Sequential involvement of two distinct CD4+ regulatory T cells during the course of transplantable tumor growth and protection from 3-methylcholanthrene-induced tumorigenesis by CD25-depletion

Methylcholanthrene-Induced Sarcomas Develop Independently from NOX2-Derived ROS

Reactive oxygen species (ROS) produced by the inducible NADPH oxidase type 2 (NOX2) complex are essential for clearing certain infectious organisms but may also have a role in regulating inflammation and immune response. For example, ROS is involved in myeloid derived suppressor cell (MDSC)- and regulatory T cell (T(reg)) mediated T- and NK-cell suppression. However, abundant ROS produced within the tumor microenvironment, or by the tumor itself may also yield oxidative stress, which can blunt anti-tumor immune responses as well as eventually leading to tumor toxicity. In this study we aimed to decipher the role of NOX2-derived ROS in a chemically (by methylcholanthrene (MCA)) induced sarcoma model. Superoxide production by NOX2 requires the p47(phox) (NCF1) subunit to organize the formation of the NOX2 complex on the cell membrane. Homozygous mutant mice (NCF1*/*) have a functional loss of their super oxide burst while heterozygous mice (NCF1*/+) retain this key function. Mice harboring either a homo- or a heterozygous mutation were injected intramuscularly with MCA to induce sarcoma formation. We found that NOX2 functionality does not determine tumor incidence in the tested MCA model. Comprehensive immune monitoring in tumor bearing mice showed that infiltrating immune cells experienced an increase in their oxidative state regardless of the NOX2 functionality. While MCA-induced sarcomas where characterized by a T(reg) and MDSC accumulation, no significant differences could be found between NCF1*/* and NCF1*/+ mice. Furthermore, infiltrating T cells showed an increase in effector-memory cell phenotype markers in both NCF1*/* and NCF1*/+ mice. Tumors established from both NCF1*/* and NCF1*/+ mice were tested for their in vitro proliferative capacity as well as their resistance to cisplatin and radiation therapy, with no differences being recorded. Overall our findings indicate that NOX2 activity does not play a key role in tumor development or immune cell infiltration in the chemically induced MCA sarcoma model.

Tumor-infiltrating regulatory T cells inhibit endogenous cytotoxic T cell responses to lung adenocarcinoma

Immune cells comprise a substantial proportion of the tumor mass in human nonsmall cell lung cancers (NSCLC), but the precise composition and significance of this infiltration are unclear. In this study, we examined immune complexity of human NSCLC as well as NSCLC developing in CC10-TAg transgenic mice, and revealed that CD4(+) T lymphocytes represent the dominant population of CD45(+) immune cells, and, relative to normal lung tissue, CD4(+)Foxp3(+) regulatory T cells (Tregs) were significantly increased as a proportion of total CD4(+) cells. To assess the functional significance of increased Tregs, we evaluated CD8(+) T cell-deficient/CC10-TAg mice and revealed that CD8(+) T cells significantly controlled tumor growth with antitumor activity that was partially repressed by Tregs. However, whereas treatment with anti-CD25-depleting mAb as monotherapy preferentially depleted Tregs and improved CD8(+) T cell-mediated control of tumor progression during early tumor development, similar monotherapy was ineffective at later stages. Because mice bearing early NSCLC treated with anti-CD25 mAb exhibited increased tumor cell death associated with infiltration by CD8(+) T cells expressing elevated levels of granzyme A, granzyme B, perforin, and IFN-γ, we therefore evaluated carboplatin combination therapy resulting in a significantly extended survival beyond that observed with chemotherapy alone, indicating that Treg depletion in combination with cytotoxic therapy may be beneficial as a treatment strategy for advanced NSCLC.

