CAR T-cell therapies are already approved and improving outcomes in patients with some blood cancers. However, there remains scope to better the efficacy of existing products, both by improving response rates and durability in existing indications and by expanding into new settings, notably solid tumors. The use of alternative costimulatory domains is one possible way to improve outcomes.

Second-generation CARs contain 4-1BB or CD28 intracellular domains to support immune activation but it is unclear if they are the best options in all settings. To address the uncertainty, researchers at the University of California, San Francisco developed a repetitive stimulation assay to "mimic the protracted stress and exhaustion of chronic antigen exposure on T cells in difficult-to-eliminate tumors."

The scientists used the assay to evaluate a signaling domain library of 40 inhibitory and stimulatory domains from innate and adaptive immune cells. Performing a suite of pooled assays in primary human CD4 or CD8 T cells containing the CAR library generated data on the CAR T cell costimulation landscape.

Using the approach, which the team dubbed "CAR Pooling," the scientists identified B cell-activating factor receptor (BAFF-R) as a potent costimulatory domain from the tumor necrosis factor (TNF) receptor family that enhanced T-cell proliferation and cytotoxicity in vitro. The discovery that domains primarily associated with the B cell lineage enhanced T cells was a surprise enabled by the speed of CAR Pooling.

"Our method makes the process of testing new receptor-based therapeutics much faster. This not only allows the researchers to save time, but also to explore designs they wouldn't have been able to test individually. We would never have chosen to test BAFF-R if we weren't utilizing CAR Pooling," co-first author Camillia Azimi said in a statement.

The researchers validated BAFF-R in a xenotransplant mouse model of multiple myeloma. Replacing the 4-1BB intracellular signaling domain with BAFF-R in a B cell maturation antigen (BCMA)–specific CAR improved survival in the mouse model. BCMA is the target of two Food and Drug Administration (FDA)-approved CAR T-cell therapies.

Having developed CAR Pooling, the researchers plan to use the approach to test more receptors. The team could use the approach to find the best receptors for different settings, recognizing that the optimal approach may differ between glioblastoma and melanoma, for example.