By
From the * Istituto Nazionale per la Ricerca sul Cancro and Centro Biotecnologie Avanzate, 16132 Genova, Italy; and the Dipartimento di Medicina Sperimentale, Università degli Studi di Genova,
16132 Genova, Italy
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Abstract |
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After culture in interleukin (IL)-2, natural killer (NK) cells acquire an increased capability of
mediating non-major histocompatibility complex (MHC)-restricted tumor cell lysis. This may
reflect, at least in part, the de novo expression by NK cells of triggering receptors involved in cytolysis. In this study we identified a novel 44-kD surface molecule (NKp44) that is absent in
freshly isolated peripheral blood lymphocytes but is progressively expressed by all NK cells in vitro
after culture in IL-2. Different from other markers of cell activation such as CD69 or VLA.2,
NKp44 is absent in activated T lymphocytes or T cell clones. Since NKp44 was not detected in
any of the other cell lineages analyzed, it appears as the first marker specific for activated human
NK cells. Monoclonal antibody (mAb)-mediated cross-linking of NKp44 in cloned NK cells
resulted in strong activation of target cell lysis in a redirected killing assay. This data indicated
that NKp44 can mediate triggering of NK cell cytotoxicity. mAb-mediated masking of NKp44
resulted in partial inhibition of cytolytic activity against certain (FcR-negative) NK-susceptible target cells. This inhibition was greatly increased by the simultaneous masking of p46, another recently identified NK-specific triggering surface molecule. These data strongly suggest
that NKp44 functions as a triggering receptor selectively expressed by activated NK cells that,
together with p46, may be involved in the process of non-MHC-restricted lysis. Finally, we
show that p46 and NKp44 are coupled to the intracytoplasmic transduction machinery via the
association with CD3
or KARAP/DAP12, respectively; these associated molecules are tyrosine phosphorylated upon NK cell stimulation.
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Introduction |
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Human NK cells express a series of clonally distributed HLA class I-specific killer inhibitory receptors (KIR) that suppress the NK cytotoxicity upon engagement with self HLA class I alleles (1). At least in the case of HLA-C-specific receptors, an activating form (termed p50) has also been identified. Upon interaction with appropriate HLA-C alleles, p50 triggers the NK-mediated cytolytic activity (5, 6). Therefore, HLA class I-specific receptors may regulate the NK cell function both in a negative and a positive fashion. However, NK cells are known to efficiently kill target cells that do not express HLA class I molecules, thus implying the existence of triggering receptors for non-HLA ligands. Since all NK cell clones kill HLA-negative target cells (1, 7, 8), it is conceivable that triggering receptors for non-HLA ligands are broadly expressed in NK cells. However, major variability exists in the susceptibility of different HLA class I-negative target cells to NK-mediated lysis (9, 10). This phenomenon may reflect a variable expression of different non-HLA ligands. Moreover, culture in exogenous IL-2 greatly enhances the cytolytic activity of NK cells against HLA-negative target cells (11). A possible explanation could be the de novo expression in NK cells of triggering receptors specific for additional non-HLA ligands. At present, little is known on the nature of the triggering NK receptors or of their non-HLA ligands. Along this line, we recently identified a novel triggering surface molecule, termed p46, that is specifically expressed on all resting and activated human NK cells (14). mAb-mediated masking of p46 molecules resulted in a low degree inhibition of NK cell-mediated lysis of some HLA-negative target cells (14). A possible interpretation of these data is that p46 may contribute to the process of tumor cell lysis, which, however, would require the cooperation of additional receptor-ligand interactions.
In this study, we describe a novel NK-specific molecule termed NKp44 that, similar to p46, mediates triggering of all NK cell clones. However, different from p46, NKp44 is only expressed by activated NK cells, thus representing a useful marker for their identification. More importantly, NKp44 appears to function as a receptor initiating a pathway of NK cell triggering during the process of non-MHC-restricted cytotoxicity by activated NK cells.
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Materials and Methods |
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Antibodies.
