Cancer Immunotherapy Mechanism Reconsidered: PD-1 Disrupts Signaling Through Aromatic Residues, Not Phosphorylation

A long-standing theory about how PD-1, a key target in cancer immunotherapy, suppresses T-cell signaling has been overturned.

According to BioOn on the 22nd (local time), a research team led by Monica Sharma reported in the journal Science that PD-1 physically disrupts the formation of T-cell activation signaling clusters through “aromatic residues,” rather than phosphorylation.

PD-1 is an inhibitory receptor expressed on the surface of activated T cells. The “phosphorylation-dependent model” has long been accepted as the standard explanation: When tyrosines in the ITIM and ITSM inhibitory motifs on PD-1’s cytoplasmic tail are phosphorylated, they recruit phosphatases such as SHP-1/SHP-2, which turn off T-cell signaling.

In this study, however, the researchers found that PD-1’s cytoplasmic domain directly inhibits LAT clusters formed in response to T-cell receptor (TCR) stimulation. LAT clusters are assemblies of signaling molecules essential for full T-cell activation.

In particular, phosphorylation of the tyrosines abolished PD-1’s inhibitory function. By contrast, replacing the tyrosines with tryptophan, another aromatic amino acid, restored that function.

Based on these findings, the researchers said that aromatic residues, which provide hydrogen bonds within the inhibitory motifs, are the key mechanism for disrupting the signaling network—not phosphorylation.

The finding is considered a link between research on immune checkpoints and biomolecular clusters. The researchers suggested that other inhibitory receptors, including CTLA-4, BTLA and LAG-3, may also regulate signaling clusters through a similar aromatic-residue mechanism.

Clinically, the findings point to the potential development of small-molecule drugs and peptides that mimic or disrupt the interaction between PD-1’s cytoplasmic tail and LAT clusters. The state of LAT cluster formation and the characteristics of aromatic residues could also serve as biomarkers for predicting responses to PD-1 antibodies.

The researchers also predicted that removing or modifying the PD-1 cytoplasmic domain in CAR-T cells could improve their persistence and killing ability in solid tumors.