Baker, Carly E

Abstract Background Immunotherapy has yet to make significant gains in glioblastoma (GBM) treatment, due in part to GBM-mediated immune suppression. Increasing evidence points to critical roles for tumor-derived extracellular vesicles (EVs) and immunosuppressive myeloid cells as key factors in this process. Methods Immunophenotyping of the tumor-immune microenvironment was performed using ultrasonic aspirate collected during GBM resection by high-dimensional flow cytometry. EVs collected from patient-derived GBM cell lines were used to condition myeloid cells collected from healthy donors to generate immunosuppressive myeloid cells. siRNA was used to knockdown TIGIT and/or NLRP3 expression prior to EV conditioning. T cell co-culture studies were performed with donor-matched T cells. Results Immune phenotyping of the tumor microenvironment and EV-conditioned myeloid cells revealed similar immunomodulatory protein expression across myeloid cell populations, with particularly elevated TIGIT expression. Knockdown of TIGIT reduced the immunosuppressive polarization of myeloid cells, resulting in improved T cell function. This finding proceeded in an NLRP3-dependent manner, with substantial co-expression of TIGIT and NLRP3 expression prior to knockdown, and concomitant knockdown of NLRP3 abrogating the effect of TIGIT knockdown. TIGIT expression correlated with increased IL-13 expression, and IL-13 blockade unmasked a pro-inflammatory myeloid cell phenotype. Conclusion TIGIT expression in myeloid cells in the GBM microenvironment is a functional marker of immunosuppressive activity, with TIGIT knockdown reducing IL-13 expression and unmasking the pro-inflammatory activity of NLRP3. This study bolsters our understanding of the immunosuppressive complexities of the GBM microenvironment and supports attenuation of immunosuppressive myeloid cell activity as a strategy to restore immune function in GBM.