The following mAbs were produced in our lab: JT3A (IgG2a anti-CD3); c127 (IgG1 anti-CD16); KD1 (IgG2a anti-CD16); c218 (IgG1 anti-CD56); A6-220 (IgM anti-CD56); c227 (IgG1, anti-CD69); BAB281 (IgG1, anti-p46); and FES172 (IgG2a anti-p50.3). mAbs D1.12 (IgG2a anti-HLA-DR), HP2.6 (IgG2a anti-CD4), and B9.4 (IgG2b anti-CD8) were provided by Drs. R.S. Accolla (Università di Pavia, Pavia, Italy), P. Sanchez-Madrid (Hospital de la Princesa, Madrid, Spain), and B. Malissen (Centre de Immunologie INSERM/CNRS, Marseille, France). The 24-31 mAb (IgG1, anti-CD154/CD40L) was purchased from Ancell (Bayport, MN), and the Gi9 mAb (IgG1, anti-CD49b/ VLA.2) was purchased from Immunotech (Marseille, France).Production of Z231 mAb and Flow Cytofluorometric Analysis.
Z231 mAb (IgG1) was obtained by immunizing a 5-wk-old BALB/c mouse with the NK clone SA260. Analysis of the cell distribution of NKp44 molecules was performed by using one- or two-color fluorescence cytofluorometric analysis (FACScan®, Becton Dickinson, San Jose, CA), as previously described (15).Purification of Polyclonal or Clonal NK or T Cell Populations and Cytolytic Activity.
PBMCs were depleted of plastic-adherent cells. To obtain NK cell-enriched lymphocyte populations, PBMCs were incubated with anti-CD3 (JT3A) and anti-CD4 (HP2.6) mAbs for 30 min at 4°C followed by treatment with goat anti- mouse-coated Dynabeads (Dynal, Oslo, Norway) for 30 min at 4°C. The resulting CD3Biochemical Characterization of the NKp44 Molecules.
Cyanogen bromide sepharose (Pharmacia Biotech, Piscataway, NJ)-coupled Z231 or BAB281 mAbs or Sepharose-protein A-coupled KD1 and C227 mAbs were used to immunoprecipitate specific molecules from 1% NP40 lysates of cells surface-labeled with 125I (DuPont-NEN, Wilmington, DE) as previously described (5, 15). Immunoprecipitates were analyzed by discontinuous SDS-PAGE. Specific bands were cut out from dried gel and eluted proteins were used for N-glycosidase F digestion (Boehringer Mannheim, GmbH, Mannheim, Germany) (5, 16). For two-dimensional peptide mapping analysis, the purified proteins were digested with pepsin and peptides were analyzed by electrophoresis in the first dimension (Multiphor II, Pharmacia Biotech), followed by chromatography in the second dimension (5, 16).Biochemical Characterization of the p46- and NKp44-associated Molecules.
108 NK cells were stimulated or not with 100 µM Na-pervanadate (NaV, 10', 37°C), washed twice in cold PBS, and lysed in PLB (50 mM Tris-HCl, pH 8, 15 mM NaCl, 1% Digitonin, and phosphatase and protease inhibitors) (45', 4°C). Lysates were immunoprecipitated with various mAbs for 1 h at 4°C. Immunoprecipitates were washed twice in PLB and eluted samples were analyzed in 15% discontinuous SDS-PAGE under reducing conditions. Proteins were transferred to Immobilon P (Millipore Corp., Bedford, MA), and the membranes were saturated with 5% BSA and probed with TIA/2 (anti-CD3 ![]() |
Results and Discussion |
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Mice were immunized with the NK cell clone SA260 (surface phenotype: CD3, CD16+, p46+, p70+, NKG2A+). After cell fusion, an mAb termed Z231 was selected on the basis of its
ability to induce lysis of the Fc
R+ P815 target cells by a
panel of NK cell clones. The magnitude of cytolysis induced by Z231 mAb in this redirected killing assay was
comparable to that induced by anti-CD16 or by the anti-p46 mAb BAB281 (Fig. 1 a). Cell lysates, obtained from
IL-2-cultured, polyclonal NK cells that had been surface
labeled with 125I, were immunoprecipitated with Z231
mAb and analyzed by SDS-PAGE. A broad band of ~44-kD could be identified both under reducing and nonreducing conditions (Fig. 1 b). The mobility of this molecule (hereafter termed NKp44) was slightly modified by treatment with N-glycosidase F (Fig. 1 c). On the other hand,
treatment with neuraminidase followed by 0-glycanase had
no effect (data not shown). These data suggest that NKp44
may represent a molecule with a low degree of glycosylation.
