Annals of Oncology Advance Access originally published online on April 13, 2005
Annals of Oncology 2005 16(6):847-862; doi:10.1093/annonc/mdi192
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© 2005 European Society for Medical Oncology
Review |
Therapeutic vaccination in patients with gastrointestinal malignancies. A review of immunological and clinical results
1 Immune and Gene Therapy Laboratory, Department of Oncology, Cancer Center Karolinska, Karolinska University Hospital, Stockholm; 2 Department of Oncology, Uppsala University Hospital, Uppsala, Sweden
* Correspondence to: Professor H. Mellstedt, Department of Oncology, Cancer Center Karolinska, Karolinska University Hospital, S-171 76 Stockholm, Sweden. Tel: +46-8-517-74308; Fax: +46-8-318327; Email: hakan.mellstedt{at}karolinska.se
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Abstract |
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 Top Abstract IntroductionImmunogenicity of GI...Tumour antigensMechanisms of action of...Tumour escapeVaccine strategiesTherapeutic vaccines in patients...Conclusions and future...References |
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Gastrointestinal (GI) malignancies are the most common types of human cancers. Despite the introduction of new cytotoxic drugs, a large proportion remains incurable. There is a great need to develop new complementary therapeutic modalities. Strategies exploiting targeted therapies are expanding. The focus of the present article is to review active specific immunotherapy (vaccination) in patients with GI malignancies. The review comprises a description of the immunogenicity of GI malignancies, various types of tumour antigens and mechanisms of action of cancer vaccines. Tumour escape from immune surveillance, vaccine strategies and adjuvants are also described. Clinical and immunological endpoints of cancer immunotherapy are outlined. Results of therapeutic vaccine trials published mainly during the last 5 years in PubMed enrolling a minimum of six patients with GI malignancies are included. Studies presented at the two last annual meetings of the American Society of Clinical Oncology are also covered. More than 2000 patients have been vaccinated with tumour antigens (self antigens). The procedure is safe and no autoimmune disorders have been observed after >4 years follow-up in a substantial number of patients. Humoral and cellular tumour antigen-specific immune responses were induced. A correlation between immune responses and prolonged overall survival was seen in several studies. The most encouraging results were noted in randomised controlled phase II/III trials including over 1300 colorectal carcinoma patients with minimal residual disease. A statistically significantly improved disease-free or overall survival was shown either in all vaccinated or in sub-groups of patients. Promising results were also reported in pancreatic and hepatocellular carcinoma. If the results of the randomised controlled trials hold true, active specific immunotherapy may provide a new promising targeted therapeutic approach in GI malignancies with minimal toxicity. Further enlarged randomised controlled studies are warranted to confirm the results, particularly in colon carcinoma with minimal residual disease.
Key words: clinical effects, gastrointestinal tumours, immune response, therapy, vaccination
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Introduction |
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TopAbstractIntroductionImmunogenicity of GI...Tumour antigensMechanisms of action of...Tumour escapeVaccine strategiesTherapeutic vaccines in patients...Conclusions and future...References |
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Gastrointestinal (GI) malignancies constitute the highest incidence and mortality rates among human cancer worldwide. Colorectal cancer (CRC) is the second most common cancer. Pancreatic and hepatocellular carcinoma has the poorest prognosis among all cancer types [1
].
Surgery remains the primary treatment for most solid tumours. However, only 10–15% of pancreatic cancer is resectable at the time of diagnosis and the overall 5-year survival rate is only 5%. Patients in whom pancreatectomy is feasible, the overall 5-year survival rate is not more than 20% [2]. Gemcitabine treatment for metastatic pancreatic carcinoma is only associated with a marginal survival benefit [3].
The 5-year survival for patients with CRC following surgery varies between 80–90% for stage I, 70–75% for stage II, 35–50% for stage III and <5% for stage IV disease [4, 5]. Despite the fact that 80% of CRC patients have complete macroscopic clearance of the tumour by surgery, 50% relapse [6]. This is presumably due to the presence of micrometastasis at the time of surgery. The recurrence rate for stages II–III CRC varies from 20 to 60%. Peri-operative radiotherapy is recommended for stages II and III rectal cancer. 5-fluorouracil based (5-FU) adjuvant chemotherapy in stage III colon cancer shows a decrease in the odds of colon cancer related death by 25–30% (or an absolute survival benefit of 5–6%). In stage II colon cancer, the survival benefit of adjuvant chemotherapy is controversial.
