TREML4

TREML4 is a DAP12-associated TREM-family receptor of myeloid cells that links recognition of dying cells to amplification of innate immune signaling and inflammatory cell-fate programs (PMID:19155473, PMID:25848864). Its extracellular domain binds late apoptotic and necrotic cells, and it pairs selectively with the adaptor DAP12 rather than DAP10 or FcRγ, with expression concentrated on CD8α+ dendritic cells and splenic macrophages (PMID:19155473). Although dispensable for dead-cell uptake in vivo, TREML4 is required for productive antigen presentation: antigens delivered to TREML4 are efficiently presented to CD8+ and CD4+ T cells (PMID:22210914). Mechanistically, TREML4 acts as an essential positive amplifier of TLR7 signaling in macrophages, promoting recruitment of MyD88 to TLR7 and p38 MAPK-dependent STAT1 phosphorylation to drive type I interferon and inflammatory cytokine output; its loss attenuates autoantibody and cytokine responses in lupus-prone mice and blunts antiviral immunity (PMID:25848864, PMID:36264674). Beyond signaling amplification, TREML4 governs inflammatory cell-fate decisions, regulating calcium homeostasis, ER stress, and apoptotic cell death during sepsis, promoting necroptosis in infected diabetic cornea, and driving atherosclerotic plaque progression in macrophages (PMID:33020659, PMID:37279395, PMID:32292401). In the CNS, microglial TREML4 propagates neuroinflammation through a Lyn/Syk/ERK signaling axis (PMID:42159801). Across these settings TREML4 genetic ablation is generally protective, marking it as a pro-inflammatory node rather than a homeostatic restraint.

1. 2009 Medium

   Established TREML4 as a myeloid surface receptor that physically recognizes dead cells and signals through a defined adaptor, framing it as a candidate sensor of cell death.

   Evidence Fc-fusion binding to apoptotic/necrotic cells, co-IP of adaptors, and antibody-based flow cytometry across DC and macrophage subsets

   PMID:19155473

   Open questions at the time

   • The molecular ligand on dead cells is not identified
   • DAP12-dependent downstream signaling output was not demonstrated
2. 2011 Medium

   Resolved the in vivo role question by showing TREML4 is redundant for dead-cell uptake but required for antigen presentation, separating recognition from immunological function.

   Evidence Treml4 knockout mice plus antibody-targeted antigen delivery with CD8+/CD4+ T cell priming readouts and a tumor model

   PMID:22210914

   Open questions at the time

   • Mechanism linking TREML4 engagement to enhanced presentation is unresolved
   • Which DC/macrophage subset mediates the effect in vivo is not pinpointed
3. 2015 High

   Defined the core intracellular mechanism: TREML4 amplifies TLR7 signaling by promoting MyD88 recruitment and p38-driven STAT1 phosphorylation, connecting the receptor to type I IFN and autoimmunity.

   Evidence Genome-scale shRNA screen validated in Treml4-/- macrophages with phospho-STAT1/p38 and MyD88-TLR7 co-IP, plus MRL/lpr lupus and influenza models

   PMID:25848864

   Open questions at the time

   • How a surface receptor controls cytosolic MyD88-TLR7 assembly is not structurally defined
   • Whether DAP12 is required for the TLR7-amplifying function is not established
4. 2020 High

   Extended TREML4 function to control of inflammatory cell fate and metabolism, showing it shapes calcium, ER stress, and apoptosis in sepsis and drives atherosclerotic inflammation.

   Evidence Whole-genome CRISPR screen and Treml4-/- mice in CLP sepsis and Apoe-/- atherosclerosis models with transcriptomic, metabolomic, calcium, MPO, ER-stress and apoptosis readouts

   PMID:32292401 PMID:33020659

   Open questions at the time

   • Direct receptor-proximal signaling driving ER stress and calcium changes is not mapped
   • Whether these effects depend on the TLR7 axis is unclear
5. 2022 Medium

   Placed TREML4 in a transcriptional circuit by showing Nr4a1 controls its expression in patrolling monocytes that hyper-respond to TLR7, reinforcing the TLR7-amplification model in autoimmunity.

   Evidence Pristane lupus model with Nr4a1-/- mice, flow cytometry, and TLR7-stimulated TNFα measurement

   PMID:36264674

   Open questions at the time

   • Direct Nr4a1 binding to the Treml4 locus is not shown
   • Causal contribution of TremL4-high monocytes to disease is correlative
6. 2023 Medium

   Implicated TREML4 in necroptotic cell-death control during infection, broadening its cell-fate role beyond apoptosis.

   Evidence siRNA knockdown in diabetic mouse cornea during Pseudomonas keratitis with phospho-RIPK3/caspase-8 markers and infection susceptibility

   PMID:37279395

   Open questions at the time

   • Single knockdown method without genetic confirmation in this tissue
   • Receptor-proximal link to the necroptosis machinery is not defined
7. 2026 Medium

   Identified a distinct CNS signaling output in which microglial TREML4 drives neuroinflammation through Lyn/Syk/ERK, extending its pro-inflammatory role to the brain.

   Evidence CLP sepsis model with Treml4 knockout, BV-2 siRNA knockdown, Lyn-agonist rescue, phospho-Lyn/Syk/ERK Western blots, and behavioral testing

   PMID:42159801

   Open questions at the time

   • How TREML4 engages the Lyn/Syk kinase cascade is not defined
   • Relationship between this axis and the macrophage TLR7/p38 pathway is unknown

• The endogenous ligand on dead cells and how surface ligand engagement is mechanistically transduced into divergent outputs (TLR7-MyD88 amplification, ER-stress/apoptosis, necroptosis, Lyn/Syk/ERK) remain unresolved.
• No defined molecular ligand
• No unifying model connecting DAP12 association to the multiple downstream pathways
• No structural data on the receptor or its complexes