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Cytofluorimetric analysis revealed that NKp44 was expressed on virtually 100% of cells in activated polyclonal
NK cell populations (Fig. 2, box A). Moreover, it was expressed in all NK cell clones analyzed. Fig. 2 (boxes B-E)
shows four representative clones that illustrate the heterogeneity in the levels of NKp44 expression among different
clones. On the other hand, none of the numerous T cell
clones or polyclonal T cell populations analyzed expressed NKp44 (Fig. 2, boxes F-L). This was also true for selected
T cell clones (either TCR-/
+ or -
/
+) expressing NK
cell markers including CD56, CD16, and KIRs (p58, p70,
or CD94/NKG2A). In addition, Z231 mAb did not stain
fresh PBMCs (isolated from different donors), granulocytes,
monocyte/macrophages, enriched T, B, or NK cells isolated from peripheral blood, EBV-induced B cell lines, or
various nonhematopoietic cell lines (data not shown).
These data indicate that NKp44 is specifically expressed by
activated NK cells, thus differing from other molecules
such as p46 or CD16, which are expressed also in resting
NK cells. We next analyzed the surface expression of
NKp44 in unfractionated PBMCs cultured in IL-2. The
expression of NKp44 or other informative markers including CD56, p46, CD69, and VLA-2 was analyzed at various time intervals by double fluorescence against CD3. As
shown in Fig. 2 (day 0), at the onset of the culture NKp44
molecules were undetectable in both CD3+ and CD3
cells. After 10 d of culture, NKp44 was expressed by a portion of CD3
cells, whereas after 20 d, virtually all CD3
cells were NKp44+. No expression of NKp44 could be
detected in CD3+ T cells even after 30 d of culture (data
not shown). These data support the notion that NKp44
represents a novel marker of cell activation specifically expressed by NK cells. In agreement with previous data, the
other markers of cell activation, used for comparison, were
not confined to NK cells (17, 18). Thus, CD69 (Fig. 2) and
VLA-2 (data not shown) were expressed both in CD3+ and
CD3
cultured cells. In addition, CD69 reached maximal
expression after 24-48 h of culture (17), whereas expression of NKp44 only started after 3 d and progressively increased thereafter (data not shown). In agreement with the
data that NKp44 is not expressed in fresh NK cells, mAb
Z231 failed to trigger target cell lysis by fresh CD3
cells in
a redirected killing assay. Increments in cytolytic activity
induced by Z231 mAb paralleled the progressive expression of NKp44 in cultured NK cells. Moreover, triggering
of cytolytic activity in a redirected killing assay was detectable only by using the intact mAb and not the F(ab')2 fragment (data not shown). This data indicates that NKp44 requires efficient cross-linking to induce NK cell triggering.
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Both the finding that NKp44 mediates triggering of NK
cell cytotoxicity and the finding that its expression is confined to NK cells are reminiscent of the previously described p46 molecule. The two molecules also display a
similar molecular mass (44 and 46 kD, respectively). In addition, as with p46, cross-linking of NKp44 led to Ca2+
mobilization and lymphokine (TNF- and IFN-
) release
in cloned NK cells (data not shown). A major difference
between the two molecules is that NKp44 is expressed only
upon NK cell activation. However, the possibility exists
that the epitope recognized by the Z231 mAb, although
carried by p46 molecules, may be exposed only upon cell
activation. To verify this possibility, 125I-labeled cell lysates
from either polyclonal or clonal (IL-2-cultured) NK cell
populations were precleared with anti-p46 mAb followed by immunoprecipitation with anti-NKp44 mAb (and vice
versa). These experiments clearly indicated that the two
molecules were distinct (Fig. 3 a). These data were further
confirmed by two-dimensional peptide map analysis: as
shown in Fig. 3 b, none of the spots visualized in the
NKp44 map correspond to those of the p46 map and vice
versa.