Adjuvant chemotherapy and radiotherapy are limited by lack of specificity and toxicity. Despite the introduction of effective new drugs, novel treatment strategies are warranted, especially for optimal adjuvant therapy to improve the prognosis of patients with GI malignancies [7]. Research efforts are focused on the development of targeted therapeutics. Novel strategies include the blockage of the epidermal growth factor receptor, inhibition of Ras-mediated signal transduction, targeting vascular growth factors and receptors, inhibition of matrix metalloproteinase or cyclooxygenase-2, gene therapy and immunotherapy [8–10].
In the past decade, the expansion in the understanding of immunology and molecular biology, the development of recombinant DNA technology, the molecular identification of tumour antigens and immunodominant epitopes have opened new avenues for the development of rationally designed, potent immunotherapeutic strategies against cancer.
Anti-tumour immunotherapy can be divided into two major approaches: non-specific and antigen-specific immunotherapy [11, 12]. Non-specific immunotherapy makes use of exogenous immunostimulants, cytokines, adoptive transfer of non-specific immune effector cells, inhibition of negative immune regulatory pathways or tumour-derived immune suppressive molecules. Antigen-specific immunotherapy can be divided into passive (monoclonal antibodies, adoptive transfer of specific immune effector cells) and active strategies. The present review will focus on active specific immunotherapy (vaccination) in GI malignancies.
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Immunogenicity of GI malignancies |
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TopAbstractIntroductionImmunogenicity of GI...Tumour antigensMechanisms of action of...Tumour escapeVaccine strategiesTherapeutic vaccines in patients...Conclusions and future...References |
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Tumour antigens spontaneously inducing an immune response might be potential targets for antigen-specific cancer immunotherapy. There is evidence that spontaneous humoral and cellular immune responses against tumour antigens expressed by GI tumours are induced in cancer patients. However, the prognostic significance of the immune response is unclear [13
]. Natural humoral and cellular immune responses have been reported against tumour antigens in patients with CRC [14, 15]. The presence of anti-CEA (carcinoembryonic antigen) IgM antibodies was associated with improved survival of CRC patients [16]. High titre anti-MUC-1 IgG antibodies predicted a favourable prognosis for patients with pancreatic cancer [17]. Tumour infiltrating dendritic cells (DC) [18], natural killer (NK) cells [19, 20], CD8+ T cells [21–23], macrophages [22] and eosinophils [24] have been reported to positively correlate with prognosis in carcinomas of the GI tract. Downregulation of MHC class I antigen on tumour cells had a negative impact on survival [25], whereas MHC class II expression appeared to be a favourable prognostic marker [13]. The data suggest that immune mechanisms may play a role in the control of GI malignancies. |
Tumour antigens |
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TopAbstractIntroductionImmunogenicity of GI...Tumour antigensMechanisms of action of...Tumour escapeVaccine strategiesTherapeutic vaccines in patients...Conclusions and future...References |
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Carcinogenesis is associated with a number of genetic and epigenetic changes leading to the expression of neoantigens on tumour cells, but only a minority of tumours express truly foreign proteins. A large number of tumour antigens and their immunodominant epitopes have been described in GI malignancies [26
]. The list of these antigens is rapidly expanding. Examples of different groups of tumour antigens expressed by GI malignancies are shown in Table 1.
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The following characteristics make a tumour antigen particularly attractive as a vaccine target: lack of pre-existing tolerance, differential expression on tumour versus normal tissue and a role in tumorigenesis. Mutated oncoproteins (e.g. K-ras), tumour suppressor proteins (e.g. p53), proteins of frameshift mutations (e.g. TGFßRII; transforming growth factor ß receptor type II) or tumour associated viral antigens are expressed on tumour but not on normal cells. No tolerance has developed against such antigens that may allow the induction of a high affinity antigen-specific T cell response, and autoimmune reactions are less likely to occur. Although this group of antigens might be an ideal target for immunotherapy, the identification and isolation of mutated self antigens from individual patients is not practical [27
]. Cancer-testis antigens are selectively expressed by the tumour and testis, which makes this group of antigens an attractive target for vaccination. As cells of the testis do not express HLA class I antigens, these cells do not provide targets for an immune response. Tumour-associated antigens (TAAs) that are over-expressed on tumour cells, but also expressed on somatic cells may be feasible targets for immunotherapy (oncofetal and over-expressed self antigens). The existence of immunological tolerance and the induction of an autoimmune reaction should be taken into consideration when targeting these antigens. However, the present clinical experience using this group of antigens for vaccination does not indicate that autoimmunity is of major concern. CEA, Ep-CAM (epithelial cell adhesion molecule) and p53 are commonly over-expressed in GI malignancies and have been targeted by immunotherapy. Altered glycosylation during carcinogenesis of over-expressed mucins (e.g. MUC-1, sialyl-Tn) expose neoantigenic sites of the protein backbone, providing an interesting vaccine target. Several other TAAs, such as the complement regulatory protein CD55, the proto-oncogene Her2/neu, gastrin, ß-hCG (human chorionic gonadotropin ß) etc. have also been targeted by vaccination in clinical trials (see below). |
Mechanisms of action of cancer vaccines |
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TopAbstractIntroductionImmunogenicity of GI...Tumour antigensMechanisms of action of...Tumour escapeVaccine strategiesTherapeutic vaccines in patients...Conclusions and future...References |
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Most antigens targeted by cancer immunotherapy are non-mutated self antigens. The major challenge in developing a successful vaccination strategy is shifting the balance from tolerance to self antigens towards the generation of a long-lasting, therapeutic antitumour immunity, while avoiding the induction of autoimmune toxicity.
