Studies of chemokine receptors (CKR) in natural killer- (NK-) cells have already been published, but only a few gave detailed information on its differential expression on blood NK-cell subsets. We report on the expression of the inflammatory and homeostatic CKR on normal blood CD16+ and    NK-cells. Conventional and NK-cells present in the normal PB do express CKR for inflammatory cytokines, although with different patterns NK-cells are mainly CXCR1/CXCR2+ and CXCR3/CCR5−/+, whereas mostly NK-cells are CXCR1/CXCR2 and CXCR3/CCR5+. Both NK-cell subsets have variable CXCR4 expression and are CCR4 and CCR6. The CKR repertoire of the NK-cells approaches to that of neutrophils, whereas the CKR repertoire of the NK-cells mimics that of Th1+ T cells, suggesting that these cells are prepared to migrate into inflamed tissues at different phases of the immune response. In addition, we describe a subpopulation of NK-cells with intermediate levels of CD56 expression, which we named NK-cells. These NK-cells are CXCR3/CCR5+, they have intermediate levels of expression of CD16, CD62L, CD94, and CD122, and they are CD57 and CD158a. In view of their phenotypic features, we hypothesize that they correspond to a transitional stage, between the well-known and NK-cells populations.

1. Introduction

Natural killer- (NK-) cells were originally identified by their natural ability to kill target cells and are known for a long time as effector cells of the innate immune system, with an important role in controlling several types of tumors and infections [1]. In recent years, NK-cells have also been recognized as regulatory cells, which are able to interact with other cells of the immune system, such as dendritic cells (DC), monocytes/macrophages, and T cells, thereby influencing the innate and adaptive immune responses [25]. The role of their interaction with neutrophils in shaping the immune response is also being increasingly documented [6, 7].

The cytotoxic activity of the NK-cells is controlled by the balance between inhibitory and activating receptors, whose ligands are self-Major Histocompatibility Complex (MHC) class I molecules and molecules expressed on stressed, viral infected, and tumor cells. They comprise, among others, the killer cell immunoglobulin-like receptors (KIR), killer cell lectin type receptors (KLR), and natural cytotoxic receptors (NCR) as well as immunoglobulin Fc receptors (FcR) and complement receptors [810].

Meanwhile, the immunoregulatory properties of the NK-cells are mediated, not only by cell-to-cell contact, but also by the soluble factors they produce, which enable them to recruit and to activate other immune cells. These include chemokines (CK), such as MIP-1α (macrophage inflammatory proteins-1 alpha, CCL3) and MIP-1β (CCL4), RANTES (regulated activation, normal T cell expressed and secreted, CCL5), and ATAC (activation-induced, T cell derived, and chemokine-related cytokine, CXCL1). They also comprise cytokines, for example, IFN-γ (interferon-gamma) and TNF-α (tumor necrosis factor alpha) and growth factors, such as GM-CSF (granulocyte-macrophage colony-stimulating factor) [11, 12].

Using adhesion molecules and chemokine receptors (CKR), NK-cells are able to circulate in the blood and to distribute throughout the body, by homing into secondary lymphoid organs (e.g., lymph nodes), localizing in specific nonlymphoid organs (e.g., liver, placenta), and migrating into acute or chronic inflamed tissues, where they participate in the immune responses [1316]. In some organs, NK-cells exhibit specific phenotypes and functions [17, 18], for example, promoting decidualization of the endometrium, embryo implantation and placenta development [19, 20], and influencing the hematopoiesis [21, 22].

Two different subsets of mature CD56+ NK-cells have been described in humans, based on the levels of CD56 and CD16 expression: CD16+ and    NK-cells from now on designed and , respectively [23, 24]. While the former clearly predominates in the peripheral blood (PB), where they represent around 90% of the circulating CD56+ NK-cells, the latter are more represented in secondary lymphoid organs, chronically inflamed tissues and placenta [1316, 19, 20].

