Dendritic Cells in the Treatment of Prostate Cancer

By Georgi Pirtskhalaishvili, MD, PhD, and Michael R. Shurin, MD, PhD

Cancer of the prostate is the most commonly diagnosed cancer in men and the second most common cause of cancer death. Despite significant advances in diagnosis, surgery, and radiation therapy in the past decade, a cure is possible only when the disease is localized within the prostate gland. The only effective treatment for advanced cancer of the prostate (PCa) is hormonal therapy, introduced more than 50 years ago, which does not provide complete cure. Chemotherapy is largely ineffective, since prostate carcinoma is a slow-growing tumor; only a small fraction of cells proliferate at any given time.1 On the other hand, advanced PCa is a compelling target for immunotherapy because this approach does not require a high proliferative index. Prostate cells express more than 500 unique gene products which could serve as potential therapeutic targets. PCa thus offers potentially unique antigens, which may be particularly suited for the generation of specific antitumor immune responses following cell-based and/or cytokine-based immunotherapies.

Background

It is well known that cytotoxic T lypmhocytes (CD8+ T cells, CTL) are the most powerful effector antitumor component of the immune system. Therefore, the aim of each antitumor vaccination is to induce a strong cytotoxic T cell response against the tumor antigens. However, T cells are unable to recognize unprocessed proteins. For activation, they require the presentation of antigens in conjunction with MHC molecules. The stimulation of naïve T cells also requires the presence of costimulatory molecules. Antigen-presenting cells such as B cells and macrophages cannot induce primary immune response without prior activation. However, dendritic cells (DC), the most potent antigen-presenting cells, are able to stimulate naïve T cells and mount primary immune responses.

DC, first described in 1973,2 are significantly more potent than macrophages and B cells in their ability to stimulate T cells. Unlike monocytes, DC are able to induce the generation of cytotoxic T cells in the absence of CD4+ helper cells.3 A single DC can stimulate 100-3000 T cells and requires only a minimal amount of superantigen to generate a significant lymphocytic response.4 Thus, a great interest in DC biology for tumor immunology during recent years is understandable. When the problem of DC generation in sufficient numbers was solved,5,6 DC-based therapy was explored for cancer treatment. Pulsing of DC with synthetic tumor-associated peptides may induce an effective antitumor immune response.7,8 DC can also be pulsed with a tumor lysate9 or RNA10 in order to induce specific immune reaction against tumors. All of these approaches have certain limitations when considered for use in humans, since the preparation of tumor lysates or extraction of tumor antigens requires a large amount of solid tumors. Another approach is a direct injection of DC into the tumor, where DC pick up tumor antigens in situ and present them to T cells. This approach confirmed its effectiveness in animal models including prostate cancer model (Nishioka et al. 1999, unpublished data).

Since PCa expresses a number of antigens, different peptides can be used for DC pulsing. Peshwa and associates employd prostatic acid phosphatase (PAP) peptides to pulse DC.11 Activated cells were used to generate prostate-tumor specific CD8+ cells, which were able to lyse prostate tumor cells in vitro. Prostate specific antigen (PSA) derived peptides were used by others for the generation of CD8+ T cells which were able to lyse PSA-expressing cells.12