To initiate an immune response, the antigen has to be presented by MHC molecules (peptide–MHC complex; signal 1) with the appropriate immune activating signals (costimulatory molecules; signal 2) in the context of a danger signal [28]. Tumour cells do not usually express costimulatory molecules and are unable to prime T cells. Tumour antigens need to be presented by professional antigen presenting cells (APCs). DCs are the most potent type of APCs that activate naive T cells and play an important role in the induction of an immune response [29, 30]. DCs deliver exogenous antigens (e.g. derived from lysed tumour cells or protein-based vaccines) into the MHC class II processing pathway to activate helper T cells, and present endogenous antigens (e.g. derived from viral vector-based vaccines) by MHC class I molecules to CD8+ T cells. DCs are also capable of delivering exogenous antigens into the MHC class I processing pathway to activate CD8+ T cells (cross-presentation and cross-priming) [31]. DCs stimulate both the innate and adaptive immune system and have the ability to interact with CD4+ and CD8+ T cells, NK cells and NKT cells. The cytokine milieu may direct the immune response towards immunity or tolerance [29]. Type-1 cytokines (IL-2, IL-12, IL-15, IFN-
, IFN-
and IFN-ß) are involved in T helper 1 (Th1) immune responses, primarily inducing cell-mediated immunity. In contrast, type-2 cytokines (IL-4, IL-5, IL-6, IL-10 and IL-13) are associated with a Th2 immune response promoting humoral immunity [30, 32].
It is most likely that successful cancer vaccines should be able to activate the cellular and humoral arms of the innate and adaptive immune systems [30]. There is a growing consensus that induction of CD8+ cytotoxic T cells (CTLs) and a Th1-biased CD4+ T-cell response are crucial for an efficient antitumour response [27]. A schematic illustration of potential antitumour effector functions that might be induced by vaccination are shown in Figure 1.
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Tumour escape |
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TopAbstractIntroductionImmunogenicity of GI...Tumour antigensMechanisms of action of...Tumour escapeVaccine strategiesTherapeutic vaccines in patients...Conclusions and future...References |
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Despite the presence of tumour antigen specific T cells (either natural or vaccine-induced), the immune response in cancer patients may not correlate with a clinical anti-tumour response. Tumours might develop several strategies allowing them to escape immune surveillance and destruction [33
]. APCs presenting tumour antigens may fail to reach T cell areas of the lymph nodes and activate T cells. As a consequence, tumour cells are ignored. Tolerance induction is one of the major mechanisms by which tumour cells might evade immune recognition. Tolerance may be induced by several mechanisms. The immune response might be skewed towards a Th2 response or T cells might be rendered anergic or deleted. Increasing evidence suggests that CD4+CD25+ regulatory T cells play an important role in tolerance induction [34]. Mutation or downregulation of immunodominant tumour antigens, MHC molecules or molecules involved in the antigen processing machinery may also explain escape from the initial immune response. Expression of anti-apoptotic molecules, down-regulation or mutation of pro-apoptotic molecules renders tumour cells resistant to apoptosis. Tumour cells may also acquire mechanisms that actively contribute to escape from immune surveillance. Fas ligand (FasL)-expressing tumours can deliver an apoptotic signal to activated T and NK cells expressing the Fas receptor. Tumour cells may also produce soluble immunosuppressive factors such as TGF-ß, IL-10, monocyte colony-stimulating factor (M-CSF) and prostaglandins. Counteracting tumour escape mechanisms is a key issue for successful immunotherapy. |
Vaccine strategies |
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TopAbstractIntroductionImmunogenicity of GI...Tumour antigensMechanisms of action of...Tumour escapeVaccine strategiesTherapeutic vaccines in patients...Conclusions and future...References |
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Progress in the understanding of mechanisms of tumour-specific immune responses and immune escape has encouraged the development of new generations of cancer vaccines. Each of the numerous vaccination strategies [11
] exhibit advantages and disadvantages (reviewed elsewhere) [35]. A summary of the most frequently used vaccine preparations in clinical trials is depicted in Table 2. Most of these modalities have been applied in GI malignancies.
