Background
Dendritic cells (DC) are specialized antigen presenting cells that can initiate a primary immune response on encountering foreign antigens [
1]. There has been much focus aimed at harnessing their potency in several clinical applications including cancer, infectious and inflammatory diseases [
2‐
4]. However, there has been limited success in the treatment of many cancers using dendritic cell based immunotherapy [
5]. DC are capable of ingesting dead and dying (apoptotic) tumour cells which potentially expose the DC to an array of tumour-associated antigens for processing and presentation to T cells via HLA class I and II pathways [
6‐
8]. Whilst several methods for loading DC
ex vivo with tumour antigens are currently used, including DNA, RNA, peptides and apoptotic tumour cells, the optimal approach has yet to be determined. The reasons why individual loading strategies may fail to successfully induce anti-tumour immunity are still not fully understood. However, an understanding of the natural mechanisms by which DC acquire tumour antigens and mature
in situ has resulted in the improvement of several DC-based immunotherapy strategies.
It has been proposed that
in situ destruction of tumour cells using chemotherapy, radiotherapy or other physical methods releases suitable antigenic material which can lead to enhanced antigen acquisition and stimulation of an immune response [
9]. The chemotherapeutic agent paclitaxel (Paclitaxel) induces cancer cell death by promoting the polymerisation of tubulin, thereby causing cell death and apoptosis by disrupting the normal microtubule dynamics required for cell division [
10]. Paclitaxel has been shown to be highly immunosuppressive at cytotoxic doses. There is clinical evidence to show that systemic administration of cytotoxic compounds such as paclitaxel can have a detrimental effect on the number of systemic DC [
11].
However, at lower concentrations there is also evidence to suggest that paclitaxel may be immunostimulatory which may contribute to the overall antitumour effects in the clinical setting [
12‐
17]. Several murine cancer models have demonstrated that combined chemotherapy and DC-based immunotherapy can lead to complete tumour regression in contrast to partial regression in response to each element used individually [
18].
The maturation status of DC is a key factor required for the induction of a specific immune response and is reliant on the presentation of antigens by fully mature DC. Paclitaxel has been shown to interact with TLR-4, a know receptor for lipopolysaccharide (LPS), on murine myeloid cells [
19,
20]. However, the effects of paclitaxel on DC maturation remain to be clarified.
The aims of this study were to evaluate the immunomodulatory effects of paclitaxel applied to ex vivo generated DC in terms of phenotype, function and cytokine expression. The findings of this study may have important implications in treating malignancies with chemotherapy and concomitant administration of ex vivo DC.
Discussion
Several chemotherapeutic agents have been shown to have dose dependent immunostimulatory activity [
23]. However, there is conflicting data to suggest that these compounds are also capable of inhibiting the generation and maturation of cells capable of antitumour effects, especially in cancer patients who are already debilitated by their disease [
24]. Paclitaxel is a widely used cytotoxic agent which targets microtubules and induces cellular death via apoptosis. The doses of paclitaxel under evaluation in this study reflect clinical practice: paclitaxel disappearance from plasma after intravenous infusion is biphasic; half-lives of the first and second phases occur approximately 0.3 and 8 hours, respectively. The peak plasma concentration with a high dose such as 275 mg/m
2, occurs immediately post-infusion, and is approximately 8 μM [
25]. We have demonstrated that, at similar plasma levels as in clinical practice, the mitochondrial function of human DC is less susceptible to inhibition by paclitaxel when compared to cancer cell lines. This is in accordance with data published on murine DC which were more resistant to paclitaxel compared to the murine melanoma cell line B16 [
26]. The same report demonstrated that there was no induction of necrosis or apoptosis in murine DC treated with paclitaxel for 24 hours. A recent publication indicated significant effects of paclitaxel on DC at a dose range of 1-5 μM albeit on murine DCs [
27]. We found, by two different methods, that paclitaxel at 100 μM does not induce either immediate cellular death or reduce DC viability for up to 5 days in culture. In contrast, it has been reported that human DC can undergo apoptosis following incubation with Paclitaxel at concentrations above 10 μM when continually exposed for 24 hours but these cells still maintaining mitochondrial function [
28].