Combining a peptide vaccine with oral ingestion of Lentinula edodes mycelia extract enhances anti-tumor activity in B16 melanoma-bearing mice

New anticancer vaccines must overcome regulatory T cell (Treg)-mediated immunosuppression. We previously reported that oral ingestion of Lentinula edodes mycelia (L.E.M.) extract restores melanoma-reactive T cells in melanoma-bearing mice via a mitigation of Treg-mediated immunosuppression. In this study, we investigated the effect of oral ingestion of the extract on peptide vaccine-induced anti-tumor activity. The day after subcutaneous inoculation in the footpad with B16 melanoma, mice were freely fed the extract and were vaccinated with a tyrosinase-related protein 2(180-188) peptide. The peptide vaccine was repeated thrice weekly. Melanoma growth was significantly suppressed in mice treated with both the peptide vaccine and L.E.M. extract compared with mice treated with vaccine or extract alone, and the effect was CD8(+) T cell-dependent. The combination therapy increased H-2K(b)-restricted and B16 melanoma-reactive T cells in the draining lymph nodes and spleen. Flow cytometric and immunohistological analyses revealed that the combination therapy significantly decreased the percentage of Tregs in the draining lymph nodes and spleen of melanoma-bearing mice compared to treatment with vaccine or extract alone. Kinetic analyses of peptide-specific T cells and Tregs revealed that induction of peptide-specific T cells by the peptide vaccine alone was transient, but when combined with L.E.M. extract, it efficiently prolonged the duration of peptide-specific T cell induction without increasing the percentage of Tregs. These results indicate that combination therapy enhances peptide vaccine-induced anti-tumor activity due to attenuation of the increase in the percentage of Tregs in tumor-bearing hosts.

Oral ingestion of Lentinula edodes mycelia extract inhibits B16 melanoma growth via mitigation of regulatory T cell-mediated immunosuppression

Mitigation of regulatory T cell-mediated immunosuppression is crucial for optimal in vivo anti-tumor immune responses. In this study, we examined the anti-tumor effect induced by oral ingestion of an immunomodulating diet comprised of Lentinula edodes mycelia (L.E.M.) extract. C57BL/6 mice were inoculated subcutaneously in the footpad with B16 melanoma and fed L.E.M. extract. Ingestion of L.E.M. extract significantly inhibited tumor growth, and this in vivo anti-tumor effect was not observed in nude mice, suggesting a T cell-dependent mechanism. In addition, ingestion of L.E.M. extract led to significant restoration of H-2K(b) -restricted and melanoma-reactive T cells in the spleen and draining lymph nodes of melanoma-bearing mice. Flow cytometry analysis revealed that the percentage of Foxp3(+) CD4(+) T cells increased in spleen and draining lymph nodes in melanoma-bearing mice, but decreased significantly with ingestion of L.E.M. extract. Importantly, selective depletion of CD8(+) T cells abolished the L.E.M.-induced anti-tumor effect, whereas that of CD4(+) T cells or CD25(+) cells showed no additive influence on the effect. Real-time PCR analysis revealed that ingestion of L.E.M. extract by melanoma-bearing mice decreased the level of Foxp3 mRNA within the tumor tissues, and lowered plasma transforming growth factor (TGF)-β. Furthermore, an in vitro assay revealed that an immunosuppressive activity of CD4(+) T cells from melanoma-bearing mice was canceled by ingestion of L.E.M. extract. Our results indicate that oral ingestion of L.E.M. extract restores immune responses of class I-restricted and melanoma-reactive CD8(+) T cells in melanoma-bearing mice, presumably by a mitigation of regulatory T cells-mediated immunosuppression.

Conditional regulatory T-cell depletion releases adaptive immunity preventing carcinogenesis and suppressing established tumor growth

Foxp3 is a central control element in the development and function of regulatory T cells (Treg), and mice expressing a diphtheria toxin (DT) receptor-enhanced green fluorescent protein fusion protein under the control of the foxp3 gene locus (DEREG mice) allow conditional and efficient depletion of Foxp3(+) Treg by DT injection. Herein, we use DEREG mice and a mouse model of carcinogenesis to show that conditional and effective Treg depletion can both protect mice from carcinogenesis by innate control, yet permanently eradicate a proportion of de novo-established tumors in mice in a largely CD8(+) T-cell- and IFN-γ-dependent manner. Tumors displayed a heterogeneous response to Treg depletion, and suppression of established tumors was accompanied by an increase in the tumor-infiltrating CD8(+) T-cell/B-cell ratio. Tumor rejection occurred in the absence of overt autoimmunity, suggesting that effective transient Treg depletion strategies may be therapeutic in at least a proportion of spontaneous tumors developing in the host.

