WT1 has features appropriate for a cancer antigen
WT1-directed immune response is detected in patients with high tumor burden & in patients after HSCT. When tumor cells are damaged spontaneously or by treatment, they release WT1 protein, which elicits WT1-specific humoral and cellular immunity [2]. The former is the production of WT1-specific antibodies and the latter is the induction and/or activation of WT1-specific CTLs or helper T lymphocytes. A series of investigations has demonstrated the high immunogenicity of WT1 protein. As for humoral immunity, it was demonstrated that a considerable proportion of leukemia patients responded to the leukemia cell-derived WT1 protein and produced IgM- and IgG-type WT1 antibodies, indicating that not only WT1-specific B cells but also T cells that induce immunoglobulin class-switching of WT1 antibodies were activated in them [5]. Furthermore, it was recently found that the disease-free survival rate of non-small-cell lung cancer (NSCLC) patients with high titers of WT1 IgG antibodies was better than that of NSCLC patients with low titers, which suggests that WT1 plays an important role as a target antigen in tumor immunity for NSCLC [6]. It was also shown that high values of anti-WT1 antibody constitute predictors of longer survival for myelodysplastic syndrome (MDS) patients, which is consistent with the above result regarding NSCLC [7]. Several investigations have also provided direct evidence of spontaneous elicitation of WT1-specific CD8+ CTL response, which is a principal effector response in tumor immunity [1,8]. These findings suggest that WT1-specific immune response for the elimination of tumors is induced in many cancer patients, although the induced response is not strong enough to control the tumor growth or is diminished by tolerance mechanisms.
WT1 has features appropriate for a cancer antigen. It has been demonstrated that T-cell-mediated immunity has the power to control cancer. An example is hematopoietic stem cell transplantation (HSCT), which confers a powerful T‑cell-mediated graft-versus-leukemia (GVL) or graft-versus-tumor (GVT) effect. The undisputed effect of GVL/GVT led to the idea that autologous T lymphocytes, as well as donor T lymphocytes, may eradicate cancer cells, provided that cytotoxic T lymphocytes (CTLs) that recognize cancer cell-derived antigens, so-called ‘cancer antigens’, are appropriately activated. To date, hundreds of cancer antigens have been identified, one of which is the Wilms tumor gene (WT1) product, which has two features appropriate for a cancer antigen [1]. First, WT1 gene possesses oncogenic functions. This is a theoretical advantage for using its product, the WT1 protein, as a target antigen, because WT1 expression is thought to be essential for maintaining the transformed characteristics of cancer cells, and tumor escape from immune surveillance as a result of its downmodulation is unlikely to occur [1,2]. Second, wild-type WT1 is expressed in many kinds of hematological and solid malignancies, which indicates that WT1-targeting immunotherapy can be used for a variety of malignancies [1,2]. On the basis of these features and other findings described later, WT1 has been ranked as the most promising cancer antigen in a recent prestigious paper [3]. In addition, leukemia stem cells have been shown to express WT1, which supports the superiority of WT1 as a cancer antigen, because it indicates the possibility that WT1-directed immunotherapy can target cancer stem cells [4].
WT1-directed immune response is detected in patients with high tumor burden & in patients after HSCT
When tumor cells are damaged spontaneously or by treatment, they release WT1 protein, which elicits WT1-specific humoral and cellular immunity [2]. The former is the production of WT1-specific antibodies and the latter is the induction and/or activation of WT1-specific CTLs or helper T lymphocytes. A series of investigations has demonstrated the high immunogenicity of WT1 protein. As for humoral immunity, it was demonstrated that a considerable proportion of leukemia patients responded to the leukemia cell-derived WT1 protein and produced IgM- and IgG-type WT1 antibodies, indicating that not only WT1-specific B cells but also T cells that induce immunoglobulin class-switching of WT1 antibodies were activated in them [5]. Furthermore, it was recently found that the disease-free survival rate of non-small-cell lung cancer (NSCLC) patients with high titers of WT1 IgG antibodies was better than that of NSCLC patients with low titers, which suggests that WT1 plays an important role as a target antigen in tumor immunity for NSCLC [6]. It was also shown that high values of anti-WT1 antibody constitute predictors of longer survival for myelodysplastic syndrome (MDS) patients, which is consistent with the above result regarding NSCLC [7]. Several investigations have also provided direct evidence of spontaneous elicitation of WT1-specific CD8+ CTL response, which is a principal effector response in tumor immunity [1,8]. These findings suggest that WT1-specific immune response for the elimination of tumors is induced in many cancer patients, although the induced response is not strong enough to control the tumor growth or is diminished by tolerance mechanisms. In other words, if the spontaneous WT1-specific CD8+ CTL response can be suitably boosted, the enhanced response may result in a successful attack on tumor cells. In addition to spontaneous elicitation of WT1-specific CTL response in cancer patients, this response seems to be involved in GVL or GVT effect after HSCT [9]. This finding suggests that boosting the WT1-specific CTL response after HSCT may enhance GVL or GVT effect, thereby leading to a decrease in relapse rate after HSCT. If this can be confirmed, HSCT followed by WT1-targeting immunotherapy may become a standard therapy for patients with a high risk of relapse following treatment with HSCT alone. “…if the spontaneous WT1-specific CD8+ [cytotoxic T lymphocyte] response can be suitably boosted, the enhanced response may result in a successful attack on tumor cells.”
