3.1. Passive Immunotherapy
Passive immunotherapy makes use of in vitro
produced immunologic effectors that are capable of influencing tumour cell growth. The most common form of passive immunotherapy is called monoclonal antibody therapy. It consists of humanized monoclonal antibodies that are investigated in several human malignancies. Monoclonal antibodies can target cells directly [15
] or indirectly. Monoclonal antibodies are also used as immune modulators to inhibit immune suppressive molecules/cells or activate immune stimulatory molecules. Efficacy of this approach can sometimes be enhanced by linking a toxin to these antibodies (e.g., radionucleotides or anticancer drugs).
In mesothelioma, preclinical studies targeting mesothelin with immunotoxins CAT-5001 (formerly SS1P) and amatuximab (previously known as MORab-009) were promising [16
] and therefore progressed to clinical trials. CAT-5001, administered to mesothelioma patients, among other cancer types, showed only modest clinical responses [17
]. Amatuximab failed to demonstrate any radiological responses in a phase I trial in mesothelioma and other cancer types [19
]; however preclinical studies demonstrated significant antitumour efficacy using combination of amatuximab and chemotherapy treatment [20
] justifying a multicenter phase II clinical trial utilizing cisplatin/pemetrexed with amatuximab in mesothelioma patients. This trial has been completed and results are expected soon. More recently a phase I study of SS1(dsFv)PE38, a recombinant antimesothelin immunotoxin, was commenced which is ongoing at this moment (ClinicalTrials.gov Identifier: NCT00575770).
Another method of passive immunotherapy uses adaptive transfer of (autologous or allogeneic) antigen-specific effector cells (like T cells and NK cells) that can be expanded and/or activated ex vivo
and subsequently administered to the patient to attack the tumour [21
]. This approach showed the potential to reconstitute host immunity against pathogens, like Epstein-Barr virus (EBV) in immune suppressed patients, but more importantly also provides evidence that adaptive T cell transfers can prevent the induction of EBV-associated lymphomas [22
]. This led to the concept that antigen-specific T cell transfer can be used as an antitumour therapy to eradicate established tumours. The approach of adaptive T cell transfer to eradicate malignancies is challenging [23
3.2. Active Immunotherapy
Active immunotherapeutic approaches aim at inducing or boosting immune effector cells in vivo against tumour cells, through the administration of immune mediators capable of activating the immune system.
Several cytokines are capable of activating and recruiting specific immune cells that can enhance antitumour immunity (e.g., IL-2, IL-12, IL-15, TNF-α, GM-CSF). These cytokines can be used as single agent or in combination with other immunotherapeutic strategies.
Defined TAA epitopes have been used to vaccinate cancer patients [24
]; however this approach is limited by the relatively low number of identified specific peptides and by the requirement of MHC typing. By using the whole TAA protein for immunization, the need of peptide identification can be circumvented. These proteins can be taken up by APCs and endogenously processed into epitopes for presentation to T cells. Adjuvants need to be added to induce APCs activation and avoid tolerance induction [25
DNA sequences coding for specific TAAs can be directly injected into the skin. DNA then needs to be taken up, transcribed into mRNA, translated into a protein, and processed into peptides by APCs.
In mesothelioma, the TAA's mesothelin and Wilms tumour-1 (WT-1) are highly expressed and thought to be physiologically relevant to this tumour type [26
]. In the Memorial Sloan-Kettering Cancer Center a phase I peptide vaccination clinical trial in mesothelioma patients is ongoing (ClinicalTrials.gov Identifier: NCT01265433). In these patients, inoculation with WT-1 peptide elicited WT-1-specific CD4 and CD8 T-cell responses, with minimal toxicity [26
]. TroVax has been shown to stimulate an immune response to a particular protein widely found on mesothelioma cells called 5T4; a clinical trial testing the effectiveness of TroVax is currently active in the Wales Cancer Trials Unit (ClinicalTrials.gov Identifier: NCT01569919).
