American Journal of Respiratory and Critical Care Medicine

Malignant pleural mesothelioma is an uncommon tumor largely confined to the thoracic cavity, which is resistant to conventional therapies, therefore prompting an intensive search for effective treatment alternatives. This study focuses on dendritic cell (DC) vaccination for malignant pleural mesothelioma and evaluates the in vitro efficacy of antigen-loaded DC-based vaccines for the induction of major histocompatibility complex Class I-restricted antimesothelioma cytotoxic T lymphocyte responses. The source of tumor-associated antigens for HLA-A2+ DCs from healthy donors was apoptotic HLA-A2 mesothelioma cells either lacking or expressing heat shock protein 70 according to whether tumor cells were heat shocked or not before ultraviolet-mediated apoptosis. Our results show that both apoptotic preparations were equivalent regarding the responsiveness of DCs to combined treatment with tumor necrosis factor-α and poly(inosinic-cytidylic) acid, as determined by similar increased expression of costimulatory molecules and interleukin-12 production. However, only DCs loaded with apoptotic heat shock protein 70-expressing cells were found to be potent in vitro inducers of cytotoxic T lymphocyte activity against HLA-A2+ mesothelioma cells. Such elicited cytotoxic T lymphocytes also exhibit cytotoxic activity against an HLA-A2+ melanoma cell line, suggesting recognition of shared antigens. These findings therefore carry the potential of offering an alternative, promising approach for the therapy of patients with malignant pleural mesothelioma.

Malignant pleural mesothelioma (MPM) is expected to increase in most industrialized countries because of the widespread use of asbestos over the past century (13). MPM is an aggressive tumor generally arising from serosal cavities and whose latency period is between 15 and 40 years (4). The prognosis for patients with this disease is poor, as the overall median survival ranges from 1 to 9 months (5). In spite of the effectiveness of lovastatin and talc to kill mesothelioma cells in vitro (6, 7), therapy of MPM remains challenging because conventional treatments such as surgical resection followed by radiotherapy and/or chemotherapy do not significantly improve the outcome of the disease (811). Likewise, alternative therapeutic strategies based on pleural injections of recombinant cytokines (e.g., interleukin [IL]-2, IL-12, and IFN-γ) remain quite unsatisfactory as they have shown little potential for improving the overall survival of patients with MPM (1215).

One possibility for improved treatment may be the design of new immunotherapeutic strategies. Such an approach involves the activation of tumor-specific T cells and their migration to the tumor site, where the recognition of relevant elements leads to the elimination of tumor cells (16). To date, much attention has been focused on active immunotherapy involving dendritic cells (DCs) as vectors for antigenic targets (17, 18). DCs are now commonly described as highly potent professional antigen-presenting cells that are uniquely capable of priming naive T cell responses (19, 20). Indeed, DC-based vaccination strategies have yielded encouraging clinical data in patients with metastatic malignant melanoma (2123) or renal cell carcinoma (24, 25).

The source of tumor-associated antigens (TAA) for DCs remains a critical issue that will determine the efficacy of DC-based vaccination. Most current clinical vaccination protocols are based on pulsing DCs with MHC Class I–restricted peptides of known sequence, therefore requiring previous identification and characterization of antigenic epitopes. To date only a few TAAs for MPM have been defined, such as those belonging to the cancer testis antigens (26).

For this reason, we focused on another approach to TAA delivery, based on the uptake of dead cells (necrotic or apoptotic cells) by immature DCs, which offers several advantages over vaccinating with a single or only a few identified antigens. Indeed, feeding DCs with apoptotic tumor cells provides a full array of antigenic peptides that rapidly gain access to both MHC Class I (cross-presentation) and MHC Class II pathways, therefore leading to a diversified immune response involving cytotoxic T lymphocytes (CTLs) as well as CD4+ T cells (27, 28). We have already shown the potential of apoptotic cell-pulsed DCs in eliciting specific MHC Class I–restricted cytotoxic T cell responses both in vivo (29, 30) and in vitro (31).

