What Is FMS-Like Tyrosine Kinase 3 Ligand? Definition and What Research Reports

Definition

FMS-like tyrosine kinase 3 ligand — abbreviated Flt3L, FL, or FLT3LG after its gene — is a naturally occurring hematopoietic cytokine (a signalling protein, not a short synthetic peptide) that binds and activates FLT3, also called CD135, a class III receptor tyrosine kinase found on early bone marrow progenitor cells and on dendritic cell precursors. It is produced as a type I transmembrane glycoprotein that can also circulate in a soluble, cleaved form, and both forms engage the same receptor. In immunology its defining property is that it drives the generation, expansion and survival of dendritic cell lineages, including plasmacytoid dendritic cells (pDCs) and conventional dendritic cells, which is why the term appears constantly in papers on antigen presentation, vaccine design and innate immunity. This page is for educational purposes only and is not medical advice; consult a licensed physician with any question about a medical condition or an investigational compound.

What Class of Molecule Is It, and Where Does It Come From?

Flt3L belongs to the family of hematopoietic growth factors — the same broad functional class as other colony-stimulating cytokines — rather than to the short-chain peptide category that dominates consumer peptide discussion. Because it is a folded glycoprotein with a defined receptor-binding structure, it is typically produced for laboratory work by recombinant expression in cell systems rather than by solid-phase peptide synthesis.

In the body, Flt3L is expressed by a range of cell types, including bone marrow stromal cells, T lymphocytes and fibroblasts, and soluble Flt3L can be measured in serum and synovial fluid. That measurability is why it also appears in the literature as a candidate biomarker: one pilot investigation of pre-clinical rheumatoid arthritis reported that survivin, but not Flt3L, was up-regulated in samples taken before the onset of disease (PMID 24495510).

Related terminology

• FLT3 / CD135 — the receptor tyrosine kinase that Flt3L binds.
• FLT3LG — the human gene symbol for the ligand.
• pDC / cDC — plasmacytoid and conventional dendritic cells, the populations most often measured in Flt3L studies.
• Flt3L-derived BMDCs — dendritic cells grown from bone marrow in culture using Flt3L as the differentiating cytokine.

How the Term Is Used in Peptide and Protein Research

Across the published record, the phrase "FMS-like tyrosine kinase 3 ligand" is used in several distinct ways, and readers encountering the term benefit from knowing which sense is meant:

1. As a culture reagent. Researchers add Flt3L to bone marrow cultures to generate dendritic cells in vitro. One 2018 report examined aryl hydrocarbon receptor ligands in this system and reported that indoxyl 3-sulfate and indole-3-carbinol inhibited Flt3L-induced bone marrow-derived plasmacytoid dendritic cell differentiation (PMID 30402330).
2. As an administered agent in animal models. Studies have given Flt3L to mice to expand dendritic cell compartments and then measured disease outcomes, as in a post-burn pneumonia model where the study reported attenuated local and systemic infection (PMID 28796661).
3. As a genetic condition. "Flt3L-deficient" or "Flt3-independent" describes animals or cell populations lacking the signalling axis, used to ask what the pathway contributes; researchers reported that absence of Flt3L signalling protected against collagen-induced arthritis (PMID 24064002).
4. As a component of engineered constructs. Flt3L sequences have been fused to antigens in experimental vaccine design, as in a therapeutic vaccine combining a rearranged human papillomavirus type 16 E6/E7 fusion protein with Flt3L that reportedly induced CD8+ T cell responses and an antitumour effect in the reported model (PMID 29029939).

What the Published Literature Reports

The verified papers summarised here are laboratory and animal studies; none of them is presented on this page as evidence for any human use, and no dosing information is provided. The findings below are descriptive summaries of what each study reported in its own model.

Infection and burn injury

Two related reports examined Flt3L in burn models. Researchers reported that Flt3-like tyrosine kinase-3 ligand increased resistance to burn wound infection through effects on plasmacytoid dendritic cells (PMID 28228109), and a companion line of work reported attenuation of local and systemic infection in a model of post-burn pneumonia (PMID 28796661).

