Phosphatidylserine binding regulates TIM-3 effects on T cell receptor signaling

Co-signaling receptors for the T cell receptor are important therapeutic targets, with blocking co-inhibitory receptors such as PD-1 now central in immuno-oncology. Advancing additional therapeutic immune modulation approaches requires understanding ligand regulation of other co-signaling receptors. One poorly understood therapeutic target is TIM-3 (T cell immunoglobulin and mucin domain containing-3). Which ligands are relevant for TIM-3 signaling is unclear, and different studies have reported it as co-inhibitory or co-stimulatory. Here, we show that TIM-3 promotes NF-{kappa}B signaling and IL-2 secretion following T cell receptor stimulation in Jurkat cells, and is regulated by phosphatidylserine (PS) binding. TIM-3 signaling is stimulated by PS exposed constitutively in cultured Jurkat cells, and can be blocked by mutating the PS-binding site or by occluding this site with an antibody. We also find that TIM-3 signaling alters CD28 phosphorylation. Our findings help clarify conflicting literature results with TIM-3, and inform its exploitation as a therapeutic target.