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In human NK cells, triggering receptors
such as CD16 or p50 (i.e., the activating forms of the
HLA-C-specific receptors; references 5, 6, 16) are coupled
to the intracytoplasmic transduction machinery via the association with polypeptide subunits characterized by the
presence of immunoreceptor tyrosine-based activation motifs (ITAMs). CD16 associates with CD3/Fc
RI
polypeptides (19, 20), whereas p50 molecules have recently
been shown to be part of a multimeric complex formed by
the association of these receptors with a newly identified
transducing molecule, termed KARAP/DAP12 (21, 22).
Both CD3
and KARAP/DAP12 molecules are tyrosine phosphorylated after engagement of the associated receptors. We analyzed whether NKp44 and p46 molecules
were associated with one or another signal-transducing
molecules. Cell lysates derived from NK bulk populations,
stimulated or not with Na-pervanadate, were immunoprecipitated with anti-NKp44, anti-p46, anti-p50.3, or anti-CD16 mAbs; samples were analyzed in SDS-PAGE, transferred to PVDF membranes, and then immunoblotted with
anti-CD3
(Fig. 4 a) or anti-PTyr (Fig. 4 b) mAbs. No tyrosine phosphorylation of either p46 or NKp44 molecules
could be detected under these experimental conditions
(data not shown). On the other hand, as shown in Fig. 4 a,
p46, like CD16, was associated with CD3
molecules that
became tyrosine phosphorylated upon cell stimulation (Fig. 4 b). On the contrary, NKp44 did not associate with
CD3
molecules (Fig. 4 a). However, after stimulation, tyrosine phosphorylated molecules displaying a molecular
mass of ~14-16 kD could be detected in NKp44 immunoprecipitates (Fig. 4 b). These molecules displayed a molecular mass similar to the p50.3-associated KARAP/DAP12
molecules (Fig. 4 b). In addition, the 14-16-kD NKp44-
associated molecules were still detectable after extensive
preclearing with anti-CD3
/Fc
RI
Abs. This experiment
ruled out the possibility that the NKp44-associated 14-16-kD tyrosine phosphorylated molecules were either CD3
or Fc
RI
, and suggested that they correspond to the recently identified KARAP/DAP12 molecules.
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Since NKp44 represents a triggering surface molecule that is selectively expressed by activated NK cells, it is possible that it represents
a receptor involved in the process of non-MHC-restricted tumor cell lysis by activated NK cells. To analyze this possibility, (NKp44+) NK cell clones were selected for their
ability to kill a panel of (FcR
) human target cell lines.
Selected clones were next assessed for cytotoxicity either in
the presence or absence of Z231 mAb. Although in the absence of mAb all targets were efficiently lysed, in the presence of Z231 mAb a minor inhibitory effect was detectable against the A549 (lung adenocarcinoma), RPMI 8866, and
LCL 721.221 (EBV-transformed B cell lines) target cells
(Fig. 5). A similar inhibitory effect was obtained with the
anti-p46 mAb BAB281. These data suggested that NKp44
and p46 molecules could cooperate in the process of NK
cell triggering, resulting in killing of A548, 221, and 8866 cells. To assess this possibility, anti-NKp44 and anti-p46
mAbs were used in combination. Indeed, a sharp inhibition of target cell lysis was obtained (Fig. 5), thus suggesting that the two molecules may function as receptors that act synergistically in the process of non-MHC-restricted lysis by activated NK cells. No inhibition of cytotoxicity was detected in the presence of a control, isotype-matched antibody
(CD56, IgG1; data not shown) or in the presence of an antibody directed to another triggering molecule expressed by
NK cells after activation (anti-CD69, IgG1). In addition,
the inhibitory effects of anti-NKp44 or p46 mAb were not
increased when used in combination with anti-CD69 mAb
(Fig. 5).