Targeted inhibition of IL-10-secreting CD25- Treg via p38 MAPK suppression in cancer immunotherapy

Cancer-induced immunotolerance mediated by inducible Treg (iTreg) is a major obstacle to cancer immunotherapy. In a basic study of immunotolerance, injection of an endogenous superantigen, i.e. the minor lymphocyte stimulatory (Mls)-1(a), into specific TCR Vbeta8.1-Tg mice enabled generation of anergic CD25(-) iTreg, the immunosuppressive function of which was maintained by IL-10 production via p38-MAPK activation. Interestingly, although p38-chemical inhibitor (p38-inhibitor) is capable of breaking CD25(-) iTreg-induced immunotolerance, the p38-inhibitor had hardly any immunotolerance breaking effect when CD25(+) Treg were present, suggesting that depletion of CD25(+) Treg is necessary for p38-inhibitor to be effective. Peptide OVA(323-339) iv.-injection into its specific TCR-Tg (OT-II) mice also induced adaptive tolerance by iTreg. Peptide immunotherapy with p38-inhibitor after CD25(+) Treg-depletion was performed in an OVA-expressing lymphoma E.G7-bearing tolerant model established by adoptive transfer of OT-II CD25(-) iTreg, which resulted in suppression of tumor growth. Similarly, the antitumor immunity induced by peptide immunotherapy in colon carcinoma CT26-bearing mice, in which the number of IL-10-secreting iTreg is increased, was augmented by treatment with p38-inhibitor after CD25(+) Treg-depletion and resulted in inhibition of tumor progression. These results suggest that simultaneous inhibition of two distinct Treg-functions may be important to the success of cancer immunotherapy.

Multiple antitumor mechanisms downstream of prophylactic regulatory T-cell depletion

Several reports have shown that prophylactic depletion of regulatory T cells (Treg) using various monoclonal antibodies (mAb) in mice can stimulate potent antitumor immune responses and prevent tumor development. These same depletion methods do not significantly suppress tumor growth in a therapeutic setting. Although different strategies to deplete FoxP3(+) Treg have been used, no study has systematically compared these qualitatively for the effector mechanisms they each liberate. Herein, using prophylactic depletion of FoxP3(+) Tregs with either anti-CD4, anti-CD25, or anti-FR4 mAbs, we have compared the cellular and effector requirements for elimination of the renal carcinoma RENCA and prevention of methylcholanthrene-induced fibrosarcoma. Collectively from these two models, it was clear that CD8(+) T cells and natural killer cells played an important role downstream of Treg depletion. However, whereas all three mAbs quantitatively depleted FoxP3(+) T cells to a similar extent, subtle differences in the downstream mechanisms of tumor control existed for all three approaches. In general, neutralization of any lymphocyte subset or effector mechanism was insufficient to alter tumor suppression initiated by Treg depletion, and in some settings, the neutralization of multiple effector mechanisms failed to prevent tumor rejection. These studies reveal that Tregs control multiple redundant elements of the immune effector response capable of inhibiting tumor initiation and underscore the importance of effectively targeting these cells in any cancer immunotherapy.

Glucocorticoid-induced tumor necrosis factor receptor stimulation enhances the multifunctionality of adoptively transferred tumor antigen-specific CD8+ T cells with tumor regression

We have reported for the first time the significance of effector T-cell multifunctionality in antitumor immunity, suggesting that the appearance of multifunctional/polyfunctional tumor-specific CD8(+) T cells in vivo is a critical determinant of the success of antitumor immunotherapy, and a strategy to induce multifunctionality in effector cells is required for the successful immunotherapy of hosts with progressing tumor. Glucocorticoid-induced tumor necrosis factor receptor (GITR) stimulation has been shown to enhance antitumor immune response. However, its functional impact on adoptively transferred T cells remains unclear. Here, we analyzed the impact of GITR stimulation in vivo on the functional profiles of adoptively transferred CD8(+) T cells specific for murine fibrosarcoma CMS5. GITR stimulation was found to enhance multifunctionality (interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha production and CD107a mobilization as a degranulation marker) in transferred cells at the single-cell level. These cells exhibited upregulated expression of CD25 in draining lymph nodes and increased infiltration in tumor. Mice that received T-cell therapy with GITR stimulation showed reduced Foxp3(+)CD4(+) T cells among tumor infiltrating lymphocytes and increased in vivo cytotoxic T lymphocytes (CTL) activity even with progressing tumor, resulting in enhanced tumor regression. These data strengthen the idea that effector T-cell multifunctionality is a sensitive immune correlate for successful immunotherapy against malignancy and provide an immunological rationale for effective T-cell therapy combined with GITR stimulation.

