Dual-State Structures Reveal How Drugs Switch the Immune Receptor GPR84

A receptor long viewed as one of the more enigmatic members of the lipid-sensing G protein-coupled receptor family is now coming into sharp molecular focus. New research published in Experimental & Molecular Medicine describes how structural snapshots of GPR84, captured in distinct conformational states, can be paired with functional assays in immune cells to explain how synthetic ligands push the receptor toward activation, toward repression, or into a strikingly balanced middle ground. The work, presented under the title Pharmacological modulation of GPR84 revealed by dual states structures and immune functional assays, offers one of the most complete pictures to date of how a single receptor can be tuned in opposite directions by chemically related molecules, and why that tuning matters for inflammatory disease.

GPR84 belongs to a subgroup of GPCRs that respond to medium-chain fatty acids, the metabolic fragments released when fats are broken down. Because these fragments accumulate in tissues under metabolic stress, GPR84 is thought to act as a metabolic sensor for the immune system, translating changing lipid levels into altered cellular behavior. Decades of pharmacological studies have linked the receptor to macrophages, neutrophils and other innate immune cells, where its activation has been associated with amplified production of inflammatory signaling molecules. That association made GPR84 an attractive drug target for companies pursuing anti-inflammatory therapies, and it also made the receptor a cautionary tale: several clinical candidates targeting it were halted, in some cases because the biology proved more complicated than early animal studies suggested.

The central problem has been that GPCRs are not simple on-off switches. They are dynamic molecular machines that sample a range of conformations, and different ligands can stabilize different subsets of those conformations. A ligand that locks the receptor into a fully active shape will recruit signaling proteins robustly, while a ligand that favors inactive shapes will silence the pathway. But many ligands do something subtler: they stabilize partially active conformations, or they favor active shapes in one signaling branch while leaving others untouched. Without structural information, medicinal chemists were essentially adjusting molecular shapes blindly, hoping that small changes in a ligand scaffold would produce predictable changes in receptor behavior.

The new study addresses that gap by capturing GPR84 in two distinct functional states, allowing the researchers to compare the receptor’s architecture when it is being activated against its architecture when it is being blocked or modulated. Structures determined in multiple states are technically demanding, because a membrane-embedded receptor must be stabilized in each conformation long enough to be imaged at atomic resolution. Achieving this typically requires engineered variants, stabilizing antibodies or nanobodies, and carefully chosen ligands that preferentially hold the receptor in the desired state. The resulting paired structures function like two frames of a molecular movie, revealing which helices shift, which side chains rotate, and which structural water molecules rearrange as the receptor transitions between resting and signaling-competent forms.

Comparing the two states highlights the allosteric heart of the receptor. In GPCRs, ligand binding at a pocket nestled among the transmembrane helices is transmitted through a conserved relay of hydrogen bonds, salt bridges and hydrophobic contacts to the intracellular face, where G proteins and other effectors dock. The dual-state GPR84 structures delineate how agonist binding contracts this relay into the canonical active arrangement, with an inward movement of the extracellular portion of a key transmembrane helix and a corresponding outward swing on the intracellular side that opens the effector-binding cavity. In the inactive or antagonist-bound state, that same helix relaxes outward, the intracellular cavity collapses, and the network of polar interactions reconfigures into a pattern incompatible with effector coupling. The residue-by-residue map of these changes gives chemists concrete positions to target when they want to bias the equilibrium toward one state or the other.

Crucially, the structural work is not left to stand alone. The authors couple it with functional assays performed in immune cells, measuring how well each ligand drives or suppresses downstream signaling and, importantly, how it affects inflammatory outputs such as cytokine release. This combination is what elevates the study from structural description to pharmacological instruction. A structure can suggest that a compound should be an agonist, but only cellular assays reveal the potency, the efficacy ceiling, and whether the compound behaves as a full agonist, a partial agonist or a biased ligand in a physiologically relevant context. By testing ligands across the spectrum, the researchers could correlate specific structural features of the binding pocket with specific functional consequences, effectively building a translation table between atomic geometry and immune cell behavior.

One of the more interesting implications concerns ligand bias, the phenomenon in which a receptor signals preferentially through one intracellular pathway over another. For GPR84, whose activation intersects with inflammatory programs in macrophages, a biased ligand could in principle dampen harmful signaling while preserving useful functions, or vice versa. The dual-state structures provide a mechanistic handle on bias: ligands that engage only part of the activation relay, or that fail to complete certain helical movements, may recruit one effector but not another. The immune functional assays then quantify what that partial engagement means for cytokine production, giving drug developers a rational framework for designing compounds with tailored signaling profiles rather than accepting whatever profile a scaffold happens to produce.

The study also speaks to the thorny issue of species differences, which has plagued GPR84 drug development. Synthetic agonists developed against the human receptor have often shown sharply different potency in mouse models, complicating the interpretation of preclinical efficacy studies and contributing to the sector’s clinical disappointments. Although the structures presented are of the human receptor, the detailed pocket architecture allows researchers to pinpoint which residues differ across species and to predict, before expensive animal work, whether a given compound is likely to translate. That kind of structural triage could save years of effort and redirect resources toward candidate molecules with a realistic chance of reproducing human biology in animal systems.

Beyond immediate drug design, the work contributes to a broader shift in GPCR pharmacology toward state-based thinking. Rather than classifying ligands simply as agonists, antagonists or inverse agonists, the field increasingly describes them by the conformational ensembles they stabilize and the signaling outputs those ensembles produce. GPR84, with its metabolic ligands, its immune cell expression and its checkered clinical history, is an ideal test case for this framework. The demonstration that paired structures plus immune assays can jointly explain and predict pharmacological behavior provides a template that other lipid-sensing and metabolite-sensing receptors could follow, particularly those where immune modulation is the therapeutic goal.

For patients, the significance lies in what this molecular clarity could eventually enable. Chronic inflammatory and metabolic diseases, including conditions involving macrophage-driven tissue damage, remain areas of substantial unmet need, and receptors that sense the metabolic environment of inflamed tissue are logical points of intervention. By showing exactly how ligands of different pharmacological classes occupy the GPR84 binding pocket and drive or block its conformational changes, the study turns a previously opaque target into an addressable one. The next steps, testing structure-guided ligand designs and validating their immune effects in disease models, will determine how quickly this structural knowledge moves from the pages of a journal toward the clinic, but the roadmap for modulating GPR84 rationally is now drawn.

Subject of Research: Structural and pharmacological modulation of the lipid-sensing immune receptor GPR84

Article Title: Pharmacological modulation of GPR84 revealed by dual states structures and immune functional assays

Article References: Choi, M. K., Park, D. J., Kim, P., Choi, H. S., Myung, S., Yoo, Y., Chang, N., Yoon, G.-Y., Kang, H. J., Ha, S.-J., & Cho, H.-S. (2026). Pharmacological modulation of GPR84 revealed by dual states structures and immune functional assays. Experimental & Molecular Medicine. https://doi.org/10.1038/s12276-026-01841-w

Image Credits: AI Generated

DOI: 10.1038/s12276-026-01841-w

Keywords: GPR84, G protein-coupled receptor, dual-state structures, immune functional assays, ligand bias, inflammatory signaling, macrophages, medium-chain fatty acids, structural biology, drug discovery, Pharmacological, modulation