Generation and characterization of monoclonal antibodies to prostate secretory protein

Development of new prostate specific monoclonal antibodies

BACKGROUND

Despite the need for new prostate-specific diagnostic and therapeutic targets, very few unique prostate (cancer) specific antigens have been characterized. Monoclonal antibody (mAb) technology is a powerful tool to identify specific antigenic markers, which could be potential targets for cancer diagnostics or therapy.

METHODS

Splenocytes from mice immunized with prostate cancer (PCa) homogenates of different origin were fused using standard techniques. Employing a differential high-throughput screening method followed by immediate screening in immunohistochemistry (IHC) a large number of hybridomas were screened for prostate (cancer) specificity.

RESULTS

From 25 successful fusions approximately 300 clones were identified excreting PCa-reactive antibodies. Subsequent immunohistochemical fine-specificity analysis reduced this number to 26. Eventually, after extensive fine-specificity analysis, the number of mAbs appearing to define prostate-specific antigenic structures that might serve as new diagnostic or therapeutic targets was reduced to three.

CONCLUSIONS

Using mAb technology combined with a high throughput screening method we have developed three mAbs (1.8, 2.26, and 3.10) directed against prostate associated antigens that might identify potential new therapeutic targets.

Factor H binding to bone sialoprotein and osteopontin enables tumor cell evasion of complement-mediated attack

Metastatic cancer cells, like trophoblasts of the developing placenta, are invasive and must escape immune surveillance to survive. Complement has long been thought to play a significant role in the tumor surveillance mechanism. Bone sialoprotein (BSP) and osteopontin (OPN, ETA-1) are expressed by trophoblasts and are strongly up-regulated by many tumors. Indeed, BSP has been shown to be a positive indicator of the invasive potential of some tumors. In this report, we show that BSP and OPN form rapid and tight complexes with complement Factor H. Besides its key role in regulating complement-mediated cell lysis, Factor H also appears to play a role when "hijacked" by invading organisms in enabling cellular evasion of complement. We have investigated whether BSP and OPN may play a similar role in tumor cell complement evasion by testing to see whether these glycoproteins could promote tumor cell survival. Recombinant OPN and BSP can protect murine erythroleukemia cells from attack by human complement as well as human MCF-7 breast cancer cells and U-266 myeloma cells from attack by guinea pig complement. The mechanism of this gain of function by tumor cell expression of BSP or OPN has been defined using specific peptides and antibodies to block BSP and OPN protective activity. The expression of BSP and OPN in tumor cells provides a selective advantage for survival via initial binding to alpha(V)beta(3) integrin (both) or CD44 (OPN) on the cell surface, followed by sequestration of Factor H to the cell surface and inhibition of complement-mediated cell lysis.

In situ hybridization study of PSP94 (prostatic secretory protein of 94 amino acids) expression in human prostates

BACKGROUND

The prostatic secretory protein of 94 amino acids (PSP94), also named beta-microseminoprotein, is one of the major proteins secreted by the human prostate. However, its value as a prognostic marker for prostate cancers is still under debate. The aim of the present study was to examine the expression pattern of this protein in fetal, pubertal, and aged human prostates.

METHODS

Nonisotopic in situ hybridization using a digoxigenin-labeled riboprobe for PSP94 and immunohistochemistry were used to demonstrate the expression of PSP94 in different regions or zones of fetal, pubertal, and adult human prostates. Its localization pattern was also compared with those of two other major secretory proteins, prostate-specific antigen (PSA) and prostatic acid phosphatase (PAP), by immunohistochemistry.