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Adjuvants
Vaccination with a tumour antigen alone is usually not sufficient to induce a potent immune response. Adjuvants are often crucial to improve the efficacy. Adjuvants may have different mechanisms of action. Some adjuvants have a depot effect for the vaccine. Other adjuvants might recruit and activate professional APCs and/or immune effector cells, whereas others help to deliver antigens to the cytosol and facilitate subsequent cross-priming by mimicking a danger signal [32
, 36]. Preferably, a cancer vaccine adjuvant should promote a Th1-type response [32].
One of the most commonly used adjuvant, aluminium-based salts (alum), acts as a depot formulation and promotes a Th2-polarised immune response [37]. Incomplete Freund's adjuvant (IFA) does not seem to be a powerful adjuvant in humans. In contrast, cytokines, particularly GM-CSF (granulocyte-macrophage colony-stimulating factor), have been shown to augment both humoral and cellular immunity by multiple mechanisms of action and facilitate cross-presentation [38, 39]. Bacterial DNA containing unmethylated CpG oligodeoxynucleotides is another powerful adjuvant and seems to be safe in humans [40]. Products of microorganisms, such as lipopolysaccharide (LPS), Newcastle-disease virus (NDV) and Bacillus Calmette–Guerin (BCG) also exert adjuvant activity. Ex vivo expanded DCs pulsed with tumour antigens are powerful immunogens. Biodegradable microshperes, virus-like particles, heat-shock proteins (HSPs), and various oil- or lipid-based chemical adjuvants (such as liposomes, ISCOMs, QS21 or AF) promote cross-presentation of the antigen. Attenuated viral vectors also belong to this category. Helper peptide epitopes (e.g. tetanus toxoid) may also be useful in enhancing and skewing immunity towards a Th1 response [32, 36].
Clinical and immunologic endpoints of cancer vaccine trials
Survival duration should be the unambiguous, ‘gold standard’ endpoint of therapeutic cancer vaccine trials. However, most oncology drugs have been approved on the basis of other endpoints than survival [41]. Tumour response (tumour reduction) [42] and delay in time to tumour recurrence or progression have been the primary end-points of many cancer treatment trials [41]. Tumour reduction cannot, however, be measured in minimal residual disease when cancer vaccines are most likely to be of clinical benefit. Moreover, tumour response does not always correlate with an improved progression-free or overall survival. It is also difficult to interpret time-to-progression in patients with slow growing malignancies. The majority of cancer vaccine studies in GI malignancies have been phase I or II non-randomised trials, which have not primarily been designed to address clinical efficacy. Furthermore, vaccine trials have often been performed in patients with metastatic disease, when an improved survival is less likely to occur. Due to the favourable toxicity profile of cancer vaccines, improvement in quality of life may also be a viable endpoint.
Identification of surrogate markers with a good correlation to the clinical outcome is an ongoing dilemma. Serum tumour markers for GI malignancies remain an unvalidated endpoint [43]. Analysis of micrometastasis in bone-marrow or peripheral blood before and after vaccination might be a promising surrogate marker in minimal residual disease [44]. Assays are, however, yet to be validated to achieve reliable results [45, 46].
Immunity induction has been discussed as a potential surrogate endpoint in cancer vaccine trials. However, it is not clear what a therapeutically relevant immune response is. Probably a Th1-biased tumour-specific immune response is of importance, which involves the activation of several components of the immune system. Induced immunity should most likely also be of sufficient magnitude and long-lasting. It has been questioned whether there is a relation between the antitumour immune response and clinical prognosis. Previous data and the present review suggest that there might be a correlation between immune parameters and the clinical outcome [35, 47, 48]. If that proves to be the case, immune responses might be good surrogate markers in vaccine trials, especially when applied in the adjuvant setting.
