Authors: Monica Sharma, Zachary B. Katz, David C. DeWitt, Kiersten M. Ruff, Akashdip Singh, Huan Lan, Wilson Phung, Jeffery Tom, Aimin Song, Jawahar Sudhamsu, John Davies, John G. Quinn, Rohit V. Pappu, Andrey S. Shaw
PD-1 blockade has become a foundation of cancer immunotherapy, but the molecular explanation for how PD-1 suppresses T-cell activation is still being refined. Most models have centered on phosphorylation of the receptor’s intracellular tyrosines and the inhibitory signaling that follows.
T-cell activation, however, also depends on how signaling proteins organize in space. TCR engagement promotes formation of biomolecular condensates containing linker for activation of T cells (LAT), a scaffold that helps assemble the signaling machinery needed to propagate the immune response.
In a study published in Science, Monica Sharma and colleagues asked whether PD-1 could act at this level by interfering directly with LAT condensate formation. Their results point to an unexpected mechanism: the cytoplasmic domain of PD-1 inhibited LAT condensation through unphosphorylated tyrosines in its two inhibitory motifs, while phosphorylation removed this particular inhibitory effect. (Sharma et al., 2026).
PD-1 Directly Inhibited LAT Condensate Formation
The researchers used in vitro reconstitution experiments to test whether the intracellular region of PD-1 could directly affect the formation of LAT signaling condensates.
It could.
The cytoplasmic domain of PD-1 inhibited LAT condensation without requiring the effect to be explained solely through a downstream phosphorylation-dependent pathway. The finding places PD-1 directly at the level of signaling-network assembly: the receptor can interfere with the formation of structures generated downstream of TCR engagement. (Sharma et al., 2026).
This matters because LAT condensates are not simply collections of signaling proteins. They create a local environment in which molecules involved in T-cell activation are concentrated and organized. Preventing their formation offers another way to weaken the signal before it develops into a full immune response.
Two Tyrosines Were Required for PD-1 Inhibition
The inhibitory activity mapped to the two tyrosine-containing motifs in the cytoplasmic region of PD-1.
Both tyrosine residues were required for the interaction that suppressed LAT condensation. When the relevant tyrosine chemistry was altered, the inhibitory effect was lost. (Sharma et al., 2026).
That result was unexpected because these residues are usually considered important mainly after they become phosphorylated. Here, the opposite state mattered: the unphosphorylated tyrosines themselves were required for inhibition.
The tyrosines therefore appeared to be doing more than serving as future phosphorylation sites. Their chemical properties were contributing directly to how PD-1 disrupted the signaling network.
Phosphorylation Removed the Condensate-Blocking Effect
The most striking result came when the investigators phosphorylated the PD-1 tyrosines.
Rather than strengthening the effect on LAT condensation, phosphorylation eliminated it. (Sharma et al., 2026).
This does not mean that phosphorylated PD-1 loses all inhibitory function. It means that the specific mechanism identified in this study depends on the receptor being in an unphosphorylated state.
That distinction changes how the two inhibitory motifs can be viewed. They are not necessarily functional only after phosphorylation. Before phosphorylation occurs, the tyrosines themselves can participate in an interaction that suppresses signaling-condensate formation.
The receptor may therefore move between molecular states with different inhibitory functions rather than operating through one fixed mechanism.
Tryptophan Restored PD-1 Inhibition
To determine what property of tyrosine was responsible, the researchers replaced it with tryptophan.
The substitution restored PD-1 inhibition in T cells. (Sharma et al., 2026).
Tyrosine and tryptophan are chemically different, but both contain aromatic side chains capable of donating a hydrogen bond. The rescue therefore argued against phosphorylation being responsible for the effect and instead pointed toward the chemistry of the unmodified amino acid.
The authors propose that hydrogen bond-donating aromatic residues within inhibitory motifs can disrupt signaling networks and block immune activation.
That conclusion extends the paper beyond PD-1 itself. If the same molecular principle is present in other inhibitory receptors, unphosphorylated motifs may have signaling functions that have been overlooked when attention is focused primarily on their phosphorylated state.
PD-1 May Control the Architecture of T-Cell Signaling
Taken together, the experiments support a different view of PD-1 inhibition.
TCR stimulation promotes LAT condensation. The cytoplasmic region of PD-1 can directly inhibit that process. The effect requires the two unphosphorylated tyrosines, disappears when they are phosphorylated, and can be restored by replacing tyrosine with another hydrogen bond-donating aromatic residue. (Sharma et al., 2026).
The important shift is from thinking only about the activity of individual signaling proteins to considering the architecture of the signaling network.
A T cell needs the right molecules to be activated, but it also needs those molecules to assemble in the right place and in the right molecular environment. The findings suggest that PD-1 can interfere with that organization itself.
What Does This Mean for Cancer Immunotherapy?
The study does not show that anti-PD-1 or anti-PD-L1 treatment should be used differently, and it does not identify a new clinical biomarker or mechanism of patient-level resistance.
Its relevance to immuno-oncology is more fundamental.
Checkpoint inhibition is built around reversing suppressive signals in T cells. Understanding exactly how those signals are generated matters for explaining why immune activation succeeds in some contexts and fails in others.
Sharma and colleagues add another mechanism to that biology: PD-1 may suppress immune activation partly by preventing signaling proteins from organizing into functional condensates.
The next question is whether the same principle operates across other inhibitory receptors and whether differences in this condensate-regulating behavior influence immune-cell function in tumors.
For now, the paper changes the molecular picture of PD-1 in a subtle but important way. Its inhibitory motifs are not only sites waiting to be phosphorylated. In their unphosphorylated state, they can actively participate in shutting down the organization of the T-cell signaling network. (Sharma et al., 2026).