Cells dying from apoptosis are thought to be weakly immunogenic as they may significantly impede exogenous stimuli-driven DC maturation (32, 33), thereby modulating the immune response toward tolerance rather than immunity (34). However, reports have argued the feasibility of overriding the inhibitory effects of apoptotic cell ingestion on DC maturation through triggering apoptosis in the presence of “danger signals” such as increased expression of heat shock proteins (HSPs) (35). Indeed, HSPs have been reported to be involved in (1) the induction of DC maturation on binding of several cell surface receptors (3638) and (2) the protection of antigenic peptides from degradation along the MHC Class I pathway (39). Such key roles therefore emphasize the relevance of providing both HSPs and TAAs for DCs, with the aim of generating efficient tumor-specific immune responses.

The purpose of the present study was to design an in vitro model for the development of a therapeutic vaccine against MPM based on apoptotic tumor cell–pulsed DCs. Because the harvest of both tumor cells and peripheral blood mononuclear cells from the same individual with MPM proved to be particularly difficult, an HLA-A2 allogeneic mesothelioma cell line was used for DC-loading experiments. We observed that only DCs fed with apoptotic HSP70-overexpressing mesothelioma cells, generated from an HLA-A2 mesothelioma cell line, were capable of cross-priming naive T cells obtained from HLA-A2+ healthy donors for tumor-specific cytotoxic T cell responses against HLA-A2+ mesothelioma cells. These findings show the potential of DCs, when pulsed with apoptotic HSP70-expressing mesothelioma cells, for future immunotherapeutic strategies in the treatment of MPM.

Media and Cell Lines

The mesothelioma cell lines Meso13 (HLA-A2) and Meso11 (HLA-A2+) were established in our laboratory from tumor pleural fluids of patients with histologic diagnosis of epithelioid MPM according to standard methods (Sapede and coworkers, unpublished data). The melanoma cell lines M17 (HLA-A2+) and M136 (HLA-A2) were a kind gift from F. Jotereau (U463 INSERM Unit, Nantes, France). Culture medium consisted of RPMI 1640 (Life Technologies, Cergy Pontoise, France) supplemented with 10% fetal calf serum (Eurobio, Les Ulis, France), 1% penicillin–streptomycin, and 1% l-glutamine (Life Technologies).

DC Preparation

Immature monocyte-derived DCs were generated from leukapheresis products of HLA-A2+ healthy donors (Nantes Etablissement Français du Sang, Nantes, France) after obtaining informed consent. Monocytes were enriched by adding monocyte RosetteSep cocktail (StemCell Technologies, Vancouver, BC, Canada) and then separated from leukapheresis products by Ficoll-Paque (Amersham Biosciences, Uppsala, Sweden) centrifugation and cultured in X-VIVO 15 medium (Cambrex Bio Science Walkersville, Walkersville, MD) with granulocyte-macrophage colony-stimulating factor (Leucomax, 500 IU/ml; Novartis, Rueil-Malronion, France) and IL-4 (50 ng/ml; AbCys, Paris, France).

Induction and Detection of Apoptosis

Meso13 tumor cells were induced to undergo apoptosis by exposure to ultraviolet (UV)-B (25 kJ/m2) using a UV-stratalinker 2,400 (Stratagene, Amsterdam, The Netherlands). In some experiments, UV-mediated apoptosis was consequently induced after heat shock (30 min, 42°C). Cell death was measured with an annexin-V kit (BD Pharmingen, Le Pont de Claix, France) and was quantified by a caspase-3 colorimetric protease assay (CaspACE assay system; Promega, Madison, WI). Additional details are provided in the online supplement.

RNA Isolation and Reverse Transcription-Polymerase Chain Reaction

One microgram of TRIzol-prepared total RNA was used for reverse transcription with oligo(dT) primers and avian myeloblastosis virus reverse transcriptase (Roche Diagnostics, Mannheim, Germany). Polymerase chain reaction (PCR) amplification of the GAGE-1, -2, and -7; GAGE-3–6 and -8; and β2-microglobulin sequences was performed with specific primers (see online supplement).

