Adoptive T cell therapy of solid cancers

Abstract

The development of immune-based approaches for the treatment of cancer has been actively investigated for many years. One strategy that has emerged as a potentially effective strategy for the treatment of aggressive established malignancies is adoptive T cell therapy. The power of this approach has been repeatedly observed in preclinical animal models. However, moving from homogeneous animal models to the heterogeneous human clinical setting has been very difficult. It is only in recent times that we have been able to pinpoint the problems of the clinical translation of adoptive T cell therapy. Some of the major problems are sources of tumor-specific T cells, ex vivo expansion, persistence, and anti-tumor activity. This review overviews the nature of these problems and some of the emerging solutions.

Introduction

Adoptive T cell therapy has been envisioned as a future tumor therapy strategy for a few decades. In some disease settings such as bulky established malignancy, adoptive T cell therapy may be a better choice than therapeutic vaccines for treating established disease. Adoptive T cell therapy is a strategy that potentially gives better control over both the quantity and quality of the T cell response. The potential for clinical efficacy of adoptive T cell therapy in humans has been repeatedly confirmed in animal models, but translation of the approach from animals to humans has been problematic. In recent years, however, many of the key problem areas of adoptive T cell therapy have been identified, which will ultimately lead to improving the feasibility and efficacy of the approach in years to come. These problem areas include [1] ex vivo expansion of tumor antigen specific T cells; [2] improving the persistence of the T cells; and [3] augmenting in vivo antitumor activity [1, 2]. This review assesses these problems and evaluates some emerging techniques aimed at overcoming them.

Ex vivo expansion

The feasibility of generating T cells ex vivo is limited by the initial numbers of tumor antigen-specific T cell frequency in the starting bulk cultures, which can be very low for most tumor antigens. This difficulty has led predominantly to the use of cloning from PBMC to select out and expand the rare tumor-specific T cells. Clones of T cells with a single peptide specificity and affinity, however, can greatly limit the efficacy of adoptive T cell therapy. Polyclonal T cell lines have been shown to be more efficacious in animal models, which may be related to delivery, multiple specificities, affinities and destructive T cell functions to the tumor site [3]. Thus, to overcome the problems with clone generation, some investigators have focused on the generation of tumor antigen-specific T cell lines from enriched sources such as the tumor infiltrating lymphyocyte (TIL) population and the tumor draining lymph nodes. The feasibility of generating T cell lines that demonstrate clinical efficacy has recently been established by Dudley and colleagues at the NIH, who harvested and expanded melanoma TIL that were able to mediate an antitumor response in advanced stage patients under conditions in which T cell clones were ineffective [4]. The translation of TIL-based adoptive T cell therapy to other cancers may be problematic due to tumor and TIL acquisition. As an alternative, peripheral blood mononuclear cells (PBMC) can be a source of tumor antigen-specific T cells in patients where the isolation of TIL is not possible. The T cell repertoire of cancer patient individuals contains tumor-antigen-specific T cells capable of recognizing tumor antigens, although in many at levels too low to be detected directly ex vivo. Furthermore, the frequency of these tumor antigen specific T cells may be elevated compared to normal healthy donors as a result of exposure of the T cell repertoire to tumor antigens in cancer patients [5–7]. The levels of these tumor antigen-specific T cells, however, will likely need to be elevated prior to harvesting the blood for ex vivo expansion as they are still relatively rare. We compared the outcome of ex vivo expansion of HER-2/neu-specific T cells in short-term cultures in patients vaccinated with a HER-2/neu peptide vaccine and nonvaccinated individuals, both populations having HER-2/neu-overexpressing tumors. The generation of short-term T cell lines with measurable HER-2/neu-specific immunity was improved with prior vaccination (Knutson et al., unpublished observations). A number of vaccine strategies are useful for elevating levels such as peptide epitopes, proteins, naked DNA, viral constructs, whole tumor cells or antigen-loaded dendritic cells, all of which have been extensively tested in phase I/II clinical trials and have successfully elevated the levels of antigen-specific T cells [8]. One potential largely unexplored source of higher frequencies of tumor-reactive T cells has been found in the bone marrow of breast cancer and multiple myeloma patients, which may obviate the need for prior vaccination in adoptive T cell therapy [9, 10]. For example, Feuerer and colleagues have found that bone marrow of breast cancer patients contains much higher proportions of memory T cells than bone marrow of healthy individuals, and detected in one individual a high frequency of HER-2/neu specific CTL [10].

