Aishwarya Atmakuri: Androgen pathway modulation-resistant prostate cancer, APMC, previously known as castration-resistant prostate cancer, has been historically considered immunologically cold. This dogma is based on immune profiling studies done in the metastatic setting, which have shown low levels of immune infiltration and high levels of immunosuppressive cells. This has resulted in poor antitumor immunity and poor response to immunotherapy.
However, this understanding might not apply at earlier stages of disease. The composition of the local immune microenvironment and localized prostate cancer remains poorly defined, especially in relation to Gleason scoring, which is the gold standard pathological scoring system of prostate cancer. We hypothesized that the local microenvironment of prostate cancer might be more heterogeneous than previously described, with distinct differences between low and high Gleason prostate cancer. High Gleason prostate cancer is defined as a score of about eight to 10, and this is associated with worse outcomes and more resistance than low Gleason prostate cancer, which is a score of six to seven. We hypothesized that there would be distinct differences in the immune composition between low and high Gleason prostate cancer in the localized setting.
Andrea Miyahira: Wonderful. So talk about what types of samples and methods that you used in this study.
Aishwarya Atmakuri: Our samples were collected from 29 treatment-naive patients who had undergone radical prostatectomy. 15 of these patients had low Gleason tumors and 14 had high Gleason tumors. Whole prostate samples were sectioned and stained using multiplex immunofluorescence, and data was analyzed using tissue-based cyclic immunofluorescence. We were able to get single cell-level resolution in our data to analyze exactly where cells were and what specific types of cells we had, and then we could compare differences in cell composition between low and high Gleason prostate cancer.
Andrea Miyahira: Okay. What were some of your key findings and did they surprise you?
Aishwarya Atmakuri: Our key findings were surprising because they challenged that dogma that prostate cancer was uniformly immunologically cold. Our first main finding is that we found that high Gleason tumors had increased levels of immune infiltration and they had high levels of immunosuppressive cell populations, like regulatory T-cells and M2-like macrophages. And we also found that high Gleason tumors had increased levels of tertiary lymphoid structures, which I'll be referring to as TLSs.
TLSs are ectopic aggregates of immune cells, and we were really interested in their relationship with other immune cells. We found that in the peri-TLS environment, specifically at the TLS tumor boundary in high Gleason tumors, there were high levels of T-cell exhaustion. We then did a deeper dive into the composition of TLSs and we found that in high Gleason tumors, TLSs had increased levels of T-cell exhaustion, and TLSs in general had higher levels of immunosuppressive cells. Our TLS findings were surprising because TLSs are typically considered a positive prognostic biomarker in many other types of cancers. However, in our studies, we've been finding the opposite, where they're associated with high-grade disease and immunosuppressive features.
Andrea Miyahira: Okay, thank you. And talk a little bit more about what tertiary lymphoid structures are and what mechanisms drive their development in tumors.
Aishwarya Atmakuri: Tertiary lymphoid structures are ectopic aggregates of immune cells, and these are typically found in states of chronic inflammation and tumors. They form in response to chronic exposure to neoantigens, and they're driven by cytokine and chemokine signaling. TLSs have different maturation states. Initial TLSs are just groups of T-cells and B-cells, and as they mature, they get more structural organization. TLSs can function like mini lymph nodes and coordinate an immune response.
Andrea Miyahira: So what mechanisms do you think might be driving the appearance of tertiary lymphoid structures in higher grade prostate tumors? And are the immunosuppressive phenotypes you observed here typical?
Aishwarya Atmakuri: So high-grade prostate cancer is associated with increased levels of chronic antigen exposure. This could be what's driving increased levels of immune infiltration, but also creating a dysregulated response, with high levels of exhaustion and immunosuppressive cells. For the TLSs in particular, this is currently an area of active investigation. TLS function is based on location, composition, and maturity of the TLSs, and our lab is trying to better characterize these factors to understand the biological mechanisms behind what we're seeing. While TLSs are typically considered to be a positive prognostic factor, recent evidence suggests that there's more nuance in the role of TLSs in tumors. Collectively, it's clear that high-grade prostate cancer is able to elicit a strong immune response, but it could be dysregulated and dysfunctional rather than protective.
Andrea Miyahira: And how do you think these findings might change how we should be approaching immunotherapy in prostate cancer?
Aishwarya Atmakuri: Our findings suggest that high-grade prostate cancer is far more immunologically active than previously though. We're seeing high levels of immunosuppressive cells and complex interactions between TLSs, immunosuppressive cells, and tumors might be creating a pro-tumorigenic environment. However, we can attempt to flip the switch using novel immune-based therapies to try to activate the immune system. This is a hypothesis that Dr. Patnaik is actively investigating in neoadjuvant trials in the localized prostate cancer setting.
Andrea Miyahira: Thank you. What are your next steps in your studies?
Aishwarya Atmakuri: So the findings of our studies are currently in manuscripts. One is in the process of revisions and one is in the process of being prepared for submission. Our current goals are to focus on better characterizing the qualities and the functional capacity of immune cells in the microenvironment, in addition to their spatial localization. Ultimately, the goal would be to conduct more translational research to see if neoadjuvant immunotherapy has a benefit in high-grade cases of localized disease. This would ultimately give patients better outcomes and more options when it comes to their prostate cancer.
Andrea Miyahira: Well, thank you so much for joining us and sharing all of this with us today.
Aishwarya Atmakuri: Thank you.