Apoptotic Cell Loading and DC Maturation

Ingestion of apoptotic tumor cells was assessed by both flow cytometry and confocal microscopy as previously described (40) (see online supplement). Maturation was induced by 48 hours of treatment of a combination of tumor necrosis factor (TNF)-α (20 ng/ml; R&D Systems, Abingdon, UK) and poly(inosinic-cytidylic) acid [poly(I:C); 50 μg/ml; Sigma, St. Louis, MO].

Flow Cytometric Analysis

Phenotypic marker expression associated with maturation was assessed by flow cytometry, using DCs labeled with the fluorescein isothiocyanate (FITC)-conjugated CD80, CD83, HLA-ABC (Immunotech, Marseille, France), CD86, and HLA-DR (Caltag, Burlingame, CA) antibodies (see online supplement).

Cytokine Detection

IL-10 and IL-12 p70 production was determined by ELISA (BD Biosciences Pharmingen) assays according to the manufacturer's protocol.

In Vitro Sensitizations and Cytotoxicity Assay

The first stimulation was performed in 96-well culture plates by mixing 3 × 104 mature DCs with 3 × 105 responder naive T cells in RPMI 1640 medium containing IL-6 (5 ng/ml). The two subsequent stimulations were performed at 7 days in medium containing IL-2 (25 U/ml) and IL-7 (5 ng/ml) (R&D Systems). The method for measuring cytotoxicity activity is described in the online supplement.

Immunoblotting Analysis

Forty micrograms of protein from each cell extract was electrophoresed in sodium dodecyl sulfate–polyacrylamide gels and immunoblotted with anti-GRP94 (glucose-regulated protein 94), anti-HSP70, anti-HSP60, anti-HSP27 (StressGen Biotechnologies, Victoria, BC, Canada), and anti-actin (Chemicon International, Temecula, CA) monoclonal antibodies (see online data supplement).

HSP Content in Viable, Stressed, and Killed Mesothelioma Cells

On the basis of more recent findings reporting that HSP may play a key role in promoting antigen DC presentation (37, 38), we first sought to determine whether these proteins were present in Meso13 cells to be used in immunization experiments. As indicated in Figure 1A

, Western blotting analysis for HSP revealed high levels of basal expression for GRP94, HSP60, and HSP27 under control conditions, whereas only a weak signal was detected for HSP70. However, exposing the cells to an elevated temperature of 42°C followed by a recovery period of 5 hours at 37°C resulted in a substantial increase in the level of HSP70 protein expression (Figure 1A). To the naked eye, the expression pattern of GRP94, HSP60, and HSP27 in heat-shocked cells seemed quite similar to that noted for control cells.

To better quantify the heat shock effects on HSP expression, we compared the signal intensities of autoradiographs from Western blot analyses of each HSP with those of β-actin, using standard densitometry scanning methods. Hence, normalization experiments with β-actin therefore confirmed that GRP94, HSP60, and HSP27 accumulate only to slightly higher levels as a result of heat treatment whereas a sharp 10-fold increase in HSP70 content was observed as compared with control cells (Figure 1B). This therefore suggests that HSP70 is likely the major HSP to be maximally synthesized in response to a heat shock of 42°C in these cells. However, to further check whether the increased expression of HSP70 might be preserved in the apoptotic process, Western blots were performed on UV-treated cells (as a negative control) and on UV-treated cells after heat shock (HS+UV). As shown in Figure 1C, HS+UV treatment did not affect HSP content, which remained highly abundant as judged by comparison with that detected in UV-treated cells. Collectively, these findings underscore the requirement for heat shocking tumor cells before UV exposure to obtain killed mesothelioma cells expressing high levels of HSP70 as a source of TAA for DCs.

