Cancer immunotherapy: synthetic and natural peptides in the balance

Novel prostate acid phosphatase-based peptide vaccination strategy induces antigen-specific T-cell responses and limits tumour growth in mice

Treatment options for patients with advanced prostate cancer remain limited and rarely curative. Prostatic acid phosphatase (PAP) is a prostate-specific protein overexpressed in 95% of prostate tumours. An FDA-approved vaccine for the treatment of advanced prostate disease, PROVENGE® (sipuleucel-T), has been shown to prolong survival, however the precise sequence of the PAP protein responsible for the outcome is unknown. As the PAP antigen is one of the very few prostate-specific antigens for which there is a rodent equivalent with high homology, preclinical studies using PAP have the potential to be directly relevant to clinical setting. Here, we show three PAP epitopes naturally processed and presented in the context of HHDII/DR1 (114-128, 299-313, and 230-244). The PAP-114-128 epitope elicits CD4(+) and CD8(+) T-cell-specific responses in C57BL/6 mice. Furthermore, when immunised in a DNA vector format (ImmunoBody®), PAP-114-128 prevents and reduces the growth of transgenic adenocarcinoma of mouse prostate-C1 prostate cancer cell-derived tumours in both prophylactic and therapeutic settings. This anti-tumour effect is associated with infiltration of CD8(+) tumour-infiltrating lymphocytes and the generation of high avidity T cells secreting elevated levels of IFN-γ. PAP-114-128 therefore appears to be a highly relevant peptide on which to base vaccines for the treatment of prostate cancer.

Cancer vaccines. Any future?

The idea that vaccination can be used to fight cancer is not new. Approximately 100 years ago, researchers attempted to stimulate a tumor-specific, therapeutic immune response to tumors by injecting patients with cells and extracts from their own tumors, or tumors of the same type from different individuals. During the last decade, great efforts have been made to develop immunotherapeutic approaches for the treatment of malignant diseases as alternatives to traditional chemo- and radiotherapy. A quintessential goal of immunotherapy in cancer is treatment with vaccines that elicit potent anti-tumor immune responses without side effects. In this article, we have attempted to review some of the most problematic issues facing the development of cancer vaccines. With the prospect of immunosuppression, an ill-designed cancer vaccine can be more harmful than a no-benefit therapy. We have noted that "immunoediting" and "immunodominance" are the premier setbacks in peptide-based vaccines and therefore it appears necessary not only to manipulate the activity of a vast number of principal components but also to finely tune their concentrations in time and space. In the face of all these quandaries, it is at least doubtful that any reliable anti-cancer vaccine strategy will emerge in the near future.

DC-based vaccine loaded with acid-eluted peptides in acute myeloid leukemia: the importance of choosing the best elution method

Tumor-associated peptides isolated by acid elution are frequently used for therapeutic immunization against various tumors both in mice and in humans. In acute myeloid leukemia (AML), the frequent accessibility of a large tumor burden allows for extraction of peptides from leukemia cells by using either citrate-phosphate (CP) or trifluoroacetic acid (TFA) buffer. To develop an optimal immunotherapeutic protocol for AML patients, we evaluated both in mice and in humans, the immunogenicity of peptides eluted from leukemia cells with the two acids (TFA or CP). Although ex vivo studies in mice showed that both prophylactic immunizations with mature dendritic cells (DC) loaded with TFA-peptides (DC/TFA), or CP-peptides (DC/CP), were able to stimulate specific antileukemia immune responses, only vaccination with DC/TFA was able to prevent leukemia outgrowth. Moreover, in humans, only DC/TFA generated significant antileukemia CD4(+) and cytotoxic CD8(+) T cell responses in vitro. In summary, these data demonstrate that the choice of the acid elution procedure to isolate immunogenic peptides strongly influences the efficacy of the antileukemia immune responses. These finding raise essential considerations for the development of immunotherapeutic protocols for cancer patients. In our model, our results argue for the use of the TFA elution method to extract immunogenic AML-associated peptides.