Clinical and Experimental Data

A clinical trial using DC therapy for PCa was undertaken at Northwest Hospital in Seattle, WA.13 Autologous DCs were cultured from the peripheral blood mononuclear cells obtained from prostate cancer patients. The authors used two peptides for loading into DCs in this study. They were derived from prostate-specific membrane antigen (PSMA) and designated as PSM-P1 and PSM-P2. Five groups were formed from 51 patients with hormone-refractory prostate cancer. Two groups of patients received only peptides (PSM-P1 and PSM-P2), respectively, a third group was treated with DC only, and 4th and 5th groups received DC pulsed with peptides PSM-P1 and PSM-P2, respectively. Patients received four or five doses of the tested substance at 6-8 weeks intervals. Seven partial responders were identified based on National Prostate Cancer Project (NPCP) criteria. Two patients were from peptides-only groups (Groups 1 and 2), and five were from peptide-pulsed DC groups (Groups 4 and 5). There was no responder in the DC-only treated group. Side effects were minor. Immunologic monitoring studies showed an increase of T cell response to the appropriate PSMA peptides in patients in groups 4 and 5. No significant response was observed in groups 1, 2, or 3. A Phase II clinical trial started in January 1997 involving 107 patients.14 Participants received a total of six infusions of autologous DC pulsed with PSM-P1 and PSM-P2 cocktail at six-week intervals. Overall, out of 95 evaluable patients, 11 (11.58%) had a complete response and 25 (26.32 %) had a partial response. Twelve of 19 patients (63%) with hormone-refractory metastatic PCa (stage D2) survived for more than 600 days. These results, though remarkable since effect was achieved in patients usually insensitive to the conventional modes of treatment, still demonstrate somewhat limited efficacy of the administered therapy.

One reason of the limited efficacy of immunotherapy in PCa patients is associated with the local or systemic suppression of the DC system by the PCa-derived factors, resulting in inhibition of immune responsiveness. Troy and colleagues examined the presence of DC in the prostate cancer and found significantly less DC in cancer tissue in comparison to normal prostate or benign prostatic hyperplasia (BPH).15 Bigotti and coworkers found inverse correlation between the number of prostate cancer infiltrating DC and the histopathological grade of the PCa, with grade 5 tumors (which carry the worst prognosis) virtually devoid of antigen-presenting cells.16 It is a well-known fact for many tumors that the presence of DC is correlated with a better prognosis.17 The low number of DC within the prostate tissue may be due to low immunogenicity of PCa, (most prostate cancer cells lack MHC Class I antigens18), as well as the result of active suppression of DC by PCa-derived factors. Indeed, the coincubation of prostate cancer cells with DC resulted in death of DC19, which was documented in both human and animal models. The local or systemic suppression of the generation, function, and survival of immune cells induced by the PCa-derived factors and resulting in inhibition of immune responsiveness may limit the efficacy of immunotherapy in PCa patients.

Since prostate cancer caused active suppression of DC, the possible mechanisms of this protection were examined. So far, interleukin 12 (IL-12), interleukin 15 (IL-15), CD40 ligand (CD154), and TNF-a were studied for this purpose. IL-12 is a strong proinflammatory protein, stimulating INF-g production by T and NK cells.20-22 IL-15 induces T cell proliferation and promotes IL-12 production.23 Together with IL-12, IL-15 has been shown to stimulate NK cells to produce INF-g and TNF-a.24,25 CD154 binds to CD40 on DC leading to the increased survival in cultures, probably secondary to increased expression of the anti-apoptotic proteins of Bcl-2 family.26,27 TNF-a is involved in the regulation of cell death and proliferation, and induces the production of proinflammatory cytokines IL-1b and IL-6.28,29 We found that the stimulation of DC with CD154, TNF-a, IL-12, or IL-15 resulted in the increased resistance of DC to the PCa-induced apoptosis.30 Animal experiments are in progress to explore the effectiveness of modified DC therapy in murine prostate cancer.

Conclusion

DC have demonstrated their efficacy in the treatment of advanced prostate carcinoma, and their use will expand during the coming years. Until now, peptide pulsed DC were used mainly systematically. It is possible that intratumoral administration of DC will also be tested in the nearest future, which may produce a more specific response for a given patient, although experimental studies need to be carried out before the conducting of clinical trials. Since it was established that PCa causes active supression of DC, we expect that in coming clinical trials DCs will need to be modified before infusion to protect them from PCa-induced apoptosis. Alternatively, DCs may be administered together with cytokines, which may also enhance the antitumor activity of the host immune system. (Dr. Pirtskhalaishvili is a resident in oncology and Dr. Shurin is on staff at Departments of Urology and Surgery, University of Pittsburgh Medical Center, Pittsburgh, PA.)

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