Prostate cancer is the second leading cause of death in men, with approximately 220,000 new cases and an expected 28,000 deaths in the year 2003. A decrease in prostate cancer related deaths has been attributed to early prostate-specific antigen (PSA) detection, more effective chemotherapy treatments, and immunotherapies. Although tumors can often evade an immune response by modulating their tumor antigens, reducing major histocompatibility complex-1 (MHC-I) expression or inhibiting cytotoxic T-cell activity, the use of immune modulation for prostate cancer is a relatively new concept because the prostate is not generally considered a site where immune processes typically occur. Since tumors arise when cancer cells evade the immune system, the prostate is an ideal target for immunotherapy
The four most common types of lesions associated with the prostate are acute/chronic prostatitis (bacterial/abacterial), proliferative inflammatory atrophy (PIA), benign prostatic hyperplasia (BPH), and prostate carcinoma. The types of proliferative lesions that occur in the prostate are in different regions of the prostate. Most hyperplasias are prevalent in the transitional and periurethral zones, whereas carcinomas are found mostly in the peripheral zone.
PIA, a newly recognized prostate lesion, is hypothesized to be a precursor to prostatic intraepithelial neoplasia (PIN) and to prostate cancer. Proliferative inflammatory atrophy lesions, which contain proliferating epithelial cells that fail to fully differentiate into columnar secretory cells, are typically present in the peripheral zone of the prostate, where prostate cancers arise, and are often directly juxtaposed to PIN and/or prostate cancers. PIA may link inflammation with prostatic carcinogenesis.
Inflammation
Virtually 100% of prostate specimens contain histological and immunological evidence of chronic inflammation. Inflammation is a physiological response to a variety of stimuli such as infection, tissue injury, growth factors, or chemokines. The distribution, location and histology of leukocytes determine the type of inflammation. Persistent immune activation resulting in chronic inflammation often has pathological consequences. Acute/chronic inflammation is characterized by distinct cellular changes whereas precancerous lesions are associated with the change in the balance in the angiogenic and apoptotic cell cycle process.
During the inflammation process, activated macrophages release various hydrolytic enzymes and reactive oxygen and nitrogen species that may contribute to tissue damage. Chemokines, plasma enzyme mediators (bradykinen, fibrinopeptidases), opsonins, leukotrienes, and prostaglandins are all mediators that have some role in the inflammatory process. The normal prostate is populated by αβ T-cells, B-cells and macrophages, with the T-cells being evenly distributed throughout the interstitium and between the epithelial cells. There is some indication that the number of T-cells increases with age, which correlates with the incidence of prostate inflammation during the aging process.
The proliferation and differentiation of prostate tissue are modulated by growth factors as well as hormonal androgen therapy Evidence of hormonal impact on the prostate is seen in atrophy of the prostate following castration, as well as treatment of BPH samples with a 5a-reductase inhibitor. When this inhibitor is given to patients, there is a regression of dihydrotestosterone (DHT) levels and a reduction in prostate volume.
Recent data suggests that mutations in RNase L predispose men to an increased incidence of prostate cancer, which in some cases reflect more aggressive disease and/or decreased age of onset compared with non-RNase L linked cases. It is proposed that RNase L functions in counteracting prostate cancer by virtue of its ability to degrade RNA, thus initiating a cellular stress response that leads to apoptosis. RNase L is a uniquely regulated endoribonuclease that requires 5′-triphosphorylated, 2′, 5′-linked oligoadenylates (2-5A) for its activity. The presence of both germline mutations in RNase L segregating with disease within HPC-affected families, and loss of heterozygosity (LOH) in tumor tissues suggest a novel role in the pathogenesis of prostate cancer. The association of mutations in RNase L with prostate cancer cases further suggests a relationship between innate immunity and tumor suppression.
Microbial activators may contribute to acute/chronic inflammatory processes, which could then lead to malignancy Differentiating between acute/chronic bacterial prostatitis and chronic abacterial prostatitis is done by quantification of bacterial cultures and microscopic examination of urine. Depending on the severity and duration of the inflammation, acute bacterial prostatitis displays histological evidence of stromal leukocytic infiltration accompanied by increased elaboration of prostatic secretion or leukocytic infiltration within the glandular spaces.
