Engineering Synergistic Adoptive Cell Therapies for Enhanced Treatment of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is the most fatal type of leukemia, with high rates of resistance to standard treatments and relapse when remissions are achieved. Adoptive cell therapy (ACT) is a promising therapeutic approach for AML that uses genetically modified T cells to generate a “living drug” that can recognize and destroy tumor. However, the hostile AML bone marrow environment commonly hinders the success of these T-cell therapies. The leukemic bone marrow impedes T-cell anti-tumor activity in multiple ways, including suppression of T-cell metabolism, upregulation of inhibitory ligands against T-cell and rendering a hypoxic microenvironment. Rational engineering of T cells could overcome multiple AML barriers simultaneously.

We innovate transmembrane Transform Switch Receptors (TSRs): fusion proteins that combine the ectodomain of an inhibitory receptor with a stimulatory signaling endodomain. TSRs engineered into T cells simultaneously decoy inhibitory signals while giving T cells a boost. Specifically, our lab developed a series of novel TSRs that combine the ectodomain of the inhibitory Fas receptor, known for transmitting death signals upon interaction with Fas ligand (FasL), with costimulatory endodomains from the tumor necrosis factor receptor superfamily (TNFRSF). FasL expression is highly increased in AML blasts, and we previously showed that T cells bearing a Fas receptor fused with the 4-1BB costimulatory endodomain successfully enhance anti-tumor T cell function in a mouse AML model. We hypothesized that a mixture of T cells, each carrying different Fas-TSR vectors, would leverage different costimulatory signal-programmed functions to produce more robust anti-tumor responses and prolonged protection against AML recurrence.

In in vitro studies, our novel Fas-TSR constructs stably expressed in T cells and endowed T cells with improved expansion over time, improved proliferation in both normoxic and AML-like hypoxic environments, and significantly enhanced tumor lysis upon multiple tumor re-challenges overtime. While all Fas-TSR constructs improved T cell function compared to TCR-only control, some Fas-TSR candidates differed in their capacity to enhance T cell expansion, serial tumor lysis ability, and cytokine production upon tumor stimulation. Our in vitro findings support the hypothesis that T cells bearing different Fas-TSR can contribute to tumor-killing by leveraging their distinct costimulatory endodomain-programmed phenotypes. Using an in vivo immunocompetent murine AML model, we also found that Fas-TSR T cells improved survival of mice. After identifying functional Fas-TSR T cells acting as a single-therapeutic agent, we further tested the serial-killing ability Fas-TSR T cells in vitro. We identified three different Fas-TSR T cell combinations with synergistic tumor-lysing ability. We are now advancing leading Fas TSRs T cell combinations to test in vivo in our immunocompetent murine AML model. Overall, we show that Fas-TSRs can transform FasL inhibitory signals into distinct functional signals within T cells that depend on the costimulatory endodomain fused to Fas. Differences in the functional outcome of Fas-TSR T cells have the potential to synergize to significantly improve T cell therapeutic efficacy against AML.