WT1 vaccination-associated disappearance
Various kinds of malignancies were successfully treated with WT1 peptide vaccine. These reports are being accumulated
Theoretically, in vivo activation of naturally occurring WT1-specific CTLs by vaccination with WT1 CTL peptide should damage tumor cells [2]. This hypothesis prompted us to start clinical trials for WT1 peptide vaccination [10,11]. Our report, published in 2003, on two MDS patients who received WT1 peptide vaccination is the first to describe its clinical application [10]. Successful reduction of leukemic blast and/or WT1 expression levels that reflect tumor burden, as well as leukocytopenia, was induced. It is considered that most of the normal-appearing blood cells in these MDS patients are derived from WT1-expressing transformed hematopoietic stem or progenitor cells, which were attacked by WT1-specific CTLs activated by the vaccination. Subsequently, other groups also reported clinical responses in MDS patients [1,12,13]. As for hematological malignancies other than MDS, WT1 vaccination-induced clinical responses were reported for acute myeloid leukemia and multiple myeloma [1,11,13]. A recent investigation reported an interesting case of acute myeloid leukemia, for whom WT1 peptide vaccination resulted in the disappearance of trisomy 8, a molecular marker, and an improvement of anemia [13]. It is considered that WT1 peptide vaccination eradicated the abnormal clones, which then led to the recovery of normal hematopoiesis.
WT1 vaccination-associated disappearance of a molecular marker other than WT1 expression level should be considered robust evidence for the vaccine’s therapeutic potential. A chronic myelogenous leukemia patient successfully treated with WT1 peptide vaccine in combination with imatinib, a molecular-target-based drug, was also reported [14]. WT1 peptide vaccination has been applied to solid cancers. To date, clinical responses in patients with breast cancer, lung cancer, renal cancer, ovarian cancer, and glioblastoma have been reported [1,2,11,15,16]. Successful WT1 peptide vaccination for childhood malignancies has also been reported [17]. One criticism of cancer vaccines is that response rates induced by these vaccines, that is, percentage of complete response plus partial response, are very low compared with those induced by chemotherapy. However, the action mechanisms of chemotherapy and cancer vaccine are quite different [1,2]. The former attacks tumor cells directly, while the latter activates the cancer antigen-directed immune system, which then attacks tumor cells. Furthermore, cancer vaccination is not as rapid-acting as chemotherapy. Such difference could explain the relatively low response rate of cancer vaccine. However, since cancer vaccine-associated systemic side effects are usually very mild, the vaccination can be repeated, which may result in disease stabilization or tumor shrinkage after the initial growth (late response), or long-term disease stabilization with good quality of life. In fact, there have been reports of patients with such clinical course [1,15,16,18].
We should develop ways to further enhance WT1 peptide vaccine’s efficacy
Here, we summarize the prospects for WT1 peptide vaccine, based on the present situation. Identification of WT1 CTL epitopes, performed by us and other groups, was first published in 2000 [1,19–22]. Subsequently, accumulating evidence has demonstrated that vaccination of patients with a ‘single kind’ of WT1 peptide can induce WT1-specific immunological responses and the associated clinical responses.
Thus, clearly indicating that WT1 peptide vaccination has a therapeutic potential. As a next step, we should develop ways to enhance this vaccine’s therapeutic efficacy [1,2]. From an immunological point of view, the following possibilities merit consideration [1,2]:
Vaccination with multiple kinds of WT1 CTL peptides;
Covaccination with WT1 helper peptides;
Using a stronger adjuvant locally injected together with WT1 peptide;
Using systemic cytokines;
Combined use with anticostimulatory molecule antibodies.
As previously mentioned, successful results have been reported for WT1 peptide vaccination in combination with imatinib [14]. Previously, successful treatment of chronic myelogenous leukemia patients with a combination of multiple peptides targeting BCR–ABL fusion protein and imatinib had been reported [23]. These findings suggest that administration of a combination of a molecular target-based drug and cancer vaccine constitutes a promising treatment strategy. The use of a combination of WT1 peptide vaccine and chemotherapy drugs is also interesting, since it was reported that the removal of regulatory T cells or blockade of their function by coadministration of cyclophosphamide or gemcitabine-containing chemotherapy may improve the efficacy of peptide vaccine. Certain types of molecular-target-based drugs and chemotherapy drugs should therefore constitute promising partners for WT1 peptide vaccine [1,2].
In order to enhance the usefulness of WT1 peptide vaccine, it is important to identify suitable target patients for the vaccine.
For early-phase clinical trials, patients with advanced-stage disease are usually recruited. Next, we should proceed to clinical trials in an adjuvant setting, in which we treat patients with minimal residual disease (MRD) after operation, chemotherapy, radiation or HSCT [1,2,24]. In these patients, the effector:target ratio is high and an immunosuppressive environment induced by a large tumor burden is unlikely to occur, which leads us to expect that immunotherapy should be more effective for patients with MRD than for those with a high tumor burden. Treatment of MRD patients with WT1 peptide vaccine may not be popular among clinical oncologists, because it is difficult to observe short-term effects. However, the importance of WT1 peptide-based clinical trials in the adjuvant setting warrants recognition.
We therefore need to establish the optimal timing and method for WT1 peptide vaccination, that is, for patients at advanced stages or with MRD and with the vaccine alone or in combination with other treatments. Identification of immunological markers that predict the vaccination-associated clinical response, which can target the most suitable patients more precisely, is also useful. If these things can be achieved, WT1 peptide vaccine can be expected to play an important role in the treatment of malignancies.