The limitations of our current methods of ex vivo expansion have also impeded the clinical translation of adoptive T cell therapy. In recent years, however, a number of techniques have been developed that can improve the quality and quantity of ex vivo expanded T cells. These improvements include the ability to predict tumor antigen-derived MHC class II and class I tumor, and cellular cofactors. With the use of computer algorithms, our group has identified several MHC class II peptides derived from HER-2/neu, which we have found useful for generating T cells ex vivo [11, 12]. In our initial approach, we observed that the traditional approaches of culturing with peptide and IL-2, however, were ineffective at promoting the expansion of the CD4 T helper cells specific for these MHC class II peptides. Thus, we developed strategies that incorporated the use of IL-12 to expand HER-2/neu-specific CD4 T helper cells [13]. IL-12 is a heterodimeric inflammatory cytokine, produced by B cells, macrophages, and professional APC, which has multiple reported effects on CD8 T cell function when added together with low-dose IL-2 [14–19]. As a model for MHC class II antigen to evaluate the utility of IL-12 for the expansion of CD4 T helper cells, we chose p776–790, a naturally processed peptide derived from the intracellular domain of HER-2/neu [12, 20]. Using T cells derived from patients that had been immunized with the peptide, we found that immunity to p776–790 could be readily measured in short-term cultures. However, cell lines generated by in vitro stimulation with peptide and IL-2 as the only added cytokine resulted in no antigen-specific T cell expansion. The inclusion of IL-12, along with IL-2, restored antigen-specific responsiveness in a dose-dependent fashion and the resulting p776–790-specific T cells responded readily to antigen by proliferating and producing type I cytokines (IFN-γ and TNF-α)[13]. The increased proliferative response of the cultures was due in part to an increase in the number of HER-2/neu-specific T cells as assessed directly by ELIspot analysis. Inclusion of IL-12 into the cultures also resulted in a significant decrease of nonspecific cellular proliferation indicating that IL-12 suppresses nonspecific T cell proliferation induced by IL-2. Other cytokines are also being examined for utility in ex vivo expansion of T cells, including IL-7 and IL-15. IL-7 has also shown promise for the expansion of CTL under selected ex vivo conditions. IL-7 is a stromal cell-derived cytokine and is associated with the early development of lymphoid cells. IL-7 activates the proliferation of naïve T cells and has been implicated as a key cytokine in maintaining homeostatic proliferation and T cell survival in vivo [21]. Under some conditions, IL-7 can promote preferential expansion of antigen-specific T cells [22]. For example, CTL derived from TIL specific for follicular lymphoma (FL) cells requires the inclusion of IL-7 along with IL-2. The expanded T cells have greatly enhanced FL-specific CTL activity. The effects of IL-7, however, depend on the ex vivo expansion environment and may not be useful in bulk cultures of peripheral blood mononuclear cells. In our lab, we have found that when IL-7 is included along with an influenza matrix peptide and IL-2, peptide-specific lysis is reduced significantly compared to cells cultured with soluble peptide and IL-2 alone (Fig. 1). The background, nonspecific lysis was also increased threefold. However, when DC are used as the antigen presenting cells, IL-7 can expand peptide-specific CTL [23]. These discrepancies in outcome point to the need to optimize the use of cytokines in preclinical studies prior to clinical trials.

Fig. 1.

IL-7 can reduce the antigen-specificity during ex vivo expansion. Cytolytic T cell responses to flu matrix peptide-loaded (squares) or nonloaded (circles) tumor cells were measured following ex vivo expansion of human T cells in the presence (open symbols) or absence (closed symbols) of IL-7. Activity was measured using a standard 4-h chromium release assay

IL-15 can also be effective in ex vivo expansion of peptide- and protein-specific T cells. Like IL-2, IL-15 is a pleiotropic cytokine and induces proliferation and functional changes of multiple hematopoietic cells including αβT cells, γδT cells, DC, and NK cells [24]. In vivo, IL-15 preserves memory T cells and displays some properties similar to IL-2 [25]. Some of the memory preservation properties might be observed in vitro as well. For example, we have found that IL-15 promotes the preservation of antigen-specific T cells, when included with IL-2 but in the absence of antigen (Fig. 2) [26]. Recently, IL-15 has also been found to expand naïve T cells when given with IL-21, demonstrating the pleiotropic activities of cytokines and that altering conditions may greatly impact the outcome of ex vivo expansion [27].