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Remarkably, killing of IGROV cells was not affected by anti-p46 and anti-NKp44 mAbs even when used in combination. This may be explained as follows. (a) IGROV cells do not express, or express in low density, the ligands recognized by NKp44 or p46 receptors. (b) IGROV cells do express the p46 and NKp44 ligands; however, their recognition by the corresponding NK receptors does not contribute significantly to NK cell triggering. In any case, it is conceivable that IGROV cells express additional ligands for still undefined triggering NK receptors. Interactions between these receptors and their ligands would play a predominant role in the NK-mediated lysis of these target cells. These data are consistent with the idea that NK cell triggering may be mediated by several different receptor- ligand interactions (23). Expression, lack of expression, or even different levels of expression of given ligands for triggering receptors may influence the susceptibility of tumor cells to NK cell-mediated killing. In this context, the use of target cells such as A549, 8866, and 221 gave useful results in this study in identifying the role of NKp44 molecule as a triggering receptor in the NK-target cell interactions.
Regarding the still undefined ligand for NKp44, the use
as target cells of the HLA class I 221 cell line transfected
or not with different HLA class I alleles ruled out the possibility that HLA class I molecules could trigger NK cells via
NKp44 (data not shown).
The NK-mediated target cell lysis may also depend upon the type (and density) of receptors on different effector cells. For example, since NKp44 molecule is selectively expressed by activated NK cells, it is conceivable that it may play at least a partial role in the enhanced cytolytic activity of these cells. Although both NKp44 and p46 are homogeneously expressed on activated NK cells, it is possible that other triggering receptors may be expressed selectively by subsets of NK cells (as is the case for the HLA-C-specific p50 molecules).
In humans, other surface molecules, including CD69 (24) and CD40L (25), have been reported to be involved in triggering of cytotoxicity mediated by in vitro activated NK cells. However, these molecules are not NK-specific and do not appear to interfere with non-MHC-restricted target cell lysis. Regarding the CD40L, we found that it is expressed only by some NK cell clones and we failed to induce any substantial NK cell triggering in redirected killing assays (Fig. 1 a).
Gp42, a 42-kD, NK-specific rat molecule, is reminiscent of NKp44 (26, 27). Indeed, it was detected in IL-2-induced NK cells and was shown to trigger cytotoxicity mediated by the RNK-16 rat NK cell line. However, Gp42 failed to activate IL-2-induced NK cells. In addition, Gp42 is a glycosylphosphatidylinositol-anchored surface molecule, which is not the case for GPI NKp44 since its surface expression is not modified by cell treatment with phosphatidylinositol-specific phospholipase (data not shown).
In conclusion, this study revealed a novel surface molecule selectively expressed by human activated NK cells that appears to function as a triggering receptor involved in non-MHC-restricted cytotoxicity by activated NK cells. The identification of NKp44 as a putative NK receptor may lead to a more precise definition of the nature of the receptor-ligand interactions occurring in the NK-mediated lysis of tumor cells.
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Footnotes |
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Address correspondence to Lorenzo Moretta, Centro Biotecnologie Avanzate, Largo R. Benzi 10, 16132 Genova, Italy. Phone: 39-10-57-37-217; FAX: 39-10-35-41-23; E-mail: miriello{at}sirio.cba.unige.it
Received for publication 3 March 1998 and in revised form 8 April 1998.
We thank Mrs. Cinzia Miriello for typing the manuscript.
This work was supported by grants awarded by the Associazione Italiana per la Ricerca sul Cancro (A.I.R.C.), Istituto Superiore di Sanità (I.S.S.), Consiglio Nazionale delle Ricerche (C.N.R.), and Progetto Finalizzato Applicazioni Cliniche per la Ricerca Oncologica.
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