Rational Modifications to CAR Antigen Binding Domain Improve Sensitivity to Low-Density Target Antigens

Chimeric antigen receptor (CAR) T cells have demonstrated significant success in treating relapsed or treatment-refractory hematologic malignancies in the clinic, inducing remissions in 70-90% of pediatric patients with B-lineage acute lymphoblastic leukemia (B-ALL). Unfortunately, up to half of patients will eventually relapse, often due to poor CAR T cell expansion/persistence or modulation of the target antigen by tumor cells. One mechanism of antigen modulation is a decrease in surface expression of the CAR-targeted antigen, as seen in clinical trials with CAR T cells targeting the CD22 antigen in B-ALL, as well as in trials targeting solid tumors. Based on clinical data from our anti-CD22 CAR trial, we believe that sensitivity to low expression of the CD22 target antigen is important for efficacy of CARs targeting this antigen, and have shown in xengraft models that CAR T cells are less effective in responding to leukemia with low expression of the target antigen (CD22Lo).  Antigen downregulation thus represents a major mechanism of resistance to CAR T cell therapy and an area for improvement. Recent work from our lab and others have implicated the CAR antigen binding domain as an important factor in the efficacy of CARs responding to the CD22 target antigen. We have set out to characterize the effects of multiple modifications to the antigen binding domain on CAR responses to CD22Lo leukemia as a means to enhance overall CD22 CAR response rates. Collectively, these investigations will deepen our understanding of the factors important to designing CARs which are capable of targeting tumors with weakly-expressed antigens.

Learning Objectives:

1. Describe the impact of leukemic target antigen density on CAR T cells. 

2. Identify a way in which the CAR antigen binding domain can be modified. 

3. Identify one T cell function which is impacted by leukemic target antigen density.