Apart from the different expression of CD16, the low affinity receptor for IgG (FcγRIIIA) and CD56, the neural cell adhesion molecule (NCAM), the conventional CD56+ NK-cell subsets also differ in the expression of other adhesion, homing, and costimulatory molecules as well as on the repertoires of NCR, KIR and KLR, and receptors for cytokines, chemokines, and growth factors [2529]. In addition, these NK-cell subsets exhibit distinct sialylated forms of CD43 and posttranslational modifications of the P-selectin glycoprotein ligand-1 (PSGL-1) [30, 31].

From the functional point of view, NK-cells are essentially cytotoxic, with a greater level of antibody dependent cell mediated cytotoxicity (ADCC) [32], whereas NK-cells have a high proliferative response to low doses of interleukin- (IL-) 2 (IL-2) and C-kit ligand [33]. In addition, the latter display a more important immunomodulatory role associated with cytokine production in response to IL-2 and monokines [33]. More recently it became apparent that upon target cell recognition, NK-cells are more prominent cytokine and chemokine producers than NK-cells [34]. These diverse functional properties would suggest that and NK-cells could be naturally prepared to act in different sites and at different phases of the immune response.

The exact relationship between these NK-cell subsets still remains unclear. Some studies have shown that bone marrow progenitor cells give rise to or NK-cells depending on being cultured in the presence of IL-15 alone or in combination with IL-21, respectively [35, 36]. However, more recent data would favor a possible maturation relationship between these NK-cell subsets and suggest that NK-cells originate from NK-cells [3742].

Chemokines are small proteins that control a number of biological activities, including cell development, differentiation, tissue distribution, and function [43]. They act by binding chemokine receptors (CKR), a family of seven-transmembrane proteins that are classified by structure according to the number and spacing of conserved cysteines into four major groups given the names CXCR, CCR, CX3CR, and XCR to which four groups of CK correspond: CXCL, CCL, CL, and CX3CL [44]. In addition, CXCL chemokines have been further subclassified into glutamic acid-leucine-arginine tripeptide (ELR) positive or negative, based on the presence or absence of the ELR motif N-terminal to the first cysteine. From a functional point of view, two distinct types of CK have been considered: inflammatory/inducible CK, which are regulated by proinflammatory stimuli and dictate migration to the inflamed tissues and homeostatic/constitutive CK, which are responsible for the homing of the immune cells to the lymphoid organs and tissues. Similarly, two distinct groups of CKR have been described: those that interact mainly with inflammatory/inducible CK and have overlapping specificities and those that are relatively specific for homeostatic/constitutive CK [43, 44].

To the best of our knowledge only a few studies analyzed in detail the CKR repertoire on and NK-cells and the results obtained were somewhat divergent [45, 46]. For instance, Campbell et al. have found that CD56+/CD16+ (primarily ) NK-cells uniformly express high levels of CXCR1, CXCR4, and CX3CR1 and low levels of CXCR2 and CXCR3 but no CCR1–6, CCR9, CXCR5, and CXCR6; they also found that CD56+/CD16 (primarily ) NK-cells do express CXCR3, CXCR4, CCR5, and very low levels of CX3CR1, but no CXCR1, CXCR2, CXCR5, CCR1–4, 6, and 9 [45]. In contrast, Berahovich et al. observed that NK-cells are CXCR1+, CXCR3+, and CXCR4+ and contain subsets expressing CCR1, CCR4, CCR5, CCR6, CCR9, CXCR5, and CXCR6 [46]; according to their work, with the exception of CCR4, these CKR are expressed at higher percentages by NK-cells [46]. Additionally, both authors have found CCR7 to be restricted to NK-cells, which has been proved to regulate its selective homing into the lymph nodes (LN) [47, 48], where these cells establish the link between innate and adaptive immunity [47, 48].