In contrast, bacterial/abacterial chronic prostatitis shows histological evidence of aggregation of numerous lymphocytes, plasma cells, macrophages, and neutrophils within the prostatic substance. Chemokines act as chemoattractants, which activate lymphocytes and other immune modulators into neighboring tissues via extravasation. Since antibodies penetrate the prostate with poor efficiency, this type of inflammation is difficult to treat. The normal aging process in the prostate results in aggregations of lymphocytes, which are prone to appear in the fibromuscular stroma of the gland. Frequently, this histology of the aging prostate is diagnosed as chronic prostatitis even though the macrophages and neutrophils are absent.
Stromal-epithelial interactions are crucial for normal growth and homeostasis within the prostate. These interactions are thought to influence the rate of development of benign prostatic hyperplasia and prostate carcinoma. BPH is characterized by diffuse infiltrates of activated T-lymphocytes in fibroblastic, fibromuscular, and stromal nodules. Histological evidence of nodular hyperplasia or BPH is present in 20% of men 40 years of age, 70% by age 60, and 90% in men 70 years of age. The usual benign prostatic hyperplasia nodule weighs between 60-100 grams with some nodules weighing over 200 grams. Studies indicate that BPH samples also display chronic mononuclear inflammation, which contain CD3+ T-lymphocytes and express the T-cell receptor. The epithelial cells associated with the inflammatory infiltrate were observed in the periglandular stroma and were almost exclusively activated T-cells expressing CD45RO, and producing IL-2 and IFNΎ. Expression of IFN-Ύ, IL-2, and IL-4 mRNA in benign prostatic hyperplasia suggests that the disease is associated with Th1 and Th2 response.
The Immune System
The goal for cancer immunotherapy is to induce antibody and/or T-lymphocyte immune response targeted to the cancer cells. There are several branches of the immune system that can be targets for immunotherapy. They include antibody producing B-cells, CD8+ cytotoxic T-cells, CD4+ T-helper cells, natural killer (NK) cells, natural killer T (NKT)-cells, and monocytes. B-cells produce antibodies that kill antigen presenting cells via complement, antibody dependent cellular cytotoxicity (ADCC), or apoptosis. Cell mediated response appears to play a major role in a tumor immune response. Many tumors induce a specific cytotoxic T-cell response that recognizes antigens presented by MHC-I, which can elicit a higher response.
NKT-cells share several features with NK cells, such as CD 161 and CD 122 expression. These cells display intermediate levels of T-cell receptor (TCR) and are CD4+ or CD4-/CD8-. NKT-cells produce IL-4, a pro-inflammatory cytokine, in response to engagement of the T-cell receptor. In the presence of IL-18 and IL-12, NKT-cells will produce IFN-Ύ and kill target cells in a Fas ligand (FasL) dependent manner without engagement of the TCR.
NK as well as NKT-cells recognize tumor cells through cell-cell contact and mediate killing with Fas/FasL or with the induction of cytokines or lytic enzymes. NK cells recognize target cells based on expression of activating or inhibitory receptors. Since NK cells do not recognize target cells based on MHC expression, a decrease in MHC expression does not limit their activity. Also, some Fc receptors on NK cells can bind to antibody coated tumor cells leading to ADCC.