An important restriction of this method is the relatively inefficient delivery into APCs. Viruses engineered to express TAAs can be injected directly into the patient. The virus then infects the host cell, leading to cell death and presentation of antigenic epitopes to the immune system. A wide variety of viral vectors are available. Currently, a trial using intrapleural administration of a vaccine with a measles virus strain is performed at the Mayo Clinic (ClinicalTrials.gov Identifier: NCT01503177). However there are concerns regarding the immune dominance of viral antigens over TAAs, resulting in a strong anti-virus response leading to virus eradication and attenuation of the antitumour immune response [27
DCs have emerged as the most powerful initiators of immune responses. In the natural activation of the adaptive immune system against tumour cells, DCs play a crucial role since they are capable to engulf tumour antigens and activate lymphocytes in an antigen-specific manner. Therefore, the application of DCs to therapeutic cancer vaccines has been prompted [28
The research group of Dr. Robinson published a very interesting trial, in which they used an autologous tumour lysate vaccine that was manufactured from surgically resected mesothelioma material and administered subcutaneously together with granulocyte-macrophage colony stimulating factor (GM-CSF). GM-CSF facilitates APCs recruitment and survival in vivo
which in turn may generate tumour-specific immunity after uptake of the TAA from the lysate. Twenty-two patients were enrolled onto this trial. Of these, five developed positive delayed type hypersensitivity skin tests, and five showed evidence of altered antibody specificities by western blotting, proving that GM-CSF could induce tumour-specific immunity, both cellular and humoral responses. 32% of the patients developed at least one type of anti-MM immune response. Furthermore, the therapy was safe and was associated with stable disease; however no major tumour regressions were observed [29
While this study showed potential for GM-CSF as immunotherapeutical approach, in vivo
stimulation of APCs is also a very attractive method. Sipuleucel-T is an active cellular immunotherapy consisting of autologous peripheral-blood mononuclear cells (PBMCs), including APCs. Recently, Kantoff et al. published a phase III trail where they used ex vivo
activated Sipuleucel-T with a recombinant fusion protein (PA2024). PA2024 consists of a prostate antigen, prostatic acid phosphatase that is fused to GM-CSF, an immune-cell activator. Sipuleucel-T prolonged survival among men with asymptomatic or minimally symptomatic metastatic castration-resistant prostate cancer [30
], providing evidence for cell-based immunotherapeutic agents in solid tumours.
In mesothelioma, the source of the TAA for DC loading remains a critical issue that will determine the efficacy of the DC-based vaccination. A careful identification and characterization of antigenic epitopes is needed when peptides want to be used. However, the ideal source of TAAs may be the tumour itself, since it expresses all the TAAs that can be targeted.
Incubating DCs with dead tumour cells (necrotic or apoptotic cells), as was shown in a pioneering article by the research group of Dr. Gregoire, DCs were exposed to 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. In their paper they successfully demonstrated in vitro
culture and antigen loading in a human mesothelioma model, resulting in a specific CTL response [31
One of the advantages of an ex vivo
culture model is that DCs can be generated in large amounts and pulsed with tumour antigens under optimal conditions. In mesothelioma, we previously investigated the effect of DC-based immunotherapy on the outgrowth of mesothelioma in a murine model [32
]. We established that DC-based immunotherapy induced strong tumour-specific CTLs responses leading to prolonged survival in mice. The efficacy of immunotherapy was dependent on the tumour load; most beneficial effects were established at early stages of tumour development.
On the basis of these preclinical animal studies, we have performed the first clinical trial in which autologous tumour lysate-pulsed DCs were administrated in mesothelioma patients [33
]. Patients were eligible for the study when sufficient tumour cells could be obtained from pleural effusion or tumour biopsy material at the time of diagnosis. DC-immunotherapy was planned after completion of the cytoreductive therapy provided that during chemotherapy no major side effects occurred and there was no progressive disease. Patients received three immunizations with mature DCs, loaded with autologous tumour lysate. Each immunization, consisting of 50 × 10e
6 cells, was administered intradermally and intravenously (). Overall, the vaccination regimen with loaded DCs was well tolerated, and a successful immune reaction was induced by the DC vaccinations.
Figure 2 Schematic drawing showing the administration of ex vivo cultured mature dendritic cells into a patient (1), resulting in antigen presentation in the lymph node (2) and a specific antitumour cytotoxic antitumour response (3). Tumour cells are depicted (more ...)
The University Hospital of Antwerp has started a similar protocol in mesothelioma and several other solid tumours but is using WT-1 as antigen loading for the DCs (ClinicalTrials.gov Identifier: NCT01291420), circumventing the need for patient's tumour material.
Another method to load DCs is to make use of measles-virus-infected mesothelioma cells. It was shown that this method induced spontaneous DC maturation and that priming of autologous T cells by DCs loaded with measles-virus-infected mesothelioma cells led to a significant proliferation of tumor-specific CD8 T cells [34