Depletion of CD4(+)CD25(+) regulatory T cells can promote local immunity to suppress tumor growth in benzo[a]pyrene-induced forestomach carcinoma

AIM

To elucidate the distribution of CD4(+)CD25(+) regulatory T cells (Tregs) in different lymphoid tissues and its local enhancement on tumor growth before and after depletion of CD4(+)CD25(+) Tregs.

METHODS

Female ICR mice were gavaged with benzo[a]pyrene (BaP) to induce forestomach carcinoma. CD4(+)CD25(+) Tregs were intraperitoneally depleted with monoclonal antibody PC61. These mice were divided into BaP-only, BaP + IgG, BaP + PC61, and control groups. The forestomach of mice was dissected for histological analysis, and tunnel test was performed for apoptosis of tumor cells. CD4(+)CD25(+) Tregs were sorted from different lymphoid tissues and expression of Foxp3, IL-10, and chemokine receptors was analyzed by flow cytometry, semi-quantitative and real-time polymerase chain reaction.

RESULTS

The mice gavaged with only BaP showed increased forestomach papilloma and carcinoma at wk 16 and 32. The proportion of CD4(+)CD25(+) Tregs was significantly higher in peri-stomach regional lymph nodes than in other lymphoid tissues. These CD4(+)CD25(+) Tregs in regional lymph nodes expressed higher levels of Foxp3 and IL-10, enriched in the CD62L-subset, and CCR1 and CCR5 chemokine receptors. In mice gavaged with BaP + PC61, the number of tumor nodules and tumor volume decreased significantly with massive infiltrating cells and apoptosis of tumor cells. In the draining regional lymph nodes, the number of CD4(+)CD25(+) Tregs also decreased significantly.

CONCLUSION

Inducible and activated CD4(+)CD25(+) Tregs in the draining regional lymph nodes suppress host local immunity during tumor growth. Depletion of CD4(+)CD25(+) Tregs can promote host local immunity to suppress tumor growth.

Identification of a CD4 T-cell epitope in tumor rejection antigen RLakt on BALB/c radiation-leukemia RL male 1

We have previously shown that the RLakt antigen was predominantly recognized by CD8 cytotoxic T lymphocytes (CTL) in RL male 1-bearing or -rejected syngeneic BALB/c mice. CD8 CTL were directed to the octamer pRL1a peptide IPGLPLSL of which recognition was H-2L(d)-restricted. In this study, we identified a CD4 T-cell epitope peptide in the tumor rejection antigen RLakt on BALB/c radiation-leukemia RL male 1. Analyses of the recognition of a bulk CD4 T-cell line using several recombinant RLakt proteins suggested the presence of multiple CD4 T-cell epitopes in the molecule. However, cloning from a bulk CD4 T-cell line resulted in only two clones from 200 wells seeded at three cells per well, and those two CD4 T-cell clones recognized the same epitope peptide in RLakt. The epitope peptide was 14-mer p12-25, AYREETLSIIPGLP, and its recognition was H-2IA(d)-restricted. This sequence overlapped with the CD8 T-cell epitope pRL1a in its N-terminal 5 amino acid residues. The relationship of the epitope to the pRL1a peptide predominantly recognized by CD8 CTL suggests that the 14-mer epitope is predominantly recognized by CD4 T-cells.

