HPK1 Influences Regulatory T Cell Functions

Hematopoietic progenitor kinase 1 (HPK1) is a negative regulator of TCR-initiated signal transduction. Both the HPK1^2/2 mice and the genetically engineered mice with a point mutation that disrupts the catalytic activity of HPK1 possess enhanced antitumor immunity, especially when these mice are treated with anti–PD-L1 immune checkpoint Ab. Because CD4⁺FOXP3⁺ regulatory T cells (Tregs) play an important role in suppressing tumor immunity, we investigated whether the loss of HPK1 expression could result in the reduction of Treg functions. We found that the number of HPK1^2/2 Tregs is elevated relative to the number found in wild-type C57/BL6 mice. However, HPK1^2/2 Tregs lack the ability to carry out effective inhibition of TCR-induced proliferative responses by effector T cells. Furthermore, HPK1^2/2 Tregs respond to TCR engagement with an elevated and sustained Erk MAPK and p65/RelA NF-kB phosphorylation in comparison with wild-type Tregs. Also, a multiplex cytokine analysis of HPK1^2/2 Tregs revealed that they demonstrate an aberrant cytokine expression profile when stimulated by anti-CD3e and anti-CD28 crosslinking, including the uncharacteristic expression of IL-2 and antitumor proinflammatory cytokines and chemokines such as IFN-g, CCL3, and CCL4. The aberrant HPK1^2/2 phenotype observed in these studies suggests that HPK1 may play an important role in maintaining Treg functions with wider implications for HPK1 as a novel immunotherapeutic target.