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Oncologia Sperimentale C, Immunologia, Istituto Nazionale Tumori Fondazione Pascale, Naples, Italy;
Centro di Endocrinologia ed Oncologia Sperimentale del CNR, Naples, Italy; || Division of Immunology, The Netherlands Cancer Institute, Amsterdam, Holland; and ¶ Microbiology and Tumorbiology Center, Karolinska Institutet, Stockholm, Sweden
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NK cells represent a distinct lineage of lymphocytes that are able to kill a variety of tumor (1), virus-infected (2), bone marrow transplanted (3), and allogeneic target cells (4). NK cells do not express T cell receptors or immunoglobulins and are apparently normal in mice with defects in the recombinase machinery (5, 6).
Our knowledge about NK cell specificity has increased considerably in the last years. NK cells can probably interact with target cells by a variety of different cell surface molecules, some involved in cell adhesion, some activating the NK cytolytic program (7, 8), and other ones able to inhibit this activation by negative signaling (as reviewed in reference 9).
A common feature of several inhibitory NK receptors is the capability to bind MHC class I molecules (10, 11), as predicted by the effector inhibition model within the missing self hypothesis of recognition by NK cells (12–14). Interestingly, the MHC class I receptors identified so far belong to different gene families in mouse and man; these are the p58/p70/NKAT or killer cell inhibitory receptors (KIR)1 of the immunoglobulin superfamily in man and the Ly49 receptors of the C-type lectin family in the mouse. There is also evidence that MHC class I molecules can be recognized as triggering signals in NK cells of humans, rats as well as mice (13). The inhibitory receptors allow NK cells to kill tumor or normal cell targets with deficient MHC class I expression (12, 14). This does not exclude that other activating pathways can override inhibition by MHC class I molecules (15) and, even in their absence, there must be some activating target molecules that initiate the cytolytic program. Several surface molecules are able to mediate positive signals in NK cells. Some of these structures, like NKRP1 (16), CD69 (17), and NKG2 (18) map to the NK complex region (NKC) of chromosome 6 in mice and of chromosome 12 in humans (13). CD2 (19) and CD16 (20) molecules can also play a role in the activation pathway.
NK cells resemble T cells in many respects, both may arise from an immediate common progenitor (21, 22), and share the expression of several surface molecules (23). NK cells produce cytokines resembling those secreted by some helper T cell subsets (24) and contain CD3 components in the cytoplasm (21). The expression of some surface structures, involved in TCR-dependent T cell costimulation, like CD28 in human (25), has been described on NK cells, but the functional relevance of these molecules for NK activation processes has not been fully established.
Another T cell molecule of interest is CD40L, which interacts with CD40, a 50-kD membrane glycoprotein expressed on B cells (26), dendritic cells (27), and monocytes (28). CD40 is a member of the tumor necrosis factor/nerve growth factor receptor family (29) which includes CD27 (30), CD30 (31), and FAS antigen (32). Murine and human forms of CD40L had been cloned and found to be membrane glycoproteins with a molecular mass of
NK cells have also been suggested to play a role in B cell differentiation and immunoglobulin production (45). Therefore, it was of interest to investigate whether NK cells could use a CD40-dependent pathway in their interactions with other cells. Therefore, we have investigated the ability of target cells expressing CD40 to induce activation of NK cytotoxicity.
Monoclonal Antibodies, Immunofluorescence, and Flow Cytometry.
NK Polyclonal Populations, NK Lines, and NK Clones.
Cytotoxicity Assay.
39 kD induced on T cells after activation (33). Also mast cells (34), eosinophils (35), and B cells (36) can be induced to express a functional CD40L. The CD40L–CD40 interaction has been demonstrated to be necessary for T cell–dependent B cell activation (33, 37). Mutations in the CD40L molecule cause a hyper-IgM immunodeficiency condition in man (38, 39, 40). On the other hand, CD40–CD40L interactions also orchestrate the response of regulatory T cells during both their development (41, 42) and their encounter with antigen (43, 44).
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Materials and Methods
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Abstract
Materials and Methods
Results
Discussion
References
Cell Lines.
K562, an MHC class I–negative human erythroleukemia cell line, T2 and T2/TAP1+2, transfectants of the T2 T-B lymphoblast hybrid line (46), were cultured in RPMI 1640 (Biochrom K.G., Berlin, Germany) supplemented with 5% heatinactivated FCS and 2 mM glutamine (Biochrom) at 37°C in 5% CO2/95% air. In the T2/TAP1+2 culture the medium was supplemented weekly with 600 µg/ml G-418 (Sigma Chemical Co., St. Louis, MO). Murine mastocytoma P815 cells stably transfected with human CD40 cDNA containing plasmid expression vectors (43, 47), were a generous gift of Dr. L. Lanier (DNAX, Palo Alto, CA). These cell lines are referred to as CD40-P815 throughout.