Fig. 2.

IL-15 preserves memory T cell responses during ex vivo expansion of T cells. T cell responses to either CMV lysate or a CMV pp65 peptide were measured preexpansion (a) and postexpansion (b, c) using IFN-γ ELIspot analysis. T cells (b, c) were incubated with IL-2 (10 U/ml) without (b) or with (c) IL-15 (10 ng/ml) for 12 days followed by incubation with anti-CD3/anti-CD28 beads and IL-2 for an additional 12 days. Note that the cells preincubated with IL-15 demonstrated a better preservation of CMV lystate and pp65-specific memory T cell immunity. Each bar is the mean and SEM of six replicates

In addition to cytokines, a number of other molecular strategies are being defined to augment ex vivo expansion. The inclusion of anti-CD28 antibodies has also proven useful by providing effective co-stimulation during antigen stimulation. A well-studied approach has been to co-mobilize anti-CD28 antibody along with anti-CD3 antibodies, which can be used for nonspecific activation and growth of T cells [28]. While the major use of these anti-CD3/anti-CD28 beads is to stimulate T cells in a nonspecific fashion, the beads have been incorporated into antigen-specific strategies where, under the appropriate conditions, preferential expansion of antigen-specific T cells can be achieved [29]. We have tested a technique in our laboratory that combines an antigen-specific T cell activation phase followed by subsequent bead stimulation. In that study, we used evaluated expansion of HER-2/neu-specific T cells in cultured PBMC. The cells were first stimulated with a HER-2/neu T cell helper peptide along with IL-2 and IL-12 [13] followed by rapid expansion with anti-CD3/anti-CD28 beads. Over the course of the 25 day ex vivo expansion, the antigen-specific T cells increased up to 500-fold, while the bystander cells grew only about 30-fold (Knutson, et al., unpublished observations). In another approach, Maus and colleagues coupled anti-CD28 to magnetic beads containing peptide-charged HLA class II tetramers and found effective stimulation and expansion of antigen-specific CD4 T cells [30]. Manipulating the anti-CD28 antibody can also alter the Th1/Th2 phenotype of the resulting T cell population, which may be useful for selecting specific cytokine profiles. CD28 co-stimulation apparently favors a Th1 phenotype when used in cis whereas the trans configuration favors a Th2 profile [31].

Although often overlooked, the basic elements of T cell culture medium can be adjusted to improve ex vivo expansion. We have found that the addition of uric acid to standard media can result in significantly higher levels of antigen-specific T cells after short-term culture (Fig. 3). Uric acid (crystalline) has been described recently as an immune modulator that alerts the immune system to dying tissue and tumors in the mouse [32, 33]. We have observed in our studies that uric acid may have a similar role in humans as well. As shown in Fig. 3, the inclusion of uric acid during short-term culture with tetanus toxoid resulted in doubling the numbers of tetanus-specific T cells. The atmosphere in which the T cells are grown can greatly influence both recovery and biologic activity. Haddad et al. [34] found that the best oxygen atmosphere for generating T cells ex vivo is 5% rather than standard ambient level of 20%. In their study, DNA microarray analysis revealed that an oxygen level of 20% resulted in higher expression of genes involved in stress response, cell death, and cellular repair. T cells incubated at 5%, however, demonstrated improved health with higher expression of genes involved in immune function and cell cycle progression. The optimization of conditions for the reliable and reproducible expansion of T cells for adoptive T cell therapy is complex. Many key elements can be adjusted to improve the desirable outcome. Ex vivo expansion conditions will likely need to be tailored to fit the therapeutic scheme. Animal models to define the appropriate cell culture conditions may be useful in defining the basic requirement. As with many other model systems, however, the weakness is evident when it comes to the translation of the technique to a heterogeneous human population. To move a unique ex vivo expansion condition into routine clinical use, it is likely that in vitro optimization will require samples from many individuals in order to derive the appropriate confidence to predict how the technique can be translated to a human clinical trial.

Fig. 3.