Naїve T-cells require more than one signal for activation and subsequent proliferation into an effector cell. This activation is triggered by recognition of MHC-peptide complex and a co-stimulatory signal. Frequently, tumor cells give little or no co-stimulatory signals that can inhibit the activation of cytotoxic T-cells. The co-stimulatory signal occurs by interaction of B7 on antigen presenting cells and CD28 on the T-cells. CTLA-4 and CD28 are T-surface antigens, which bind to B7-1 or B7-2 ligands on antigen presenting cells to activate a T-cell response and control proliferation. CD28 is expressed on resting and active cells while CTLA-4 is virtually undetectable on resting cells. Their ligands, B7-1 and B7-2, are two related forms of immunoglobulin superfamily members with similar extracellular domains but with different cytosolic domains. These ligands are constitutively expressed on dendritic cells and can be induced on macrophage and B-cells. Signaling through CD28 produces a positive co-stimulatory signal and increases CTLA-4 levels on the T-cells. Although CTLA-4 and CD28 are structurally similar, they act antagonistically. Surface levels of CTLA-4 are lower than CD28, but it competes favorably for B7 binding sites due to its high avidity.
Targets of Immunotherapy
The complexity of the immune system presents many legitimate targets for the induction of an immune response. One aspect of the innate immune system present throughout the body, including the prostate epithelium and stroma, is the presence of toll-like receptors (TLR). TLRs are capable of recognizing foreign antigens and act as molecules with pattern recognition capabilities and may be soluble or cell-associated receptors. Pattern recognition receptors (PRR) are extracellular or present on cell membranes and target microbes or components in tissue fluids and blood. Typically, signals transduced through a TLR result in transcriptional activation, synthesis, and secretion of cytokines. This signaling process results in the activation of antigen presenting cells, all of which are involved in or promote inflammation. For instance, TLR-5 mRNA is found in the prostate, testis, ovaries, and leukocytes. TLR-5 interacts with microbial lipoproteins leading to nuclear factor-kappa B (NF-kB) activation, cytokine secretion, and inflammation. Other TLR activation induces secretion of cytokines, such as IFN-Ύ, MAPK pathways, or acts as a target for CpG islands (found in bacterial DNA) or double stranded (ds)RNA.
Cytokines
A second potential target for immunotherapy lies in the world of cytokines. Cytokines are low molecular weight regulatory proteins or glycoproteins that regulate the immune response, hematopoiesis, control of cellular proliferation and differentiation, and are involved in wound healing. Cytokines share many properties with hormones and growth factors in that they are secreted soluble factors that elicit biological effects. As cytokines are discovered, many can be grouped into families based on protein structural homology. Several examples of cytokine families are: interferons, tumor necrosis factors, and interleukins. These molecules are redundant and have overlapping functions. Once a cytokine encounters the appropriate receptor, it acts in an antigen non-specific manner and can induce a series of protein tyrosine phosphorylations. The two main cell types responsible for cytokine secretion are the T-helper cell and macrophage.
Interferons are one of the major groups of cytokines that have been used for clinical cancer studies. IFN-α is produced by macrophages and increases MHC-I expression, activates NK cells, induces an anti-viral state, and inhibits cell division of normal or malignant transformed cells in vitro. IFN-β, produced by fibroblasts, increases MHC-I expression and activates NK cells. IFN-Ύ is produced by CD8+ T-cells and NK cells and activates macrophages, increasing both MHC-I and MHC-II expression when foreign antigen is present. Data suggest that malignant tumors display a decrease in MHC-I expression and that the interferons may be responsible for restoring MHC-I expression, thereby increasing cytotoxic T-cell activity towards the tumor. Daily injections of recombinant INF-α have been shown to induce partial or complete regression in hematological cancers (i.e. leukemia and lymphoma), as well as some solid tumors (i.e. breast and renal cancer).
Tumor necrosis factors TNF-α and TNF-(3 have been shown to display anti-tumor activity by direct killing of the tumor cells, reducing proliferation rate (while sparing the normal cells), and inhibiting angiogenesis by damaging vascular endothelial cells. Frequently, when treated with either factor, the tumor undergoes hemorrhagic necrosis and regression. Macrophages, monocytes, and other cell types including fibroblasts and T-cells secrete TNF-α. However, TNF-β is only produced by activated T-cells and B-cells and is a mediator of immune function and involved in wound healing. Both INF-Ύ and TNF-α are associated with chronic inflammation. The complexity of cytokines and how they may potentially interact with each other has been one major obstacle of this type of therapy. Many cytokines have short half-lives and depending on the circumstances can act as either a pro-inflammatory or anti-inflammatory agent (i.e. IL-7, IL-9). Systemic administration of a large amount of cytokines has led to serious consequences and has even been fatal, therefore these immunotherapies can be limiting.
