Myeloid Cells Induce Infiltration and Activation of B Cells and CD4+ T Follicular Helper Cells to Sensitize Brain Metastases to Combination Immunotherapy.

Brain metastasis (BM) is a poor prognostic factor in cancer patients. Despite showing efficacy in many extracranial tumors, immunotherapy with anti-PD-1 monoclonal antibody (mAb) or anti-CTLA-4 mAb appears to be less effective against intracranial tumors. Promisingly, recent clinical studies have reported that combination therapy with anti-PD-1 and anti-CTLA-4 mAbs has a potent antitumor effect on BM, highlighting the need to elucidate the detailed mechanisms controlling the intracranial tumor microenvironment (TME) to develop effective immunotherapeutic strategies. Here, we analyzed the tumor-infiltrating lymphocytes in murine models of BM that responded to anti-CTLA-4 mAb to anti-PD-1 mAb. Activated CD4+ T follicular helper (TFH) cells with high CTLA-4 expression characteristically infiltrated the intracranial TME, which were activated by the combination anti-CTLA-4 and anti-PD-1 treatment. Loss of TFH cells suppressed the additive effect of CTLA-4 blockade on anti-PD-1 mAb. B cell-activating factor belonging to the TNF family (BAFF) and a proliferation-inducing ligand (APRIL) produced by abundant myeloid cells, particularly CD80hiCD206lo pro-inflammatory M1-like macrophages, in the intracranial TME, induced B cell and TFH cell infiltration and activation. Furthermore, the intracranial TME of patients with non-small cell lung cancer featured TFH and B cell infiltration as tertiary lymphoid structures. Together, these findings provide insights into the immune cell crosstalk in the intracranial TME that facilitates an additive anti-tumor effect of CTLA-4 blockade with anti-PD-1 treatment, supporting the potential of a combination immunotherapeutic strategy for BM.