Combined PD-L1/TGFβ blockade allows expansion and differentiation of stem cell-like CD8 T cells in immune excluded tumors

It has been previously shown that combining immune checkpoint inhibitors with TGF & beta; blockade potentiates anti-tumor immune responses. Here the authors show that, in an immune excluded preclinical tumor model, combining therapeutic anti-PD-L1 with anti-TGF & beta; treatment promotes expansion and differentiation of stem-cell like CD8 + T cells. TGF & beta; signaling is associated with non-response to immune checkpoint blockade in patients with advanced cancers, particularly in the immune-excluded phenotype. While previous work demonstrates that converting tumors from excluded to inflamed phenotypes requires attenuation of PD-L1 and TGF & beta; signaling, the underlying cellular mechanisms remain unclear. Here, we show that TGF & beta; and PD-L1 restrain intratumoral stem cell-like CD8 T cell (T-SCL) expansion and replacement of progenitor-exhausted and dysfunctional CD8 T cells with non-exhausted T effector cells in the EMT6 tumor model in female mice. Upon combined TGF & beta;/PD-L1 blockade IFN & gamma;(hi) CD8 T effector cells show enhanced motility and accumulate in the tumor. Ensuing IFN & gamma; signaling transforms myeloid, stromal, and tumor niches to yield an immune-supportive ecosystem. Blocking IFN & gamma; abolishes the anti-PD-L1/anti-TGF & beta; therapy efficacy. Our data suggest that TGF & beta; works with PD-L1 to prevent T-SCL expansion and replacement of exhausted CD8 T cells, thereby maintaining the T cell compartment in a dysfunctional state.