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

Brain metastasis is a poor prognostic factor in patients with cancer. Despite showing efficacy in many extracranial tumors, immunotherapy with anti-PD-1 mAb or anti-CTLA4 mAb seems to be less effective against intracranial tumors. Promisingly, recent clinical studies have reported that combination therapy with anti-PD-1 and anti-CTLA4 mAbs has a potent antitumor effect on brain metastasis, highlighting the need to elucidate the detailed mechanisms controlling the intracranial tumor microenvironment (TME) to develop effective immunotherapeutic strategies. In this study, we analyzed the tumor-infiltrating lymphocytes in murine models of brain metastasis that responded to anti-CTLA4 and anti-PD-1 mAbs. Activated CD4+ T follicular helper (TFH) cells with high CTLA4 expression characteristically infiltrated the intracranial TME, which were activated by combination anti-CTLA4 and anti-PD-1 treatment. The loss of TFH cells suppressed the additive effect of CTLA4 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 proinflammatory 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 cross-talk in the intracranial TME that facilitates an additive antitumor effect of CTLA4 blockade with anti-PD-1 treatment, supporting the potential of a combination immunotherapeutic strategy for brain metastases. Significance: B-cell and CD4+ T follicular helper cell activation via BAFF/APRIL from abundant myeloid cells in the intracranial tumor microenvironment enables a combinatorial effect of CTLA4 and PD-1 blockade in brain metastases.