Gene therapy and immunotherapy of prostate cancer: adenoviral-based strategies

Evidence-based approaches to reduce cancer health disparities: Discover, develop, deliver, and disseminate

The Texas Center for Health Disparities (TCHD) at the University of North Texas Health Science Center is a National Institute on Minority Health and Health Disparities-funded, specialized center of excellence for health disparities. TCHD organized its 12^th annual conference focusing on "Evidence-Based Approaches to Reduce Cancer Health Disparities: Discover, Develop, Deliver, and Disseminate." At this conference, experts in health care, biomedical sciences, and public health gathered to discuss the current status and strategies for reducing cancer health disparities. The meeting was conducted in three sessions on breast cancer, prostate cancer, and colorectal cancer disparities, in addition to roundtable discussions and a poster session. Each session highlighted differences in the effects of cancer, based on factors such as race/ethnicity, gender, socioeconomic status, and geographical location. In each session, expert speakers presented their findings, and this was followed by a discussion panel made up of experts in that field and cancer survivors, who responded to questions from the audience. This article summarizes the approaches to fundamental, translational, clinical, and public health issues in cancer health disparities discussed at the conference.

Chimeric antigen receptor-engineered T cells for the treatment of metastatic prostate cancer

Cancer immunotherapy was selected as the Breakthrough of the Year 2013 by the editors of Science, in part because of the successful treatment of refractory hematological malignancies with adoptive transfer of chimeric antigen receptor (CAR)-engineered T cells. Effective treatment of B cell leukemia may pave the road to future treatment of solid tumors, using similar approaches. The prostate expresses many unique proteins and, since the prostate gland is a dispensable organ, CAR T cells can potentially be used to target these tissue-specific antigens. However, the location and composition of prostate cancer metastases complicate the task of treating these tumors. It is therefore likely that more sophisticated CAR T cell approaches are going to be required for prostate metastasis than for B cell malignancies. Two main challenges that need to be resolved are how to increase the migration and infiltration of CAR T cells into prostate cancer bone metastases and how to counteract the immunosuppressive microenvironment found in bone lesions. Inclusion of homing (chemokine) receptors in CAR T cells may improve their recruitment to bone metastases, as may antibody-based combination therapies to normalize the tumor vasculature. Optimal activation of CAR T cells through the introduction of multiple costimulatory domains would help to overcome inhibitory signals from the tumor microenvironment. Likewise, combination therapy with checkpoint inhibitors that can reduce tumor immunosuppression may help improve efficacy. Other elegant approaches such as induced expression of immune stimulatory cytokines upon target recognition may also help to recruit other effector immune cells to metastatic sites. Although toxicities are difficult to predict in prostate cancer, severe on-target/off-tumor toxicities have been observed in clinical trials with use of CAR T cells against hematological malignancies; therefore, the choice of the target antigen is going to be crucial. This review focuses on different means of accomplishing maximal effectiveness of CAR T cell therapy for prostate cancer bone metastases while minimizing side effects and CAR T cell-associated toxicities. CAR T cell-based therapies for prostate cancer have the potential to be a therapy model for other solid tumors.

High frequency of prostate antigen-directed T cells in cancer patients compared to healthy age-matched individuals

BACKGROUND

In order to obtain a sustained cytotoxic T lymphocyte (CTL) response against cancer cells it is preferable to have CTLs directed against multiple peptide epitopes from numerous tumor-associated antigens.

METHODS

We used a Flow Cytometry-based interferon (IFN)-gamma secretion assay with peptide-pulsed C1R-A2 as antigen-presenting cells to analyze whether CD8+ T cells directed against any of 24 HLA-A*0201-binding peptides from 15 prostate-associated proteins can be found in the peripheral blood of patients with localized prostate cancer. We also investigated whether multiple prostate antigen-specific CD8+ T cells can be generated simultaneously, from a naïve T cell repertoire. In that case, dendritic cells (DCs) from peripheral blood of healthy donors were divided in six portions and separately pulsed with six peptides. The peptide-pulsed DCs were then pooled and used to stimulate autologous T cells. The T cells were re-stimulated with peptide-pulsed monocytes.

RESULTS

We found prostate antigen-restricted CD8+ T cells in the peripheral blood in 48 out of 184 (26.1%) analyzed samples from 25 cancer patients. This is significantly higher than 17 out of 214 analyzed samples (7.9%) from 10 healthy age-matched male individuals (P = 0.0249). In the cases when antigen-specific T cells could not be detected, we were able to generate IFN-gamma-producing CD8+ T cells specific for up to three prostate antigens simultaneously from a naïve T cell repertoire.

CONCLUSIONS

CD8+ T cells directed against prostate antigen peptides can be found in, or generated from, peripheral blood. This indicates that such T cells could be expanded ex vivo for adoptive transfer to prostate cancer patients.

