Fgl2-knockout tumor cells serve as a vaccine inducing long-duration brain-resident memory T cells that reject subsequent intracranial tumor cell challenges

The failure to prevent brain tumors, including both recurrent primary and metastatic brain tumors, is the primary cause of patients' mortality. We developed a novel whole tumor-cell vaccine to rapidly induce long-duration brain-resident memory T (T_RM) cells that prevent brain tumor progression. Ten Fgl2-KO primary and metastatic tumor cell lines, generated via CRISPR/Cas9, were used to vaccinate mice and for intracranial challenges with the WT tumor cells. Not only did vaccinated mice reject these tumor cell challenges, but also more than half of these mice became long-duration survivors. Transplanting brain immune cells from vaccinated mice into naïve mice enabled this rejection of intracranial challenges in the recipient mice, whereas depleting T_RM cells impaired it. Mechanistic studies uncovered that Fgl2 KO impaired the immunosuppressive molecule CD47; reconstitution of CD47 expression in Fgl2-KO tumor cells reversed the protection. Likewise, vaccination with CD47-knockdown tumor cells produced similar effects. Proteomic analysis found that Fgl2-KO-mediated suppression of CD47 occurred through the Src and PKCα pathways; inhibition of either pathway reduced CD47 expression. This study is the first to show that disrupting the Fgl2-CD47 circuit in tumor cells impairs their tumorigenic properties and induces long-term brain T_RM cells, thereby providing new strategies for improving the efficacy of currently used whole tumor-cell vaccines.