TREM stands for Triggering Receptor Expressed on Myeloid Cells, a family of proteins found on the surface of immune cells. These receptors help regulate how your immune system responds to threats like infections and damaged tissue. TREM has gained significant attention in recent years because of its links to Alzheimer’s disease, sepsis, and other inflammatory conditions.
The TREM Family of Receptors
The TREM family includes four members, designated TREM-1 through TREM-4. They all belong to a broader group of proteins called the immunoglobulin receptor superfamily, and each one sits on the surface of specific immune cells, acting like an antenna that picks up signals from the surrounding environment.
TREM-1 is found on monocytes, neutrophils, dendritic cells, and natural killer cells, with lower levels on certain T cells and B cells. It plays a major role in amplifying inflammation. TREM-2 has the widest range of expression in the family. It appears on standard immune cells but also on specialized cells in bone (osteoclasts) and, critically, on microglia, the brain’s resident immune cells. TREM-2’s presence across tissues including the kidney, liver, heart, brain, and lungs reflects how broadly it influences the body’s maintenance and repair systems.
TREM-3 and TREM-4 exist only in mice, not in humans, which limits their direct medical relevance but makes them useful in laboratory research. The TREM gene cluster also includes related proteins called TLT-1 and TLT-2. TLT-1 is found on platelets and the cells that produce them, while TLT-2 appears on B cells, neutrophils, and macrophages.
TREM-1: The Inflammation Amplifier
TREM-1 functions as a potent amplifier of the innate immune response. When your body detects bacteria or tissue damage, immune cells ramp up their inflammatory signals. TREM-1 takes those signals and turns up the volume, making the response faster and more aggressive. In a healthy scenario, this helps you fight off infections quickly.
The problem arises when TREM-1 activation becomes dysregulated. Overactive TREM-1 signaling has been implicated in sepsis (a life-threatening overreaction to infection), inflammatory arthritis, and neurodegenerative diseases. Researchers are now investigating whether blocking TREM-1 could dial down harmful inflammation in these conditions. A soluble form of the receptor, called sTREM-1, is shed into the bloodstream during inflammation and is being studied as a potential biomarker. Measuring sTREM-1 levels could help clinicians gauge how severe an inflammatory condition is and track whether it’s getting better or worse.
TREM-2: The Alzheimer’s Connection
TREM-2 is the most heavily researched member of the family, largely because of its connection to Alzheimer’s disease. In the brain, TREM-2 sits on microglia and helps them perform essential housekeeping tasks: clearing away dead cells, removing amyloid plaques, and managing inflammation. When TREM-2 works properly, microglia can detect and clean up the protein clumps that accumulate in Alzheimer’s. When it doesn’t, those clumps build up and the surrounding brain tissue suffers more damage.
A specific genetic variant of TREM-2, known as R47H, roughly doubles the risk of developing Alzheimer’s disease (odds ratio of 2.19). To put that in perspective, carrying this variant is one of the strongest genetic risk factors for Alzheimer’s outside of the well-known APOE4 gene. The R47H mutation appears to impair the receptor’s ability to bind to the signals it needs to detect, essentially making microglia less effective at their cleanup role.
This discovery sparked interest in developing drugs that could boost TREM-2 activity. The most prominent candidate, a monoclonal antibody called AL002, was designed to activate the TREM-2 pathway and enhance microglial function. It advanced to a phase II clinical trial called INVOKE-2, a randomized, double-blind study in patients with early-stage Alzheimer’s. Unfortunately, AL002 did not significantly slow clinical progression of the disease. The failure has prompted researchers to reconsider how and when TREM-2 activation might be most effective, rather than abandoning the target entirely.
TREM-2’s Role in Metabolism and Obesity
TREM-2’s influence extends well beyond the brain. In fat tissue, macrophages use TREM-2 to identify and clean up dying fat cells, a process that becomes increasingly important during weight gain. When you consume a high-fat diet over time, fat cells can undergo a form of inflammatory cell death. This triggers enzymes called ADAM10 and ADAM17 to clip TREM-2 off the surface of nearby macrophages, a process known as shedding.
Once macrophages lose their TREM-2, they become less effective at clearing dead fat cells and more prone to promoting inflammation. This creates a damaging cycle: dying fat cells cause inflammation, inflammation strips macrophages of the receptor they need to manage the problem, and the tissue becomes more inflamed as a result. In animal studies, blocking the enzymes responsible for TREM-2 shedding reduced weight gain on a high-fat diet, improved metabolic health markers, and shifted macrophages toward an anti-inflammatory profile. This line of research suggests that TREM-2 dysfunction in fat tissue may be one mechanism linking obesity to chronic, low-grade inflammation.
Why TREM Matters for Medicine
The TREM family sits at a crossroads of some of the most pressing health challenges: neurodegeneration, sepsis, metabolic disease, and chronic inflammation. TREM-1 and TREM-2 essentially represent two sides of immune regulation. TREM-1 amplifies inflammatory responses, which is helpful during acute infection but harmful when it goes unchecked. TREM-2 supports tissue maintenance and repair, and its loss or dysfunction allows damage to accumulate in the brain, fat tissue, and other organs.
Both receptors are being explored as therapeutic targets and as diagnostic tools. Soluble forms of TREM-1 and TREM-2 that circulate in blood and cerebrospinal fluid could serve as biomarkers for tracking disease severity. On the treatment side, the challenge has been translating promising laboratory findings into drugs that work in humans. The INVOKE-2 trial’s results with AL002 illustrate how complex this translation can be, particularly for a receptor involved in so many different biological processes across so many tissues. Still, TREM remains one of the more active areas of immunology research, with implications that reach into neurology, metabolic medicine, and critical care.