We have previously characterized the immunophenotype of blood and NK-cells [29]. In order to better understand the migration pathways and cell-interactions of these NK-cell subsets and to establish the normal reference patterns for the study of the NK-cell lymphoproliferative disorders, we decided to investigate the expression of a number of CKR on these NK-cell subsets. At some point in our study, we found that blood CD56+ NK-cells include a minor population of CXCR3/CCR5+ NK-cells whose levels of CD56 expression are intermediate between those observed on and NK-cells, most of which are CD16+. These cells, from now on referred to as NK-cells, fail to display CD57 and KIR, and they have intermediate levels of CD62L, CD94, and CD122 expression. Based on the results presented herein and on the published data, we discuss the migration routes of the conventional and NK-cells and their relevance for the success of the immune response and hypothesize that NK-cells probably represent a transitional NK-cell state.

2. Material and Methods

2.1. Subjects

We first analyzed by flow cytometry the expression of a number of CKR on the CD56+ NK-cells in the PB of 15 adult healthy individuals (blood donors), 9 males and 6 females, aged from 19 to 54 years (median age of 38 years). After suspecting the existence of a subpopulation of NK-cells, these cells were further characterized using another group of 13 adult healthy individuals (blood donors), 8 males and 5 females, aged from 20 to 64 years (median age of 40 years).

2.2. Ethical Statement

This study was approved by the Ethical Committee as part of a research project aimed to characterize the CKR on normal and neoplastic T cells and NK-cells in order to better understand the biology of the T cell and NK-cell lymphoproliferative disorders. All individuals gave informed consent to participate in the study.

2.3. Flow Cytometry Studies

Immunophenotyping was performed using a whole blood stain-lyse-and-then-wash direct immunofluorescence technique using FACS lysing solution (Becton Dickinson, San José, CA) (BD) for erythrocyte lysis and cell fixation and four-color stainings with monoclonal antibodies (mAbs) conjugated with fluorescein isothiocyanate (FITC), phycoerythrin (PE), PE-Cyanine 5 (PC5) or peridinin chlorophyll protein (PerCP), and allophycocyanin (APC). These were purchased to BD, Pharmingen (PH; San Diego, CA), Beckman Coulter (BC; Miami, FL), Immunotech (IOT; Marseille, France), and CLB (Amsterdam, Netherlands). Appropriate fluorochrome-conjugated isotype matched mAbs were used as negative controls.

In order to characterize the CKR expression on the conventional and NK-cell subsets, APC-conjugated anti-CD3 (BD; mouse IgG1,κ; clone SK7), PC5-conjugated anti-CD56 (IOT; mouse IgG1,κ; clone N901/NKH-1), and FITC-conjugated anti-CD16 (IOT; mouse IgG1,κ; clone 3G8) mAbs were used in combination with PE-conjugated mAbs directed against the following CKR (PH): CXCR1 (CD181) (mouse IgG2b,κ; clone 5A12), CXCR2 (CD182) (mouse IgG1,κ; clone 6C6), CXCR3 (CD183) (mouse IgG1,κ; clone 1C6), CCR4 (CD194) (mouse IgG1,κ; clone 1G1), CCR5 (CD195) (mouse IgG2a,κ; clone 2D7/CCR5), and CCR6 (CD196) (mouse IgG1,κ; clone 11A9).

Subsequently, NK-cells (which, in most of the normal PB samples, cannot be distinguished from the conventional or NK-cells using the staining protocol mentioned above) were further characterized using APC-conjugated anti-CD3, PC5-conjugated anti-CD56, PE-conjugated anti-CXCR3 + PE-conjugated anti-CCR5, and one of the following FITC-conjugated mAbs directed against these molecules: anti-CD16 (IOT; mouse IgG1,κ; clone 3G8), anti-CD57 (BD; mouse IgM,κ; clone HNK-1), anti-CD62L (BD; mouse IgG2a,κ; clone SK11), anti-CD94 (PH; mouse IgG1,κ; clone HP-3D9), anti-CD122 (CLB; mouse IgG2a,κ; clone MIK-b1), and anti-CD158a (BD; mouse IgM,κ; clone HP-3E4).