RESULTS

PSP94 mRNA and its protein were localized to the secretory epithelium of normal pubertal and adult human prostates. No hybridization signal and immunoreactivity of PSP94 were seen in fetal prostates at 6-7 months of gestation, whereas some glandular cells were positive to PSA and PAP immunostainings. In the adult prostates, PSP94 expression was intense in the acini in the peripheral zone, less intense in the transition zone, and variable in the central zone. Such a zonal expression pattern was more apparent in the pubertal prostates. However, no obvious differential expression pattern was observed in the immunohistochemistry of PAP and PSA, which showed a uniform staining of the secretory epithelia of the acini in all anatomic zones. The hybridization signals and immunoreactivity of PSP94 became reduced or lost in premalignant prostatic intraepithelial neoplastic lesions and different grades of prostatic carcinomas.

CONCLUSIONS

Fetal prostates at 6-7 months of gestation already synthesize PSA and PAP but not PSP94. The delayed expression of PSP94 appears to correlate with the development of the prostate gland. A differential expression pattern of PSP94 is demonstrated in different anatomical zones, showing that this protein is more expressed and synthesized in the acini in the peripheral zone than in the central and transition zones. However, such a zonal pattern is not seen in the immunohistochemistry of PSA and PAP. The present study also shows that PSP94 is downregulated in different grades of prostate cancers.

Bispecific rabbit Fab'-bovine serum albumin conjugate used in hemagglutination immunoassay for beta-microseminoprotein

A polyclonal bispecific (bifunctional) antibody was prepared to develop a hemagglutination immunoassay for beta-microseminoprotein (beta-MSP), a predominant seminal protein. Three types of F(ab')2 fragments of rabbit IgG, affinity-purified anti-human red blood cell (RBC) F(ab')2 nonaffinity-purified anti-beta-MSP F(ab')2 and nonspecific (nonimmunized) F(ab')2, were mixed to obtain a F(ab')2 mixture containing 10% anti-RBC molecules and 10% anti-beta-MSP molecules. Fab' was obtained from the F(ab')2 mixture, and then reacted with maleimide-activated bovine serum albumin (BSA) at a molar ratio of 10:1. As estimated by the decrease in the maleimide content, approximately 7 Fab' molecules were introduced per one BSA molecule. The bispecific (anti-beta-MSP and anti-RBC) Fab'-BSA conjugate thus prepared was incubated successively with a human RBC suspension and with samples. In the presence of beta-MSP, RBCs become agglutinated, providing a test simple for forensic semen identification.

Diagnostic and prognostic markers for human prostate cancer

BACKGROUND

The incidence and mortality of prostate cancer are increasing at alarming rates, partially due to an aging population. Early detection of prostate cancer, using clinically sensitive procedures and/or tumor markers (e.g., prostate-specific antigen [PSA]), is of prime importance. However, the choice of therapeutic interventions for prostate cancer at the time of diagnosis is largely dependent on clinical and pathologic staging and prediction of the degree of aggressiveness of the disease. Clinically applicable prognostic markers are urgently needed to assist in the selection of optimal therapy.

METHODS

Literature review of the potential diagnostic and prognostic markers for human prostate cancer.

RESULTS

Well-established tissue prognostic indicators, including histologic grade, margin positivity, pathologic stage, intraglandular tumor extent, and DNA ploidy, are not reviewed in this paper. Recently, a number of novel markers have been identified. In this paper, we begin with a discussion of a number of well-established as well as investigational diagnostic markers and then focus on evaluation of prognostic markers. Diagnostic markers that have prognostic value and investigational prognostic markers are also discussed.

CONCLUSIONS

Currently, only PSA is utilized for early diagnosis and monitoring of prostate cancer. A number of potential prognostic markers warrant further investigation. Multimarker analysis is implicated.