Comparative methodologies of regulatory T cell depletion in a murine melanoma model

There has been recent interest in the depletion of regulatory T cells (Tregs) as part of a multi-faceted approach to the immunotherapy of melanoma patients. This is in part due recent findings that convincingly show that Tregs are an integral part of regulating and even suppressing an immune response to growing tumor cells. We therefore compared three methods of Treg depletion and/or elimination, utilizing low dose cyclophosphamide (CY), a specific antibody directed against the IL-2 receptor found on Tregs (PC61) and the use of denileukin diftitox (DD), which is a fusion protein designed to have a direct cytocidal action on cells which express the IL-2 receptor. We show that CY administration resulted in the highest reduction in Tregs among the three reagents. However, the reduction in Tregs with CY was also associated with the concomitant reduction of CD8(+) T cells and a lack of tumor antigen priming. Utilization of DD resulted in a >50% Treg cell reduction without parallel cytocidal effects upon other T cell subsets but did not enhance anti-tumor immunity against B16 melanoma. Lastly, the PC61 showed a moderate reduction of Tregs that lasted longer than the other reagents, without a reduction in the total number of CD8(+) T cells. Furthermore, PC61 treatment did not abrogate tumor antigen-specific immunity elicited by dendritic cells (DC). We therefore conclude that PC61 administration was the most effective method of reducing Tregs in a murine melanoma model in addition to providing evidence of a synergistic effect when combined with DC-based immunotherapy.

Regulatory T cells and tumour immunity - observations in mice and men

An enormous body of work supports a role for CD4+ CD25+ regulatory cells (Tregs) in shaping the immune response to tumours. Indeed, there is evidence that the cells impede effective tumour immunosurveillance, inhibit vaccine-induced antitumour immune responses and promote tumour progression. Studies exploring the impact of Tregs on tumour development are discussed in the context of manipulating this T-cell population for the purpose of cancer immunotherapy.

Increase of CD4+ CD25+ regulatory T cells in the peripheral blood of patients with metastatic carcinoma: a Phase I clinical trial using cyclophosphamide and immunotherapy to eliminate CD4+ CD25+ T lymphocytes

We determined the number and functional status of CD4+ CD25(high) regulatory T cells (Treg) in blood samples from patients with metastatic carcinoma, and evaluated their sensitivity to a single intravenous infusion of cyclophosphamide. Treg numbers were significantly higher in 49 patients with metastatic cancer (9.2% of CD4+ T cells) compared to 24 healthy donors (7.1%). These cells expressed the transcription factor forkhead box P3 (FoxP3), glucocorticoid-induced tumour necrosis factor receptor family-related protein (GITR) and intracellular CD152, and demonstrated a suppressive activity in vitro against CD4+ CD25- autologous proliferation. At a single intravenous infusion, cyclophosphamide failed, in association with a non-specific immunotherapy by intratumoral bacille Calmette-Guérin (BCG), to modulate significantly Treg numbers or function. Metastatic cancer is associated with an expansion of peripheral blood CD4+ CD25(high) FoxP3+ GITR+ CD152+ Treg cells whose immunosuppressive properties do not differ from those of healthy subjects. Moreover, cyclophosphamide administration may not represent an optimal therapy to eliminate Treg, which further underlines the need to identify specific agents that would selectively deplete these cells.

Defining the ability of cyclophosphamide preconditioning to enhance the antigen-specific CD8+ T-cell response to peptide vaccination: creation of a beneficial host microenvironment involving type I IFNs and myeloid cells

Although cyclophosphamide (CTX) has been clearly shown to enhance active specific and adoptive immunotherapies, the mechanism(s) underlying these beneficial effects have not been clearly defined. To define the impact of CTX preconditioning on the antigen-specific CD8 T-cell response to peptide vaccination, we used an adoptive transfer model based on the OT-1 T-cell receptor transgenic mouse. CTX preconditioning dramatically enhanced the antigen-specific CD8 T-cell response to peptide vaccination. Specifically, CTX significantly enhanced the expansion and function of responding CD8 T cells as demonstrated by flow cytometry and cytokine production. In parallel experiments, we attempted to define the mechanism(s) underlying these beneficial effects of CTX therapy. CTX therapy increased the relative number and activation status of myeloid dendritic cells, and was associated with the induction of significant levels of the inflammatory cytokines interferon-alpha, monocyte chemoattractant protein-1, and IL-6. Adoptive transfer experiments into type I IFNR-/- and CR3-/- mice confirmed that the beneficial effects of CTX were at least partially dependent on type I interferons and myeloid cells. Adoptive transfer of up to 150x10(6) naive spleen cells at the time of antigen-specific CD8 T-cell transfer did not abrogate the effects of CTX therapy, suggesting that the creation of a niche in the immune system may not be required. CTX decreased the absolute, but not relative number of CD4+CD25+ Treg cells, consistent with the possibility that regulatory T cells may be targeted by CTX therapy. Of note, combination therapy with CTX and a synthetic TLR3 agonist further enhanced the antigen-specific CD8+ T-cell response. Taken together, our data suggest that CTX modulates specific components of the innate immune system resulting in a beneficial host microenvironment. Specific targeting of these components may enhance the effectiveness of CTX preconditioning for adoptive immunotherapy.

