The simplest vaccine approach that has been applied to cancer is the inoculation of individuals with irradiated tumor cells. This approach has many advantages:
- Specific tumor antigens do not need to be identified or characterized prior to vaccination;
- Immune responses to multiple tumor antigens can be generated, which may protect against tumor escape variants ;
- Such vaccines are not limited by patient HLA background owing to cross-presentation of tumor antigens after uptake by dendritic cells (DCs) [37,38] – this is particularly advantageous as tumor cell lines are readily available, while the availability of autologous tumor cells may be restricted;
- The tumor cell vaccine platform can be easily modified.
For example, tumor cells can be transduced to express immunomodulatory cytokines such as granulocyte macrophage colony-stimulating factor (GM-CSF), as was performed by Jaffee et al.
in a Phase I clinical trial [39
]. In their studies, a pancreatic tumor cell vaccine induced a CD8+
T-cell response, specific to mesothelin, regardless of HLA match between the tumor vaccine and recipient – demonstrating that cross-priming had occurred [38
]. Mesothelin is a particularly promising cancer vaccine target owing to its low level of expression on nontumor tissues and high levels of expression on pancreatic as well as other cancers (i.e., ovarian) [40
]. A Phase II trial for this vaccine is ongoing in patients with resectable pancreatic cancer (NCT00389610).
Tumor cell vaccines have also be modified to express epitopes, which increase antibodymediated uptake by DCs. Normally, MUC-1 expressed on tumors is immunogenic owing to overexpression and tumor-restricted hypoglycosylation [41
]. The NewLink Genetics Corporation (IA, USA) has developed a whole-cell vaccine expressing MUC-1 modified to express α-gal epitopes, which is the focus of multiple clinical trials (NCT00255827, NCT00614601, NCT00569387 and NCT01072981) [42
]. This vaccine takes advantage of anti-α-gal antibodies that are found in most people due to exposure to gastrointestinal flora, resulting in increased uptake of the vaccine in an antibody-dependent manner [43
]. In murine studies, the addition of such α-gal epitopes to a Muc-1+
pancreatic cancer whole-cell vaccine resulted in increased production of anti-Muc-1 antibodies; enhanced tumor-specific T-cell responses and increased survival after challenge with Muc-1+
B16 cells in α-gal knockout mice, previously sensitized to α-gal [44
]. A study using similarly treated melanoma cells as vaccine resulted in the complete protection against melanoma in mice [45
Autologous DCs have also been used in tumor vaccination when pulsed with tumor lysates or peptides, transfected with whole-tumor mRNA, or transfected with mRNA or cDNA of a specific antigen [47
]. Mature DCs have the benefit of expressing high levels of costimulatory molecules in addition to both HLA class I and class II molecules, allowing for direct presentation of tumor antigens to, and enhanced activation of, both CD8+
T cells. For example, Schmidt et al
. intratumorally vaccinated with whole tumor mRNA transfected DCs and found an antitumor specific immune response and significantly decreased tumor volume in a murine pancreatic cancer model [48
]. In melanoma patients, the whole-tumor mRNA approach has been used to generate antitumor CD4+
T-cell responses [49
]. Apoptotic pancreatic tumor lysates have also been evaluated as a source of antigen and have been demonstrated to elicit stronger antitumor lytic activity when used to stimulate autologous human CD8+
T cells in vitro
compared with those stimulated with tumor lysate-pulsed DCs [51
]. Recently, a peptidepulsed autologous DC vaccine has been US FDA approved for the treatment of asymptomatic metastatic castration-resistant prostate cancer. This vaccine, known as Provenge®
(Dendreon Corp., WA, USA) or Sipuleucel-T, consists of autologous, patient-derived DCs pulsed with a fusion protein consisting of the prostate tumor antigen prostatic acid phosphatase and GM-CSF [52
]. In a Phase III clinical trial, vaccination resulted in a 3-year survival advantage in vaccinated castration- resistant prostate cancer patients (31.7% survival) compared with placebo (23%). Such a result is encouraging and gives hope that pancreatic cancer-targeted DC vaccines could produce similar effects.
In addition, autologous DCs, virally transduced to express IL-12, have also been used in cancer treatment. One pancreatic cancer patient receiving this treatment had a partial response in studies by Mazzolini et al.
]. As the treatment DCs were not loaded with tumor antigen, cross-presentation of tumor antigens must have occurred.
A variation on the whole-cell approach involves the fusions of cancer cells and DCs, with the resulting cell used as the vaccine. Such vaccines can be made with autologous DCs and autologous tumor, with allogenic DCs and autologous tumor, or with autologous DCs and allogenic tumor [54
]. This technique has been used to treat mice in a pancreatic tumor model, resulting in the generation of CD8+
T cells with tumor-specific cytolytic activity and tumor rejection [55
In cases in which an immunogenic tumor antigen is known, autologous DCs have been transfected with, or virally transduced to express, the mRNA or cDNA of a specific tumor antigen. This technique does not require that the exact immunogenic epitopes of the antigen be identified, as full-length protein is transfected. Such a vaccine consisting of autologous DCs transfected with MUC-1 cDNA was administered to ten patients with advanced breast, pancreatic or papillary cancer in a Phase I/II clinical trial. A MUC-1-specific CD8+
T-cell response was generated in four patients, with a delayed-type hypersensitivity (DTH) response found in three patients [56
]. However, all of the pancreatic cancer patients in this study developed progressive disease. Currently, a similar vaccine is in a Phase I/II clinical trial in melanoma patients using DCs transfected with the mRNA of tumor cells along with that of telomerase and survivin (NCT00961844). The use of multiple antigens may protect against tumor escape variants.
For some pancreatic tumor antigens, HLA-restricted immunogenic peptide epitopes have been identified and have been used to pulse DC vaccines. This approach allows the immune system to respond only to the immunologically relevant epitope, although its use is limited to patients expressing the corresponding HLA alleles. In an encouraging study by Lepisto et al.
, patients with resected pancreatic and biliary tumors were administered MUC-1, peptide-pulsed, autologous DCs in a Phase I/II clinical trial [57
]. Although a clear antigen-specific T-cell response was not detectable following vaccination, 25% of the patients were alive at year 4. In similar studies with a telomerase peptide-pulsed DC vaccine, an antigen-specific CD8+
T-cell response was generated in patients who developed various cancers following vaccination. This response was enhanced when DCs were also pulsed with class II telomerase peptides, illustrating the importance of CD4+
T cells in fighting tumor [58
]. Pulse can also be performed with peptides from multiple tumor antigens, as was performed in a Phase I clinical study by Carbone et al.
Patients with various cancers, including pancreatic cancer, immunized with p53 and K-Ras peptide-pulsed PBMCs saw increased survival [59