Despite the fact that prostate cancer represents the foremost noncutaneous malignancy and the second leading cause of cancer-related death for men in the United States, only a handful of targetable cell surface prostate cancer–associated proteins have been described in the literature. In addition, even though virtually all men who die of prostate cancer will do so after receiving some interval of androgen-ablative therapy, surprisingly, little is known about the influence of hormone therapy on the expression of these proteins by primary tumors or metastases. Finally, the role of many of these prostate cancer–associated proteins is poorly understood, thus making it difficult to readily ascribe a function for these proteins in modulating prostate cancer biology and disease outcomes. Consequently, the identification of any cell surface molecule that is uniformly and stably expressed by primary and metastatic prostate tumors, especially in the context of androgen-ablative therapy, encompasses a vital finding and an opportunity to improve prostate cancer treatment. This is especially true when the function of the molecule is relatively well defined and can be readily linked to mechanisms for cancer progression.
Roth et al. (19
) recently described low levels of B7-H3 expression in normal prostate epithelia as well as significantly increased B7-H3 expression in nearly all prostate cancer cells. In addition, enhanced levels of B7-H3 expression by prostate cancer cells were found to correlate with virtually every adverse clinicopathologic feature associated with this disease. Specifically, higher levels of B7-H3 expression were associated with larger tumor volume, extraprostatic extension, higher Gleason score, seminal vesicle involvement, and positive surgical margins. Moreover, multivariate adjustment for the Gleason, PSA, seminal vesicle, and margin status score showed that intense B7-H3 expression serves as a statistically significant predictor of prostate cancer progression after attempted extirpative surgery (19
). Likewise, Zang et al. (20
) recently reported an independent study validating the aforementioned observations and further reported that adenocarcinoma of the prostate also expresses high B7-H4 (B7x) ligand levels.
Herein, we show that B7-H3 expression by prostate cancer cells remains stable in response to hormone treatment. Moreover, we show that intense B7-H3 expression persists as a statistically significant predictor of prostate cancer progression after RP, a finding that is analogous to what we previously reported as well as to validation studies conducted by one other group (19
). Finally, we show that bone metastases maintained very high levels of B7-H3 expression even after prolonged androgen-ablative therapy.
Given its relatively recent discovery in 2001, the understanding of the mechanisms regulating B7-H3 expression continues to evolve. In mice, B7-H3 is constitutively expressed by resting and activated B cells, macrophages, dendritic cells, and minor subsets of CD4+ and CD8+ T cells (10
). In humans, B7-H3 mRNA expression can be detected in multiple tissues and cell lines; however, protein expression on human dendritic cells and monocytes has only been shown in response to treatment with phorbol 12-myristate 13-acetate and ionomycin. In one early study, human B7-H3 was reported as a costimulator of CD4+ and CD8+ T cells, promoting T-cell proliferation and cytokine production in vitro
). More recently, B7-H3 has been overwhelmingly implicated as a potent inhibitor of T-cell activity (29
). Specifically, single or duplicate constructs of the immunoglobulin V–like and immunoglobulin C–like domains of human B7-H3 have been shown to down-regulate both T-cell proliferation and cytokine production in response to CD3/CD28-mediated costimulatory activation (29
). Furthermore, it has been reported that B7-H3–deficient mice develop accelerated forms of induced airway inflammation and experimental autoimmune encephalitis, thus implicating B7-H3 as an inhibitor of TH1-mediated immunity (30
). In a separate study, plate-immobilized murine B7-H3 in the presence of anti-CD3 was shown to inhibit murine CD4+ T-cell activation and interleukin 2 production, an effect completely abrogated by antibody-mediated B7-H3 blockade (28
). Antibody-mediated blockade of B7-H3 also exacerbates experimental autoimmune encephalomyelitis in mice (28
). In human neuroblastoma studies, B7-H3 has been implicated as an inhibitor of natural killer cell–mediated lysis, and multiple receptors have now been postulated to explain the ability of B7-H3 to inhibit responses by both T cells and natural killer cells (29
). To date, however, cognate receptors for B7-H3 have not been elucidated. Finally, it also seems that B7-H3 ligand expression may be regulated by host factors or tumor microenvironment as is supported by differential protein expression of B7-H3 based on tumor type or even location within a given tumor.