Potent Vaccine Therapy with Dendritic Cells Genetically Modified by the Gene-Silencing-Resistant Retroviral Vector GCDNsap

Dendritic cells (DCs) genetically modified to express tumor-associated antigens (TAAs) would be promising tools in cancer immunotherapy. However, the use of retroviral vectors for such modifications is still a challenge because of low transduction efficiency and gene silencing in DCs. We have established an efficient method to prepare such DCs by in vitro differentiation of hematopoietic progenitor cells transduced with chicken ovalbumin (OVA) cDNA via the gene-silencing-resistant retroviral vector GCDNsap packaged in vesicular stomatitis virus G protein. When c-KIT(+)/lineage(-) cells were transduced with OVA followed by expansion and differentiation, more than 90% of mature DCs expressed the transgene. Mice inoculated with those cells completely rejected the OVA-expressing tumor E.G7-OVA, and the anti-tumor effects were stronger than those observed in mice inoculated with the same number of OVA peptide-pulsed DCs. The mice harbored more cytotoxic T lymphocytes (CTLs) against E.G7-OVA and produced antibody against OVA, suggesting the generation of multiple CTLs recognizing different OVA epitopes and OVA-specific CD4(+) T cells. Successive inoculations of the transduced DCs in a therapeutic setting eradicated preexisting E.G7-OVA and prevented the progression of retransplanted tumors. Thus, this vaccine therapy may represent a potent immunotherapeutic approach for various malignant tumors that express suitable TAAs.

Strategies of Antigen-Specific T-Cell–Based Immunotherapy for Cancer

The critical role of antigen-specific T-cells in the eradication of cancer has been demonstrated in numerous animal models, while significant challenges need to be conquered before antigen-specific T-cell immunotherapy can achieve true success in clinical practice. These challenges include: (1) weak or nonimmunogenicity of spontaneous tumors, (2) negative immune regulation mechanisms of the host immune system, (3) immune inhibition exerted by tumor cells, (4) physical barrier in solid tumor, and (5) escape or resistance to immune attack by tumor cells. Nonetheless, significant success has been achieved in several clinical trials recently, highlighting the possibility of successful manipulation of the immune system for control and elimination of tumor. We focused our study on summarizing the current knowledge and corresponding strategies for improving autologous cytotoxic T-cell (CTL)-based cancer immunotherapy, which include the following aspects: (1) the selection of tumor antigens for stimulation of CTL, (2) strategies of enhancing maturation and antigen presentation activity of dendritic cells (DC), (3) strategies of activation and maintenance of CTL response, and (4) recruitment of suitable immune effector cells to tumor sites. The successful manipulation of the immune system, based on the more and more detailed knowledge of tumor immunology, may finally reach the goal of "immune surveillance of malignancy."

A soluble divalent class I MHC/IgG1 fusion protein activates CD8+ T cells in vivo

CD8+ T lymphocytes recognize tumor and viral antigens bound to class I major histocompatibility complexes (MHC). Tumors and viruses may evade detection by preventing antigen presentation. The present study was designed to determine whether a soluble divalent fusion protein, containing the extracellular domains of a class I MHC molecule fused to beta2-microglobulin and the constant domains of IgG1, could induce an immune response in vivo. Administration to mice of the fusion protein loaded with a tumor peptide induced peptide-specific T cell activation and retarded tumor growth. Administration of the fusion protein loaded with a glycoprotein B (gB) peptide derived from herpes simplex virus type 1 (HSV-1) induced gB-specific cytotoxic T lymphocytes and protected mice from a lethal HSV-1 challenge. These data suggest that antigen-loaded MHC/IgG fusion proteins may enhance T cell immunity in conditions where antigen presentation is altered.

Vaccination with a synthetic nonapeptide expressed in human tumors prevents colorectal cancer liver metastases in syngeneic rats

In previous studies, the antigen CSH-275 (RTNKEASIC) was found expressed in tissue specimens from colorectal cancer but not in normal colonic mucosa. It was also naturally expressed in the DHD-K12 experimental colorectal cancer in BDIX rats. In this study, we describe the effect of vaccination with the synthetic nonapeptide CSH-275 in preventing tumor growth in a model closely mimicking the clinical situation of liver metastases, after surgical resection of primary colorectal cancer. A vaccination protocol using CSH-275, conjugated with complete or incomplete Freund's adjuvant, was carried out to determine the effect in preventing the progression of liver metastases induced by DHD-K12 cells injected in the splenic vein (preventive vaccine). An additional vaccination procedure was carried out to determine the effect on s.c. tumor growth (therapeutic vaccine). A significant improvement in survival along with the prevention of liver metastases formation and reduced growth of s.c. tumor were observed. CSH-275 vaccination resulted in a significant increase in CTL activity against autologous DHD-K12 cells in DHD-K12 tumor-bearing rats and the generation of a CTL response against DHD-K12 cells in DHD-K12 naive rats. Vaccination also induced massive infiltration of CD8(+) cells in tumor. These results demonstrate that CSH-275 is a new molecular target for colorectal cancer immunotherapy; it is also an excellent candidate for preclinical studies because it is naturally expressed on tumors in a fully competent syngeneic animal, which reproduces the clinical pattern of cancer progression.

