three of the classical cadherins (E, N, and R), which are ubiquitously expressed in vertebrates and mediate cell-cell adhesion by homotypic and heterotypic interactions [44]. It has also been postulated to be a marker of senescence [45].

Situations related to cellular stress resulting from the presence of intracellular infectious agents and changes in self-elements (the presence of damaged-self [damage-associated molecular

T cells and by a large proportion of NK cells

T cells in umbilical cord blood, as well as in a substantial

and CD8+

and CD8+

mer-forming NKG2D and the orphan receptor NKG2F activate NK cells [37–39].

tor receptor-bound protein 2 (Grb2) [37].

8 Natural Killer Cells

**8.2. MHC class I-independent receptors**

by recognition of heparan sulfate on cancer cells [41].

*8.2.1. Natural cytotoxicity receptors*

*8.2.2. C-type lectin receptors*

antigen-experienced (memory) CD4+

**9. NK cells in alarming situations**

and naive phenotype CD4+

The main current therapeutic strategies, especially in allogeneic hematopoietic cell transplantation (HCT), are based on the use of NK cells through their education to render them alloreactive against tumor targets missing self-MHC ligand. In practice, autologous or haploidentical transplantation of NK cells requires obtaining of a very large number of pure and cytotoxic cells. Additionally, a favorable mismatch of the human leukocyte antigen class I (HLA I) molecules between donor and recipient tissues, or the absence of inhibitory KIR ligands in the recipient's HLA repertoire (KIR mismatch in the receptor-ligand model) allows NK cells in the graft to reduce its rejection by the host and the attack of residual leukemia cells, as well as the best prediction of the risk of relapse (**Figure 2**) [46, 47]. Finally, other promising approaches aim to induce an increase in their cytotoxic activities in the treatment of both hematopoietic and solid cancers by blocking inhibitory receptor signal transduction.

**Figure 2.** Role of KIR ligand mismatch in killing leukemic targets. AR, activating receptor; AL, activating ligand; IR, inhibitory receptor; KIR, killer cell immunoglobulin-like receptor. (Adapted from Ref. [26]. Images of cells are provided from Servier Medical Art.).

## **11. Conclusions**

The specific structural and functional features of NK cells describe them as major players in innate antitumor immunosurveillance and in the fight against infection. Their availability at the proximity of cellular stress signals allows them to effectively control both the process of carcinogenesis and the development of infectious diseases. Nevertheless, their activities seem to be strongly immunomodulated by cell microenvironment factors. Therefore, one of the best therapeutic strategies should create an ideal microenvironment for NK cell infiltration within target tissues while decreasing functions of their inhibitory receptors and enhancing their cytotoxic activities. Such a strategy should contribute not only to substantially increase the efficacy of targeted immunotherapies but also to prevent relapse after transplantation.