One possible mode of action by which chemotherapeutic agents may be immunomodulatory is by the induction of DC maturation by the upregulation of key molecules, especially those involved in antigen presentation such as HLA class II. We found that paclitaxel selectively enhanced the expression of HLA class II on DC in a dose dependent manner. Interestingly, the upregulation of class II did not correlate with allo-stimulatory activity. Enhancement of the allo-stimulatory activity was observed at low doses of paclitaxel which was superior to LPS activated DC. This would suggest that factors other than antigen presentation molecules are crucially affected by treatment with higher doses of paclitaxel. Low concentrations of paclitaxel have been shown to have a cytostatic effect on cells by effecting both cellular mitochondrial membrane and reducing potential. However, the apoptotic pathway is stopped upstream of mitochondria permeabilization and therefore does not lead to cell death[
29]. The interactions of MHC class II and the mitochondria are central during the induction of mature DC death via cross linking of the surface molecules [
30]. The expression of MHC class II in blood mononuclear cells is also associated with the presence of Myosin-V which is itself frequently associated with the central microtubules [
31]. Therefore, alteration in MHC class II expression may be due to either paclitaxel induced disruption of mitochondrial function or the effect on microtubule arrangement via the co-localization with Myosin-V.
A reduction in the endocytic function of DC was also observed in response to paclitaxel despite no overall change in the expression of the specific surface receptor CD209. Phagocytosis of
Listeria monocytongenes by macrophages has been shown to be dependent on functional actin filaments. Depolymerising drugs such as colchicine and nocodazole greatly reduced the cells ability to phagocytose
L. monocytogenes while the stabilization of microtubles with paclitaxel had no significant effect on uptake [
32]. Paclitaxel may therefore inhibit receptor mediated endocytosis by preventing mitochondrial activity from facilitating this active process. We have demonstrated that paclitaxel was very potent at altering both DC morphology and microtubule organization.
Paclitaxel treatment in patients results in the release of a spectrum of pro-and anti-inflammatory cytokines, but most significantly IL-10, IL-8 and IL-6 [
33]. Microarray analysis of DC after paclitaxel exposure showed downregulation of most cytokine gene expression; the overall effects of paclitaxel appeared to be anti-inflammatory. Cytokine analysis from supernatants reflected these findings. In contrast, as expected, LPS effects on DC were profoundly pro-inflammatory. Of note also was the reduction in IL-12 production after exposure to paclitaxel suggests that paclitaxel-treated DC may induce a Th2 (or Th17) cell bias with implications regarding their use therapeutically. Similarly, the lack of chemokine secretion after paclitaxel exposure implies a reduction in DC migration to lymph nodes thereby reducing the possibility of priming naive T cells in a DC therapy context.
The reduction of mitochondrial potential in DC responding to glucocorticoid has been shown to inhibit DC maturation and IL-12 production [
34]. As with MHC class II expression, depolarisation of the microtubule network can severely inhibit DC production of IL-12 in response to a TLR-4 agonist due to the disruption of intracellular TLR-4 [
35]. Therefore, yet again paclitaxel-induced disruption of mitochondrial activity or microtubule arrangement may explain the inhibition of cytokine production.
Paclitaxel has been shown to mimic LPS activation of murine macrophages in a cell cycle-independent manner. Stimulation of these primary cells with paclitaxel is a TLR-4 and MyD88- dependent pathway which augments the release of TNF and nitric oxide [
36]. TLR-4 has been shown to physically associate with the MD-2 and it is this molecule that confers LPS-responsiveness on TLR-4. The species-specific activity of paclitaxel is determined by murine MD-2 and not human MD-2, suggesting that MD-2 is the crucial element for recognition [
37]. However, both we and others have demonstrated that human and murine myeloid cells respond to paclitaxel in a TLR-4 independent manner.
Conclusion
In conclusion, this study has increased our understanding of the effects of a potentially immunomodulatory chemotherapeutic agent on the modulation of DC. Paclitaxel has been shown to both enhance and inhibit many functional capabilities of DC via inhibiting mitochondrial function and microtubule polymerization. Our preliminary data suggests that these mechanisms may occur in the absence of signaling through TLR-4. In the clinical scenario, overall, we would anticipate that concomitant administration of paclitaxel would enhance DC-based vaccine through upregulation chemokine expression and increased allostimulatory activity of DC although the precise time of administration of vaccine and paclitaxel need to be carefully considered in view of paclitaxel's rapid distribution post-infusion. Combined paclitaxel and DC-based vaccines are currently under investigation in a murine model.
