Targeting of a STING Agonist to Perivascular Macrophages in Prostate Tumors Delays Resistance to Androgen Deprivation Therapy
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Background
Androgen deprivation therapy (ADT) is a frontline treatment for prostate cancer but often leads to the development of castration resistant prostate cancer (CRPC). This causes tumors to regrow and metastasize, despite ongoing treatment, and impacts negatively on patient survival. ADT is known to stimulate the accumulation of immunosuppressive cells like protumoral tumor-associated macrophages (TAMs), myeloid-derived suppressor cells and regulatory T cells in prostate tumors, as well as hypofunctional T cells. Protumoral TAMs have been shown to accumulate around tumor blood vessels during chemotherapy and radiotherapy, where they drive tumor relapse. Our aim was to see if such perivascular (PV) TAMs accumulated in ADT-treated prostate tumors prior to CRPC, and, if so, to selectively target these PV cells with a potent immunostimulant, interferon beta (IFNβ), an attempt to stimulate anti-tumor immunity and delay CRPC.
Methods
We first used quantitative, multiplex immunofluorescence to assess the effects of ADT on distribution and activation status of TAMs, CD4+ T cells, CD8+ T cells and NK cells in mouse and human prostate tumors. We then used antibody-coated, lipid nanoparticles to selectively target a STING agonist, 2′3′-cGAMP (cGAMP), to PV TAMs in mouse prostate tumors during ADT.
Results
TAMs accumulated at high density around blood vessels in ADT-treated primary mouse and human prostate tumors prior to CRPC, where they expressed markers of a protumoral phenotype, folate receptor beta (FRβ), MRC1 (CD206), SIGLEC1 (CD169) and VISTA. Additionally, higher numbers of inactive (PD-1-) CD8+ T cells and reduced numbers of active (CD69+) NK cells were also present in PV tumor areas after ADT. LNPs coated with antibody to FRβ selectively delivered cGAMP to PV TAMs in ADT-treated tumors where they activated STING and expression of IFNβ by these cells. This resulted in a marked increase in the density of active CD4+ T cells, CD8+T cells and NK cells in PV tumor areas, and significantly delayed in the onset of CRPC.
Conclusion
Together, our data indicate that targeting a STING agonist to PV TAMs could be used to extend the treatment window for ADT in prostate cancer.
KEY MESSAGES
What is already known about the topic
Androgen deprivation therapy (ADT) is a frontline treatment for prostate cancer. However, tumors often develop resistance and start to regrow and metastasize – a condition called castration resistance prostate cancer (CRPC). Prostate cancer is considered to be an immunologically ‘cold’ tumor type and while ADT stimulates tumor infiltration by cytotoxic (CD8+) T cells, they are largely hypofunctional, possibly due to the immunosuppressive tumor microenvironment.
What this study adds
This study is the first to demonstrate that FRβ+ macrophages with a immunosuppressive phenotype accumulate around blood vessels in mouse and human prostate tumors during ADT, prior to the onset of CRPC. Lipid nanoparticles coated with an antibody to FRβ+ were then used to deliver a STING agonist selectively to these perivascular (PV) cells during ADT. This triggered STING signalling and the release of the potent immunostimulant, interferon beta, by PV macrophages, which then activated tumour-infiltrating CD4+ and CD8+ T cells, and delayed the onset of CRPC.
How this study might affect research, practice or policy
The delivery of an immunostimulant specifically to PV regions of tumors represents a new, more targeted form of immunotherapy that ensures the activation of T cells as soon as they cross the vasculature into tumors. This new approach could be used to extend the treatment window for neoadjuvant ADT in men with localised prostate tumors. In doing so, it would delay/circumvent the need for additional treatments like radiotherapy and/or or prostatectomy.
