Cancer immunity and clinical oncology - Cancer immunotherapy in clinical practice
Antigen-specific immunotherapy in cancer patients focuses either on antibodies or on T-cell mediated approaches. Targeting with antibodies generally requires the expression of molecular targets at the cancer cell surface, like peptides, proteins or lipids and glycosylated variants or sugar moieties. Since their development in the late 1970s, monoclonal antibodies have become established and integrated into many treatment regimens in cancer therapy (Table 1).
|Table 1 Antibodies in cancer medicine|
|Drug||Target||Type||Type of cancer||Year of FDA Approval|
|Gemtuzumab ozogamicin||CD33||Humanized||Acute myeloid leukaemia||2000|
|Alemtuzumab||CD52||Humanized||Chronic lymphocytic leukaemia||2001|
|Ibritumomab tiuxetan||CD20||Murine, radiolabelled||Non-Hodgkin’s lymphoma||2002|
|Tositumomab-131||CD20||Murine, radiolabelled||Non-Hodgkin’s lymphoma||2003|
|Head and neck cancer||2006|
|Non-small-cell lung cancer||2006|
|Advanced breast cancer||2008|
Crucial for an effective cancer vaccine is the use of an antigen that is immunogenic in cancer patients. This simple requirement, however, involves many issues that are still the subject of intense fundamental and clinical research. For instance, it is not known whether a monovalent (i.e. one antigen) or a polyvalent vaccine is better, or which form of antigen (peptide, protein, tumour lysate, recombinant attenuated pathogen that expresses tumour antigens) is most effective.
Also, which adjuvants stimulate innate immunity such that a sustained and effective antitumour immune response is generated is largely unknown. The duration and frequency of vaccination, as well as the phase of disease in which the patient is vaccinated, may influence its efficacy.
Importantly also, the choice of parameter to be monitored is not easy, because it is not really known which of many parameters actually correlates with clinical response. A number of current cancer vaccine strategies are summarized in Bullet list.1, and some of the above-mentioned questions are considered in further detail below.
Bullet list 1 Vaccine formulations
- Whole-cell or lysate cancer vaccines
- Gene-modified cancer cells
- Ex vivo activated lymphocytes
- Gene-modified lymphocytes
- Heat shock proteins
- Viral vectors
- Naked DNA
- Dendritic cells—APCs
An effective adjuvant treatment in bladder cancer is intravesical BCG (bacille Calmette–Guérin, heat-killed Mycobacterium tuberculosis). Multiple clinical trials have shown that BCG can give high response rates in recurrent superficial transitional cell carcinoma with significantly prolonged duration of remission. The mechanism of action of intravesical BCG is presumably the altered local cytokine production.
Unmethylated CpG (cytosine-phosphatidyl-guanosine) motifs are relatively common in bacterial genomic DNA, but not in human DNA. CpG interacts with TLR9 in the endosomal vesicles of APC, which licences the latter to activate T cells. Synthetic CpG has been used as a single agent in clinical trials in non-Hodgkin’s lymphoma, non-small-cell lung cancer, melanoma, and renal cell carcinoma with marginal success. The efficacy of the combination of tumour-associated antigen plus CpG as a strong adjuvant is further investigated in various types of cancer.
A variety of cytokines are or have been used alone or together with cancer vaccines in the clinic. Two examples are interferon-α (IFN-α) and the granulocyte–macrophage colony stimulating factor (GM-CSF). IFN-α has profound and diverse effects on gene expression: it up-regulates the MHC class I molecules, tumour antigens, and adhesion molecules. Most importantly, it promotes the activity of B cells, T cells, macrophages, and dendritic cells and increases the expression of Fcγ-receptors.
INF-α is currently used for the treatment of malignant melanoma, follicular lymphoma, hairy-cell leukaemia, Philadelphia-positive chronic myelogenous leukaemia, condylomata acuminata, cutaneous T-cell lymphoma, and AIDS-related Kaposi’s sarcoma.
High-dose IFN-α has become a standard treatment option for adjuvant therapy in stage III melanoma patients. GM-CSF is used as an adjuvant because it may increase the number of APCs and thus enhances the ability to prime an immune response in the patient. A phase I trial comparing the cytotoxic T lymphocyte (CTL) reactivity after immunization with melanocyte differentiation antigen-derived peptides alone or with additional systemic GM-CSF as an adjuvant showed enhanced DTH reactions, CD8+ CTL responses and objective tumour regressions in patients that were treated with vaccine plus GM-CSF.
The effects of GM-CSF were also tested with irradiated autologous melanoma cells engineered to secrete GM-CSF. Although only a few patients had major tumour-specific responses in classical terms, resected tumour nodules often demonstrated fibrosis and immune cell infiltrates attributed to the GM-CSF–transduced tumour cell therapy.
The intracellular processing of proteins results in the generation of short peptide epitopes of 8 to 10 and 13 to 20 amino acids that bind MHC class I and class II respectively. The majority of peptides derived from tumour antigens are presented in association with MHC class I molecules and are recognized by CD8+ T cells.