Methods
Antibodies
The following antibodies were purchased from Serotec, Oxford, UK; HLA-ABC FITC HLA-DR,-DP,-DQ FITC; CD80 PE; CD86 PE, CD40 PE and anti-tubulin. The following antibodies were purchased from Becton Dickinson, Oxford, UK; CD83 FITC, CD197 PE and CD209 PE. The Alexa 488 conjugated anti-rat secondary antibody was purchased from Molecular Probes, Paisley, UK. Anti-TLR4 blocking antibody was purchased from Axxora, Nottingham, UK.
Cell lines
The MCF7-pR human breast cancer cell line (a gift from Dr. Kay Colston, St George's University of London, UK) was grown in culture medium (CM) RPMI 1640; 10% FCS; 1% Penicillin/Streptomycin and 2 mM L-Glutamine all purchased from Sigma-Aldrich, Dorset, UK) and incubated at 37°C in a humidified incubator at 5% CO
2. The melanoma cell line MJT-3 was previously established in our laboratory from a brain metastasis of a 45 yr old female patient [
38] and cultured as outlined for MCF7-pR.
Generation of monocyte-derived dendritic cells (DC)
Immature DC were prepared from peripheral blood mononuclear cells (PBMCs) isolated from buffy coats purchased from the blood bank. A minimum of 3 different buffy coats were used for each experiment. PBMCs were incubated with anti-CD14 microbeads (Miltenyi BiotecBergish Gladbach, Germany) as specified in the manufacturer's instructions. The isolated monocytes were resuspended at 106 cells/ml in medium supplemented with 10% human AB serum, 100 ng/ml GM-CSF (Leucomax; Novartis, Camberley, UK) and 50 ng/ml IL-4 (Peprotech EC Ltd, London, UK), cultured in tissue culture flasks. On day 7 the cells were defined as immature DC with low/intermediate HLA-DR expression
In vitro cell drug-sensitivity assay
MCF7-pR and MJT-3 cells were plated in 96 well flat bottom plates at 5 × 104 cells/well in CM. Paclitaxel (Bristol Myers Squibb, Princeton, US) was serially diluted in CM and added to the cells to give a final concentration range of 0-100 μM. Day 7 DC were plated at 105 cells/ml in CM. The cells were incubated for 2 h and then washed twice by centrifugation with 200 μl fresh CM to remove the presence of the drug. The cells were allowed to recover in fresh medium for a further 48 h before the addition of MTS/PMS (Promega, Madison, USA). Optical density (O.D.) at 490 nm was measured after a further 2 h incubation using a Dynex technologies MRX-II plate.
The percentage cell viability was calculated as;
% Viability = 100 × (absorbance of cells incubated with drug containing medium)/(absorbance of cells incubated with medium alone).
DC viability assessment
DC viability was assessed after 48 h post incubation in the presence of paclitaxel at various concentrations for 2, 24 or 48 h of the treatment. At the end of the 48 h the cells were harvested, washed twice and resuspended in PBS for TB enumeration by light microscopy or PBS containing PI (Sigma-Aldrich) for analysis by flow cytometry. Viable cells were verified and enumerated depending on their ability to exclude TB or PI. The percentage of viable DC was calculated as outlined above.
Direct surface staining for immunophenotyping
DC were harvested and washed in wash buffer (PBS containing 0.5% BSA and 0.1% sodium azide, all purchased from Sigma-Aldrich). Cells were labeled with the relevant fluorochrome-conjugated antibody at the concentration recommended by the manufacturer for 30 min on ice in the dark. DC were also incubated with an irrelevant isotype-matched control antibody to compensate for non-specific binding. The cells were then washed in wash buffer and the cell pellet fixed with 200 μl CellFix (Becton Dickinson). Samples were either analysed immediately or within 24 h (stored at 4°C in the dark) on a flow cytometer. Routinely 10,000 events were collected with dead cells and debris being gated out on the basis of their light scatter properties.