Immunomonitoring in cancer vaccine trials is still not optimal [35, 47, 48]. Quality control and assay validation for serological testing is less problematic, but the accuracy and reproducibility of T-cell-based methods should be improved. Defining standards for T-cell assays are important to ensure consistency, and to be able to compare results between different trials.
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Therapeutic vaccines in patients with GI malignancies |
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TopAbstractIntroductionImmunogenicity of GI...Tumour antigensMechanisms of action of...Tumour escapeVaccine strategiesTherapeutic vaccines in patients...Conclusions and future...References |
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Clinical trials are summarised in Tables 3–5. Special attention has been paid to the immune response and clinical effects. Mainly studies published during the last 5 years in PubMed enrolling a minimum of six patients with GI malignancies were included. If the study has been reported previously in part, only the last publication is included. Studies presented at the two last annual meetings of the American Society of Clinical Oncology (ASCO) are also included unless the study has been presented as a full report.
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Randomised controlled trials
The majority of randomised controlled therapeutic vaccine trials in GI malignancies were based on using autologous tumour cells (Table 3). Autologous tumour cells mixed with BCG as adjuvant (OncoVAX) were used for vaccination of stage II/III colorectal carcinoma patients after surgery and compared to surgery alone. A total of 704 patients were enrolled [49
–51]. Recurrence-free survival was significantly improved in patients vaccinated with the planned quality and dose of the vaccine [52]. A positive effect was only seen in colon cancer [49] and was more pronounced in patients with stage II as compared to stage III disease [51, 52]. Patients that received additional booster vaccine doses seemed to do better [51, 52]. The magnitude of the delayed-type hypersensitivity (DTH) response against autologous tumour cells was significantly correlated with an improved prognosis [50].
In another large randomised study including 567 stage I–IV colorectal carcinoma patients, autologous tumour cells mixed with NDV as an adjuvant was used for vaccination after surgery and compared to surgery alone [53]. A significant prolongation of overall survival duration was seen in the vaccine group. Patients mounting a DTH response to the vaccine survived significantly longer. Six of the 25 patients (24%) with stage IV disease had complete or partial responses in the vaccine group.
In hepatocellular carcinoma (HCC) a tumour-cell-based vaccine combined with adjuvant cytokines significantly improved overall and disease-free survival compared to resection alone. Recurrences were less frequent in patients mounting a DTH response [54].
In another randomised trial in metastatic colorectal carcinoma, an anti-idiotypic antibody mimicking the three-dimensional structure of Ep-CAM was used as a surrogate TAA [55]. This particular vaccine delayed disease progression and prolonged overall survival in immune responders as compared to the control group. In another approach, the murine monoclonal antibody (mAb) 17-1A (anti-Ep-CAM) conjugated to alum was used as an active vaccine in colorectal carcinoma patients. mAb17-1A may elicit anti-idiotypic antibodies in vivo, which carry epitopes mimicking Ep-CAM. Induced anti-idiotypic antibodies might in turn evoke an anti-anti-idiotypic response against the bona fide antigen [56]. The vaccination induced antibodies against Ep-CAM and was associated with a significant reduction of circulating Ep-CAM positive cells in the peripheral blood. Interim analysis showed that immune responders survived significantly longer than non-responders [57, 58]. Vaccination with another anti-idiotypic antibody mimicking the TAA, CD55, did not have an impact on survival in patients with advanced CRC, but only 50% of patients received the planned vaccine dose [59].
G17DT (a synthetic gastrin-like peptide linked to difteria toxoid) is a novel immunogen inducing neutralising antibodies against gastrin that acts as a growth factor for GI malignancies. Vaccination with G17DT significantly prolonged the overall survival of patients with advanced pancreatic cancer as compared to placebo [60]. G17DT has also been combined with chemotherapy in patients with metastatic colorectal, gastric and gastroesophageal cancer. An anti-G17 humoral response was shown to be a predictor of tumour response and survival [61, 62].
Correlation between induced immune response and prognosis
Several therapeutic vaccines in GI malignancies have been shown to induce immune responses that might be associated with tumour response or prolongation of progression-free and/or overall survival (Table 4).
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A significant correlation was found between prolonged overall survival and tumour antigen-specific antibodies after vaccination with SCV106, an anti-idiotypic antibody mimicking Ep-CAM [55
]. Similar results were observed after vaccination with mAb17-1A, which is able to induce anti-idiotypic antibodies mimicking Ep-CAM [58].
