Inflammation Resolution in Tissue Regeneration

Key Takeaways

Resolution is an active biological program that limits further leukocyte recruitment, clears inflammatory cells and debris, and helps restore tissue homeostasis. [1] [2]
Macrophage clearance of dying cells, called efferocytosis, both removes potentially inflammatory material and changes macrophage signaling and metabolism. [3] [4]
Regeneration depends on timing: an early inflammatory response can be necessary, whereas delayed or incomplete resolution can extend injury and favor fibrosis. [5] [6]
Resolving signals can also act on progenitor, epithelial, vascular, and stromal cells, linking immune shutdown to tissue rebuilding rather than treating them as separate events. [5] [7]
Most direct mechanistic evidence comes from animal injury models; whether a pathway improves regeneration depends on the tissue, injury, species, and timing. [8] [9]

Acute inflammation is one of the first responses to tissue injury. It recruits cells that contain threats, remove damaged material, and signal to local tissue cells. Successful recovery also requires a coordinated resolution phase that prevents those same defensive processes from continuing after their useful window and helps the tissue move toward repair or regeneration. [1] [5]

Who This Is Useful For

This page is useful for readers who want to understand why inflammation is neither simply beneficial nor simply harmful during regeneration. It focuses on how inflammatory responses end, how resolution communicates with rebuilding tissue, and why evidence from one injury model cannot automatically be generalized to another.

Resolution Is Not Passive Disappearance

Resolution was once described mainly as the fading of inflammatory stimuli. Experimental work instead identifies regulated processes that restrict additional neutrophil entry, promote neutrophil death or departure, clear apoptotic cells, reprogram macrophages, and restore vascular and tissue barriers. Endogenous lipid and protein signals participate in these changes. [1] [2]

This distinction matters because suppressing an inflammatory pathway and completing resolution are not equivalent. Resolution retains functions such as phagocytosis, antimicrobial defense, and tissue repair while changing the duration and composition of the response. The process is therefore better understood as an organized transition than as a universal off-switch. [1] [10]

A Coordinated Sequence After Injury

Process	Resolution Function	Connection to Regeneration	Possible Failure
Leukocyte control	Limits further neutrophil recruitment and removes cells already present	Reduces continuing protease, oxidant, and cytokine exposure	Persistent leukocytes can extend collateral injury
Efferocytosis	Phagocytes engulf apoptotic cells before secondary necrosis	Reprograms macrophage metabolism and pro-resolving signaling	Uncleared cells can release intracellular inflammatory material
Macrophage transition	Changes inflammatory, clearance, and remodeling functions over time	Coordinates progenitor behavior, vascular growth, and matrix turnover	Mistimed states can impair regrowth or sustain fibrosis
Barrier and matrix restoration	Reduces vascular leakage and remodels provisional matrix	Re-establishes organized tissue structure and function	Incomplete repair can leave a fragile barrier or excessive scar

These processes overlap and differ among organs; they are recurring functions rather than a fixed set of stages. [2] [5]

Efferocytosis Links Clearance to Repair

Apoptotic cells preserve membrane integrity for a limited period and display signals that attract and identify them to phagocytes. Their timely engulfment prevents secondary necrosis and the release of intracellular contents that can prolong inflammation. Efferocytosis is performed mainly by macrophages in injured tissues, although other professional and non-professional phagocytes can contribute. [3]

Engulfment is also an information-rich event. Lipids, amino acids, nucleotides, and cholesterol derived from apoptotic-cell cargo alter macrophage metabolism and signaling, supporting further clearance and production of resolving mediators. Mouse experiments have shown that metabolism of apoptotic-cell arginine can enable continual efferocytosis, illustrating how debris clearance can sustain its own completion. [4] [11]

Macrophages Change With the Repair Environment

Macrophages do not move through one universal inflammatory-to-repair sequence. Resident and recruited populations respond to dying cells, cytokines, metabolites, oxygen, extracellular matrix, and signals from regenerating tissue. These inputs produce overlapping states whose functions can change during the course of injury. [2] [5]