Heat Shock Does Not Modify the Sensitivity of Tumor Cells to UV-mediated Apoptosis

Because HSP70 is also thought to be a protein preventing apoptosis, we further assessed the apoptotic features of viable cells (as a negative control) and of cells undergoing UV, HS, or HS+UV treatments. Annexin-V/propidium iodide (PI) staining indicates that most of the HS-treated cells were annexin-V/PI by dot-blot analysis using flow cytometry, whereas the majority of UV- and HS+UV-treated cells were annexin-Vhigh/PIhigh, namely late apoptotic, after 24 hours of incubation (Figure 2A)


Cell death was also assayed by measuring the cellular caspase-3 activity in lysates from viable cells (as a negative control) and from UV-, HS-, and HS+UV-treated cells. The representative results shown in Figure 2B confirm that HS-treated cells did not undergo apoptosis, as determined by the cleavage of Ac-DEVD-AMC representing capase-3 activity, which remained very low and constant over time. Conversely, activation of Ac-DEVD-AMC–specific caspases could be seen as early as 12 hours postincubation in UV-treated cells. Meanwhile, the main peak of caspase activity in HS+UV-treated cells occurred only at 24 hours incubation. This delayed caspase-3 activity might be partially explained by the overexpression of HSP70, which is thought to modulate multiple apoptotic pathways (41). However, the increased caspase-3 activities detected in lysates from both UV- and HS+UV-treated cells within 24 hours therefore indicate that these cells were late apoptotic, but not necrotic, cells. This further underlies the feasibility of producing late apoptotic cells either lacking HSP70 (apo[UV]) or expressing HSP70 (apo[HS+UV]).

DC Engulfment of Apoptotic Mesothelioma Cells

To check whether Day 7 immature DCs may be capable of capturing apoptotic mesothelioma cells, DCs were cocultured with apoptotic cells stained with PKH26 at a ratio of 2:1 for 24 hours. Interactions of HLA-DR–labeled DCs with PKH26-labeled apoptotic cells were then determined by flow cytometry as the percentage of FITC–HLA-DR+ DCs that expressed the red dye PKH26. As shown in Figure 3A

, most apoptotic mesothelioma cells were efficiently taken up by immature DCs within 24 hours at 37°C, as the whole population consists mainly of PKH26/HLA-DR double-positive cells. In contrast, less than 3% of DCs exhibited PKH26 staining at 4°C, indicating that the process of engulfing apoptotic material was almost completely inhibited (Figure 3B). It should be noted that heat shocking the tumor cells before triggering apoptosis did not affect the capacities of DCs to engulf dying cells because the rates of phagocytosis between apo(UV) and apo(HS+UV) were quite similar, regardless of the level of HSP70 protein expression (Figure 3A, left and right, respectively). Confocal laser-scanning microscopy, which is the only technique permitting discrimination between ingestion and simple binding, further confirms the efficient internalization of apoptotic tumor cells by immature DCs, whatever the death-inducing strategy used (Figure 3A).

Apoptotic Mesothelioma Cells Exert Slight Inhibitory Effects on DC Maturation Driven by TNF-α and Poly(I:C)

To ascertain whether apoptotic cell ingestion may impair DC maturation in response to exogenous inflammatory stimuli, DC activation was monitored by evaluating the level of expression of CD80, CD83, CD86, and MHC Class I and II before and after the uptake of these various forms of apoptotic cells. A typical experiment is shown in Figure 4

and a summary of three experiments is shown in Table 1

TABLE 1. Dendritic cell maturation after uptake of apoptotic mesothelioma cells||

DC Phenotype

 Unpulsed DCs20.135.6 39.78135.21142.36
 apo(UV)-pulsed DCs15.65 3.85 26.93139.32133.73
 apo(HS+UV)-pulsed DCs14.22 4.33 24.65105.8196.4
 Unpulsed DCs49.3460.54728.31420.87197.7
 apo(UV)-pulsed DCs42.0733.31||658.97303.81||159.97
 apo(HS+UV)-pulsed DCs

*Day 7 immature DCs were cocultured for 24 hours with or without apoptotic mesothelioma cells.

The combination of TNF-α and poly(I:C) was added to DCs for 48 hours.

Expression of CD80, CD83, CD86, HLA-ABC, and HLA-DR on DCs was evaluated by flow cytometry.

§Results are expressed as mean fluorescence intensity.