Data acquisition was carried out in a FACSCalibur flow cytometer (BD) equipped with a 15 mW air-cooled 488 nm argon ion laser and a 625 nm neon diode laser, using the CellQUEST software (BD). Information on a minimum of 2 × 105 events was acquired and stored as FCS 2.0 data files for each staining. For data analysis the Paint-a-Gate PRO (BD) and the Infinicyt (Cytognos, Salamanca, Spain) software programs were used.

Using the first staining protocol, NK-cells were first gated based on their CD3/CD56+ phenotype; then, the conventional and NK-cell subsets were selected based on their levels of CD56 expression and on their differential positivity for CD16 and separately analyzed for the expression of CXCR1, CXCR2, CXCR3, CCR4, CCR5, and CCR6. Using the second staining protocol, in which the anti-CXCR3 and CCR5 mAbs used have the same fluorochrome, we were able to distinguish three populations of CD56+ NK-cells: CXCR3/CCR5, CXCR3/CR5+, and CXCR3/CR5+. These were separately analyzed for the expression of CD16, CD56, CD57, CD62L, CD94, CD158a, and CD122.

The percentage of positive cells, the mean fluorescence intensity (MFI, expressed as arbitrary relative linear units scaled from 0 to 10,000), and the coefficient of variation of the MFI (CV, expressed as percentage) were recorded for each molecule tested.

2.4. Statistical Analysis

For all quantitative variables under study, mean, standard deviation, median, and range values were calculated. The statistical significance of the differences observed between groups was evaluated using the Mann-Whitney -test (SPSS 10.0, SPSS, Chicago, IL, USA). values less than 0.05 were considered to be associated with statistical significance.

3. Results

3.1. Chemokine Receptors on Blood and NK-Cells

Conventional and NK-cells present in the normal PB have different CKR repertoires (Figure 1 and Table 1).

3.1.1. Chemokine Receptors on Conventional NK-Cells

Most NK-cells are CXCR1/CXCR2+; that is, the majority expresses high levels of CXCR1 (93.0 ± 4.5%) and CXCR2 (91.9 ± 3.4%), whose ligands are CXCL8 (IL-8) and other ELR motif containing chemokines involved in inflammation and angiogenesis [49] (Table 1 and Figure 1). In addition, these NK-cells are CXCR3/CCR5−/+, which means that a variable proportion of them have low levels of CXCR3 and/or CCR5 (15.6 ± 11.1% and 13.3 ± 8.8%, resp.) (Table 1 and Figure 1). CXCR3 binds IFN-γ inducible cytokines, such as CXCL9 (monokine induced by gamma-interferon, MIG), CXCL10 (interferon-induced protein of 10 kD, IP-10), and CXCL11 (interferon-inducible T cell alpha chemoattractant, I-TAC) and mediates Ca++ mobilization and chemotaxis [5052]. On the other hand, CCR5 has affinity to CCL3 (MIP-1), CCL4 (MIP-1), CCL5 (RANTES), and CCL8 (monocyte chemotactic protein-2, MCP-2) [53, 54].

Concerning the expression of constitutive/homeostatic CKR and the fraction of NK-cells that expresses CXCR4, a CKR present on most hematopoietic cell types that binds to CXCL12 (stromal cell derived factor type 1, SDF-1) [55, 56] and has been shown to play a pivotal role in hematopoiesis [57] is variable (21.8 ± 8.7%) (Table 1 and Figure 1). In contrast, CCR4 is expressed in only a very small percentage of the NK-cells (0.8 ± 0.4%) (Table 1 and Figure 1). This CKR has been reported to be a marker for T helper 2 (Th2) lymphocytes [58] and promotes homing of memory T cells to inflamed skin [59] by means of interaction with CCL17 (thymus and activation-regulated chemokine, TARC) and CCL22 (macrophage-derived chemokine, MDC) [60, 61]. Similar results were obtained for CCR6, which is expressed in only 0.6 ± 0.4% of the NK-cells (Table 1). This CKR mediates responsiveness of memory T cells to CCL3 (MIP-1α) [62] and CCL20 (liver- and activation-regulated chemokine, LARC) [63] and has also been implicated in the homing of Langerhans’ cells to the epidermis [64].