A smaller number of peptide epitopes have been defined for MHC class II molecules, and are recognized by CD4+ T cells. The major disadvantages of short peptides as a vaccine are the short in vivo half-life and the fact that they will be presented by other cells besides professional APCs, both of which result in limited immunogenicity.
In addition, the use of peptides requires knowledge of the epitopes derived from the tumour-associated antigen and is often limited to patients with particular MHC alleles, such as HLA-A2. Nevertheless, phase I and II clinical studies in which melanoma patients were vaccinated with peptides plus different adjuvants showed the induction of peptide-specific CD8+ T cell responses.
For example, in a recent phase I clinical trial, 12 HLA-A2+ patients with progressing NY-ESO-1-expressing metastatic tumours of different types were vaccinated intradermally with NY-ESO-1 peptides first alone and then in combination with GM-CSF as a systemic adjuvant.
Five out of seven vaccinated patients, who were initially NY-ESO-1 antibody negative, developed stabilization or regression of individual metastases after induction of NY-ESO-1 specific CD8+ T-cell responses. In addition, there was disease stabilization following NY-ESO-1 immunization in three of five antibody-positive patients, indicating that vaccination may also result in clinical benefit in patients with baseline spontaneous immunity to NY-ESO-1.
Long peptides, recombinant proteins, tumour cell lysates, and DNA vaccines
In order to reduce the risk of immune escape, an immune response against a broad range of MHC class I and II epitopes is required. Vaccination with full-length antigens or (overlapping) long peptides has the potential to broaden the response and is not limited to patients with particular HLA alleles.
In a fairly recent clinical trial evaluating the safety and immunogenicity of recombinant NY-ESO-1 protein with ISCOMATRIX adjuvant, 46 patients with resected NY-ESO-1-positive tumours received 3 doses of vaccine intramuscularly at monthly intervals. The majority of vaccinated patients demonstrated high-titre antibody responses, strong delayed-type hypersensitivity reactions, and circulating CD8+ and CD4+ T cells specific for a broad range of NY-ESO-1 epitopes, including known and previously unknown epitopes.
Autologous and allogeneic tumour cells have also been used as tumour vaccines, with mixed results. In theory, the main advantage of tumour cell vaccines is that they have all the relevant tumour antigens for the immune system to mount an effective antitumour response. In addition, tumour cell-based immunization allows the development of cancer vaccines without prior knowledge of specific antigens. However, the lack of knowledge of specific antigens responsible for anti-tumour immunity severely limits antigen-specific immunological monitoring.
Naked DNA vaccination introduces tumour antigens into dendritic cells for cross-presentation to cytotoxic T cells in draining lymph nodes. The advantages of this approach include simplicity, stability, and low cost. However, DNA vaccination has poorer efficacy than vaccination with recombinant viruses.
In an attempt to improve the antitumour immune responses of DNA vaccines, several strategies have been employed including transdermal or mucosal delivery, gene-gun delivery of DNA-coated gold beads, DNA–liposome complexes, and the generation of chimeric recombinant constructs of antigen and IgG Fc, HSP 70, FLT3-L, and cholera toxin.
Recombinant pathogens that express full-length tumour-associated antigens have the same advantage as recombinant proteins or long peptides, namely that they are exclusively presented by professional APCs. In addition, recombinant pathogens have natural adjuvant activity. Recombinant vaccinia virus, adenovirus, and fowlpox virus vaccines have been evaluated in preclinical models as cancer vaccines.
Clinical trials with recombinant vaccinia virus vaccines expressing CEA, NY-ESO-1 or HPV E6 or E7 showed that such vaccines induce specific immune responses. Recent studies show that priming with one virus and boosting with another, the so-called prime–boost strategy, is well tolerated and is superior to priming and boosting with the same virus with respect to the induction of immune responses.
Dendritic-cell based vaccines
Dendritic cells are the most potent and efficient professional Antigen-presenting cells (APCs) and play a critical role in the initiation of the immune response through the uptake, processing, and presentation of antigens, including tumour antigens, to T cells. Dendritic cells pulsed with tumour lysates, tumour protein extracts, and synthetic peptide tumour antigen epitopes generate protective immunity to subsequent tumour challenge in mouse models.
Studies of dendritic cell-based immunotherapy have been conducted in several tumour types including melanoma, colon cancer, prostate cancer, lymphoma, and multiple myeloma. All of these and other studies indeed suggest that monocyte-derived dendritic cells are capable of eliciting antigen-specific immune responses in humans, some of which have been associated with clinical responses.
A major drawback of dendritic-cell based immunotherapy is the dependence on specialist preparation of cellular products, and this is likely to make the approach unavailable for widespread use.
Adoptive T-cell therapy
As cancer antigen specific T-lymphocyte expansion ex vivo under Good Manufacturing Practice conditions is feasible today, adoptive transfer of in vitro expanded effector lymphocyte populations may become an effective treatment modality. Positive results in infectious diseases have been published and early results in cancer point to a promising, yet challenging, modality in cancer therapy.