Growth Factors
Many tumors display high levels of growth factor receptors on their membranes making growth factor receptors a likely target for immunotherapy Inappropriate expression of either a growth factor or its receptor can result in uncontrolled proliferation. Vascular endothelial growth factor (VEGF) is a potent mitogen for cells and is one of the most well studied growth factors. VEGF mRNA expression is seen in breast cancer and is associated with poor prognosis of colon cancer and non-small cell lung carcinoma. VEGF can be activated by ras oncogene causing inactivation of p53 and Von Hippel Landau (VHL), as well as cause activation of PKC. VEGF is expressed in the epithelial and stromal areas of the human prostate, however, hyperplastic glands stain very poorly for the growth factor. Several studies have shown that prostate cancer specimens display 32% staining in the stroma and 56% staining in the epithelium. In contrast, staining for VEGF in benign prostatic hyperplasia displayed 73% staining of the stroma and 50% staining in the epithelium. Of note is the use of the 5α-reductase inhibitor, Finasteride®, which has been shown to decrease expression of VEGF in prostatic tissue.
Tumor Antigens
Tumor specific antigens may result from mutations that cause altered cellular proteins or may be normally expressed at certain stages of differentiation encoded by a variant form of the normal gene or may be exclusively expressed by the tumor. Tumor associated glycoprotein-72 (TAG-72) is a mucin found on many adenocarcinomas including colorectal, pancreatic, gastric, ovarian, endometrial, and mammary, as well as some prostate cancers.
Tumor antigens, while being specific for tumor tissue, can vary widely from tumor to tumor. The use of tissue specific antigens is usually undesirable, as normal tissue would also be targeted with the tumor. The case of prostate cancer is unique in that the prostate is not a vital organ and could be targeted without serious harm to the patient. This allows for the targeting of tissue specific antigens in the prostate. Several prostate specific antigens have been discovered and are targets of immunotherapy These tissue specific antigens include prostate-specific antigen, prostate alkaline phosphatase (PAP) and prostate specific membrane antigen (PSMA). Although these self-proteins are not always immunogenic they do provide a basis for further development and testing.
Monoclonal Antibody Therapy
Antigenic modulation in the treatment of many diverse cancers has been used for a number of years. In fact, the Food and Drug Administration has approved several monoclonal antibodies for treatment of various cancers and non-malignant diseases. Ideally, by treating tumor cells with an antibody, one would hope for complete destruction of the tumor without recurrences. However, tumors seem to regenerate once the antibody treatment has ceased. Passive administration of antibodies or active vaccination to induce antibodies can target cells that express antigenic proteins on their cell membranes. Monoclonal antibodies are often conjugated to chemotherapeutic agents, biological toxins, radioactive compounds, or immunotoxins. These immunoconju-gates target the neoplastic cells expressing tumor specific or tumor-associated markers. Problems with antibody specificity, delivery, and cost are often hurdles for therapy. Unlike antibodies to Her2-neu for breast cancer or antibodies to CD20 for non-Hodgkin’s lymphoma, there are limiting numbers of antibody targets for the treatment of prostate cancer.
CC49, a murine lgG1 antibody, recognizes TAG-72 and shows disease response when coupled to a radioisotope in ovarian cancer and has been shown to be expressed in prostate cancer cells. A clinical trial utilizing I-CC49 failed to show any clinically relevant data. However, when I-CC49 was used in conjunction with INF-Ύ, up-regulation of TAG-72 and enhancement of the response was seen. The trial included 16 patients with androgen independent prostate cancer (AIPC), of which 12 patients had antibody localization to the tumor. None of the patients had a >50% decline in their PSA or a radiologic response, however several had moderate pain relief from bone metastases. Rapid production of anti-mouse antibodies and development of thrombocytopenia precluded further dosing. In a subsequent clinical study, 14 patients were treated with IFN-α prior to the administration of I-CC49. Two patients had a minor radiographic response while 3 had a ≥50% reduction of their serum PSA levels. Therefore, IFN-α may be acting as an adjuvant yielding a greater response in comparison to just I-CC49 therapy.