Results are expressed as a percentage of positive cells.

||Represents significant phenotypic changes in comparison with DCs cultured alone that have been subjected to a 48-hour treatment of TNF-α and poly(I:C) (unpaired Student t test, p < 0.01). Results are representative of three independent experiments.

Definition of abbreviations: apo(HS+UV) = apoptotic cells treated with heat shock and ultraviolet light, and expressing HSP70; apo(UV) = apoptotic cells treated with ultraviolet light, and lacking HSP70; DCs = dendritic cells; I:C = inosinic-cytidylic; TNF-α = tumor necrosis factor-α.

. DCs not exposed to apoptotic tumor cells efficiently progressed toward maturity within 48 hours in response to combined treatment with TNF-α and poly(I:C), as determined by the high-level expression of all phenotypic markers. It was interesting to note that coculturing Day 7 immature DCs with apoptotic cells for 24 hours did not result in spontaneous DC maturation whatever the apoptosis-inducing strategy used, as indicated by the low level of costimulatory molecules and the lack of CD83 (Figures 5A and 5B and Table 1). However, pulsing DCs with apoptotic tumor cells partially affected DC maturation in response to combined treatment with TNF-α and poly(I:C) as indicated by the intensity of surface expression (i.e., mean fluorescence intensity), which remained low for both MHC Class I and II molecules (Figure 5 and Table 1). These results are consistent with precedent findings reporting the inhibitory effects of apoptotic cell ingestion on DC maturation (33).

As shown in Figure 5 and Table 1, regarding CD80, CD83, and CD86 molecule expression DCs loaded with apo(HS+UV) progress in maturation to a similar extent as seen with unpulsed DCs when exposed to 48-hour combined treatment with TNF-α and poly(I:C). Likewise, activating DCs that have ingested apo(UV) with TNF-α plus poly(I:C) led to increased expression of costimulatory molecules, but not CD83, whose upregulation was slightly but significantly reduced by about 15%. Taken together, these results underscore the slight immunosuppressive activity exerted by apoptotic cell ingestion by specifically preventing the upregulation of MHC molecules despite exposure to strong combined treatment with TNF-α and poly(I:C). Nonetheless, and in contrast to previous data (42), no significant differences could be detected in costimulatory molecule upregulation between apo(UV)- and apo(HS+UV)-pulsed DCs, which are known to play a crucial role in T cell stimulation, the expression levels of which remained unaffected. It may be due to the use of complete medium supplemented with 10% fetal calf serum in these experiments, which is thought to ease the inhibitory effects of apoptotic cell ingestion on DC maturation (unpublished observations).

Apoptotic Cell-pulsed DCs Drive a Helper T Cell Type 1 Cytokine Response

To further investigate the effects of apoptotic cell loading on cytokine production by DCs, supernatants from unpulsed DCs (which served as a control) and DCs pulsed with various forms of apoptotic cells were quantified for IL-12 p70 and IL-10 before and after a 48-hour incubation with a combination of TNF-α and poly(I:C).

IL-12 p70 heterodimer is the bioactive form of IL-12 that has been shown to play a key role in the induction of IFN-γ-secreting helper T Type 1 cells. As indicated in Figure 6A

, IL-12 p70 values from DCs that had not been stimulated with the combination of TNF-α plus poly(I:C) were low, ranging from 0.5 to 0 pg/ml (i.e., near or below the detection limit) in all situations. These data are consistent with the interpretation that IL-12 synthesis is not performed by DCs exhibiting an immature phenotype, as determined on the basis of cell surface markers. By contrast, treatment with TNF-α and poly(I:C) for 48 hours resulted in a significant increase in bioactive IL-12 production by both unpulsed and pulsed DCs (Figure 6A). It should be noted that the IL-12-inducing capacity of the combination treatment with TNF-α plus poly(I:C) was similar for both apo(UV)- and apo(HS+UV)-pulsed DCs.

