EXTH-44. ENHANCING GLIOBLASTOMA CLEARANCE: EMPOWERING ANTI-EGFRVIII CAR T CELLS WITH A PARACRINE SIRPΓ-DERIVED CD47 BLOCKER

Abstract

Abstract A major challenge for chimeric antigen receptor (CAR) T cell therapy against glioblastoma (GBM) is the immunosuppressive microenvironment (iTME), which is densely populated by protumoral glioma-associated microglia and macrophages (GAMs). Blockade of CD47, a “don’t-eat-me” signal overexpressed by GBM cells, disrupts the CD47-SIRPα axis, and regulates the phagocytic function of GAMs. However, systemic anti-CD47 monotherapy has been ineffective against human solid tumors to date. Here, we combined local CAR T therapy with paracrine GAM modulation to synergistically eliminate GBM. We engineered an armored CAR T cell against EGFRvIII that constitutively secretes a SIRPγ-related protein (SGRP) with high affinity to CD47. Anti-EGFRvIII-SGRP CAR T cells eliminated EGFRvIII+ GBM in vitro, and eradicated EGFRvIII-heterogeneous (EGFRvIII+ and EGFRvIII- mosaic) orthotopically xenografted tumors by locoregional application in vivo. This resulted in significant tumor-free long-term survival, followed by partial tumor control upon re-challenge. Combination of anti-CD47 antibodies with anti-EGFRvIII-CAR T cells failed to achieve synergy, pointing towards the importance of sustained paracrine GAM conversion. Paracrine SGRP enhanced CAR T infiltration, tumor clearance and GAM reconversion to an antitumor phenotype as shown by multidimensional brain immunofluorescence microscopy and in-depth spectral flow cytometry multiplexing. Moreover, the plasma of anti-EGFRvIII-SGRP CAR T-treated mice displayed a distinct signature of innate immune activation including TNFα and CCL3. Validation of the anti-EGFRvIII-SGRP CAR T in human explants is ongoing. Furthermore, armoring anti-CD19 CAR T cells with SGRP resulted in superior efficacy in a peripheral lymphoma mouse model compared to anti-CD19 CAR T treatment alone, most problably due to innate immune activation. Thus, local anti-EGFRvIII-SGRP CAR T cell therapy combines the potent antitumor effect of engineered T cells with the modulation of the surrounding innate iTME, inducing a synergistic clearance of GBM in a manner that overcomes known mechanisms of CAR T therapy evasion, such as tumor immune suppression and antigen escape.