Since prostate-specific antigen is found in the serum, many researchers have used the antigen for a potential target for immunotherapy In vitro data show that the generation of antibodies that recognize PSA and CD3 on T-cells would direct non-specific CD3+ T-cells to PSA, and in turn, this would re-direct preactivated peripheral mononuclear cells to lyse PSA expressing cells. Although demonstrated in vivo, a human trial is necessary. Since PSA is in the serum, directing antibodies to the prostate tissue would be difficult.
Prostate specific membrane antigen is an ideal target for monoclonal antibody therapy since the target cell is always internalizing the protein and its internalization is augmented by monoclonal antibody contact and is strongly expressed on nearly 100% of prostate tumors. Prostacint® (7E11 from Cytogen) is an anti-PSMA antibody used to image the prostate. Prostacint® has been found to bind an intracellular epitope of prostate specific membrane antigen and, therefore, likely binds areas of tumor necrosis. Second-generation anti-PSMA antibodies have been developed to target the extracellular domain of PSMA due to the fact that internal domain binding antibodies, such as 7E11 and PM2J0004.5 (Hybritech) do not bind viable cells.
Most antibody therapies to prostate specific membrane antigen have used J591, a mouse monoclonal antibody that is immunogenic. J591 has been genetically modified to eliminate the mouse antigens and is now fully “humanized,” allowing repeated dosing without generating anti-mouse antibodies. The unmodified antibody could focus the immune system on tumor sites to complement activation, however, dramatic responses to naked antibodies are infrequent. J591 binding to PSMA is rapidly internalized into the cell and has been quite useful for imaging of known sites of metastasis.
Antibodies to the extracellular domain of prostate specific membrane antigen and coupled to toxins or radioisotopes have been shown to have some effect in prostate cancer cell lines and murine models. In one study, J591, PEQ226.5, and PM2P079.1 were conjugated with ricin A chain (RTA), a holotoxin containing an α subunit that inactivates protein synthesis and facilitates intra-cellular trafficking of RTA. Since J591 and PEQ226.5 recognize the same epitopes that are related to PSMA, a lower cytotoxic effect was observed of the antibodies in cell mono layers in comparison to treatment with RTA alone, while the specificity of prostate specific membrane antigen expression of the tumors was increased.
A study performed by Dr. Bander and colleagues at the College of Medicine of Cornell University enrolled 53 patients into a phase-I study to assess disease staging, metastatic or recurrent disease. Twenty-nine patients received 111In/90Y-DOTA-J591 while 24 patients received 177LU/DOTA-J591. The results indicated 98% of the patients had successful targeting of J591 to the bone and soft tissue with 87% having radiographic evidence of metastasis and 13% had zero visible lesions. Remarkably, 16 of 18 patients with no evidence of metastasis showed positive J591 staining. Other biotechnology companies have developed external domain specific anti-PSMA antibodies using mice genetically engineered to express human antibodies, resulting in the development of monoclonal antibodies that are non-immunogenic. These anti-PSMA antibodies have demonstrated significant activity in clinical trials.
Modulation of T-Cells
Modulation of the co-stimulatory signals required for T-cell activation has been shown to be an effective therapy through blocking cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) with antibodies and prolonging a T-cell response. Anti-CTLA-4 blocks B7 binding to CD28, preventing stimulation and decreasing expression of T-cells. Anti-CTLA-4 antibody treatment in animal models can induce tumor rejection in immunogenic tumors. Coupled with an anti-tumor vaccination, it can induce rejection of minimally immunogenic tumors in the TRAMP animal model. In a study of CTLA-4 blockade administered immediately after primary tumor resection, a reduction of metastatic relapse from 97.4% to 44% was observed. Consistent with this, lymph nodes obtained 2 weeks after treatment reveal marked destruction or complete elimination of C2 metastases in 60% of mice receiving adjunctive anti-CTLA-4 whereas 100% of control antibody-treated mice demonstrated progressive C2 lymph node replacement.