Uric acid increases antigen-specific T cells during short-term T cell culture. PBMC from a normal healthy blood donor were incubated in tetanus toxoid (TT) with and without uric acid (25 μg/ml) for 8 days. The cells were then restimulated with either no antigen or TT and analyzed using IFN-γ ELIspot. Shown are the TT-specific T cells determined by subtracting out the background spots in the no-antigen wells. Each bar is the mean and SEM of three replicates. Uric acid was able to double the number of antigen-specific T cells

Promoting the in vivo persistence of ex vivo expanded T cells

The therapeutic efficacy of transferred T cells is likely dependent on the longevity of infused T cells. Initial studies of adoptive T cell therapy focused on the infusion of CD8 T cell clones by Yee and colleagues at the Fred Hutchinson Cancer Research Center and Dudley and colleagues at the National Cancer Institute [35–37]. In these studies, the mean survival of the infused CD8 T cell clones was 3 weeks or less. A number of theories have surfaced to explain the lack of persistence with the goal of identifying techniques to promote the integration of the transferred T cells into the normal mechanisms of homeostatic proliferation and survival (i.e., immunologic memory). Infused CD8 T cell effectors have a finite lifespan and, based on murine infectious disease models of persistence, a signal must be generated to promote the differentiation of the effectors into memory cells that proliferate and can subsequently give rise to effectors capable of destroying the tumor tissue over an extended period of time [38]. Powell et al. [39] at the NCI studied the effector to memory transition of infused T cells in a clinical trial of T cell therapy that resulted in significant regressions of melanoma deposits. Patients studied had either partial responses (≥50% reduction at 4 weeks) or mixed responses (mix of reduction in some lesions and increases in others at 4 weeks). Characteristic transition profiles were observed in the infused T cell population, including the acquisition of the ability to respond to IL-7 (i.e., upregulation of the IL-7R on the T cells), a cytokine pathway well known to be involved in the memory cell homeostasis [26]. The question remains as to how the transition occurs and what effector T cell markers, which can be observed in vitro, correlate with the memory transition. Studies in the mouse by Wang et al. [40] demonstrate that in vivo antigen exposure is not required for the successful transition of the effector T cells suggesting that additional antigen-independent steps need to be taken during T cell culture. One possibility is that the T cells could be exposed to IL-15 in vitro following antigen exposure in order to create or restore a memory phenotype, as described above.

Alternatively, in vivo conditioning prior to infusion may be suitable to convert a portion of the infused effectors into memory T cells in vivo. Conditioning with chemotherapeutic agents such as cytoxan and fludarabine was suggested to play a key role in promoting persistence. However, the evidence for a role of chemotherapy is contradictory. In one recent study, Dudley et al. [4] observed that infused melanoma-specific T cell lines persisted for several months, in vivo, which may have been attributable to prior treatment of the patients with fludarabine and cytoxan. However, in another study, the same group reported that CD8 T cell clone persistence was minimal, despite prior treatment with a similar chemotherapy regimen [35]. It is probable that chemotherapy may be necessary but not sufficient to ensure longevity of the infused T cells. This theory is supported by evidence in mice that the cytoxan supports in vivo expansion of memory T cells through a mechanism involving IFN α and β [41]. An alternative explanation is that cytoxan chemotherapy supports adoptive T cell therapy by reducing regulatory T cells and regulatory cytokines (i.e., IL-10), which are elevated in the periphery and concentrated in the tumor tissues in patients with solid tumors (discussed below) [42–46].

Other studies have suggested that CD4 T cell infusion is required for CD8 T cells to persist [47]. The role of CD4 T cell help, in the persistence of CD8 T cells, is thought to be mediated by the elaboration of a plethora of cytokines involved in long-term persistence. Recent evidence from our laboratory suggests that tumor antigen-specific CD4⁺ T helper cells could prolong the life of CTL in vivo [11]. We observed that greater than 60% of patients immunized with HER-2/neu helper epitopes, each containing an encompassed HLA-A2 epitope, were able to develop HER-2/neu specific CD8⁺ T cell immunity. The CD8⁺ T cell response was maintained, in some patients, for at least 1 year following vaccination. In contrast, 2/5 (40%) patients immunized with a single HER-2/neu HLA-A2 9-mer peptide, p369–377 (E75), developed HER-2/neu CD8 T cell immunity that declined to undetectable levels within 5 months of the last vaccination [48]. These data are consistent with the findings in murine viral models where the longevity of CD8⁺ T cells is critically dependent on concurrent CD4⁺ T cell immunity [49]. Although mechanisms by which CD4⁺ T cells support CD8⁺ T cell longevity in vivo are unclear, it is likely that IL-2 is a critical component. Yee et al. [37] observed in their clinical trial of T cell therapy that the administration of low dose IL-2 could enhance the longevity of CD8 T clones. The mechanism of IL-2 action is unclear and the improved longevity of transferred CD8 T cells may have been due to enhancing the survival of the infused T cells rather than inducing transition to a memory state.

