Context-restricted PD-(L)1 checkpoint agonism by CTLA4-Ig therapies inhibits T cell activity.

T cell surface CTLA4 sequesters the costimulatory ligands CD80 and CD86 on antigen-presenting cells (APCs) to prevent autoimmunity. Therapeutic immunosuppression by recombinant CTLA4-immunoglobulin (Ig) fusion proteins, including abatacept, is also attributed to CD80/CD86 blockade. Recent studies show that CTLA4-Ig binding to APC surface cis-CD80:PD-L1 complexes can release the inhibitory ligand PD-L1, but whether this contributes to T cell inhibition remains unclear. Here, we show that PD-L1 liberation by CTLA4-Ig is strictly limited, both in extent and context, relative to PD-L1-competing anti-CD80 antibodies. At APC surface CD80:PD-L1 ratios exceeding 2:1, CTLA4-Ig therapies fail to release PD-L1 regardless of their CD80 affinity. Additionally, introducing flexibility into CTLA4-Ig by modifying its rigid homodimer interface produces biologics that retain bivalent CD80 binding without dissociating cis-bound PD-L1. These findings demonstrate that CTLA4-Ig therapies liberate PD-L1 through a CD80 reorientation mechanism that imposes a strict context dependence to their PD-1 checkpoint agonism and resultant T cell inhibition.

Acute myeloid leukemia maturation lineage influences residual disease and relapse following differentiation therapy.

Acute myeloid leukemia (AML) is a malignancy of immature progenitor cells. AML differentiation therapies trigger leukemia maturation and can induce remission, but relapse is prevalent and its cellular origin is unclear. Here we describe high resolution analysis of differentiation therapy response and relapse in a mouse AML model. Triggering leukemia differentiation in this model invariably produces two phenotypically distinct mature myeloid lineages in vivo. Leukemia-derived neutrophils dominate the initial wave of leukemia differentiation but clear rapidly and do not contribute to residual disease. In contrast, a therapy-induced population of mature AML-derived eosinophil-like cells persists during remission, often in extramedullary organs. Using genetic approaches we show that restricting therapy-induced leukemia maturation to the short-lived neutrophil lineage markedly reduces relapse rates and can yield cure. These results indicate that relapse can originate from therapy-resistant mature AML cells, and suggest differentiation therapy combined with targeted eradication of mature leukemia-derived lineages may improve disease outcome.

Oncogenic cooperation between TCF7-SPI1 and NRAS(G12D) requires ß-catenin activity to drive T-cell acute lymphoblastic leukemia.

Spi-1 Proto-Oncogene (SPI1) fusion genes are recurrently found in T-cell acute lymphoblastic leukemia (T-ALL) cases but are insufficient to drive leukemogenesis. Here we show that SPI1 fusions in combination with activating NRAS mutations drive an immature T-ALL in vivo using a conditional bone marrow transplant mouse model. Addition of the oncogenic fusion to the NRAS mutation also results in a higher leukemic stem cell frequency. Mechanistically, genetic deletion of the ß-catenin binding domain within Transcription factor 7 (TCF7)-SPI1 or use of a TCF/ß-catenin interaction antagonist abolishes the oncogenic activity of the fusion. Targeting the TCF7-SPI1 fusion in vivo with a doxycycline-inducible knockdown results in increased differentiation. Moreover, both pharmacological and genetic inhibition lead to down-regulation of SPI1 targets. Together, our results reveal an example where TCF7-SPI1 leukemia is vulnerable to pharmacological targeting of the TCF/ß-catenin interaction.