Background Immune checkpoint blockade has proven effective in targeting exhausted T-cells to reactivate the immune system against cancer. However, the majority of patients fail to respond to currently available therapies, which primarily target PD-1. Thus, a key challenge for checkpoint blockade therapy is to identify and understand new therapeutic targets. Another immune checkpoint receptor is TIM-3, which – like PD-1 – is expressed on exhausted T-cells in the tumor microenvironment.^{1, 2} TIM-3 belongs to a family of phosphatidylserine (PS) receptors, including TIM-1 and TIM-4, which have well-documented roles in the engulfment of apoptotic cells by phagocytes.³ However, the role of PS in regulating TIM-3 function is less clear. We therefore investigated how TIM-3 modulates T-cell signaling and how PS influences TIM-3 activity, with the ultimate goal of improving the translation of candidate TIM-3 therapies to the clinic.

Methods Surface plasmon resonance (SPR) was used to quantify the interaction between human TIM-3 and PS. A Jurkat T-cell model was used to investigate the role of TIM-3 in T-cell receptor (TCR) signaling and to determine the role of PS in regulating TIM-3 function.

Results TIM-3 bound PS-containing membranes with low micromolar affinity in vitro. In the Jurkat cell model system, high – but not low – surface levels of TIM-3 promoted T-cell signaling, suggesting a threshold of receptor expression needed to modulate T-cell signaling, similar to what has recently been reported for PD-1.⁴ However, chimeric receptors that maintained the TIM-3 cytoplasmic tail but were unable to bind PS failed to enhance T-cell signaling like the full-length TIM-3 receptor. Cells expressing mutant TIM-3, which displayed reduced PS binding as quantified by SPR, also displayed reduced T-cell signaling compared to cells expressing wild-type TIM-3. Importantly, treatment of TIM-3-expressing cells with a functional TIM-3 antibody that blocks PS binding also reduced T-cell signaling compared with untreated TIM-3-expressing cells.

Conclusions Our results support a role for PS as a ligand capable of modulating TIM-3 activity. Using chimeric receptors, TIM-3 mutants, changes in receptor expression, and a functional TIM-3 antibody, we show that preventing the interaction between TIM-3 and PS blocks TIM-3 activity. These data suggest that blocking the PS-TIM-3 interaction is a key mechanism for functional antibodies targeting TIM-3. Ultimately, this work supports the development and use of clinical antibodies that block the interaction of TIM-3 with PS and provides new mechanistic insight into how TIM-3 modulates TCR signaling.

Acknowledgements This work was supported by the PhRMA Foundation Pre-Doctoral Fellowship in Pharmacology/Toxicology.

References

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