CD4+CD25+ regulatory T-cell-inactivation in combination with adenovirus vaccines enhances T-cell responses and protects mice from tumor challenge

Cancer vaccines are a promising approach to treating tumors or preventing tumor relapse through induction of an immune response against tumor-associated antigens (TAA). One major obstacle to successful therapy is the immunological tolerance against self-antigens which limits an effective antitumor immune response. As a transient reduction of immunological tolerance may enable more effective vaccination against self-tumor antigens, we explored this hypothesis in a CEA tolerant animal model with an adenovirus expressing CEA vaccine in conjunction with inactivation of CD4(+)CD25(+) regulatory T cells. This vaccination modality resulted in increased CEA-specific CD8(+), CD4(+) T cells and antibody response. The appearance of a CD4(+) T-cell response correlated with a stronger memory response. The combined CD25(+) inactivation and genetic vaccination resulted in significant tumor protection in a metastatic tumor model. Non-invasive tumor visualization showed that not only primary tumors were reduced, but also hepatic metastases. Our results support the viability of this cancer vaccine strategy as an adjuvant treatment to prevent tumor relapse in cancer patients.

Tumor-infiltrating T lymphocytes: friends or foes?

The prognostic significance of tumor-infiltrating lymphocytes (TILs) has been a longstanding topic of debate. In cases where TILs have improved patient outcome, T lymphocytes are recognized as the main effectors of antitumor immune responses. However, recent studies have revealed that a subset of CD4(+) T cells, referred to as CD4(+)CD25(+) regulatory T cells (Treg), may accumulate in the tumor environment and suppress tumor-specific T-cell responses, thereby hindering tumor rejection. Hence, predicting tumor behavior on the basis of an indiscriminate evaluation of tumor-infiltrating T cells may result in inconsistent prognostic accuracy. The presence of infiltrating CD4(+)CD25(+) Treg may be detrimental to the host defense against the tumor, while the presence of effector T lymphocytes, including CD8(+) T cells and non-regulatory CD4(+) helper T cells may be beneficial. Enhanced recruitment of antitumor effector T lymphocytes to tumor tissue in addition to inhibition of local Treg, may therefore be an ideal target for improving cancer immunotherapy. This article reviews the antitumor functions of T-lymphocytes, with special attention given to CD4(+) regulatory T-cells within the tumor environment.

Cancer Immunosurveillance and Immunoediting: The Roles of Immunity in Suppressing Tumor Development and Shaping Tumor Immunogenicity

Cellular transformation and tumor development result from an accumulation of mutational and epigenetic changes that alter normal cell growth and survival pathways. For the last 100 years, there has been a vigorous debate as to whether the unmanipulated immune system can detect and eliminate such altered host derived cells despite the fact that cancer cells frequently express either abnormal proteins or abnormal levels of normal cellular proteins that function as tumor antigens. In this review, we discuss the current state of this argument and point out some of the recent key experiments demonstrating that immunity not only protects the host from cancer development (i.e., provides a cancer immunosurveillance function) but also can promote tumor growth, sometimes by generating more aggressive tumors. The terminology "cancer immunoediting" has been used to describe this dual host protective and tumor promoting action of immunity, and herein we summarize the ever-increasing experimental and clinical data that support the validity of this concept.

The requirement of multimodal therapy (vaccine, local tumor radiation, and reduction of suppressor cells) to eliminate established tumors

PURPOSE

Numerous immune-based strategies are currently being evaluated for cancer therapy in preclinical models and clinical trials. Whereas many strategies look promising in preclinical models, they are often evaluated before or shortly following tumor implantation. The elimination of well-established tumors often proves elusive. Here we show that a multimodal immune-based therapy can be successfully employed to eliminate established tumors.