Adjuvants are often used with various immunotherapies because they can increase B7 co-stimulation and activate macrophages. Activated macrophages can then cluster around tumors and are better T-helper activators. These increase both humoral and cell-mediated responses and correlate with tumor regression. The Fc portion of human IgG has been fused to the B7 binding domain of CTLA-4 to produce a chimeric molecule, CTLA-Ig. The human Fc portion gives the molecule a longer half-life and the B7 binding domain allows binding to CD28. A humanized antibody for CTLA-4, designated MDX-101 (Medarex, Inc.) has recently been tested in a Phase-I trial, which included 14 patients with AIPC. The study showed successful blocking of the co-stimulatory signaling with no T-cell activation occurring. The therapy was well tolerated and 2 patients had a >50% decline in prostate-specific antigen levels.
Vaccines
Tumor vaccines cause induction of a cell-mediated response to antigens and are often composed of tumor-associated proteins mixed with a nonspecific antigen. Demonstrating that antigen specific T-cells are up-regulated by a particular vaccine strategy is important for immunologic therapies. Antigen presentation is critical for any immunization technique and its enhancement can modulate tumor immunity. Anti-tumor vaccines that activate cytotoxic lymphocytes (CTLs) and human tumor infiltrating lymphocytes (HTLs) would be most desirable since HTLs induce CTLs. Human tumor infiltrating lymphocytes produce both IFN-Ύ and granulocyte macrophage colony stimulating factor (GM-CSF) and kill tumor cells. Therefore, vaccines that induce anti-tumor CTLs include MHC-II restricted epitopes, which would trigger a HTL response to tumor-associated antigens. A method of activating these T-cells may be to use the antigen presenting dendritic cells to initiate the immune response.
Dendritic cells are the most potent antigen-presenting cells, capable of presenting antigen to CD8+ (MHC-I restricted) and CD4+ (MHC-II restricted) T-cells. Dendritic cell-based vaccines use a patient’s bone marrow derived antigen presenting cells that are able to sensitize naive T-cells to new antigens. By combining dendritic cells with tumor antigens, the therapy supposes that the dendritic cells will then activate T-cells with tumor antigen.
Mouse dendritic cells, pulsed with tumor fragments and incubated with GM-CSF, were re-infused into the mice to activate the TH and CTL response to the tumor antigen. Mouse tumor cells are immunogenic, therefore animals injected with killed tumor cells do not grow tumors when challenged with live tissue, a term designated protective immunity. The same is true for humans. When tumor cells were transfected with GM-CSF and given back to the patient, they were able to secrete more GM-CSF and enhance the differentiation and activation of the host antigen presenting cells. As the dendritic cells surround the tumor cells, GM-CSF is secreted by the tumor and enhances presentation of antigen to the TH and CTL cells.
Denedron Corporation has developed Provenge®, a recombinant fusion protein with GM-CSF fused to prostate acid phosphatase (PAP), a prostate specific isozyme of acid phosphatase that is secreted by prostate cells. This strategy uses autologous dendritic cells combined with human GM-CSF. Thirty-one patients with prostate cancer were enrolled in the clinical study and received three monthly infusions and one final boost at 24 months if the disease had not progressed. Results showed 38% of the patients had a T-cell response against native prostate acid phosphatase while some had a decline in their PSA levels. T-cells collected after the treatment revealed the presence of IFN-Ύ, a reflection of successful activation.
The use of prostate acid phosphatase as a vaccine has also been studied, since serum prostate alkaline phosphatase levels increase with prostate cancer progression, from 33% up to 92%, making it a more important marker for advanced disease. One study used a xenogenic homologue of PAP (mPAP), which was given to patients with metastatic prostate cancer. The homologous mPAP possessed sufficient differences from self-antigen to render it immunogenic, but similar enough that they would cross-react with human prostate acid phosphatase. Seven out of 21 patients had stable disease following the vaccination beyond one year while three patients had stable disease beyond three years. All of the patients had T-cell immunity to mPAP and 11 out of 21 had induced immunity to human prostate acid phosphatase.