We next determined the concentration of IL-10, which is an immunosuppressive cytokine, in the same 48-hour culture media. As shown in Figure 6B, unpulsed DCs exposed to combined treatment with TNF-α and poly(I:C) for 48 hours did not produce higher levels of IL-10 than did immature DCs. Interestingly, the IL-10-inducing capacity of combined treatment with TNF-α and poly(I:C) was restricted to DCs that have ingested apoptotic material, regardless of the death-inducing strategy used. Indeed, significant levels of basal secretion of IL-10 were detected in DCs that had ingested apo(UV) or apo(HS+UV); these levels were further augmented after the addition of the maturation-inducing agents.

DCs Loaded with Apoptotic Mesothelioma Cells Expressing HSP70 Prime Naive CD8+ T Cells to Differentiate into Mesothelioma-specific CTLs

We next investigated the ability of DCs loaded with apoptotic cells to induce CTLs with cytolytic activity for HLA-matched and -mismatched target mesothelioma cells. In vitro sensitizations of autologous CD3+ T cells by pulsed or unpulsed DCs were established as described in Methods.

After weekly stimulation for 3 weeks, T cells were harvested and CD8+ T cells were depleted from the cell suspension, using magnetic bead-conjugated anti-human CD4 monoclonal antibody, and tested for their cytotoxic activity against either Meso11 (HLA-A2+) or Meso13 (HLA-A2) (which served as a negative control) mesothelioma cell lines.

As shown in Figure 7

, CD8+ T cells derived from T lymphocytes sensitized with DCs loaded with apoptotic cells lacking HSP70 did not show any cytotoxicity against either Meso11 (HLA-A2+) cells or Meso13 (HLA-A2) cells. In contrast, CTL lines generated with DCs and apoptotic cells expressing HSP70 enhanced cytotoxic activity to Meso11 cells (Figure 7A). As shown in Figure 7B, Meso13 cells were killed to a minor extent only, as the lysis was similar to that obtained by CD8+ T cells that had been cultured with unpulsed DCs, therefore emphasizing that the cytotoxic activity was HLA-A2 restricted. This implies that the cytolytic capacities of the expanded CD8+ T cells are likely not attributable to the release of soluble antigens from apoptotic cells that have failed to be taken up by DCs. The cytotoxic activity to Meso11 cells was substantially reduced when tumor cells were preincubated with an MHC Class I-blocking antibody (W6/32) before adding effector cells, therefore confirming that tumor cell lysis was specific and MHC Class I restricted (Figure 7C). Unspecific lysis mediated by natural killer cells was also excluded as no significant lysis could be detected (less than 5%) when cytotoxicity was directed against natural killer cell–sensitive K562 cells (data not shown).

DCs Loaded with Apo(HS+UV) Efficiently Sensitize CD8+ T cells to Kill HLA-A2+ Melanoma Cells

Sigalotti and coworkers suggested that the pattern of expression of cancer testis antigens belonging to the MAGE, GAGE, and SSX gene families in MPM cells was quite consistent with that reported in metastatic melanomas (26). This prompted us to analyze Meso11 and Meso13 mesothelioma cells, as well as M17 and M136 melanoma cells (as a positive control), for gene expression using PCR primers designated to give cDNA-specific bands (all GAGE primer pairs and β2-microglobulin). As expected, mRNA for all GAGE subtypes was clearly evident in M17 and M136 melanoma cell lines (Figure 8A)

. Interestingly, mRNAs for GAGE-1, -2, and -7 and for GAGE-3–6 and -8 were significantly expressed by Meso13 cells, which were used for immunization experiments. By contrast, little or no expression of the GAGE genes was found in Meso11 cells (Figure 8A). In all cases, RT-PCR amplification of β2-microglobulin mRNA, a housekeeping gene, was included as a control to ascertain the appropriate quality and quantity of the various RNA samples. Otherwise, it is of note that we failed to detect MAGE and SSX genes in both Meso13 and Meso11 mesothelioma cell lines (data not shown), thereby emphasizing that the GAGE gene was the predominantly expressed shared antigen between the two tumors.