Mixed Lymphocyte Reaction (MLR)
The stimulatory function of the drug-treated DC was assessed by their ability to induce proliferation in allogeneic non-adherent PBMCs. Day 7 DC were incubated in the presence of paclitaxel for 2 h as previously described. The cells were washed, resuspended in CM and graded numbers of DC co-cultured with 106 allogeneic non-adherent PBMCs. Proliferation was measured on day 5 following 18 h of pulsing with 1 μCi [3H]-Thymidine per well. Mean values of triplicates were measured and expressed as stimulation indices (SI).
SI = (mean counts per minute for test sample)/(mean counts per minute of background)
The endocytic capacity of DC was determined as followed. Briefly, DC were treated as before with various concentrations of paclitaxel for 2 h prior to incubation at 4°C or 37°C for 10 min in CM to equilibrate the temperature. Dextran-FITC (40,000 MW- Sigma-Aldrich) was added to each sample and incubated for a further 60 min at the same temperature. Washing the cells twice in ice cold PBS quenched the endocytic activity and removed any free dextran-FITC. Cells were fixed in 200 μl of CellFix and routinely 5000 cells were analysed by flow cytometry. Surface only binding of dextran-FITC was deemed to occur when the DC were incubated at 4°C while endocytosis was judged to occur at 37°C.
Measurement on cytokine production
IL-12p70, TNFα, IL-10 and IL-1β were analysed using the cytometric bead array (Becton Dickinson). Supernatants were collected from DC after treatment with LPS (1 μg/ml) or Paclitaxel for 2 h followed by re-culture for 24 h. The supernatants were frozen at -20°C until analysis.
Actin-remodelling in DC
Coverslips were washed in 70% ethanol for 10 min, followed by brief washes in 95% and then 100% ethanol before air drying. Coverslips were inverted in 50 μg/ml fibronectin solution (Sigma-Aldrich) for 2 hours. Coverslips were rinsed extensively in PBS, placed in the wells of a tissue culture plate and covered with CM. Immature DC or LPS-matured DC were seeded on the fibronectin-coated coverslips for 1 hour. Cells were then treated with paclitaxel at a final concentration of 0-100 μM as required for 2 hours prior to fixation and immunocytochemistry. DC were washed twice in PBS before incubating in -20°C methanol for 5 min. Cells were blocked for 30 min in blocking buffer (PBS containing 1% non-fat dried milk) before incubation with anti-tubulin antibody for 1 hour. DC were washed in PBS before incubation with an Alexa 488 conjugated anti-rat secondary antibody for 1 h. Coverslips were washed before mounting on glass slides using Mowiol (Calbiochem, Nottingham, UK). Images were captured using a Zeiss Axiovert 200TV microscope connected to a Hammamatsu camera and imported into Imaris. Images were exported to and figures assembled in Adobe Photoshop.
Blocking of TLR-4 receptor
DC were cultured with LPS (1 μg/ml) or paclitaxel (100 μM) for 2 h in the absence or presence of 4 μg/ml of a blocking antibody directed towards TLR-4. The cells were washed before being returned to culture for a further 24 h. Cells were assessed for the expression of HLA DR as outlined above.
Microarray analysis of the effects of Paclitaxel and LPS on DC
Total RNA was extracted from day 7 DC which had been treated for 2 h with 1 μg/ml LPS or 100 μM paclitaxel for 2 h then washed and returned to culture for 24 h. This was used as a template to generate Cy3-labelled cRNA, using the Low RNA Input Linear Amplification Kit (Agilent). This was used as a probe on the Whole Human Genome Microarray (4 × 44 K) slide (Agilent). Slides were scanned using the Agilent scanner and data extracted using Feature Extraction Software 9.5.3 (Agilent). Subsequent data analysis was performed using GeneSpring GX software.
Statistical analysis
The statistical significance of experimental data was evaluated using the Student's t-test where P < 0.05 was considered as statistically significant.
Authors' contributions
JJ participated in experimental design and implementation. MI was involved in generation of DCs. CR carried out in vitro drug sensitivity assays. JA was involved in experimental techniques. RA was involved in statistical analysis. AM carried out a critical review of the manuscript. HP conceived the study and participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.