Combined inhibition of myeloid cell PI3Kgamma and regulatory T cell integrin alphavbeta8 promotes durable tumor suppression

The immunosuppressive tumor microenvironment (TME) remains a critical barrier to effective immunotherapy. Immune suppressive myeloid cells and regulatory T cells can each prevent effective anti-tumor immune responses. The integrin alphavbeta8 promotes immune evasion by serving as a key activator of latent TGF-beta in regulatory T cells, while the lipid kinase PI3Kgamma promotes immunosuppressive signaling in myeloid cells. Genetic and pharmacological inhibition of PI3Kgamma stimulates integrin alphavbeta8 expression on regulatory T cells both in mouse models of cancer and in patients enrolled in cancer clinical trials. Inhibition of either alphavbeta8 or PI3Kgamma significantly suppressed head and neck tumor growth, suggesting they each play important roles in modulating anti-tumor immunity. To gain mechanistic insights into the impacts of these therapeutic approaches, we performed single-cell RNA sequencing (scRNA-seq) on HPV+ HNSCC tumors from wild-type (WT) and PI3Kgamma knockout (KO) mice, with or without anti-alphavbeta8 treatment, to investigate the individual and combined impacts of PI3Kgamma inhibition and alphavbeta8 blockade in the TME. RNA velocity and cell-cell communication inference analysis was performed to uncover dynamic regulatory interactions among cell populations. Antagonism of integrin alphavbeta8 led to increased CD8+ T cell infiltration and a reduction in TGF-beta-responsive transcriptional programs. PI3Kgamma inhibition promoted a proinflammatory shift in myeloid cells, characterized by diminished M2-like polarization and enhanced antigen presentation, and increased CD8+ T cell recruitment. Together combined inhibition of alphavbeta8 and PI3Kgamma reversed T cell exclusion and promoted durable tumor suppression.