Vaccination with a ß-hCG peptide or gastrin-17, both linked to difteria toxoid (DT) showed a significant survival benefit for antibody responders [63, 64]. Peak IgM titres against the tumour antigen TA90 were shown to be a significant predictor of overall survival in CRC patients immunised with CancerVax, an allogeneic tumour cell vaccine preparation expressing the TA90 antigen [65]. A positive correlation between the anti-CEA IgG titre and overall survival of CRC patients vaccinated with a recombinant CEA protein was also recently demonstrated [66]. A large number of patients with various adenocarcinomas, including colon cancer have been vaccinated with Theratope (the sialyl-Tn antigen conjugated to keyhole limpet hemocyanin [67]). Patients generating high IgG titres against mucin sialyl-Tn (STn) epitopes survived longer, and anti-STn IgM titre was shown to be an independent positive prognostic factor for colon cancer [68, 69].
A DTH response correlated with delayed relapse, prolonged disease-free or overall survival in patients vaccinated with tumour-cell-based vaccines [50, 53, 54, 65, 70, 71] or ras peptides [72]. A proliferative T-cell response against ras peptides was also associated with a positive clinical outcome [72].
It has been suggested that IFN- production might be a surrogate marker for circulating CTL precursors [73]. Vaccination with tumor-derived heat-shock protein gp96 induced in vivo expansion of MHC class I restricted IFN- producing T cells recognising CRC cells, which was related to improved disease-free and overall survival [74]. Disease-free survival was also increased in pancreatic carcinoma patients mounting an IFN- T-cell response against an MHC class I restricted tumour antigen after vaccination with GM-CSF transfected tumour cells [71, 75]. Immunisation with a CEA-peptide loaded onto ex vivo expanded DCs induced a dramatic tumour regression in 17% of the patients, and a further 25% of the patients had mixed response or stable disease. A strong correlation was demonstrated between the clinical outcome and the magnitude of CD8+ T cell response as defined by peptide–MHC tetramer staining [76]. CEA-specific IFN- response was also associated with improved survival of patients treated with a prime-boost immunisation approach using CEA expressed in vaccinia and ALVAC viral vectors [77, 78].
Additional vaccine trials
Further trials utilising therapeutic vaccines are summarised in Table 5. These studies have usually included a limited number of patients that precludes conclusion about clinical efficacy. The results clearly demonstrate that immunisation with various formulations of self antigens reproducibly induce tumour antigen-specific immune responses. Despite advanced disease and a poor immune status of the patients in most studies, immune responses were detected. Several vaccine strategies induced a broad Th1-polarised immune response. Occasionally, tumour responses, disease stabilisation and decrease in serum tumour markers were noted. A progression-free interval comparable to that achieved by chemotherapy was observed in some studies.
An increased 2-year survival rate was seen in CRC patients vaccinated with NDV-infected autologous tumour cells as compared to historical controls, whereas no beneficial effect of BCG as adjuvant was noted [79]. Vaccination with tumour-cell lysate-pulsed DCs of patients with various adenocarcinomas, including pancreatic and hepatocellular cancer, induced tumour-specific cellular immune responses. Clinical and tumour marker responses were noted in some cases [67, 80]. Disease stabilisation and Th1-polarised immune responses were seen in patients immunised with tumour cells fused to DCs [81].
The most frequently targeted TAAs in GI malignancies are CEA and Ep-CAM. Different formulations of vaccines induced antigen-specific humoral and/or cellular immune responses. DC-based vaccination targeting CEA induced CEA-specific CD4+ helper T-cell and CD8+ CTL responses. Clinical and tumour marker responses were observed [76, 82–85].
Besides CEA, other TAAs have been targeted by DC-based vaccines. DTH responses and CTL induction were accompanied by occasional clinical responses in patients with gastric carcinoma or other GI tumours vaccinated with MAGE-3 or HER-2/neu peptides [86, 87]. Vaccination with mutant p53 peptides induced specific immune responses and patients had a longer overall survival than expected [88, 89]. MUC-1 peptide-pulsed DCs were immunogenic in patients with pancreatic cancer or biliary tumours [90].