We thus assessed the ability of CTLs generated by sensitizations with DCs loaded with apo(HS+UV) mesothelioma cells to kill melanoma cell lines. Hence, after three weekly stimulations, CD8+ T cells were sorted from the cell suspension and incubated with 51Cr-labeled M17 or M136 melanoma cells. The result, reproduced in Figure 8B, shows that CTLs were able to kill M17 (HLA-A2+) cells with up to 40% specific lysis, but not M136 (HLA-A2) cells. When experiments were performed in the presence of the MHC Class I–blocking antibody W6/32, CTL-mediated cytotoxic activity toward M17 cells was effectively blocked (Figure 8C). Again, these effector cells were incapable of lysing the natural killer cell-sensitive cell line K562 (data not shown). These results demonstrate as a novel finding that priming of CD8+ T cells with DCs loaded with apoptotic Meso13 mesothelioma cells can generate CTL lines specific for antigens expressed by M17 melanoma cells.

Because MPM responds only poorly to conventional therapies, new immunization strategies may represent highly promising therapeutic options. Indeed, DC-based vaccines may offer such an approach, because encouraging results have been achieved in patients with various tumor diseases such as malignant melanoma (2123) or renal cell carcinoma (24, 25). In this study, we have investigated the ability of monocyte-derived DCs of HLA-A2+ healthy donors loaded with antigen preparations from the allogeneic HLA-A2 Meso13 mesothelioma cell line to induce an antitumor T cell response in a cross-presentation in vitro model.

To the best of our knowledge, the present report is the first to demonstrate, in human MPM, that apoptotic tumor cell-pulsed DCs are able to induce a Class I–restricted cytotoxic T cell response against MPM tumor cells. Consistent with the report of Feng and coworkers (35), heat shocking the tumor cells before apoptosis induction was required to induce potent cross-priming of CTLs with antitumor activity. Indeed, only DCs fed apoptotic HSP-overexpressing cells were capable of inducing a strong cytotoxic T cell response, specific for MPM tumor cells. However, it should be noted that the involvement of HSP in the induction of immunity has not been addressed experimentally, but only correlated. Indeed, one cannot exclude that HS-associated genes other than those belonging to HSP family might account for the induced cross-priming of CTLs. Nonetheless, and as reported by Schena and colleagues (43), most of the genes upregulated after HS are thought to encode factors that function either as “molecular chaperones” (i.e., HSP) or as mediators of protein degradations. Hence, in our study, it is highly conceivable that HS might be essential thanks to the increased expression of HSP70, which was the HSP maximally synthesized. In agreement with this statement, a study has shown that tumor-specific T cell responses could be achieved in a lung cancer model, provided that antigenic peptides were associated with HSP70 (44).

Nevertheless, the accurate mechanisms by which apoptotic cell-derived HSP70 targets DCs to lead to potent stimulation of antitumor activity remain unclear. Evidence is accumulating to suggest that HSP70 is a direct activator of DCs, inducing a conversion to a mature phenotype highly efficient in T cell activation (38). Indeed, a variety of cell surface proteins have been reported to stimulate the immune system on binding of HSP70 in model cell systems, including CD91 (45) or Toll-like receptor (TLR)-2 and TLR-4 (4648). In contrast with previous data (42), the uptake of apoptotic HSP70-expressing cells did not result in spontaneous DC maturation. It is noteworthy that both apoptotic preparations used in this study were almost equivalent regarding the responsiveness of DCs to classic maturation-inducing agents, whatever the death-inducing strategy used. Hence, in regard to costimulatory molecules, apoptotic cell-pulsed DCs exposed to combined treatment with TNF-α and poly(I:C) for 48 hours resulted in a profile that is characteristic of mature DCs able to trigger an efficient immune response. It is thus highly unlikely that the failure of apo(UV)-loaded DCs to efficiently cross-prime tumor-specific CTLs may be attributed only to the selective and weak defect of such DCs in upregulating CD83 molecules.

