A CD8αβ co-receptor modified to contain an intracellular CD28 signaling tail enhances TCR-engineered T cell function independent of solid-tumor-associated co-stimulatory ligands

Adoptive therapies using T cells genetically modified with T cell receptors (TCR)s have shown limited efficacy in the solid tumor setting. Although functional CD4 ⁺ and CD8 ⁺ T cells transduced with a TCR specific for HLA-A2-restricted melanoma-associated antigen A1 (MAGE-A1, T _{TCR−MA1−CD8αβ} ) could be detected post-transfer and were safe in one patient who subsequently progressed, T _{TCR−MA1−CD8αβ} were insufficient to sustain antitumor activity in “stress” mouse tumor models. Leveraging the obligate co-expression of CD8αβ required for engagement of CD4 ⁺ T cells expressing the TCR, we screened positive co-stimulatory signals tethered to the intracellular tail of CD8β and identified that CD28 reduced exhaustion, enhanced tumor infiltration and improved murine tumor control. Further modifications of the CD28 intracellular domain produced a mutant CD8β-CD28 construct that conferred superior therapeutic control across tumor models. Thus, integrating co-stimulatory signals downstream of the TCR signaling complex can enhance TCR-engineered T cell function, independent of tumor-associated co-stimulatory ligand expression.