Enhancing the anti-tumor activity of transferred T cells

Ideally, T cells transferred into a tumor-bearing host would preferentially home to tumor to mediate an antitumor response without homing to other tissues. However, very little evidence exists that support the notion that there needs to be specific homing mechanisms for T cells to get into tumor tissues. It is known that the conditions under which the cells are cultured can influence their trafficking to lymphoid tissue. For example, antigen-primed murine CD8⁺ T cells cultured in IL-15, but not IL-2, preferentially home to lymphoid tissue such as spleen and lymph nodes, while IL-2 cultured CD8⁺ T cells home to sites of inflammation but not lymphoid tissue [50]. IL-15 cultured cells home to sites of inflammation to a lesser extent but mediate a robust antigen recall response. Currently, it is unknown if these findings can be extrapolated to human T cells, but it could have important implications in designing expansion conditions to generate T cells capable of targeting lymph node disease. Inducing an inflammatory response at the tumor site with exogenous agents (e.g., viruses and anti-apoptotic agents) may establish appropriate conditions to attract T cells into the tumor microenvironment. In another study, Palmer et al. [51] demonstrated that antigen-specific T cells traffic indiscriminately and ubiquitously throughout the body and therefore antitumor activity did not depend on specific homing mechanisms. The levels of antigen expression on the tumor, however, were important for the activation of the antigen-specific T cells to elicit an antitumor response. These findings point to the caveat that an understanding of the levels of antigen expression and MHC presentation in the tumor bed is critical for interpreting clinical outcome in adoptive T cell therapy trials. Loss of antigen expression has been observed in human clinical trials of adoptive T cell therapy by Yee et al. [37] using T cell clones. In that study, disease progression appeared to have some link with the loss of antigen expression. Without an understanding of the levels of antigen expression, the results of this study would have been more difficult to interpret. The tumor may also lose the capacity to process and present the antigen in MHC despite continue expression. This appears to be a major mechanism by which many tumors become resistant to the immune response in most, if not all, advanced tumors [52–55]. Often times it has been observed that as tumors progress, one or more components of the antigen-processing machinery become disabled, mutated, or altered in expression. Nonetheless, the efficacy of adoptive T cell therapy will likely be significantly impacted by the loss of HLA expression. Strategies are available that could be combined with adoptive T cell therapy to improve tumor recognition through the pharmacological upregulation of HLA molecules. IFN-γ potentiates the immune response by upregulating the HLA class I and class II antigen expression on antigen presenting cells (APC), T cells, and tumor cells [56]. When used as monotherapy complete clinical responses have been observed in several tumor settings. In patients with ovarian cancer, IFN-γ has produced complete responses; in patients, in whom previous chemotherapy had failed. In one study, 31/98 (32%) achieved a surgically documented response, including 23 patients (23%) with a complete response, defined as the complete disappearance of all macroscopic and microscopic disease with a negative IP histology [57]. The responses were generally better in younger patients (<60 years old) with smaller tumors (<2 cm). Although IFN-γ has direct effects on tumor growth, a likely mechanism for the action of the cytokine in ovarian cancer patients is upregulation of HLA expression thereby improving targeting by the T cell response. Using subcutaneous administration of IFN-γ in patients with metastatic melanoma, IFN-γ has also been shown to upregulate HLA class I expression on tumor cells and cause complete clinical responses (WHO criteria), albeit at a much lower rate (13%) [58]. Another strategy that may be effective and operate through increasing antigen processing and presentation would be to combine monoclonal antibody therapy with T cell therapy. There is some in vitro evidence that T cell therapy could be enhanced when combined with trastuzumab monoclonal antibody therapy of breast cancer. Trastuzumab is a humanized murine monoclonal antibody that binds to the extracellular domain of human HER-2/neu. Zum Buschenfelde and colleagues found that pretreatment of HER-2/neu-overexpressing tumor cells with trastuzumab enhanced the cytolytic activity of HER-2/neu-specific T cells against the HER-2/neu-overexpressing tumors in vitro [59]. Although the mechanism by which trastuzumab enhances cytolytic activity is unclear, it is possible that trastuzumab promotes the internalization and degradation of HER-2/neu, resulting in increased presentation of HER2 MHC class I epitopes, which may lead to greater activation and expansion of HER-2/neu-specific T cells. If this is the true mechanism of action, then this could potentially mean that there may be enhanced MHC class II presentation of antigens, since the MHC class II antigen presentation mechanism is also known to intersect with the receptor internalization pathways.