Besides, both apo(UV)- and apo(HS+UV)-loaded DCs did not fail to produce significant amounts of IL-12 in response to combination treatment with TNF-α plus poly(I:C). On the basis of findings reporting that DCs progressively lose their responsiveness to autocrine IL-10 during the maturation process (49), we further reason that increased IL-10 production may not contribute to convert apoptotic cell-pulsed DCs from immunostimulatory to tolerogenic cells. Altogether, these results strongly suggest that the mechanisms by which HSP70 targets apoptotic cell-pulsed DCs to efficiently cross-prime tumor-specific T cells did not occur through an improved responsiveness of DCs to exogenous maturation stimuli. Otherwise, it is also conceivable that HSP70 targets MPM-derived antigens to LOX-1, which is a scavenger receptor also involved in the trafficking of antigens toward the MHC Class I pathway (50).

The use of apoptotic tumor cells, as an antigen-delivery approach, has already been successfully addressed in various cancer models including melanoma (51), leukemia (31), as well as colorectal and prostate cancers (30, 52). Here, our in vitro investigations therefore confirm the suitability of such a source of TAAs and further extend its applicability to MPM, the management of which continues to defy curative options. We reason that the apparent unrestricted effectiveness of apoptotic cells in generating antitumor activity is likely to occur through the supply of multiple antigens leading to DC cell surface expression of varied MHC Class I–peptide complexes. Hence, such a diversity of antigen presentation implies the stimulation of a wide range of tumor-specific CTLs, which may therefore significantly improve immune efficacy and prevent possible epitope escape mutation. Indeed, it now seems clear that a single antigen will not suffice for efficient clearance of tumors consisting of polyclonal cells with a range of antigens expressed or lost.

Another interesting finding of our experiments was the detection of the GAGE gene family in the Meso13 mesothelioma cell line, which was used for immunization. From our data, we note that the GAGE transcripts were the most frequently expressed cancer testis antigens among human MPM specimens, as compared with MAGE and SSX genes (Sapede and colleagues, unpublished data). Hence, mRNAs for GAGE-1, -2, and -7 and for GAGE-3–6 and -8 were easily detected by RT-PCR in mesothelioma cells without requiring any DNA-demethylating treatments known to upregulate cancer testis antigen gene expression, as previously reported (26, 53).

On the other hand, these results are thought to be fully consistent with the fact that DCs loaded with apo(HS+UV) Meso13 cells could also elicited an MHC Class I–restricted cytotoxic T cell response against M17 melanoma cells. Indeed, such HLA-A2–restricted reactivity may conceivably be explained by the shared expression of GAGE genes from both mesothelioma and melanoma cells. We further suggest that such a result may not only be attributed to the use of this particular Meso13 cell line, as GAGE gene expression is homogeneously expressed among various mesothelioma cell lines. It is noteworthy that the specific killing capacity of the CTL lines against any GAGE antigenic epitopes could not be verified because of the lack of well defined HLA-A2–restricted peptides. Up to now, only two antigenic peptides, YRPRPRRY (54) and YYWPRPRRY (55), which are encoded by the GAGE gene family, are known to be recognized by CTLs when presented on Class I molecules HLA-Cw6 and HLA-A29, respectively. Hence, determining the accurate frequency of GAGE-specific CD8+ T cells generated with such a priming strategy may not be considered using either peptide-loaded T2 target cells or HLA Class I tetramers. The search for known tumor antigens whose peptides have already been identified as CTL epitopes among MPM cell lines is currently under way.

As previously argued, the key application of these findings is in the prospect of vaccinating patients with MPM using new immunotherapeutic approaches (56). In addition, the study of the applicability of this method under autologous conditions, which remains the ultimate question, is currently under way in our laboratory. In this article, we have established an in vitro model for MPM vaccination using DCs loaded with allogeneic apoptotic mesothelioma cells. The fact that only DCs pulsed with apoptotic HSP70-expressing cells displayed a substantial capacity to stimulate autologous antitumor T cell responses emphasizes the importance of heat shocking the tumor cells before apoptosis induction in immunotherapy protocols based on DCs. Our data might therefore be relevant regarding future clinical trials of active immunotherapy involving DCs in patients with MPM.

The authors are grateful to Romain Oger for excellent help in RNA isolation and RT-PCR experiments.

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Correspondence and requests for reprints should be addressed to Marc Grégoire, Ph.D., INSERM U601, Institut de Biologie, 44093 Nantes Cedex 01, France. E-mail:


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