Skeletal muscle provides causal evidence for the importance of this transition. In mice, inflammatory monocytes recruited after muscle injury developed into macrophages with later anti-inflammatory functions; early macrophages supported myogenic-cell proliferation in culture, whereas later cells supported differentiation and fusion. Disrupting the recruited population impaired regeneration. [6]

A separate mouse muscle study identified annexin A1 signaling through FPR2/ALX and AMPK as one pathway that promoted a repair-associated macrophage state. Loss of annexin A1 in all cells or in myeloid cells delayed inflammatory resolution and muscle-fiber regeneration in that model. [8]

Specialized Pro-Resolving Mediators

Lipoxins, resolvins, protectins, and maresins are families of locally produced lipid signals collectively described as specialized pro-resolving mediators. Across experimental systems, members of these families can limit further neutrophil recruitment, support macrophage phagocytosis and efferocytosis, and regulate barrier restoration. Individual mediators have distinct receptors and actions, so the label does not imply that all members are interchangeable. [10] [12]

Some resolving signals also act directly on tissue cells. In a mouse colonic biopsy-wound model, resolvin E1 was produced after mucosal injury and increased epithelial migration and proliferation; local nanoparticle delivery also accelerated wound closure. This is evidence for a pro-repair action in one preclinical setting, not evidence of general tissue regeneration in humans. [7]

Neutrophil Removal and Departure

Neutrophils can be removed by apoptosis followed by macrophage efferocytosis, and in some tissues they can leave an injury site through reverse migration. Which route dominates depends on the model and tissue. Both routes reduce the local burden of cells capable of releasing proteases, oxidants, and inflammatory signals. [1] [9]

Regenerating zebrafish spinal cord offers an example of reverse migration. Live imaging showed that most recruited neutrophils departed the injury, while experimental inhibition of CXCR4 increased neutrophil resolution and improved cellular and functional regeneration. The finding identifies a mechanism in zebrafish and does not establish that manipulating the same pathway would improve mammalian spinal-cord repair. [9]

When Resolution Fails

Non-resolving inflammation can arise from continued injury, infection, defective apoptotic-cell clearance, persistent leukocyte recruitment, or failure of macrophages and stromal cells to change state. The consequences may include additional cell death, prolonged barrier disruption, and continued fibroblast activation. [1] [3] [5]

Fibrosis is not simply the presence of inflammation: provisional matrix deposition can stabilize an injury. Pathology develops when inflammatory and remodeling signals remain active, matrix accumulates excessively, and normal architecture is not restored. Whether an injury resolves through regeneration, adaptive scar formation, or progressive fibrosis depends on tissue capacity and injury context as well as immune timing. [5]

Ageing and Resolution Capacity

Ageing can alter leukocyte recruitment, macrophage responses, efferocytosis, mediator production, and the tissue environments that instruct immune cells. These changes occur alongside reduced progenitor function, vascular changes, altered extracellular matrix, and greater disease burden, making it difficult to assign delayed repair to one resolution defect. [3] [5]

In mouse skeletal muscle, loss of macrophage SREBP1 delayed resolution and regeneration by altering macrophage mitochondrial function, lipid composition, and accumulation at the injury. This supports a metabolic contribution to resolving macrophage function, but it does not establish a general mechanism of human age-related regenerative decline. [13]

Evidence Quality and Interpretation

Confidence is strong that resolution is actively regulated and that efferocytosis and changing macrophage functions contribute to repair. This conclusion is supported by imaging, cell-depletion, genetic, biochemical, and pharmacological experiments across multiple animal injury models. [3] [6] [8] [11]

Confidence is lower about which resolving pathway is decisive in a given human tissue. Mouse muscle, mouse intestine, zebrafish spinal cord, and chronic human disease differ in immune composition, regenerative capacity, injury scale, and experimental accessibility. Biomarker changes or cell states observed in a biopsy can support a mechanism without proving that they caused recovery. [5] [7] [9]

Specialized pro-resolving mediators have extensive mechanistic and preclinical literature, but translation requires stable measurement, appropriate timing, receptor specificity, and clinically meaningful outcomes. Evidence that a mediator accelerates resolution in an experimental model should not be read as evidence for a broadly effective regenerative intervention. [1] [10]