Lung and airway immunity

Flt3L features heavily in respiratory immunology. A 2009 study reported that Flt3L regulated the migratory pattern and antigen uptake of lung dendritic cell subsets in a murine model of allergic airway inflammation (PMID 19917684). A later report described an Flt3L/lung dendritic cell axis that contributed to the regulation of pulmonary fibrosis (PMID 31076499). Complicating the picture, a 2020 study reported that Flt3-independent dendritic cells were major mediators of Th2 immune responses in allergen-induced asthmatic mice (PMID 33327561), indicating that not all disease-relevant dendritic cells depend on this pathway.

Joint inflammation

In arthritis models the reported direction is largely pro-inflammatory. Intra-articular Flt3L expression was reported to be a driving force in the induction and progression of arthritis (PMID 18982072), and the absence of Flt3L signalling was reported to protect against collagen-induced arthritis (PMID 24064002). In human samples, the pilot biomarker study reported that Flt3L was not up-regulated before the onset of rheumatoid arthritis (PMID 24495510).

Progenitor and transplantation biology

A 2018 stem cell study reported that Flt3L contributed to the development and function of the CD8α+ plasmacytoid precursor dendritic cell subpopulation within CD8+/TCR− facilitating cells (PMID 30004616), linking the ligand to cell populations studied in the context of hematopoietic transplantation.

Negative and null findings

Not every model showed benefit. In experimental rapidly progressive glomerulonephritis, the study reported that Flt3L treatment did not ameliorate disease (PMID 25849330). Reporting the null results alongside the positive ones is part of an accurate reading of this literature.

Reported Effects by Model at a Glance

Tolerability and Adverse Events: What Studies Report

The verified studies listed here were designed around immune-cell and disease-model endpoints rather than formal safety evaluation, and their abstracts do not establish a human tolerability profile. What can be said from them is directional: because Flt3L expands dendritic cell populations, contexts in which those cells amplify pathology have shown worse outcomes, as when intra-articular Flt3L expression was reported to drive induction and progression of arthritis (PMID 18982072), and some models have shown no benefit at all (PMID 25849330). Readers should treat this entry as a definitional reference rather than a summary of clinical safety data.

References

• FMS-like tyrosine kinase 3 ligand treatment does not ameliorate experimental rapidly progressive glomerulonephritis (PloS One, 2015)
• Fms-Like Tyrosine Kinase-3 Ligand Attenuates Local and Systemic Infection in a Model of Post-Burn Pneumonia (Shock, 2018)
• Aryl Hydrocarbon Receptor Ligands Indoxyl 3-sulfate and Indole-3-carbinol Inhibit FMS-like Tyrosine Kinase 3 Ligand-induced Bone Marrow-derived plasmacytoid Dendritic Cell Differentiation (Immune Network, 2018)
• Fms-Like Tyrosine Kinase 3-Ligand Contributes to the Development and Function of the Subpopulation of CD8α+ Plasmacytoid Precursor Dendritic Cells in CD8+/TCR− Facilitating Cells (Stem Cells, 2018)
• The FMS-like tyrosine kinase-3 ligand/lung dendritic cell axis contributes to regulation of pulmonary fibrosis (Thorax, 2019)
• Survivin but not Fms-like tyrosine kinase 3 ligand is up-regulated before the onset of rheumatoid arthritis: a pilot study (Arthritis Research & Therapy, 2014)
• Fms-like tyrosine kinase 3 ligand regulates migratory pattern and antigen uptake of lung dendritic cell subsets in a murine model of allergic airway inflammation (Journal of Immunology, 2009)
• Fms-like tyrosine kinase-3 ligand increases resistance to burn wound infection through effects on plasmacytoid dendritic cells (BMC Immunology, 2017)
• Fms-Like Tyrosine Kinase 3-Independent Dendritic Cells Are Major Mediators of Th2 Immune Responses in Allergen-Induced Asthmatic Mice (International Journal of Molecular Sciences, 2020)
• A novel therapeutic vaccine composed of a rearranged human papillomavirus type 16 E6/E7 fusion protein and Fms-like tyrosine kinase-3 ligand induces CD8+ T cell responses and antitumor effect (Vaccine, 2017)
• Absence of Fms-like tyrosine kinase 3 ligand (Flt3L) signalling protects against collagen-induced arthritis (Annals of the Rheumatic Diseases, 2015)
• Intra-articular fms-like tyrosine kinase 3 ligand expression is a driving force in induction and progression of arthritis (PloS One, 2008)