Recombinant PSP94 (prostate secretory protein of 94 amino acids) demonstrates similar linear epitope structure as natural PSP94 protein

PSP94 has the potential to be a useful diagnostic marker and therapeutic agent in prostate cancer. Recently, different immunoassay systems for quantitative analysis of PSP94 in clinical samples have been developed, but the epitope structure of PSP94 protein has not been elucidated. In this study, we report an Escherichia coli expression system for recombinant GST-PSP94 fusion protein. GST-PSP94 contains antigenic determinants similar to natural PSP94 protein (determined both by Western blotting experiments and by ELISA) and can be used to study the structure of natural PSP94 antigen. Since GST-PSP94 was expressed in E. coli and purification involved a denaturing process, we propose that the epitope structure of PSP94 is linear and largely dependent on the primary amino acid sequence, rather than conformational structure. This hypothesis was supported by reciprocal competition in ELISA among natural, GST-PSP94 fusion protein, and purified recombinant PSP94 protein. The results demonstrate that the various forms of PSP94 can compete with each other in binding to rabbit PSP94 polyclonal antibody, although the natural PSP94 has a slightly higher affinity. When natural and recombinant PSP94 protein were denatured in vitro with urea and alkali, no effect on the binding to antibody was found. The epitope activity of natural PSP94 was also shown to be resistant to the treatment of detergent and reducing agent. The location of one of the linear epitopes recognized by the PSP94 antibody was determined to be in the N-terminus by using two synthetic peptides representing N- and C-terminal sequences. Competitive ELISA between the N-terminal peptide and PSP94 protein indicate that both natural and GST-PSP94 have similar immunoactive N-termini.

Prostate inhibin peptide (PIP) in prostate cancer: a comparative immunohistochemical study with prostate-specific antigen (PSA) and prostatic acid phosphatase (PAP)

Prostate inhibin peptide (PIP) is a polypeptide synthesized by the prostate gland that is involved in prostatic growth and differentiation. The objective of this study was to evaluate PIP as an immunocytochemical marker for prostatic adenocarcinoma (PCA) by comparing it with PSA and PAP. A total of 71 cases of primary PCA and 5 cases of metastatic PCA were studied. Primary tumors were specially selected to include a disproportionate number of high-grade tumors. The distribution of cases by Gleason score was 2-5, 14 cases; 6-7, 24 cases; and 8-10, 33 cases. Four metastases were to bone (decalcified tissue) and one to soft tissue. All 71 cases of primary PCA stained positively for the three antibodies tested, with none demonstrating obvious superiority, although individual case variability was seen. In one bone metastasis, staining for PSA was negative, with both PAP and PIP giving positive results. All non-prostatic carcinomas tested were negative. These results indicate that PIP is as sensitive and specific an immunohistochemical marker as PSA and PAP in untreated prostate adenocarcinomas. Further, the androgen-independent nature of PIP may give it an advantage over PSA/PAP in tumors exposed to androgen ablating agents.

A comparative study on expression of prostatic inhibin peptide, prostate acid phosphatase and prostate specific antigen in androgen independent human and rat prostate carcinoma cell lines

Prostatic inhibin peptide (PIP), consisting of 94 amino-acid residues is synthesized and secreted by the prostate gland. Previous studies on immunohistochemical localization of PIP in primary prostatic tumor and their metastasis, have documented the value of this peptide as a tumor marker for diagnosis of prostate cancer (PCa). The present study was undertaken to compare the expression of PIP with that of prostate specific antigen (PSA) and prostatic acid phosphatase (PAP) in androgen independent human PCa cell lines (PC-3, DU-145 and TSU-Prl) by immunoperoxidase technique. The results of the study indicated that the staining for PIP was more intense than that of PSA and PAP. The PSA staining was either weakly positive (PC-3) or totally absent (TSU-Prl and DU-145) while PAP staining was intense in PC-3 and moderate in the other two human cell lines. The intense staining observed for PIP in all of the androgen independent cell lines suggests that the synthesis and secretion of PIP is not primarily dependent on androgens. Furthermore, expression of these markers in Dunning rat cultured adenocarcinoma cell lines and tumors were studied. Positive staining for all three human tumor associated antigens (PIP, PSA and PAP) cross-reacting with the Dunning rat PCa cell lines and the tumors, suggest the suitability of this model for preclinical screening of various therapeutic agents.

