Macrophage M1 to M2 polarisation
This page describes a pathway reported in the literature. It is not a claim about any product and does not recommend any procedure.
Mechanisms
What is the role of macrophage polarization in dermal remodeling?
When a particulate material is placed in the dermis, macrophages arrive as part of the foreign-body response. The literature describes a shift in their phenotype — from the inflammatory M1 state toward the M2 state associated with repair — and reports that this shift accompanies fibroblast activation and matrix deposition. Animal work has reported this sequence for both PDLLA and PLLA. The same axis does not always end in organised remodelling: review literature describes synthetic materials as tending toward chronic inflammation and fibrous encapsulation, in contrast to matrix-derived scaffolds.
Reported sequence
| Step | What the literature reports | Basis |
|---|---|---|
| Particle placed | A particulate material in the dermis is recognised as foreign and macrophages are recruited to it. This is the foreign-body response, not a side effect of it. | — |
| Phenotype shift | Macrophages are described as shifting from the inflammatory M1 state toward the M2 state associated with repair. In aged animal skin this shift was reported alongside PLLA administration. | PMID 37174720 |
| Fibroblast activation | Activated macrophages are reported to drive fibroblast activity, with TGF-beta identified as a mediator in the reported sequence. | PMID 37174720 |
| Matrix deposition | Extracellular matrix increases in the reported models. For PDLLA, one animal study attributes the increase to modulation of both macrophages and adipose-derived stem cells rather than macrophages alone. | PMID 37371934 |
Evidence that reads the other way
A review of matrix-mediated macrophage response describes synthetic materials as tending to induce a chronic inflammatory response and a fibrous capsule, while extracellular-matrix scaffolds are described as inducing a pro-regenerative immune environment. The M2 route is therefore not an automatic outcome of placing a particle: the same recruitment step can end in encapsulation rather than organised remodelling, and which way it goes is reported to depend on the material. Read PMID 36595269 alongside the animal findings above, not after them.
Materials where this is cited
Evidence
| PMID | Study type | Summary |
|---|---|---|
| 37174720 | in vivo | PLLA improved dermal collagen synthesis by modulating M2 macrophage polarisation in aged animal skin — Cells, 2023 |
| 37371934 | in vivo | PDLLA increased extracellular matrix by modulating macrophages and adipose-derived stem cells in aged animals — Antioxidants (Basel), 2023 |
| 36595269 | review | Review of macrophage response mediated by extracellular matrix, contrasting synthetic materials with matrix scaffolds — Biomed Mater, 2023 |
| 40580932 | review | Systematic review of PLLA in aesthetic dermatology, framing it as a biostimulatory scaffold rather than a volumiser — Aesthet Surg J, 2025 |
Why this pathway is cited so often
Almost every particulate biostimulator in this atlas is described the same way: the material does not supply collagen, it provokes a response that ends in collagen. Macrophage polarisation is the part of that response that has been given a name. It is the step where the account moves from “something foreign is here” to “tissue is being rebuilt”.
What the animal work actually shows
Two studies cited here place the material in aged animal skin and report both the phenotype shift and the matrix increase. That is stronger than an in vitro observation and weaker than a human histology series. The PDLLA study is worth reading carefully because it does not attribute the effect to macrophages alone — adipose-derived stem cells appear in the reported mechanism as well, which means “PDLLA works through M2” is a compression of what was measured.
The label is a spectrum, not a switch
M1 and M2 name the ends of a range of macrophage behaviour. Studies assign them with different marker panels, so two papers reporting “M2 polarisation” have not necessarily measured the same thing. When a claim about polarisation is carried from one material to another, the marker panel is the first thing to check.
Limitations & open questions
The polarisation findings cited here come from aged animal models, not from human dermis. M1 and M2 are useful labels for ends of a spectrum rather than two discrete cell populations, and the markers used to assign them differ between studies. The step from a phenotype shift to a clinical result is not measured in these studies — they report an association within the model, and no citation here demonstrates that modifying polarisation changes a clinical outcome.