EXPERIMENTAL DESIGN

This therapy consists of vaccines directed against a self-tumor-associated antigen, the use of external beam radiation of tumors to up-regulate Fas on tumor cells, and the use of a monoclonal antibody (mAb) to reduce levels of CD4+CD25+ suppressor cells.

RESULTS

We show here for the first time that (a) antigen-specific immune responses induced by vaccines were optimally augmented when anti-CD25 mAb was given at the same time as vaccination; (b) anti-CD25 mAb administration in combination with vaccines equally augmented T-cell immune responses specific for a self-antigen as well as those specific for a non-self antigen; (c) whereas the combined use of vaccines and anti-CD25 mAb enhanced antigen-specific immune responses, it was not sufficient to eliminate established tumors; (d) the addition of external beam radiation of tumors to the vaccine/anti-CD25 mAb regimen was required for the elimination of established tumors; and (e) T cells from mice receiving the combination therapy showed significantly higher T-cell responses specific not only for the antigen in the vaccine but also for additional tumor-derived antigens (p53 and gp70).

CONCLUSIONS

These studies reported here support the rationale for clinical trials employing multimodal immune-based therapies.

Intratumor depletion of CD4+ cells unmasks tumor immunogenicity leading to the rejection of late-stage tumors

Tumor environment can be critical for preventing the immunological destruction of antigenic tumors. We have observed a selective accumulation of CD4(+)CD25(+) T cells inside tumors. In a murine fibrosarcoma L(d)-expressing Ag104, these cells made up the majority of tumor-infiltrating lymphocytes at the late stage of tumor progression, and their depletion during the effector phase, rather than priming phase, successfully enhanced antitumor immunity. We show here that CD4(+)CD25(+) T cells suppressed the proliferation and interferon-gamma production of CD8(+) T cells in vivo at the local tumor site. Blockade of the effects of IL-10 and TGF-beta partially reversed the suppression imposed by the CD4(+) cells. Furthermore, local depletion of CD4(+) cells inside the tumor resulted in a change of cytokine milieu and led to the eradication of well-established highly aggressive tumors and the development of long-term antitumor memory. Therefore, CD4(+)CD25(+) T cells maintained an environment in the tumor that concealed the immunogenicity of tumor cells to permit progressive growth of antigenic tumors. Our study illustrates that the suppression of antitumor immunity by regulatory T cells occurs predominantly at the tumor site, and that local reversal of suppression, even at a late stage of tumor development, can be an effective treatment for well-established cancers.

Murine leukemia RL male 1 and sarcoma Meth A antigens recognized by cytotoxic T lymphocytes (CTL)

Peptide elution and expression cloning methods have been used to identify T cell-recognized antigens for which no molecular information is available. We identified a unique tumor antigen peptide pRL1a, IPGLPLSL that is recognized by CTL on BALB/c RL male 1 leukemia by peptide elution. The sequence of the peptide corresponded to the normally untranslated 5' region of akt. Cytotoxicity was generated in BALB/c spleen cells by in vivo and in vitro sensitization with pRL1a peptide in the form of multiple antigen peptide (MAP), but not the original form. pRL1a MAP immunization had a significant growth-inhibitory effect. pRL1a MAP was mostly internalized into the endosomal compartment of antigen-presenting cells, leaked to the cytosol, and degraded, and the pRL1a peptide produced was presented through the MHC class I pathway. In vivo depletion of CD4 T cells from tumor-inoculated BALB/c mice caused RL male 1 regression. Overexpression of the RLakt molecule seemed to induce CD4 immunoregulatory cells, which resulted in progressive RL male 1 growth in BALB/c mice. In vivo administration of anti-CD25 mAb (PC61) caused regression of RL male 1, suggesting that CD4(+) CD25(+) immunoregulatory cells were involved in the tumor growth. Recently, we improved the sensitivity and the efficacy of T cell antigen cloning from cDNA expression libraries by using large- and small-scale ELISPOT assays. Using the IFN-gamma ELISPOT method, we obtained a cDNA clone S35 of 937 bp recognized by AT-1 CTL on BALB/c Meth A sarcoma. S35 was a part of the retinoic acid-regulated nuclear matrix-associated protein (ramp). AT-1 CTL recognized the peptide LGAEAIFRL, which was derived from a newly created open reading frame due to the exon 14 extension.