Of particular relevance to prostate cancer, B7-H3 is highly expressed in developing bone during embryogenesis, and its expression increases as osteoblast precursor cells differentiate into mature osteoblasts. Specifically, B7-H3 seems to be required for osteoblast differentiation (31
). Interestingly, osteoblastic hyperdense bone lesions represent a signature of metastatic prostate cancer. Related to this, our evaluation of prostate cancer bone metastases obtained from 34 patients revealed a trend toward increased B7-H3 expression in patients who received prior androgen deprivation therapy. This finding may indicate that hormonal therapy up-regulates B7-H3 expression in bone metastases but not primary prostate tumors. Alternatively, androgen deprivation may facilitate the outgrowth of high-grade tumor cells in bone metastases, resulting in increased B7-H3 expression. However, larger case-matched studies will be needed to confirm these relatively preliminary studies pertaining to B7-H3 expression by prostate cancer or other osteoblastic metastases.
B7-H3 may provide some theoretical advantages over other reported markers pertaining to prostate cancer. Distinct from non–membrane-bound PSA, which is secreted by malignant prostate cancer cells and down-regulated by androgen-ablative therapy, B7-H3 is highly expressed on the outer cytoplasmic membrane surface of treated or untreated prostate cancer cells, making B7-H3 an ideal target for therapy. In this regard, B7-H3 is analogous to six-transmembrane epithelial antigen of the prostate and prostate-specific membrane antigen, which are also expressed on the surface of malignant prostate cells (32
). However, unlike prostate-specific membrane antigen, in which expression levels decline with metastases, reported as 44% (8 of 18) to 67% (6 of 9) of bone metastases retaining prostate-specific membrane antigen expression, we find that B7-H3 is expressed by nearly all tumor cells within 100% (34 of 34) of bone metastases that we evaluated (34
). Our study further suggests that B7-H3 expression within bone metastases may actually increase after androgen-ablative therapy. Collectively, these observations are consistent with a potential role for B7-H3 in mediating metastatic progression during androgen-ablative therapy, a finding that contrasts with the reported observation that prostate-specific membrane antigen functions to abrogate prostate cancer invasiveness (36
Although the presented data provide compelling evidence that B7-H3 may serve as potential target in castrate-resistant prostate cancer, the current data are not without limitations. When generating a cohort of matched patients that were not treated with NHT, one of the parameters utilized was Gleason score. The accuracy and application of Gleason score noted in RP specimens after NHT has previously been questioned (37
). However, when Gleason score after NHT is assigned by an experienced genitourinary pathologist, as was done in the current series, it remains a significant prognostic measure (38
). Another potential limitation is the immunohistochemical evaluation of decalcified tissue. The decalcification process may alter the immunoreactivity; however, the staining intensity and distribution of B7-H3 in the decalcified specimens in the current study are comparable with those of the primary prostatic tumors in the current and previous reports (19
In summary, we have found that B7-H3 expression persists after NHT and remains a predictor of PSA progression after RP. These results, together with data that show continued expression in castrate-resistant metastases, suggest that B7-H3 expression may be one mechanism by which select prostate cancer cells survive androgen deprivation therapy and may therefore represent a potential target for future therapies.
Although future studies will be required to better understand the precise events that govern B7-H3 expression and functionality, our past and present studies suggest that B7-H3 ligand may help to render prostate tumors more aggressive by imbuing such tumors with a capability to neutralize local antitumoral immune responses. As such, targeting of B7-H3 could prove useful to potentiate responses to current forms of systemic treatment for advanced prostate cancer, treatments that are primarily immunotherapeutic or antiangiogenic in nature and which, in their present form, yield relatively low rates of durable disease remission. Additionally, our data indicate that B7-H3 may prove useful as a prognostic marker to identify patients with localized tumors who are at high risk for post-surgical disease progression and who might benefit from adjuvant therapy in the clinical trial setting.
We found that B7-H3 expression by prostate cancer is unchanged by NHT. Moreover, B7-H3 expression remains predictive of PSA progression after RP in patients receiving NHT. In addition, B7-H3 is strongly expressed in bone metastases and hormone refractory biopsy specimens. Given its role as a marker of adverse prognosis and the lack of change in expression with androgen deprivation therapy, B7-H3 may represent an important target for future combined therapy approaches, particularly for patients with high-risk prostate cancer.