What This Does Not Mean

It does not mean inflammation should be eliminated; early inflammatory functions can be required for debris clearance, defense, and regenerative signaling. [5] [6]
It does not mean resolution is a single pathway or a fixed macrophage subtype. [1] [2]
It does not mean complete regeneration follows whenever inflammation resolves; intrinsic tissue architecture and regenerative capacity still constrain the outcome. [5]
It does not mean fibrosis is always evidence of failed healing; scar tissue can provide essential mechanical stability after severe injury. [5]
It does not mean preclinical manipulation of a resolving pathway establishes safety or benefit in people. [7] [9]

Summary

Inflammation resolution connects injury control to tissue rebuilding. It limits further inflammatory recruitment, removes dying cells, changes macrophage functions, and coordinates signals affecting progenitors, barriers, blood vessels, fibroblasts, and extracellular matrix. Resolution is therefore not a passive end point or the opposite of immunity. It is a tissue-specific transition whose timing helps determine whether an injury returns toward homeostasis, regenerates functional structure, or proceeds toward persistent inflammation and fibrosis. [1] [2] [5]

References

Fullerton, J. N., Gilroy, D. W. "Resolution of inflammation: a new therapeutic frontier." Nature Reviews Drug Discovery (2016). https://pubmed.ncbi.nlm.nih.gov/27020098/
Murray, P. J., Wynn, T. A. "Protective and pathogenic functions of macrophage subsets." Nature Reviews Immunology (2011). https://pubmed.ncbi.nlm.nih.gov/21350564/
Doran, A. C., Yurdagul, A. Jr., Tabas, I. "Efferocytosis in health and disease." Nature Reviews Immunology (2020). https://pubmed.ncbi.nlm.nih.gov/31822793/
Schilperoort, M. et al. "The role of efferocytosis-fueled macrophage metabolism in the resolution of inflammation." Immunological Reviews (2023). https://pubmed.ncbi.nlm.nih.gov/37158427/
Eming, S. A., Wynn, T. A., Martin, P. "Inflammation and metabolism in tissue repair and regeneration." Science (2017). https://pubmed.ncbi.nlm.nih.gov/28596335/
Arnold, L. et al. "Inflammatory monocytes recruited after skeletal muscle injury switch into antiinflammatory macrophages to support myogenesis." Journal of Experimental Medicine (2007). https://pubmed.ncbi.nlm.nih.gov/17485518/
Quiros, M. et al. "Resolvin E1 is a pro-repair molecule that promotes intestinal epithelial wound healing." Proceedings of the National Academy of Sciences (2020). https://pubmed.ncbi.nlm.nih.gov/32300016/
McArthur, S. et al. "Annexin A1 drives macrophage skewing to accelerate muscle regeneration through AMPK activation." Journal of Clinical Investigation (2020). https://pubmed.ncbi.nlm.nih.gov/32015229/
Tsarouchas, T. M. et al. "Neutrophil immune profile guides spinal cord regeneration in zebrafish." Brain, Behavior, and Immunity (2024). https://pubmed.ncbi.nlm.nih.gov/38925414/
Serhan, C. N., Levy, B. D. "Resolvins in inflammation: emergence of the pro-resolving superfamily of mediators." Journal of Clinical Investigation (2018). https://pubmed.ncbi.nlm.nih.gov/29757195/
Yurdagul, A. Jr. et al. "Macrophage metabolism of apoptotic cell-derived arginine promotes continual efferocytosis and resolution of injury." Cell Metabolism (2020). https://pubmed.ncbi.nlm.nih.gov/32004476/
Bannenberg, G., Serhan, C. N. "Specialized pro-resolving lipid mediators in the inflammatory response: an update." Biochimica et Biophysica Acta (2010). https://pubmed.ncbi.nlm.nih.gov/20708099/
Oishi, Y. et al. "Macrophage SREBP1 regulates skeletal muscle regeneration." Frontiers in Immunology (2024). https://pubmed.ncbi.nlm.nih.gov/38259495/

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This content is provided for educational purposes only and does not constitute medical advice.