A PAP peptide (termed PAP-5) capable of binding the HLA-A2 molecule was used to pulse an antigen presenting cell fraction containing dendritic cells isolated from a healthy HLA-A2 donor. The cells were expanded and employed to elicit a CD8+ CTL response. The peptide lysed prostate tumors in an antigen specific manner. CTLs were evaluated for peptide specific activity and potency in an in vitro chromium release assay. The assay revealed that the CTLs generated after stimulation of PAP-5 peptide loaded dendritic cells were able to endogenously process the PAP-5 antigen.
Human prostate cancer cells were removed at the time of surgery and expanded in culture. They were transfected to secrete a high amount of GM-CSF via ex vivo retroviral transduction with GM-CSF cDNA. Eight of 11 patients were then irradiated and given a subcutaneous injection of their corresponding vaccine every 21 days (3-6 doses). Biopsies showed the presence of macrophages, dendritic cells, T-cells, and eosinophils. Delayed type hypersensitivity (DTH) vs. irradiated, unmodified, autolo-gous tumor cells and recall antigen were tested pre/post treatment to assess specific tumor cells and recall antigens to determine if a tumor specific response was achieved. Two of eight patients had a DTH response prior to the vaccination while seven out of eight patients had a DTH response post vaccination. Biopsies of the DTH sites showed that 80% of the T-cells expressed CD45RO with the presence of Th1 and Th2 cells. Expression of CD45RO indicates that the T-cell has switched isoforms and is now acting as an effector cell.
A vaccine study targeting prostate specific membrane antigen (PSMA) enrolled twenty-six patients with various stages of prostate cancer. Patients were given either a cDNA plasmid encoding the extracellular domain of PSMA (with or without CD86), an adenoviral vector expressing prostate specific membrane antigen, or both in a prime-and-boost strategy trial. Some of the patients received GM-CSF in addition to their treatment. A DTH response to the PSMA expressing plasmid was seen in some of the patients including all 10 patients receiving the adenoviral vector. PSA decline was seen in some patients receiving vaccination only. Due to the various stages of disease and GM-CSF combination treatments, the results of this study are difficult to interpret.
T-Bodies
A T-cell receptor that has been modified so the intra/extracellular part of the domain is the same but the most distal part of the receptor is replaced with a single chain antibody, is known as a T-body. The distal portion of the receptor being modified is the portion that would normally recognize the peptide antigen complex in the MHC cleft. A T-cell could then be activated to attach to a tumor using a specific antibody to a tumor specific antigen. Sadelain et al. have created an artificial T-cell receptor (Pz-1) that is composed of an external PSMA-specific single chain antibody, linked to the CD 8 hinge and transmembrane domain, followed by the cytoplasmic T-cell receptor signal transduction domain. The receptor is capable of redirecting the specificity of the T-cell to target PSMA expressing cells, independent of MHC. In vitro data shows successful lysis of PSMA expressing prostate cancer cells lines and no effect on the non-PSMA expressing cells. These results indicate proliferation of modified T-cells in response to the presence of PSMA expression.
Summary
More than 80% of prostate carcinoma tissue consists of tumor cells at advanced stages, with minor infiltration of inflammatory cells. This indicates that the immune system is not involved. As a result, researchers have the opportunity to tap into a powerful natural defense system that can be augmented to involve prostate cancers. Immunotherapy can focus the immune system on a particular cancer with a wide range of alternatives that can be used singly or in concert to provide a tremendous benefit to the patient. By combining therapies involving biological response modifiers (i.e. cytokines and growth factors), conjugated monoclonal antibodies (including toxins and radiolabels), and cancer vaccines (tumor marker proteins with or without dendritic cell augmentation), the future of immunotherapeutic treatment of prostate cancer looks very promising.