Downregulation of the proteasome subunits, transporter, and antigen presentation in hepatocellular carcinoma, and their restoration by interferon-gamma

BACKGROUND

In our previous study, expressions of human histocompatibility leukocyte antigens class I molecules (HLA-I) and the transporter associated with antigen processing (TAP) 1/2 genes were investigated in seven hepatocellular carcinoma (HCC) cell lines. Two cell lines, Hep-3B and HuH-7, showed a reduced level of TAP, which might cause the low surface expression of HLA-I. In order to understand the downregulation mechanism of antigen presentation in tumors, the two cell lines were further investigated.

METHODS

Expressions of HLA-I and antigen presentation-related genes were analyzed by flow cytometry and polymerase chain reaction, respectively. Antigen presentation was tested in 51Cr-release assays.

RESULTS

Flow cytometric analyses revealed low surface expression of HLA-I on Hep-3B and HuH-7 cells. Introduction of HLA-A2 gene did not result in a high surface expression of HLA-A2. This suggested the downregulation of HLA-I expression might be related to defects in the antigen presentation machinery. We then examined expression levels of various antigen presentation-related genes. Hep-3B and HuH-7 demonstrated low expression of the low-molecular-weight protein (LMP) 2, LMP7, TAP1, and HLA-I heavy-chain transcripts. The downregulation of these genes was dissolved by treatment with gamma-interferon. Furthermore, allo-specific cytotoxic T lymphocyte (CTL) lines failed to recognize Hep-3B and HuH-7 cells, while they killed IFN-gamma-treated Hep-3B and HuH-7 cells.

CONCLUSIONS

Our results suggest that defects in the antigen presentation-related molecules might cause downregulation of HLA-I expression, antigen presentation, and subsequently, escape from specific CTL killing. The downregulation could be restored by IFN-gamma treatment, suggesting the potential use of IFN-gamma for therapeutic purposes.

Carcinoembryonic antigen as a target for therapeutic anticancer vaccines: a review

PURPOSE

To describe the features of carcinoembryonic antigen (CEA) that are important for its use in vaccination approaches and review the clinical experience with therapeutic vaccines targeting CEA.

METHODS

A PubMed search was performed on CEA, along with various qualifiers such as cancer vaccines, epitopes, and function. Relevant articles were reviewed.

RESULTS

CEA is a member of the immunoglobulin supergene family and may play a role in tumorigenesis. CEA protein is processed and presented on major histocompatibility complex (MHC) proteins for multiple alleles, including HLA A2, A3, and A24. T lymphocytes from healthy volunteers and cancer patients can recognize the processed epitopes of CEA and can become activated to lyse CEA-expressing tumors. Therapeutic vaccination approaches that have targeted CEA include vaccination with recombinant CEA protein, CEA anti-idiotype antibodies, and dendritic cells pulsed with agonist epitopes of CEA. Humoral responses have predominantly been induced with the first two approaches, whereas CD4 and CD8 responses, disease stabilization, and even objective clinical responses have been seen with the dendritic cell approach. Recently, CEA-poxvirus vectors encoding CEA and costimulatory molecules such as B7.1 have been shown to be safe and to induce increases in the frequency of T-cell precursors that recognize processed epitopes of CEA presented on MHC class 1 molecules. Disease stabilization has been seen in up to 37% of patients treated with these vaccines.

CONCLUSION

Tolerance to CEA in patients with cancer can be overcome with several different vaccination approaches, and such vaccinations are safe and immunologically active. Poxvirus-based vaccines can reproducibly generate T-cell responses to CEA and to tumors expressing CEA. Clinical activity has been seen with poxvirus or dendritic cell approaches. Other approaches are also being explored.

