New approaches to identification of antigenic candidates for future prostate cancer immunotherapy

Antibody responses to prostate-associated antigens in patients with prostatitis and prostate cancer

BACKGROUND

An important focus of tumor immunotherapy has been the identification of appropriate antigenic targets. Serum-based screening approaches have led to the discovery of hundreds of tumor-associated antigens recognized by IgG. Our efforts to identify immunologically recognized proteins in prostate cancer have yielded a multitude of antigens; however, prioritizing these antigens as targets for evaluation in immunotherapies has been challenging. In this report, we set out to determine whether the evaluation of multiple antigenic targets would allow the identification of a subset of antigens that are common immunologic targets in patients with prostate cancer.

METHODS

Using a phage immunoblot approach, we evaluated IgG responses in patients with prostate cancer (n = 126), patients with chronic prostatitis (n = 45), and men without prostate disease (n = 53).

RESULTS

We found that patients with prostate cancer or prostatitis have IgG specific for multiple common antigens. A subset of 23 proteins was identified to which IgG were detected in 38% of patients with prostate cancer and 33% patients with prostatitis versus 6% of controls (P < 0.001 and P = 0.003, respectively). Responses to multiple members were not higher in patients with advanced disease, suggesting antibody immune responses occur early in the natural history of cancer progression.

CONCLUSIONS

These findings suggest an association between inflammatory conditions of the prostate and prostate cancer, and suggest that IgG responses to a panel of commonly recognized prostate antigens could be potentially used in the identification of patients at risk for prostate cancer or as a tool to identify immune responses elicited to prostate tissue.

IgG responses to tissue-associated antigens as biomarkers of immunological treatment efficacy

We previously demonstrated that IgG responses to a panel of 126 prostate tissue-associated antigens are common in patients with prostate cancer. In the current report we questioned whether changes in IgG responses to this panel might be used as a measure of immune response, and potentially antigen spread, following prostate cancer-directed immune-active therapies. Sera were obtained from prostate cancer patients prior to and three months following treatment with androgen deprivation therapy (n = 34), a poxviral vaccine (n = 31), and a DNA vaccine (n = 21). Changes in IgG responses to individual antigens were identified by phage immunoblot. Patterns of IgG recognition following three months of treatment were evaluated using a machine-learned Bayesian Belief Network (ML-BBN). We found that different antigens were recognized following androgen deprivation compared with vaccine therapies. While the number of clinical responders was low in the vaccine-treated populations, we demonstrate that ML-BBN can be used to develop potentially predictive models.

Inducible expression of a prostate cancer-testis antigen, SSX-2, following treatment with a DNA methylation inhibitor

BACKGROUND

Active immunotherapies are one approach being developed as novel treatments for prostate cancer. Critical to the success of these therapies is the identification of appropriate target antigens. We have been seeking to identify immunologically recognized proteins, cancer-testis antigens (CTA) in particular, in patients with prostate cancer that would be rational target antigens.

METHODS

Using a previously reported panel of 29 different CTA, we used sera from 98 patients with prostate cancer and 50 healthy male blood donor controls to detect CTA-specific IgG. We then further evaluated the expression of one antigen, SSX-2, in prostate cancer cell lines and tissues.

RESULTS

We identified IgG specific for NY-ESO-1, LAGE-1, NFX-2, and SSX-2 in at least 1/98 individuals with prostate cancer. We demonstrated that SSX-2 is a prostate CTA, and its expression is associated with metastatic prostate cancer. In addition, we report that the treatment of at least two human prostate cancer cell lines with the DNA methylation inhibitor 5-aza-2'-deoxycytidine induced the expression of SSX-2. In contrast, treatment of a normal prostate epithelial cell line (RWPE-1) with 5-aza-2'-deoxycytidine did not induce SSX-2 expression.

CONCLUSIONS

Our findings suggest that SSX-2 could be further pursued as an immunotherapeutic target in prostate cancer, and that treatment with 5-aza-2'-deoxycytidine could be exploited to modulate antigen expression in combination with immunotherapeutic approaches.

Cellular immunotherapies for prostate cancer

Prostate cancer is currently the most commonly diagnosed malignancy, and the second leading cause of cancer-related death, in men in the United States. Most deaths from prostate cancer are due to metastatic disease that no longer responds to androgen deprivation. There have been several advances within the last decade leading to the approval of new agents for treating patients with castration-resistant prostate cancer. However, these agents can have significant side effects, and the average survival benefit from chemotherapy has been modest. Results from several clinical trials have suggested that immune-based therapies may have clinical benefits in patients with prostate cancer with significantly fewer adverse events. These new therapies, however, pose new potential benefits and risks. We review here two cellular immunotherapies furthest in clinical development for prostate cancer, and discuss the potential challenges to the treating oncologist with the possible advent of these new therapies.