Peptides derived from MUC-1, -fetoprotein, SART3 or other tumour antigens mixed with adjuvants or linked to a carrier protein have also been used for vaccination in colorectal, gastric, hepatocellular or pancreatic carcinoma. Antibody, DTH and CD8+ T cell responses as well as Th1-type cytokine production were induced [91–95]. Vaccination with a peptide derived from the anti-apoptosis protein, survivin, induced DTH and tumour-marker responses in CRC patients. One minor clinical response was also noted [96]. Immunisation with mutant ras peptide combined with GM-CSF and IL-2 or mixed with DETOXTM induced vaccine-specific immune responses in patients with colorectal and pancreatic carcinoma. One complete response and disease stabilisation in several patients were observed [97, 98]. Patients with advanced pancreatic carcinoma vaccinated with a peptide derived from the reverse transcriptase subunit of human telomerase plus GM-CSF mounted T helper and CTL responses. A correlation between vaccine dose, number of immune responders and survival was reported [99].
Vaccination with DNA or recombinant vaccinia virus encoding CEA showed no objective clinical responses and the induced CEA-specific immune responses were limited [100–103]. However, vaccinia–CEA and the poxvirus vector, ALVAC, containing CEA used in a prime-boost approach resulted in an increased CEA-specific CTL precursor frequency and disease stabilisation for up to 21 months in patients with advanced CRC. GM-CSF significantly augmented the cellular response, but the role of IL-2 was less clear [77, 104]. CTL induction and stable disease were seen in patients immunised with the ALVAC–CEA vector containing the costimulatory molecule, B7.1 [105, 106]. Although fewer patients receiving GM-CSF mounted a cellular response, disease stabilisation was more frequently seen in this group [107]. The number of prior chemotherapy regimens correlated negatively, whereas the number of months from the last chemotherapy regimen correlated positively to the generation of an anti-CEA T cell response [107]. An increase in anti-CEA CTL precursor frequency was more pronounced after vaccinating with ALVAC–CEA co-expressing B7.1 [107] as compared to ALVAC–CEA alone [108]. Disease stabilisation occurred in both groups.
Prime-boost vaccination with fowlpox–CEA–TRICOM and vaccinia–CEA–TRICOM containing the transgene for CEA and a triad of costimulatory molecules (B7.1, ICAM-1 and LFA-3) was associated with clinical and tumour marker responses. A trend towards increased overall survival was seen in patients who received GM-CSF as compared with patients receiving the vaccine alone. There seemed to be a positive correlation between the magnitude of anti-CEA T cell response and overall survival of patients [109]. Vaccination with DCs modified with fowlpox–CEA–TRICOM also induced a higher CEA-specific CTL precursor frequency in patients with a minor clinical response or stable disease as compared to those who progressed. Increased frequency of CD4 + CD25+ regulatory T cells correlated inversely with the CD4+ anti-CEA immune response [110].
Vaccination with Ep-CAM expressed in ALVAC vector induced a Th1-type cellular response with a relatively high precursor frequency when administered together with GM-CSF [111]. An ALVAC-p53 vaccine as well as modified vaccinia Ankara virus encoding 5T4 (TroVax) induced humoral and cellular immune responses in CRC patients [112–114]. After TroVax vaccination, disease stabilisation and tumour marker response was noted in several patients [112].
Clinical experience with defined protein vaccines is limited, with the exception of anti-idiotype proteins. An anti-idiotypic antibody mimicking CEA (CeaVac) induced humoral and cellular CEA-specific responses in all vaccinated patients [115, 116]. Preliminary analysis of a phase III study using CeaVac showed a trend towards improved overall survival in patients with metastatic CRC receiving at least five doses of CeaVac versus placebo [115, 117]. It has been suggested that tumour regression might correlate with the presence of anti-idiotype-reactive T lymphocytes [56]. More than 100 CRC patients have been treated with different anti-idiotypic antibodies mimicking Ep-CAM [118–120]. Both humoral and cellular immune responses were induced. Cellular responses seemed to be more frequent in patients immunised with human than xeno-anti-idiotypic antibodies [118, 119].
Three studies have been carried out with recombinant Ep-CAM protein vaccination in patients with colorectal or pancreatic carcinoma [120–122]. Ep-CAM protein together with GM-CSF induced a broad Th1-type immune response in most patients. The overall immune response was stronger when GM-CSF was used as an adjuvant as compared to alum.