mAb W6/32, (an IgG2a anti-HLA mAb recognizing a class I monomorphic determinant) was purchased from DAKO (Milan, Italy); mAb OKT3 (IgG2a anti-CD3) and TIB-200 (IgM antiCD57) were obtained from hybridomas provided by the American Type Culture Collection (Rockville, MD); mAb 14G7 (IgM anti-CD40) was a gift of Dr. R. van Lear (Central Laboratory of the Blood Transfusion Service, Amsterdam, The Netherlands); TRAP-1 (IgG1 anti-CD40L) was purchased from PharMigen (San Diego, CA); OX27 (IgG1, anti-rat MHC class I) was purchased from Serotec (Oxford, England). FITC- and PE-labeled mAbs against CD3, CD4, CD8, CD14, CD19, CD56, CD2, CD16, and isotype-matched labeled controls were purchased from Becton Dickinson (Mountain View, CA) and used to characterize the cell phenotype by immunofluorescence. Immunofluorescence, flow cytometry and data analysis were performed as described (48). To detect the CD40L expression on polyclonal IL-2–activated lymphocytes, PBMC, depleted of adherent cells, were incubated for the indicated periods in round-bottomed 96-well microtiter plates (Falcon; Becton Dickinson). At the indicated times, cells were washed in PBS and incubated for 30 min at 4°C with saturating concentrations of PE-labeled TRAP-1 mAb in the presence of FITC-labeled anti-CD3 or anti-CD56 or isotypic controls in a standard double-staining technique.
PBMC were isolated by centrifugation on Ficoll Hypaque (Biochrom) gradients from normal donor buffy coats obtained from the Blood Bank of the Medical School of the Federico II University of Naples. After isolation, the PBMC were washed and incubated in complete medium, in a horizontally placed plastic flask, for 2 h at 37°C to remove adherent cells. The recovered cells were used without any pretreatment or activated with rIL-2 as indicated in the results section. In some experiments, IL-2–activated effectors were depleted of CD3-positive cells by magnetic beads (Dynal, Oslo, Norway) coated with anti-CD3 mAb and a samarium cobalt magnet. The depletion procedure was repeated twice. 98% of the remaining cells were CD56+CD3–, as assessed by FACS® analysis. CD56–CD3+ effector populations were generated using anti-CD56 mAb-coated beads and the same depletion procedure. The NK3.3 human NK cell line was obtained from a healthy donor as described (49) and cultured in the presence of 1,000 IU/ml of human recombinant IL-2 (rIL-2). The JA2 NK clone was generated and characterized as described (50).
Cytotoxicity was measured in a conventional 4-h 51Cr-release assay. Target cells were labeled with Na251CrO4 (100 µCi/2 x 106 cells) and the percent of specific lysis was calculated as ([experimental release – spontaneous release] / [maximum release – spontaneous release]) x 100. The spontaneous release never exceeded 20%. Experimental details and mAb concentrations used in the reverse ADCC test as well as in the treatment of NK effectors with plastic immobilized antibodies are indicated in Results.
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Results
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Abstract
Materials and Methods
Results
Discussion
References
A CD40-dependent Activation Pathway Is Involved in Cytotoxicity Mediated by NK Clones and IL-2–activated NK Cells.
In an effort to assess the role of costimulatory molecules in NK activation, we studied the ability of the NK line NK3.3 and clone JA2 to recognize and kill P815 mastocytoma cells and their CD40 transfectants. Fig. 1 shows that the transfection of CD40 molecule is able, alone, to induce recognition and lysis of P815 targets by NK3.3 effectors (Fig. 1 A). Similar results were obtained with the NK clone JA2 (Fig. 1 B). Five NK clones were tested using the same target systems, giving comparable results (data not shown). Any interference due to possible clonal differences in the target susceptibility could be excluded by blocking experiments in which a chromium release assay was performed in the presence of targets pretreated with anti-CD40 IgM mAb 14G7 or with the isotypic control TIB-200 mAb (Fig. 1, C). These data indicated that a CD40-dependent pathway is functionally involved in the induction of cytotoxicity of human NK cell lines and clones. In contrast, fresh polyclonal NK cells that had not been exposed to IL-2 were not able to recognize the CD40-transfected targets (Fig. 2 A). However, PBMC depleted of plastic-adherent cells and activated with rIL-2 (1,000 IU/ml) were able to recognize and kill CD40 transfectants. P815 parental cells were not killed. The effect was detectable after 18 h of culture with IL-2, reached its maximum after 48 h and was still evident after 72 h of IL-2 treatment (Fig. 2 C). Depletion by magnetic beads of CD56+ but not CD3+ lymphocytes completely inhibited the killing of CD40-transfected P815 (Figs. 3, A and B). We conclude that a CD40-dependent cytotoxic pathway is functional, not only in human NK lines and clones, but also in polyclonal rIL-2–activated NK effectors, isolated from peripheral blood of normal individuals.
