Abstract 2782: Reprogramming tumor-associated macrophages to enable CAR T cell eradication of solid tumors
Authors: Luo Weichuan , Napoleon John V. , Low Philip S. 
Published: July 2021

Abstract
Although CAR T therapies have proven effective in treating liquid cancers, similar CAR T cell therapies have been unsuccessful in treating solid tumors, in part due to their immunosuppressive tumor microenvironments (TMEs). Critical to the establishment of immunosuppressive TMEs are tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) that express checkpoint co-receptors (e.g. PD-L1), release immunosuppressive cytokines (e.g. TGFβ and IL-10), secrete growth factors (e.g. FGF, VEGF, and PDGF), and inactivate T cells by nitrosylating T cell receptors. Motivated by observations that TAMs and MDSCs can be repolarized when exposed to certain immune stimuli, we investigated strategies to reprogram TAMs and MDSCs from protumorigenic to an anti-tumorigenic state. Here, we demonstrate that targeting of a proinflammatory TLR7 agonist specifically to TAMs and MDSCs can reverse an immunosuppressive TME by converting anti-inflammatory (M2-like) TAMs to pro-inflammatory (M1-like) macrophages and by decreasing the intratumoral content of MDSCs.
Our approach to the targeting of TLR7 agonists to TAMs and MDSCs is based on the observation that tumor-infiltrating myeloid cells over-express folate receptor β (FRβ) that is essentially absent from quiescent myeloid cells as well as all other cell types. By covalently linking a TLR7 agonist to folate, we demonstrate that the resulting FA-TLR7a conjugate accumulates specifically in TAMs and MDSCs while avoiding uptake by other immune cells and healthy tissues. Because FRβ resides in the same endosome as TLR7, internalization of FA-TLR7a by the TAMs and MDSCs leads to activation of TLR7 signaling and the consequent repolarization of TAMs and MDSCs to their pro-inflammatory states. Moreover, because other immune cells express no FRβ and since our murine tumor model (4T1 cells) also expresses neither FR nor TLR7, we conclude that the effects of FA-TLR7a described below derive primarily from repolarization of TAMs and MDSCs.
To test the ability of FA-TLR7a to improve the efficacy of a classic anti-CD19 CAR T cell therapy, we transduced 4T1 cells with CD19 and tested the ability of anti-CD19 CAR-T cells to eradicate 4T1 tumors in the presence and absence of FA-TLR7a. Infusion of CAR T cells alone inhibited tumor growth only slightly, while co-administration of FA-TLR7a greatly enhanced CAR T cell tumoricidal activity. Flow cytometric analysis of cell surface markers on derived TAMs and MDSCs demonstrated that the targeted TLR7 agonists shifted TAMs from M2-like to M1-like while significantly decreasing tumor infiltration of MDSCs. Similar analyses of other immune cells revealed increased numbers of T cells and anti-CD19 CAR-T cells and enhanced expression of activation markers on T cell populations. We conclude that the FA-TLR7 agonist greatly augments CAR-T cell efficacy in solid tumors via reprogramming of FRβ+ TAMs and MDSCs to a proinflammatory state.
Citation Format: Weichuan Luo, John V. Napoleon, Philip S. Low. Reprogramming tumor-associated macrophages to enable CAR T cell eradication of solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2782.