Whole protein antigens offer the advantage of expressing multiple epitopes for both CD4+ and CD8+ T cells. Their use is not restricted to patients with a particular HLA type. In addition to the MHC class II processing pathway, exogenous soluble proteins may also utilise the endogenous MHC class I antigen presentation pathway via cross-presentation [31]. It has recently been shown that vaccination with the Ep-CAM protein or anti-idiotypic antibodies mimicking Ep-CAM induced IFN- producing T cells against various MHC class I restricted Ep-CAM-derived peptides, suggesting that these protein vaccines were cross-presented in vivo. GM-CSF was used as an adjuvant, which may facilitate cross-presentation. T cells recognising MHC class II restricted Ep-CAM-derived peptides were induced as well [120]. Evidence of cross-presentation was also demonstrated by detection of IFN- producing T cells against MHC class I restricted mesothelin epitopes after immunisation with GM-CSF transfected tumour cells [75].
Other whole protein vaccines used in GI malignancies include G17DT and the anti-idiotypic antibody mimicking CD55 (105AD7). In a phase II study, G17DT induced gastrin-specific antibody responses in a high proportion of gastric carcinoma patients [123]. The 105AD7 vaccine was able to activate NK cells as well as CD4+ and CD8+ T cells. In tumour tissues, enhanced lymphocytic infiltration, as well as tumour-cell apoptosis, was noted [124–126].
Disease stage
Active specific immunotherapy is most likely to be of benefit for patients with less advanced disease [51]. More patients at an early stage of the disease seem to mount an immune response as compared to advanced disease [59, 118, 124, 125]. Moreover, immune effector functions are unlikely to be able to control the growth of a bulky tumour. Moreover, there is a certain time period between the initiation of active immunotherapy and development of a relevant immune and clinical response. Patients with advanced disease may succumb before a therapeutic benefit of the vaccination might occur.
Safety of the vaccines
There is a reasonable concern about the induction of autoimmune diseases by vaccination, especially when the antigens are shared between normal and tumour cells. However, the current clinical experience with cancer vaccines does not indicate considerable toxicity. Neither short-term serious adverse events nor long-term autoimmune side effects have been observed using therapeutic vaccines in a large number of patients (Tables 3–5). Six vaccine trials in GI malignancies have shown no evidence of autoimmune side effects after an extended observation time (minimum 4 years) in a total of 720 patients [49–51, 53, 66, 125].
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Conclusions and future perspectives |
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TopAbstractIntroductionImmunogenicity of GI...Tumour antigensMechanisms of action of...Tumour escapeVaccine strategiesTherapeutic vaccines in patients...Conclusions and future...References |
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Cancer vaccines exhibit a favourable toxicity profile. No serious adverse events and/or evidence of autoimmune side effects have been observed in >700 patients with a substantial number followed for more than 4 years.
Active specific immunotherapy can induce a specific humoral and cell-mediated immune response against different tumour antigens expressed by GI malignancies. Twelve studies using various vaccine strategies demonstrated a statistically significant positive correlation between an immune response to the vaccine and the overall survival of patients. In a few other studies, immune responses were associated with regression of metastasis or prolongation of disease-free survival. If these results are confirmed in large randomised controlled trials, immune responses might be used as an intermediate endpoint of cancer vaccine trials.
Nearly a thousand patients with GI malignancies have been enrolled in nine phase II/III randomised controlled vaccination trials. All trials, with the exception of one, showed a statistically significantly improved progression-free or overall survival either in all vaccinated or in sub-groups of patients. Most of these patients, nearly 700, were vaccinated with autologous tumour cells after surgery. A statistically significantly longer disease-free survival for stage II colon cancer patients was demonstrated using BCG as an adjuvant. Furthermore, autologous tumour cell-based vaccines with NDV or cytokine adjuvants showed a statistically significant overall survival benefit for patients with stages I–IV CRC, as well as stages I–III hepatocellular carcinoma.
The results of active specific immunotherapy in GI malignancies are encouraging and merit further study in enlarged trials. Emphasis should be placed on therapy in the adjuvant setting, as cancer vaccines are most likely to be of clinical benefit in patients with minimal residual disease. Based on the overall present clinical experience, it is urgently warranted to perform large randomised trials with the aim to analyse whether vaccination can improve prognosis, particularly after the primary treatment of stages II–III colon cancer. Moreover, as there are indications that vaccinated patients mounting an immune response may exhibit a better prognosis, further research is needed to identify which factors influence the generation of therapeutically relevant immune responses. If the results of the randomised controlled trials hold true, active specific immunotherapy may provide a new promising targeted therapeutic approach with minimal toxicity.
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This study was supported by grants from the Swedish Cancer Society, the Cancer Society in Stockholm, the King Gustav V Jubilee Fund, the Torsten and Ragnar Söderberg Foundation, the Cancer and Allergy Foundation.
Received for publication January 31, 2005. Accepted for publication February 1, 2005.
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