Identification of new epitopes from four different tumor-associated antigens: recognition of naturally processed epitopes correlates with HLA-A*0201-binding affinity

Forty-two wild-type and analogue peptides derived from p53, carcinoembryonic Ag, Her2/neu, and MAGE2/3 were screened for their capacity to induce CTLs, in vitro, capable of recognizing tumor target lines. All the peptides bound HLA-A*0201 and two or more additional A2 supertype alleles with an IC(50) of 500 nM or less. A total of 20 of 22 wild-type and 9 of 12 single amino acid substitution analogues were found to be immunogenic in primary in vitro CTL induction assays, using normal PBMCs and GM-CSF/IL-4-induced dendritic cells. These results suggest that peripheral T cell tolerance does not prevent, in this system, induction of CTL responses against tumor-associated Ag peptides, and confirm that an HLA class I affinity of 500 nM or less is associated with CTL epitope immunogenicity. CTLs generated by 13 of 20 of the wild-type epitopes, 6 of 9 of the single, and 2 of 5 of the double substitution analogues tested recognized epitopes generated by endogenous processing of tumor-associated Ags and expressed by HLA-matched cancer cell lines. Further analysis revealed that recognition of naturally processed Ag was correlated with high HLA-A2.1-binding affinity (IC(50) = 200 nM or less; p = 0.008), suggesting that high binding affinity epitopes are frequently generated and can be recognized as a result of natural Ag processing. These results have implications for the development of cancer vaccines, in particular, and for the process of epitope selection in general.

MHC class II presentation of endogenously expressed antigens by transfected dendritic cells

Dendritic cells (DC) present immunogenic epitopes of antigens in the context of MHC class I and class II molecules in association with costimulatory molecules, and efficiently activate both cytotoxic T cells and T helper cells. Gene modified DC expressing antigen encoding cDNA represent a particularly attractive approach for the immunotherapy of disease. We previously described a gene delivery system for DC based on receptor-mediated endocytosis of ligand/polyethylenimine (PEI) DNA transfer complexes that target cell surface receptors which are abundantly expressed on DC. Employing this gene delivery system, DC were generated that express chicken ovalbumin (OVA) cDNA as a model antigen and introduce antigen into the MHC class I presentation pathway. We demonstrate here that modification of OVA cDNA as transferrin receptor (TfR) or invariant chain (Ii) fusions effectively generate MHC class II specific immune responses in addition to MHC class I responses. TfR-OVA contains the membrane anchoring region of transferrin receptor and represents a membrane-bound form of OVA for access to the MHC class II compartment. Ii-OVA fusions directly target the MHC class II processing pathway. Thus, modification of antigen encoding cDNA represents a convenient and effective means to direct antigens to MHC class II presentation and thus to generate T cell help.

Synthetic multivalent ligands in the exploration of cell-surface interactions

Processes such as cell-cell recognition and the initiation of signal transduction often depend on the formation of multiple receptor-ligand complexes at the cell surface. Synthetic multivalent ligands are unique probes of these complex cell-surface-binding events. Multivalent ligands can be used as inhibitors of receptor-ligand interactions or as activators of signal transduction pathways. Emerging from these complementary applications is insight into how cells exploit multivalent interactions to bind with increased avidity and specificity and how cell-surface receptor organization influences signaling and the cellular responses that result.

DNA vaccination prevents and/or delays carcinoma development of papillomavirus-induced skin papillomas on rabbits

Malignant progression is a life-threatening consequence of human papillomavirus-associated lesions. In this study, we tested the efficacy of papillomavirus early-gene-based vaccines for prevention of carcinoma development of papillomavirus-induced skin papillomas on rabbits. Rabbit skin papillomas were initiated by infection with cottontail rabbit papillomavirus (CRPV). The papillomas were allowed to grow for 3 months without any treatment intervention. Rabbits were then immunized by gene gun-mediated intracutaneous administration of four DNA plasmids encoding CRPV E1, E2, E6, and E7 genes, respectively. All eight control rabbits receiving vector alone developed invasive carcinoma within 8 to 13 months. In contrast, only two of eight vaccinated rabbits developed carcinoma at 12 and 15 months, respectively. Papilloma growth was suppressed in the majority of vaccinated rabbits but not completely eradicated. These results indicate that gene gun-mediated immunization with papillomavirus early genes may be a promising strategy for prevention of malignant progression of human papillomavirus-associated lesions in humans.