3.1.2. Chemokine Receptors on Conventional NK-Cells

In contrast to NK-cells, the majority of the NK-cells are CXCR1/CXCR2 and CXCR3+; that is, most of NK-cells express high levels of CXCR3 (96.9 ± 2.5%) whereas only a few are CXCR1+ (%) or CXCR2+ (%), and a large fraction of them (%) is CCR5+ (Table 1 and Figure 1).

Constitutive/homeostatic CKR are also present in NK-cells, with a variable fraction of them expressing CXCR4 (%) and only a few being CCR4+ (%) and CCR6+ (%, resp.) (Table 1 and Figure 1).

3.2. Identification of a New NK-Cell Population in the Peripheral Blood

When analyzing the conventional CD56+ NK-cell subsets, we observed that the percentage of NK-cells staining for CCR5 correlated positively with the percentage of NK-cells staining for CXCR3 (; ). In addition, we found that CCR5+ and CXCR+   NK-cells had higher levels of CD56 and lower levels of CD16, as compared to CCR5 ( and , resp.) and CXCR3 ( and , resp.) counterparts. These observations allow us to investigate if cells expressing CCR5 and/or CXCR3 could represent a specific stage in NK-cell differentiation. In accordance, using another staining protocol in which anti-CXCR3 and anti-CCR5 mAbs had the same fluorochrome, we were able to identify three NK-cell populations in the normal PB, based on the expression of CD56, CD16, and the chemokine receptors CXCR3 and/or CCR5 (Figure 2): CD16+ CCR5/CXCR3 (or simply ), CD16+/− CCR5/CXCR3+ (or simply ), and    CCR5/CXCR3+ (or simply ) NK-cells.

In the normal PB, NK-cells correspond to the majority (mean of %) of CD56+ NK-cells, whereas the and NK-cells are minimally represented (mean of % and %, resp.) (Table 2).

Despite representing a minor NK-cell population in most normal PB samples, NK-cells are largely expanded in some patients with chronic lymphoproliferative disorders of NK-cells (CLPD-NK) (Figure 3).

No differences were observed between these three NK-cell subsets concerning both the cell size and complexity, as evaluated by the forward (FSC) and side light scatter (SSC), respectively, except for a slightly larger size of NK-cells (Table 2). Nonetheless, statistically significant differences were found concerning the expression of CD56 and CD16 (Figure 2 and Table 3) as well as of the other adhesion molecules and homing, cytokine, and killer cell receptors analyzed (Figure 2 and Table 4).

3.2.1. Phenotypic Characterization of Blood NK-Cells

NK-cells do express CD56 at levels that are intermediate between those observed on and NK-cells (MFI of , , and , resp.) (Figure 2 and Table 3). They also have intermediate percentages of CD16+ cells (%), as compared to and NK-cells (% and %, resp.). In addition, the levels of CD16 expression (MFI of ) were in between those observed on and NK-cells (MFI of and , resp.).

These three CD56+ NK-cell subsets also differ in the expression of other molecules (Figure 2 and Table 4).

Concerning the KLR, only a fraction of (%) expresses dimly CD94, whereas nearly all (%) and (%) are CD94+ (Figure 2 and Table 4). Curiously, the levels of CD94 expression on NK-cells are in between those observed on and NK-cells (MFI of , , and , resp.).

With respect to the expression of KIR, an opposite pattern is observed. Indeed, a variable fraction of NK-cells is CD158a+ (%), in contrast to that found in and NK-cells, which are basically CD158a (mean percentage of CD158a cells of % and %, resp.) (Figure 2 and Table 4).

Regarding cell adhesion molecules, the percentage of CD62L+ cells is significantly lower among (%), as compared to NK-cells (%), intermediate values being observed in the NK-cells (%). Similar results were obtained for the levels of CD62L expression (MFI of , , and , resp.). In addition, a large fraction of NK-cells (%) expresses variably and heterogeneously the CD57 oligosaccharide, whereas most NK-cells fail to express this molecule and NK-cells are virtually CD57 (mean % of CD57+ cells of % and %, resp.). Once again, the levels of CD57 expression on NK-cells (MFI of ) were in between those observed on (MFI of ) and NK-cells (MFI of ).

