Comparing divergent response rates in two independent, but similar clinical trials in ALL patients treated with almost identical 4-1BB-based, CD22-CAR T cells, Singh, Frey, Engels, Maude, Gill, and Ruella et al. observed that the CAR constructs differed only in the scFv linker length (CAR22-short vs CAR22-long). 4-1BB-based CAR22-short CARs drove CAR22 clustering and antigen-independent receptor homodimerization; enhanced synapse formation; increased IL-2, IFNγ, and TNFα expression; and resulted in superior efficacy in tumor models. After validating enhanced efficacy of short linkers with anti-CD33 CARs, the team designed a new anti-CD22 CAR with novel, affinity-matured scFvs.
Contributed by Katherine Turner
ABSTRACT: While CD19-directed chimeric antigen receptor (CAR) T cells can induce remission in patients with B cell acute lymphoblastic leukemia (ALL), a large subset relapse with CD19(-) disease. Like CD19, CD22 is broadly expressed by B-lineage cells and thus serves as an alternative immunotherapy target in ALL. Here we present the composite outcomes of two pilot clinical trials ( NCT02588456 and NCT02650414 ) of T cells bearing a 4-1BB-based, CD22-targeting CAR in patients with relapsed or refractory ALL. The primary end point of these studies was to assess safety, and the secondary end point was antileukemic efficacy. We observed unexpectedly low response rates, prompting us to perform detailed interrogation of the responsible CAR biology. We found that shortening of the amino acid linker connecting the variable heavy and light chains of the CAR antigen-binding domain drove receptor homodimerization and antigen-independent signaling. In contrast to CD28-based CARs, autonomously signaling 4-1BB-based CARs demonstrated enhanced immune synapse formation, activation of pro-inflammatory genes and superior effector function. We validated this association between autonomous signaling and enhanced function in several CAR constructs and, on the basis of these observations, designed a new short-linker CD22 single-chain variable fragment for clinical evaluation. Our findings both suggest that tonic 4-1BB-based signaling is beneficial to CAR function and demonstrate the utility of bedside-to-bench-to-bedside translation in the design and implementation of CAR T cell therapies.