**4. LGL surface markers**

#### **4.1. CD3<sup>−</sup> LGL markers**

In human, the LGL population can be separated into CD3+ and CD3<sup>−</sup> subtypes. The majority of LGL TCR<sup>−</sup> CD3<sup>−</sup> expresses CD16 (or FcγRIIIA, low-affinity receptor for the Fc portion of immunoglobulin G, 90%), CD56 (neural cell adhesion molecule [N-CAM], > 95%), and some markers of T cells, such as CD7 (100%), CD11b (80–80%), CD2 (70–80%), CD4 (<5%), and CD8 (15–20%).

#### **4.2. CD19<sup>−</sup> CD3<sup>−</sup> NK cell subsets**

NK cells are CD3<sup>−</sup> CD19<sup>−</sup> LGLs [14]. They thus lack two main markers of T cells (CD3+ ) and B cells (CD19+ ). In human peripheral blood, five NK cell subpopulations can be separated depending on the relative presence and expression levels of CD16 and CD56 markers [15]:



ADCC, antibody-dependent cell-mediated cytotoxicity; LAK, lymphokine-activated killer; NK, natural killer; PBNKCs, peripheral blood natural killer cells; SLO, secondary lymphoid organs.

**Table 1.** Key features of CD16brightCD56dim/+ and CD16dim/−CD56bright NK cells.

The NK cells can also be separated into two subgroups according to the expression levels of the CD16 and CD56 markers in healthy individuals (**Table 1**).

#### **5. Immune roles of NK cells**

does not use recombination-activating gene (RAG) enzymes for rearrangement of their receptor genes or complete V(D)J recombination as is the case for T cells or B cells [10] and is considered as the only lymphocytes without a clonally specific receptor [11]. Additionally, in contrast to the conventional αβ T cells, the genesis of NK cells appears to be independent of the self-major histocompatibility complex (MHC), although they have different recognition specificities of

Mature NK cells are able to self-renew and possibly persist in the host for months or years.

and function after homeostatic proliferation, expanded NK cells return to a quiescent phenotype

NK cells are present in the bloodstream and in the lymphatic vessels [12], as well as in placenta, spleen, liver, lungs, tonsils, peripheral ganglia, and bone marrow where they act as sentinels. Nevertheless, it appears that they have no access to other tissues apart from inflammatory responses. Of note, it has been reported that the homeostasis-driven NK cells can

T cells that retain a "memory-like" phenotype

and CD3<sup>−</sup>

expresses CD16 (or FcγRIIIA, low-affinity receptor for the Fc portion of immu-

LGLs [14]. They thus lack two main markers of T cells (CD3+

). In human peripheral blood, five NK cell subpopulations can be separated

noglobulin G, 90%), CD56 (neural cell adhesion molecule [N-CAM], > 95%), and some markers of T cells, such as CD7 (100%), CD11b (80–80%), CD2 (70–80%), CD4 (<5%), and CD8 (15–20%).

depending on the relative presence and expression levels of CD16 and CD56 markers [15]:

subtypes. The majority of

) and

and CD4+

and respond with comparable kinetics against viral challenge [10].

reside in both lymphoid and nonlymphoid organs for a long time [13].

In human, the LGL population can be separated into CD3+

(50–70% of CD56bright)

**4.** CD56dim CD16bright (at least 90% of all peripheral blood NK cells)

(small proportion)

 **NK cell subsets**

CD19<sup>−</sup>

**2.** CD56bright CD16dim (30–50% of CD56bright)

CD16bright (small proportion)

allogeneic MHC molecules.

2 Natural Killer Cells

Nevertheless, unlike long-lived CD8+

**3. Localization of NK cells**

**4. LGL surface markers**

CD3<sup>−</sup>

**CD3<sup>−</sup>**

NK cells are CD3<sup>−</sup>

**1.** CD56bright CD16<sup>−</sup>

**3.** CD56dim CD16<sup>−</sup>

**5.** CD56<sup>−</sup>

B cells (CD19+

 **LGL markers**

**4.1. CD3<sup>−</sup>**

LGL TCR<sup>−</sup>

**4.2. CD19<sup>−</sup>**

After maturation, NK cells migrate to the blood to provide innate defense against tumor cells and metastases, as well as infected cells by intracellular pathogens (such as viruses, bacteria, and protozoan parasites). Additionally, NK cells are also involved in the acute rejection of bone marrow transplants [16]. Finally, in addition to their ability to secrete various cytokines and regulate the immune response, activated NK cells are also involved in tissue remodeling through their ability to secrete matrix metalloproteinases (MMPs), in both physiological and pathological abnormalities, within the tumor microenvironment, through the cleavage of CD16 from the cell surface [17].

#### **6. MHC I as molecular basis of target recognition by NK cells**

The activation of NK cells does not require prior sensitization with an antigen. However, their activities are inversely correlated with the density of MHC class I molecules (MHC I), which are expressed on the surface of certain nucleated cell lines, with the exception of red blood cells and certain tissues such as salivary glands, brain, cornea, anterior chamber of the eye, liver, testis, fetotrophoblast, hair matrix, and proximal nail matrix [18].