The low affinity receptor for IL-2 and CD122, which is present in virtually all NK-cells, also exhibit intermediate levels on cells (MFI of ), as compared to (MFI of ) and to (MFI of ) NK-cells (Figure 2 and Table 4).

4. Discussion

In the present study we show that and NK-cells that circulate in the normal blood have typical and quite different patterns of expression of receptors for inflammatory chemokines. At the same time, we identify and describe a subpopulation of NK-cells that could represent a transitional stage in between the conventional NK-cell subsets referred to above, based on their intermediate levels of CD56 and CD16 expression and on their patterns of chemokine (CXCR3, CCR5), cytokine (CD122), and killer cell (CD94, CD158a) receptors and adhesion molecules (CD62L, CD57).

Differences on the CKR repertoires make the NK-cell subsets naturally able to circulate in the blood, to home into secondary lymphoid organs, or to migrate into inflamed tissues, in different circumstances and with different partners (Figure 4), in response to constitutive and inflammatory chemokines (Table 5).

In accordance, the majority of NK-cells are CXCR3/CCR5+, a pattern of CKR expression that is typically observed in Th1 cells [65], while NK-cells do express CXCR1 and CXCR2, the only CKR specific for the ELR+ CXCL chemokines involved in inflammation, thus mimicking neutrophils [66, 67]. In addition, both NK-cell subsets have variable levels of CXCR4 and virtually no CCR4 and CCR6 expression.

NK-cells and Th1 cells, the primary cell populations responsible for IL-2, IFN-γ, and TNF-α production in response to IL-2 or certain monokines, such as IL-12 and IL-15, are attracted together to chronically inflamed tissues in response to CCR5 (MIP-1α, MIP-1β, RANTES, and MCP-2) and CXCR3 (MIG, IP-10, and I-TAC) chemokine ligands, where they orchestrate the adaptive immune response. Some of these CK, such as RANTES and MIP-1α, also attract proinflammatory CD16+ monocytes, by acting as ligands for CCR1 and CCR4, as well as for CCR5 [68].

In agreement, NK-cells accumulate within Th1-type chronic inflammatory lesions in a wide variety of pathological conditions such as rheumatoid arthritis [69], psoriasis [70], sarcoidosis [71], and allograft rejection [72] as well as in sites of intracellular bacterial infections [73], chronic viral infections [74], and tumors [75]. Inside the inflamed tissues and imbibed in the appropriate monokine environment, NK-cells are able to engage with monocytes in a reciprocal fashion [76], thereby amplifying the inflammatory response and having important antitumor and antiviral effects. In the LN, they can induce the maturation of DC via IFN-γ and TNF-α release and/or cell-cell contact-dependent mechanisms [2, 3], in that way shaping the subsequent immune response. Moreover, activated NK-cells can kill immature myeloid DC, which have insufficient amounts of MHC molecules to activate T cells properly [2, 3]. In addition, NK-cells also predominate in placenta [77], where they are involved in maternal-fetal tolerance [78, 79].

In contrast, NK-cells, which are essentially cytotoxic, and neutrophils, which are phagocytic cells by excellence, predominate in the PB and are equipped with the CXCR1 and CXCR2 chemokine receptors, making them able to comigrate into sites of acute inflammation in response to IL-8 and other ELR motif containing chemokines and to participate in the earliest phase of the innate immune response. As for the neutrophils, migration of NK-cells to inflamed tissues also depends on the interaction of different forms of PSGL-1 expressed on their membrane with the selectin molecules expressed on endothelial cells [31]. Curiously, cytotoxic T lymphocytes (CTL) have also been reported to express CXCR1 [80] and PSGL-1 [81].

