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

Current practice is largely adapted from solid-organ transplantation, even though the immunology of composite grafts can be markedly different. Treatment is commonly organized around the three signals required for T-cell activation. First, a recipient T-cell receptor recognizes donor major histocompatibility complex molecules displayed by antigen-presenting cells. Second, costimulatory interactions, including CD80/86 with CD28, provide the additional confirmation needed for full activation. Third, cytokines such as interleukin-2 drive proliferation through intracellular pathways that include mammalian target of rapamycin. Induction therapy is given around the operation to blunt the initial immune response, maintenance therapy continues indefinitely, and additional drugs are used when rejection appears. In skin-rich grafts, many centers favor rabbit antithymocyte globulin, or rATG, a polyclonal antibody preparation that rapidly depletes T cells and also affects B cells, natural killer cells and other immune populations. The approach can reduce early rejection risk, but it may cause severe leukopenia, thrombocytopenia, cytokine release syndrome and opportunistic infections. Less intensive agents such as basiliximab block the interleukin-2 receptor without depleting lymphocytes, but may provide insufficient early suppression for highly immunogenic skin.

Long-term treatment commonly relies on a three-drug combination of tacrolimus, mycophenolate mofetil and corticosteroids. Tacrolimus inhibits calcineurin, preventing activation of nuclear factor of activated T cells and reducing the transcriptional program needed for T-cell responses. It is generally preferred over cyclosporine in VCA protocols, but prolonged exposure can damage the kidneys and contribute to other metabolic complications. Mycophenolate mofetil inhibits purine synthesis, restricting the proliferation of both T and B lymphocytes, while corticosteroids broadly dampen inflammation and immune activation. Sirolimus and everolimus, which inhibit the mTOR pathway, may be used when calcineurin inhibitor toxicity becomes a concern, although evidence for their use in VCA remains limited. The review emphasizes that reducing therapy is difficult: recipients often experience multiple rejection episodes, and grafts are already maintained at substantial drug intensity. When acute rejection occurs, skin lesions, swelling and redness may provide visible warnings. High-dose intravenous methylprednisolone is usually the first treatment, sometimes supplemented by topical tacrolimus or corticosteroids because the skin component is directly accessible. More resistant episodes may prompt rATG or alemtuzumab, but lymphocyte depletion alone has not reliably prevented rejection.

The burden of this strategy is especially consequential because most hand and face transplants improve quality of life rather than directly preventing death. Lifelong systemic suppression can increase susceptibility to cytomegalovirus, herpes simplex reactivation, fungal infections, pneumonia and Clostridioides difficile colitis. Cytomegalovirus remains a particular concern in donor-positive, recipient-negative mismatches and can occur despite antiviral prophylaxis. Treatment may require combinations of antiviral drugs and immune globulin. Long-term exposure also contributes to hyperglycemia, abnormal lipid levels, low magnesium, renal tubular problems, bone effects and fracture risk. The review notes that studies have reported metabolic dysregulation or malignancy in at least 65 percent of facial transplant recipients. Reduced immune surveillance can facilitate skin cancers and post-transplant lymphoproliferative disorders, while some immunosuppressants may promote tumor-associated pathways or interfere with DNA repair. This creates a difficult cycle: recurrent rejection requires stronger suppression, yet infection, metabolic disease or cancer may force clinicians to reduce treatment, potentially triggering further immune injury. Conventional therapy can control many acute episodes, but it has not provided a dependable way to prevent chronic rejection, the leading cause of late graft failure.

Chronic rejection illustrates why a more precise approach is needed. In VCA, it may appear as premature graft aging, mottled changes in skin color, thickened or prominent suture lines, telangiectasia, dryness of mucosal surfaces, tissue atrophy, fibrosis, loss of hair or other skin structures, nail changes and small-vessel thrombosis. Standard punch biopsies of skin can detect important changes, but they may miss injury in deeper tissues and arteries. In one rejected full-face graft, immunoproteomic analysis identified donor-origin CD8-positive T cells persisting inside the graft and infiltrating deep arteries near degenerating endothelium that had been repopulated by recipient-derived cells. The findings suggest a form of vascular injury that may not be captured by surveillance methods developed for solid organs. Face transplantation can also involve lymph nodes that reject independently of the visible skin. For this reason, the review supports surveillance that combines clinical inspection with scheduled skin and mucosal biopsies, consistent pathology scoring and, ultimately, molecular monitoring. The direct accessibility of VCA grafts is an advantage: clinicians can inspect tissue and sample it with minimally invasive procedures, while sentinel donor-derived flaps and tools such as ultrasound biomicroscopy may help assess rejection or vascular disease without repeatedly disturbing the primary graft.

