Prostate cancer is a significant cause of morbidity and mortality in the United States, and remains the second leading cause of cancer-related death in men [1
]. The last decade has observed enormous progress in the treatment of metastatic prostate cancer. Docetaxel was approved by the FDA as the only contemporary chemotherapeutic agent demonstrated to prolong survival in patients with androgen-independent, metastatic prostate cancer [2
]. Despite this advance, many patients and physicians believe the small survival benefit provided by chemotherapy may not warrant its use in all individuals due to its potential negative impact on quality of life [4
]. Consequently, there is great interest in the development of treatments with fewer side effects. Immunological therapies, and vaccines (also known as “active immunotherapies”) in particular, are one such treatment option being investigated [5
]. Recent clinical trials have demonstrated an increase in overall survival of patients treated with prostate cancer vaccines [7
], superior to what has been observed with chemotherapeutic agents and FDA approval for at least one prostate cancer vaccine is anticipated in the near future.
ADT is the cornerstone of treatment for patients with metastatic prostate cancer, and the concurrent use of ADT is the standard of care for all other treatments for metastatic prostate cancer. ADT is achieved clinically through orchiectomy or with administration of chemical castrants such as gonadotropin releasing hormone (GnRH) agonists, and/or anti-androgens. GnRH agonists function by continuous stimulation of the GnRH receptor, which ultimately induces desensitization and blockade of the pituitary-gonadal axis achieving castrate levels of testosterone [9
]. The non-steroidal anti-androgens compete directly with testosterone and dihydrotestosterone for the ligand-binding domain of the androgen receptor. In addition to competitive inhibition of the androgen receptor, the steroidal anti-androgens have anti-gonadotrophic effects which suppress testosterone synthesis [10
]. While the individual therapies function through different mechanisms to inactivate the androgen receptor, the ultimate outcome is atrophy and death of androgen-dependent cells of the prostate [11
In the case of orchiectomy, prostatic glandular cells undergo apoptosis characterized by DNA fragmentation, cell surface blebbing, and the formation of apoptotic bodies [13
]. The apoptotic bodies are recognized by scavenger receptors, phagocytized and digested by macrophages [14
]. However, an expansion of dendritic cells have been reported at the prostate following orchiectomy as well [15
], implicating a possible dendritic cell role in the phagocytosis and presentation of prostate antigens. Furthermore, prostate cancer patients undergoing androgen deprivation experience a T-cell infiltration of the prostate by one month post treatment, and the infiltrates appear to be of an oligoclonal nature. These findings imply that an adaptive response to the prostate may arise following androgen deprivation [15
]. In the case of androgen deprivation achieved by chemical castration, tumor cell vacuolization and glandular atrophy are observed. However, the levels of apoptosis appear unchanged, implying the predominant form of cell death may be achieved through an alternative mechanism [11
Numerous observations have also demonstrated an effect of androgens on lymphocyte numbers and function. Briefly, thymic involution can be reversed in mouse [16
] and rat [17
] models following orchiectomy. This regrowth is characterized by an increase in the weight and cellularity of the thymus, and can be reversed when androgens are re-administered to the castrate subject [18
]. The observed thymic regrowth has been attributed to an expansion of the thymic epithelia expressing increased levels of CCL25, which promotes immigration of early thymic progenitors to the thymus [19
]. A robust expansion of the common lymphoid progenitor cells can also be observed in castrated mice given hematopoeitic stem cell transplants compared to sham-treated animals [20
], data suggesting immunosuppressive effects of testosterone. In humans, administration of GnRH agonists in prostate cancer patients have been reported to induce expansion of signal joint recent thymic emigrant cells (sj T rec) [21
]. This T cell expansion can occur late in life when sj TRECS are reported to be at their lowest levels [22
], and the risk for prostate cancer increases. These observations suggest that androgens have an immunosuppressive effect, and ADT might function to reverse this suppression late in life.
The observations that ADT can affect T lymphocytes, and elicit prostate tissue-associated inflammation, suggest ADT might be combined with active immunotherapies. In a mouse model of prostate cancer, tumor-bearing animals receiving an adoptive transfer of prostate tumor-specific T cells, followed by castration and vaccination, had greater T-cell expansion and development of an effector phenotype in the adoptively transferred cells compared with tumor bearing mice similarly treated but with sham castration [23
]. Roden et al.
demonstrated splenocytes from castrated mice receiving an ovalbumin-specific vaccine proliferated more robustly in response to ovalbumin stimulation compared with splenocytes of similarly vaccinated controls from non-castrate mice [24
]. Koh et al.
reported that mice vaccinated with a dendritic cell vaccine, and then surgically castrated, had a greater number of antigen-specific IFN γ-secreting cells compared to vaccinated mice receiving sham surgery [25
]. Taken together, these results from multiple investigator groups suggest that it might be clinically beneficial to combine active immunotherapies with androgen deprivation [26
]. However, timing the administration of these therapies to gain maximum benefit needs to be experimentally determined, and if the effects of androgen deprivation on the adaptive arm of the immune system are persistent over time.
In order to investigate the effects of ADT on the adaptive immune system, and if the effects are persistent, our analysis focused on the frequency of circulating T cell subsets collected from prostate cancer patients at various time points up to 24 months after beginning androgen deprivation. We then characterized the ability of T-cell subsets to proliferate and express cytokines after receptor or mitogen activation. Finally, given the observations that ADT elicits T-cell infiltration of prostate tissue [15
], we asked if antigen-specific responses to proteins expressed in the prostate develop following ADT, which proteins were recognized, and if these responses are persistent over the course of therapy. For these studies we employed the SEREX methodology [27
]. We report an alteration to the T-cell repertoire develops following ADT, with an expansion of naïve T cells and RTEs. This T cell expansion is detectable at least by one month after beginning ADT, and the expansion was detectable up to two years later in specific individuals. Similarly, IgG responses were elicited to prostate tissue antigens as early as one month after beginning ADT, as well as after many months of treatment. Together our findings suggest that changes in the adaptive immune system following androgen deprivation may occur early after beginning treatment, and may be persistent for long periods of time. These observations may suggest that active immunotherapies might be used in sequence with androgen deprivation and/or might be affected by the concurrent use of androgen deprivation.