Recent studies indicate that other aspects of the tumor microenvironment will likely need to be altered in order for T cells to infiltrate the tumor and demonstrate anti-tumor activity. In most cases, the unmanipulated tumor microenvironment is hostile to immune effectors, and probably induces immune tolerance, suppression, or anergy. For example, tumors can specifically recruit Tregs into the tumor microenvironment, which can suppress the effector functions of T cells and render them ineffective. Although the mechanisms involved are unclear, it is known that a variety of different tumor types are able to recruit Tregs into the tumor microenvironment [45, 46, 60]. In one recent study by Zou et al. [46], it is suggested that tumor-associated macrophages produce CCL22, which attracts Foxp3-expressing Tregs into the ovarian cancer tumor microenvironment to inhibit T cell function. Our group has been studying the role of Tregs in breast cancer using the neu-transgenic (neu-tg) mouse model. The neu-tg mouse carries nonactivated rat neu as a transgene driven by the MMTV promoter. The mice develop spontaneous breast cancer at about 1 year of age in a fashion that closely resembles human breast cancer. We have found that Tregs (CD4+CD25+CD62L+GITR+CD69−) can constitute approximately 10–15% of the tumor-infiltrating lymphocytes (Knutson, unpublished observations). In order to explore the role of Tregs, in the pathogenesis of disease in the neu-tg mouse, we blocked their function in vivo with the DAB₃₈₉IL-2, an immunotoxin that enters cells through the IL-2 receptor. It was observed that treatment of mice with DAB₃₈₉IL-2 immediately after tumor implantation resulted in significant tumor protection compared to control animals, an effect that remained apparent for up to 30 days following treatment termination, indicating the involvement of immunologic memory (Knutson, unpublished observations). Studies in other murine models, using CD25+ T cell depletion, have also suggested that Tregs play a role in disease progression [61]. In addition to regulatory T cells, tumors may also attract immature dendritic cells, which are associated with a variety of cancers and can block T cell function [62, 63]. Breast, prostate and ovarian cancers express significant amounts of the chemokine, stromal-derived factor-1 (SDF-1) [64–66]. Ordinarily, SDF-1 is involved in normal embryogenesis, cardiogenesis, and hematopoiesis [67]. However, its role becomes pathogenic when overexpressed by tumor cells because it attracts immature, immune inhibitory dendritic cells (DC). Specifically, SDF-1 attracts precursor plasmacytoid dendritic cells (pPDCs) through pPDC expression of the chemokine receptor, CXCR4 [64]. pPDCs are able to suppress the development of T cell-mediated immunity. Recruited pPDCs induce the secretion of the immunosuppressive cytokine, IL-10, from T cells into the tumor microenvironment. IL-10 is an anti-inflammatory cytokine and inhibits tumor-specific T cell proliferation, as well as infiltration and activation of antigen-presenting cells (APC) that are critical in the initiation of an immune response. Lastly, many tumors and tumor-associated Tregs can produce TGF-β, which is likely capable of blocking the function of infused effector T cells within the tumor microenvironment [68].

The combination of adoptive T cell therapy with strategies to overcome these tumor-induced suppressive mechanisms also seems to be a logical advance. As previously discussed, the therapeutic efficacy of combining cyclosphosphamide with T cell therapy may be attributable to depletion of Tregs [36]. Several other strategies are currently being tested that may reduce tumor-induced immunosuppression. As previously discussed, DAB₃₈₉IL-2 may be useful for inhibiting and depleting Tregs. Anti-CD25 monoclonal antibody therapy may also be appropriate, and although some anti-CD25 products are in clinical testing or are approved for use by the USA Food and Drug Administration, studies need to be done to determine if these agents are able to deplete Tregs in humans. Another strategy is to use anti-CTLA-4 antibody, which has been shown to elevate tumor-antigen specific immunity in cancer patients possibly by a mechanism involving inhibition of systemic and intratumoral Tregs [69]

Conclusion

Recent successes in adoptive T cell therapy have invigorated the scientific community interested in this approach as a means of treating established malignancy. Concerted efforts are now being put forth to clarify those areas that limit the feasibility and efficacy of the approach. We now feel optimistic that we can make incremental improvements in the coming years with the hope that 1 day adoptive T cell therapy may be a viable therapeutic alternative in patients with advanced cancer.