Normally, both neutrophils, which are able to neutralize efficiently the extracellular pathogens, after opsonization by antibodies, using Fc receptors for IgG and IgA and NK-cells, which mediate antibody dependent cell cytotoxicity via FcγRIIIa (CD16), circulate in the blood. In that sense, it can be hypothesized that these cells are candidates to establish a bridge between the innate immune response and the antibody mediated adaptive immune response. Evidence is being accumulated in the last years for a cross-talk between neutrophils and NK-cells [6, 7]. For instance, NK-cells promote neutrophil recruitment to the inflamed tissues and several NK-cell derived cytokines and growth factors, such as GM-CSF, IFN-γ, and TNF-α, act by enhancing neutrophil survival and by modulating cell surface expression of complement and Fc receptors in neutrophils [8284]. On the other hand, neutrophils can stimulate the production of IFN-γ by NK-cells [84].

The other anatomical sites in which NK-cells and neutrophils might be concomitantly present to modulate each other’s activity and its contribution to disease are not completely elucidated. Normal liver contains mainly NK-cells, but these cells are different from the NK-cells that circulate in the blood [85]. In addition, NK-cells also infiltrate the liver of patients with primary biliary cirrhosis, an antibody mediated autoimmune disease, following the CXCR1/IL-8 axis [85]; curiously, hepatic infiltration by neutrophils is also found in these patients [86]. Moreover, NK-cells and neutrophils colocalize in the skin of patients with Sweet’s syndrome, an acute febrile neutrophilic dermatosis that can follow viral infections, autoimmune diseases, and hematologic malignancies [84].

Also of note, in this study we confirm previous observations about the lack of expression on both and NK-cells of other CKR involved in homing to nonlymphoid organs and tissues including CCR4 (skin and lung), CCR6 (intestine and liver), CCR9 (small intestine), and CCR10 (skin) [45, 87]. This suggests that, unlike memory/effector T cells, CD56+ NK-cells may not be divided into cutaneous versus mucosal/intestinal-homing compartments, based on CKR expression [58, 88, 89].

Of special interest is also the identification of a new population of NK-cells expressing intermediate levels of CD56 that we designed as NK-cells. Similar to NK-cells, most of the NK-cells are KIR and CD57; however, the majority of them display the CD16 molecule, a marker of NK-cells, and they have intermediate levels of CD62L, CD94, and CD122 expression. Due to the fact that the identification of the NK-cell populations by flow cytometry is usually based only on CD56 and CD16 expression, NK-cells are being considered together with NK-cells on routine blood analysis.

The fact that NK-cells have phenotypic features intermediate between those of conventional and NK-cells would suggest that they could represent a transitional NK-cell maturation stage.

In line with this hypothesis, evidence for the existence of transitional NK-cell populations with phenotypic features similar to those of the NK-cells described herein has also been provided in other studies [90, 91]. In accordance, Yu et al. described a    NK-cell subset expressing CD2, CD62L, CD56, KIR, granzymes, and perforin, producing IFN-γ in response to monokines, and exhibiting CD94-mediated redirected killing at levels intermediate between those observed in    and    NK-cells [90]. In addition, Juelke el al. reported on a CD62L+ NK-cell subset with the ability to produce IFN-γ and the capacity to kill [91]. Finally, when studying the differentiation of CD94/NKG2A+ into CD94/NKG2A NK-cells, Béziat et al. found a transitional CD94/NKG2A+ NK-cell subset, expressing intermediate levels of CD62L, granzyme-K, CD27, and CD57, among other molecules [92]. Given the immunophenotypic similarities, the NK-cell population described herein, which comprises less than 10% of the CD56+ NK-cells in the PB from normal healthy individuals, probably corresponds to a subpopulation of the    NK-cell subset described by Yu et al., which accounts for half of the circulating CD56+ NK-cells [90].

Another potential interest of identifying normal NK-cells with intermediate phenotypic features relies on data interpretation in clinical settings. For instance, overrepresentation of NK-cells in the PB from patients with NK-cell lymphocytosis may erroneously be interpreted as phenotypically aberrant (and thus potentially neoplastic) NK-cells. Thus, the knowledge about the immunophenotype of the NK-cell populations that circulate in normal PB, as well as in the PB from patients with inflammatory and infectious conditions [93, 94], is essential to a better understanding of the phenotypic heterogeneity of the expanded NK-cell populations observed in patients with CLPD-NK [95], thereby contributing to distinguishing nonclonal from clonal NK-cell proliferations and reactive from neoplastic conditions [96, 97].