Comparison of prostate secretory protein with prostate specific antigen and prostatic acid phosphatase as a serum biomarker for diagnosis and monitoring patients with prostate carcinoma

Serum prostate secretory protein (PSP) levels were measured in 49 patients with benign prostatic hyperplasia (BPH), 144 patients with various stages of prostatic carcinoma (CaP), and 82 CaP patients who were followed serially. PSP values were compared with serum levels of prostate specific antigen (PSA) and prostatic acid phosphatase (PAP). In the BPH group, PSP was elevated (> 10 ng/ml) in 41% of patients, whereas PSA (> 4 ng/ml) and PAP (> 3.3 ng/ml) were elevated in 39% and 23% of the cases, respectively. PSP levels were elevated in 48% of the CaP pretreatment specimens, compared to 79% for PSA and 40% for PAP. PSP levels in cancer patients who had intracapsular disease were about two to three times higher than those observed for PAP. PSP was found to be the only marker elevated in eight (6%) pretreatment CaP patient serum specimens, while PAP was never found to be elevated when PSA was normal. PSP serum concentrations correlated with the clinical course of the disease in 79% of patients, compared with 90% for PSA and 66% for PAP. In certain patients, monitored over time, disease correlation was reflected in serum values with only a single biomarker, i.e., 1% with PAP, 8% with PSP, and 10% with PSA. This study has shown that PSP is a less sensitive serum biomarker than PSA, but more sensitive than PAP for detection and monitoring the early stages of prostate cancer. This suggests that PSP as a biomarker may be a useful adjunct for the management of a subpopulation of low-stage and -grade CaP.

The detection of prostate specific antigen, MHS-5, and other markers in invasive prostate cancer and seminal vesicle

Prostate specific antigen (PSA) is the most useful serum marker for following the disease status of prostate cancer patients after therapy. While PSA is felt to be an organ specific marker, lack of PSA expression in the seminal vesicles has not been adequately established. MHS-5 is a monoclonal antibody which recognizes an epitope on seminal vesicle specific antigen. Our objectives were to define PSA expression by the seminal vesicles, to determine whether MHS-5 could serve as an adjunct in the diagnosis of seminal vesicles invasion by carcinoma of the prostate, and to determine whether carcinoma, having invaded seminal vesicles would retain its expression of PSA and other prostate markers. Using an immunoperoxidase procedure, we studied thirteen seminal vesicles without histologic evidence of prostate cancer invasion and five seminal vesicles with locally invasive cancer. No seminal vesicles expressed PSA, whereas prostate cancer invading the seminal vesicles expressed PSA in all cases. MHS-5 expression was more variable. Only two of five cases of locally invasive tumor demonstrated seminal vesicles expression for MHS-5. Our findings further support the specificity of PSA. While MHS-5 may be helpful in delineating seminal vesicles in some instances, it is not a consistently reliable marker.

Dunning rat prostate tumors and cultured cell lines fail to express human prostate carcinoma-associated antigens

The objective of this study was to determine if human prostate carcinoma-associated tumor markers were expressed by Dunning rat prostate carcinomas. Frozen and formalin-fixed paraffin-embedded tissues from 12 different sublines of Dunning tumors were evaluated for marker expression by immunoperoxidase staining by using a panel of 9 monoclonal antibodies, including antibodies against human PAP and PSA. None of the Dunning tumors were found to express any of the human prostate tumor markers. Both fixed and live immunofluorescent assays were performed on 5 cultured Dunning tumor cell lines, evaluated either as single cells or as monolayers. As with the Dunning tumor tissues, none of the cell cultures expressed any of the 9 human prostate tumor markers. The lack of antigen expression by the Dunning tumor tissues and cell lines suggests that these human prostate tumor markers are quite species specific. These results limit the use of the Dunning prostate tumors as models to explore the preclinical application of these human prostate carcinoma-associated monoclonal antibodies and their target antigens.