Relevance of the tumor antigen in the validation of three vaccination strategies for melanoma

Many preclinical studies of cancer immunotherapy are based on the testing of a single vaccination strategy in several tumor models. Moreover, most of those studies used xenogeneic Ags, which, owing to their high immunogenicity, may not represent realistic models for the validation of cancer immunotherapies. To address these issues, we compared the vaccination efficacy of three well established strategies (i.e., naked DNA; peptide-pulsed dendritic cells (DC), or a mixture of peptide and the Escherichia coli toxin LTR72) using the xenogeneic OVA or the naturally expressed tyrosinase-related protein 2 (TRP-2) tumor Ag in the B16 melanoma model. C57BL/6 mice received one to three s.c. injections of peptide-pulsed DC or DNA, or one to four mucosal administrations of peptide-toxin mixture. One to 2 wk later, the animals were challenged s.c. with B16 or B16 cells expressing OVA (B16-OVA). Vaccination of mice with OVA induced in all cases melanoma-specific CTL and protection against B16-OVA. When TRP-2 was used, all three vaccines elicited B16-specific CTL, but only DC pulsed with the immunodominant T cell epitope TRP-2181-188 allowed protection against B16. Even more importantly, a vaccination regimen with TRP-2-pulsed DC, started 24 h after the injection of a lethal number of B16 cells, caused a therapeutic effect in 60% of the challenged animals. Our results strongly emphasize the relevance of the tumor Ag in the definition of immunotherapeutic strategies for cancer, and support the use of peptide-pulsed DC as cancer vaccine in humans.

Immunization of rabbits with cottontail rabbit papillomavirus E1 and E2 genes: protective immunity induced by gene gun-mediated intracutaneous delivery but not by intramuscular injection

We previously demonstrated that gene gun-based intracutaneous vaccination of rabbits with a combination of, but not with individual papillomavirus E1, E2, E6 and E7 genes provided complete protection against cottontail rabbit papillomavirus (CRPV) infection. In the present study, we tested whether vaccination of inbred and outbred rabbits with a combination of CRPV E1 and E2 genes could provide complete protection against virus infection. In the first experiment, gene gun-based intracutaneous vaccination with E1 and E2 genes prevented papilloma formation in the majority of inbred rabbits and promoted systemic papilloma regression in one non-protected rabbit. In contrast, needle-mediated intramuscular injection of E1 and E2 genes did not prevent papilloma formation nor promoted systemic papilloma regression, indicating an absence of strong protective immunity. In the second experiment, six outbred rabbits were immunized by gene gun-based intracutaneous administration of the E1 and E2 genes. Prevention of papilloma formation or systemic papilloma regression was observed in three vaccinated rabbits. Papillomas persisted on the remaining three rabbits, but were significantly smaller than that on control rabbits. These results suggested that gene gun-based intracutaneous vaccination with the combination of papillomavirus E1 and E2 genes induced strong protective antivirus immunity but may be insufficient for complete protection in an outbred population.

Endocytic recycling is required for the presentation of an exogenous peptide via MHC class II molecules

Exogenous antigenic peptides captured and presented in the context of major histocompatibility (MHC) class II molecules on APC, have been employed as potent vaccine reagents capable of activating cellular immune responses. Binding and presentation of select peptide via surface class II molecules has been reported. Here, a role for endocytosis and early endosomes in the presentation of exogenous peptides via MHC class II molecules is described. T cell recognition of a 14 amino acid human serum albumin-derived peptide in the context of HLA-DR4 was observed only with metabolically active APC. The delayed kinetics and temperature dependence of functional peptide presentation via APC, were consistent with a requirement for peptide internalization to early endosomal compartments prior to T cell recognition. Ablating endocytosis by exposing cells to inhibitors of ATP production completely blocked the display of functional peptide:class II complexes on the surface of the APC. Presentation of the peptide was also found to be sensitive to primaquine, a drug that perturbs the recycling of transport vesicles containing endocytic receptors and mature class II complexes. Functional presentation of the endocytosed peptide was dependent upon these mature class II complexes, as inhibitor studies ruled out a requirement for newly synthesized class II molecules. N-terminal processing of the endocytosed peptide was observed upon trafficking through endosomal compartments and linked to the formation of functional peptide:class II complexes. These findings establish a novel mechanism for regulating class II-restricted peptide presentation via the endocytic pathway.