Different VCA types also create different immunologic problems. Hand grafts contain vascularized bone marrow, which could provide donor blood-forming cells capable of producing mixed chimerism and potentially supporting tolerance, although donor marrow infusion has produced inconsistent clinical results. Hand recipients can experience acute rejection in as many as 85 percent of patients in reported series, and infections may become complex when treatment is intensified. Face grafts combine skin and mucosa with structures such as salivary glands, bone and lymphoid tissue, making rejection assessment more complicated; mucosal biopsies may show higher-grade rejection than skin and can contain infiltrating B cells and plasma cells. Uterine transplantation is different because the graft is temporary, contains no skin and must support a semi-allogeneic pregnancy. Rejection can often be detected through cervical biopsy, but immunosuppressive choices must also account for fetal safety, including replacement of mycophenolate mofetil with azathioprine before embryo transfer. Abdominal wall grafts are highly immunogenic because of their skin content, yet the skin can sometimes act as a visible sentinel for intestinal rejection. Laryngeal transplantation remains extremely rare, with fewer than 20 procedures reported and only four described as successful in the review. Its mucosal, airway and endocrine components make both treatment and surveillance difficult.

The most clinically advanced alternative discussed is costimulation blockade with belatacept, a CTLA4-Ig fusion protein that interrupts CD80/86-CD28 signaling. By targeting a specific activation step, belatacept may avoid the kidney toxicity and some metabolic and neurologic effects associated with calcineurin inhibitors. A reported hand transplant recipient received belatacept with mycophenolate mofetil and prednisone in a calcineurin-inhibitor-free regimen and maintained adequate rejection control for 18 months. However, progressive necrotizing rejection requiring amputation has also occurred during belatacept therapy, indicating that blocking costimulation alone may not be enough against skin-rich grafts. Cellular therapies offer another route. Regulatory T cells, or Tregs, naturally restrain immune activation and may be expanded or engineered to recognize donor antigens. Chimeric antigen receptor Tregs directed against donor HLA-A2 are being investigated, but their clinical use faces obstacles including limited survival after infusion, instability in inflammatory environments and the time needed to produce a patient-specific cell product. Most evidence remains preclinical or comes from isolated clinical reports rather than controlled VCA trials.

Researchers are also attempting to concentrate treatment at the graft instead of exposing the entire body. In a rat hindlimb model, a tacrolimus-eluting disk placed inside the graft supported survival for more than 200 days while maintaining low systemic drug levels; the benefit disappeared after removal of draining lymph nodes, suggesting that regional immune pathways were important. Other microparticles releasing transforming growth factor beta 1, interleukin-2 and rapamycin have prolonged graft survival in rats while enriching Tregs in draining lymph nodes and producing donor-specific tolerance. Biomaterial scaffolds and hydrogels carrying immune-regulating signals are being tested for similar purposes. Ex vivo machine perfusion could provide another platform by allowing a donor graft to be treated before implantation. Normothermic perfusion of porcine forelimbs removed approximately 49 billion donor leukocytes over six hours, potentially reducing the antigen-presenting cells that initiate early rejection. The same interval could be used to deliver drugs, gene therapy vectors or cellular products directly through the graft’s vasculature. Gene-editing tools such as CRISPR-Cas9 may eventually modify donor immune or antigen-presenting cells before transplantation, although safety and durability remain unresolved. Di-chimeric cells, made by fusing donor and recipient cell types, have extended graft survival in rodent studies without the marrow-ablating conditioning required by conventional chimerism protocols.

The review presents these approaches as promising directions, not established treatments. A major requirement for progress is standardization across the small number of centers performing VCA. Protocols, biopsy practices and outcome definitions vary, making it difficult to compare results or identify the safest drug combinations. International registries and improved diagnostic criteria for mucosal, vascular and chronic rejection could provide a common framework. Noninvasive markers, including donor-derived cell-free DNA, gene-expression profiles and blood-based inflammatory signatures, may eventually help clinicians detect injury earlier and adjust treatment according to rejection phenotype. Yet animal results have repeatedly failed to translate directly to humans, reflecting differences in immune memory, immune complexity and the particular difficulty of controlling skin allografts. Larger animal studies, multicenter collaboration and adaptive clinical trial designs will therefore be necessary. The long-term objective is not simply to suppress immunity more powerfully, but to make immune control selective: regional when possible, donor-specific when feasible and guided by the tissues under attack. If that shift succeeds, VCA may become less constrained by infection, organ toxicity, malignancy and chronic graft loss, bringing the risks of these transformative procedures closer to their quality-of-life benefits.

Subject of Research: Immune modulation and rejection prevention in vascularized composite transplantation

Article Title: Immunosuppressive modulation after vascularized composite allotransplantation: current practice and future directions

Article References: Brito, E. M., Gaston, J. O., & Hashem, A. M. (2026). Immunosuppressive modulation after vascularized composite allotransplantation: current practice and future directions. BMC Plastic and Reconstructive Surgery, 2(1), Article 24. https://doi.org/10.1186/s44452-026-00035-7

Image Credits: AI Generated

DOI: 10.1186/s44452-026-00035-7

Keywords: vascularized composite allotransplantation, transplant immunology, immunosuppression, graft rejection, tolerance induction, regulatory T cells, costimulation blockade, ex vivo perfusion, Immunosuppressive, modulation, vascularized, composite