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This study demonstrates that a CD40-mediated activation pathway is functional in the induction of cytotoxicity mediated by IL-2–activated NK cells in humans. This effect is likely to be mediated by surface-expressed CD40L on human IL-2–activated CD3–CD56+ lymphocytes. To the best of our knowledge, this receptor has not been described earlier on NK cells. We found that fresh NK cells from normal donors did not express CD40L and were unable to kill the CD40 transfectants. Activation with IL-2 induced the CD40L as well as the capability to kill CD40 expressing targets. CD40L is normally expressed by T cells 6–8 h after activation, then it is quickly downmodulated (52). It is possible that a CD40-dependent pathway is functional also in NK effectors in vivo after activation with relevant stimuli. Our findings thus add the CD40–CD40L interaction to other triggering pathways for NK cells.
Previous reports (45, 53) demonstrated that NK cells can induce B cell maturation as well as immunoglobulin secretion and isotype switching. Our data open the possibility that these phenomena might in part be mediated by a CD40–CD40L interaction. CD40L+ T cells were recently described to induce IL-12 secretion by human peripheral blood monocytes, favoring Th1 priming in vitro (54, 55). mAb directed against CD40L molecules were able to prevent Th1-mediated IFN-
secretion in the same experimental model. Therefore, one may speculate as to whether NK cells expressing CD40L can mediate immunoregulatory functions. Our results suggest that negative signals mediated by MHC class I molecules on target cells can downregulate NK killing also when CD40 costimulatory molecules are involved in the triggering of cytotoxicity. These data suggest the possibility that the NK effector encounter with an APC could play a role in the regulation of some immune responses. The final outcome of this interaction could depend on the balance between activating molecules, like costimulatory structures, and MHC class I antigens expressed on the surface of the APC. In this context, inhibitory receptors, recognizing self MHC class I molecules and expressed on NK effectors, may serve as a fail-safe mechanism to prevent inappropriate responses and destruction of normal cells expressing CD40. Under some circumstances, soluble mediators, like inflammatory cytokines, may alter the balance between MHC class I inhibitory signals and costimulatory positive stimuli, activating the NK effector function. This could imply elimination of infected cells or old APCs no longer needed and potentially harmful by giving a persistent stimulation of a response.
NK cells can express distinct receptor sets able to control NK cytotoxicity. Inhibitory receptors were described to recognize MHC class I antigens expressed on targets cell surface, however some isoforms of such kind of receptors were also able to trigger NK cytotoxicity (13). Here we propose CD40L as a new NK activating molecule. Our observations suggest that the functional pathways involved in the induction of NK cytotoxicity rather than redundant, might be able to complement each other. In this context, the presence on the surface of NK effectors of activating receptors for MHC class I molecules, could be involved in the induction of cytotoxic mechanisms during allogeneic bone marrow rejection, or be able to sense modification in the structure or peptide loading of self MHC class I molecules expressed on the surface of infected cells. At variance, a role for CD40L in mechanisms of NK-mediated immune regulation, likely involving CD40 expressing targets, as monocytes and dendritic cells, could also be proposed. In addition, high CD40 expression levels might override MHC class I–mediated inhibition of NK cytotoxicity.
Recent observations, obtained in CD40–CD40L knockout murine models, have demonstrated a critical role for CD40–CD40L interaction in the activation of T cells in vivo (44, 56). Two possible mechanisms are believed to likely account for such a role. First, CD40L might be a receptor for T cell costimulation by CD40 expressing APC (42, 43); second, CD40L may be an inducer of costimulatory activity and the induced costimulatory molecules are essential for T cell activation (57, 58). Our data indicate that CD40L triggering enables human NK effectors to recognize and kill resistant targets. On the other hand, we are not able to discriminate if CD40L engagement on NK cells can directly affect the cytotoxicity machinery or mediates the induction of distinct molecules in order to activate the NK lysis programs. Similar findings were already referred to in CD40 knockout mice, in which in vivo priming in the presence of soluble CD40 was shown to partially overcome the need for CD40 expressing B cells in order to obtain a mature antibody response and to generate memory cell populations (56).
The described CD40–CD40L NK activation pathway is disrupted in the hyper-IgM immunodeficiency syndrome. These patients have a defective CD40L expression, leading to an impairment of the Ig switch and persistency of IgM production. Many of the infections in these patients can be attributed to the resulting lack of the humoral response. Some are more typical of defective cell-mediated response. The results presented here suggest that these pathologies might be due to a defective CD40L expression on NK cells.
| Acknowledgments |
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This work was supported by grants from the Scuola Superiore di Immunologia "Ruggero Ceppellini" (Naples), the Associazione Italiana per la Ricerca sul Cancro (AIRC, Milan), and the Italian Health Ministry Project "Tubercolosi". G. Terrazzano was supported by a fellowship from Istituto Italiano per gli Studi Filosofici, Naples. E. Carbone was supported, during his stay at Karolinska Institutet (Stockholm), by a fellowship from "Progetto di Scambi Internazionali", University of Naples "Federico II".
Submitted: 20 December 1996
Revised: 14 April 1997
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