Increased therapeutic efficacy of the prostate-specific oncolytic adenovirus Ad[I/PPT-E1A] by reduction of the insulator size and introduction of the full-length E3 region

Conditionally replicating adenoviruses are developing as a complement to traditional cancer therapies. Ad[I/PPT-E1A] is an E1B/E3-deleted virus that replicates exclusively in prostate cells, since the expression of E1A is controlled by the recombinant 1.4 kb prostate-specific PPT promoter. The transcriptional integrity of PPT is maintained by the 3.0 kb mouse H19 insulator that was introduced directly upstream of the PPT sequence. In order to increase the cloning capacity to be able to reintroduce E3 sequences in the 35.7 kb Ad[I/PPT-E1A] genome, various shorter insulators were examined in a luciferase reporter gene assay. It was found that the 1.6 kb core H19 insulator (i) improves the activity of PPT, compared to the 3.0 kb full-length insulator, while still maintaining prostate cell specificity and releasing 1.4 kb of space for insertion of additional sequences. To improve the ability of the virus to efficiently lyse infected cells and persist in vivo, we inserted the adenovirus death protein (ADP) or the full-length adenovirus E3 region. The oncolytic activity of PPT-E1A-based viruses was studied using MTS, crystal violet and replication assays. The virus with the reintroduced full-length E3-region (Ad[i/PPT-E1A, E3]) showed the highest cytopathic effects in vitro. Furthermore, this virus suppressed the growth of aggressively growing prostate tumors in vivo. Therefore, we conclude that Ad[i/PPT-E1A, E3] is a prostate-specific oncolytic adenovirus with a high potential for treating localized prostate cancer.

Characterization of human prostate and breast cancer cell lines for experimental T cell-based immunotherapy

BACKGROUND

In order to develop experimental immunotherapy for prostate and breast cancer it is of outmost importance to have representative target cell lines that through human leukocyte antigen (HLA) class I molecules present relevant levels of peptides from tumor-associated antigens for cytotoxic T lymphocyte (CTL) recognition.

METHODS

We sequenced the HLA-A and HLA-B loci of eight commonly used prostate and breast cancer cell lines and analyzed the surface expression of HLA-ABC, HLA-DR, CD40, CD80, CD86, and CD54 by flow cytometry. We also analyzed the cell lines for mRNA expression from 25 genes reported to be specifically or preferentially expressed by prostate cells.

RESULTS

Among the analyzed cell lines we found that LNCaP, PC-346C and MCF-7 are HLA-A*0201 positive. However, the HLA-A2 expression level is low and only MCF-7 upregulates HLA-A2 in response to IFN-gamma stimulation. MCF-7 also expresses high levels of CD54, which further improve its value as a CTL target cell line. On the other hand, LNCaP and PC-346C express 25 and 23 out of 25 prostate-related genes, respectively, while MCF-7 expresses 16 out of 25 genes.

CONCLUSIONS

None of the analyzed prostate cancer cell lines are optimal CTL target cells. However, MCF-7 could in many cases be used as a complement to HLA-A*0201 positive prostate cancer cells. The LNCaP and PC-346C cell lines are rich sources of prostate-related antigens that may be valuable for cancer vaccine development.

Two-step amplification of the human PPT sequence provides specific gene expression in an immunocompetent murine prostate cancer model

The recombinant prostate-specific PPT sequence comprises a prostate-specific antigen enhancer, a PSMA enhancer and a TARP promoter. It is transcriptionally active in human prostate cancer cells both in the presence and absence of testosterone. However, in experimental murine prostate cancer, it has no detectable transcriptional activity. Herein, we describe that the PPT sequence in combination with a two-step transcriptional amplification (TSTA) system becomes active also in murine prostate cancer cells. An adenovirus with TSTA-amplified PPT-controlled expression of the luciferase reporter gene, Ad[PPT/TSTA-Luc], has up to 100-fold higher prostate-specific transcriptional activity than a non-amplified PPT-based adenovirus, Ad[PPT-Luc], in human cells. In addition, Ad[PPT/TSTA-Luc] confers prostate-specific transgene expression in murine cells, with an activity that is approximately 23% of Ad[CMV-Luc] in the transgenic adenocarcinoma of the mouse prostate (TRAMP)-C2 cells. Moreover, to visualize luciferase expression in living mice a charge-coupled device camera was used. Ad[PPT/TSTA-Luc] yielded approximately 30-fold higher transgene expression than Ad[PPT-Luc] in LNCaP tumor xenografts. Importantly, Ad[PPT/TSTA-Luc] also showed activity in murine TRAMP-C2 tumors, whereas Ad[PPT-Luc] activity was undetectable. These results highlight that the recombinant PPT sequence is active in murine prostate cancer cells when augmented by a TSTA system. This finding opens up for preclinical studies with prostate-specific therapeutic gene expression in immunocompetent mice.