5. Conclusions

Differences in the CKR expression on and NK-cells may determine their ability to be recruited into inflamed tissues and colocalize with other cells at sites of inflammation, which is crucial for the success of the immune response. In addition, the phenotypic heterogeneity of the conventional and NK-cells may be largely due to the presence of transitional NK-cell populations, which may be preferentially expanded in some pathological conditions.

Further investigations in this area will help to better understand the terminal differentiation of the NK-cells and the maturation relationship between the NK-cell subsets, their circulation through the body, and their participation in the immune response. In addition, they will give an important contribution to establish phenotypic criteria to differentiate reactive and neoplastic NK-cell proliferations, as well as to better identify the normal cell counterparts from which the neoplastic NK-cells originate.


ADCC:Antibody dependent cell cytotoxicity
BC:Beckman Coulter
BD:Becton Dickinson
BDB:Becton Dickinson Biosciences
BM:Bone marrow
CKR:Chemokine receptors
CTL:Cytotoxic T lymphocytes
DC:Dendritic cells
ELR:Glutamic acid-leucine-arginine tripeptide motif
FcγRIIIa:Low affinity receptor for IgG Fc and alpha chain (CD16)
FCRL:Fc receptor-like
FITC:Fluorescein isothiocyanate
FSC:Forward light scatter
ILT:Immunoglobulin-like transcript
IP-10:Interferon-induced protein of 10 kD (CXCL10)
I-TAC:Interferon-inducible T cell alpha chemoattractant (CXCL11)
KIR:Killer cell immunoglobulin-like receptors
KLR:Killer cell lectin type receptors
LARC:Liver- and activation-regulated chemokine (CCL20)
LN:Lymph nodes
MCP-2:Monocyte chemotactic protein-2 (CCL8)
MDC:Macrophage-derived chemokine (CCL22)
MHC:Major Histocompatibility Complex
MIG:Monokine induced by gamma-interferon (CXCL9)
MIP-1α:Macrophage inflammatory protein 1 alpha (CCL3)
MIP-1β:Macrophage inflammatory protein 1 beta (CCL4)
NCAM:Neural cell adhesion molecule (CD56)
NCR:Natural cytotoxicity receptors
NK:Natural killer
PB:Peripheral blood
PC5:PE-Cyanine 5
RANTES:Regulated upon activation, normal T cell expressed and secreted (CCL5)
SSC:Side light scatter
SDF-1: Stromal cell derived factor type 1 (CXCL12)
TARC:Thymus and activation-regulated chemokine (CCL17)
TGF-β:Transformed growth factor beta
Th:T helper
TNF-α:Tumor necrosis factor alpha.

Conflict of Interests

The authors declare that they have no conflict of interests.

Authors’ Contribution

Margarida Lima carried out concept and research question, literature review, study design, data collection and interpretation, statistical analysis, and report writing. Ana Helena Santos, Catarina Lau, João Moura, Magdalena Leander, Marlene Santos, Maria dos Anjos Teixeira, Maria Luís Queirós, Marta Gonçalves, and Sónia Fonseca were responsible for sample processing, data acquisition, and data analysis. Catarina Lau and Maria dos Anjos Teixeira were responsible for data analysis and interpretation. Alberto Orfao was responsible for scientific consulting, paper revision, and discussion. All authors read and approved the final paper.


This work has been partially supported by a grant of the “Associação Portuguesa Contra a Leucemia” for the project “caracterização imunofenotípica de leucemias e linfomas de células T e NK: padrão de expressão de receptores de quimiocinas como determinantes do tropismo tecidular da célula neoplásica e do comportamento clínico-biológico da doença” and by the “Forum Hematológico do Norte,” Portugal.