Chronic Inflammation: Causes, Cells, Macrophage Activation, Granulomas, Fibrosis and Clinical Features

Learning outcomes

• Define chronic inflammation and distinguish it from acute and acute-on-chronic inflammation.
• List major causes and explain why the stimulus persists.
• Describe macrophage, lymphocyte, plasma-cell, eosinophil and mast-cell roles.
• Explain M1/M2 macrophage activation, cytokine loops and fibrosis.
• Recognise granulomatous inflammation and important causes.
• Relate chronic inflammation to systemic disease, cancer, organ failure and emergency flares.
• Outline diagnostic and management principles.

1. Hallmarks

Hallmark	What is seen?
Mononuclear infiltrate	Macrophages, lymphocytes and plasma cells; variable eosinophils and mast cells.
Ongoing tissue destruction	Necrosis, epithelial injury, parenchymal loss or vascular damage from the cause and inflammatory cells.
Repair and fibrosis	Angiogenesis, fibroblast activation, collagen deposition and remodelling.
Persistent mediator network	TNF, IL-1, IFN-γ, IL-17, TGF-β, chemokines and growth factors.

2. Causes

Cause	Examples	Why it persists
Persistent infections	Tuberculosis, fungi, parasites, chronic viral hepatitis.	Organism resists eradication or hides intracellularly.
Autoimmunity	Rheumatoid arthritis, SLE, inflammatory bowel disease.	Self-antigen and memory immune responses continue.
Prolonged toxin exposure	Silica, tobacco smoke, asbestos, drugs.	Continued exposure or retained particles.
Foreign bodies	Sutures, splinters, implants, crystals.	Material cannot be degraded or removed.
Metabolic stress	Obesity, atherosclerosis, urate and cholesterol crystals.	Lipotoxicity, inflammasomes and repeated tissue stress.
Unresolved acute inflammation	Chronic abscess, obstructed organ, persistent necrotic tissue.	Source control is incomplete.

3. Macrophages: the central chronic cell

Monocytes enter tissue and differentiate into macrophages under the influence of chemokines and growth factors. Resident macrophages also expand locally. They ingest microbes and debris, present antigen and direct repair or fibrosis.

3.1 Classical (M1-like) activation

IFN-γ, microbial products and TNF produce an inflammatory phenotype. M1-like macrophages release ROS, NO, IL-1, TNF, IL-6, IL-12 and proteases. They are effective at microbial killing but can destroy host tissue.

3.2 Alternative (M2-like) activation

IL-4, IL-13 and other signals produce a repair/fibrotic phenotype. M2-like macrophages release TGF-β, VEGF, growth factors and matrix-remodelling enzymes. They support angiogenesis and wound healing but can cause excessive fibrosis and tumour support.

3.3 Macrophage self-amplification

Macrophages release chemokines that recruit monocytes and cytokines that activate lymphocytes. Lymphocytes release IFN-γ, IL-17 and other signals that further activate macrophages. This positive feedback loop sustains chronic inflammation after the initial trigger becomes difficult to detect.

4. Lymphocytes and other cells

Cell	Function in chronic inflammation
T lymphocytes	CD4 subsets activate macrophages or recruit neutrophils; CD8 cells kill target cells.
B cells/plasma cells	Produce antibodies; organise tertiary lymphoid structures; may drive immune complexes.
Eosinophils	Helminth defence and allergy; granule proteins injure tissue.
Mast cells	Release histamine, proteases and cytokines; participate in allergy and fibrosis.
Neutrophils	Persist in chronic infection, smoking-related disease and acute-on-chronic flares.
Fibroblasts/endothelium	Produce matrix, angiogenesis and remodelling signals.

5. Granulomatous inflammation

Granulomas are organised collections of epithelioid macrophages, often surrounded by lymphocytes and giant cells. They form when a stimulus is difficult to eradicate or when T-cell responses strongly activate macrophages.

Type	Examples	Diagnostic approach
Caseating infectious	Tuberculosis, histoplasmosis and other deep fungi.	AFB/fungal stains, culture, PCR and exposure history.
Non-caseating immune	Sarcoidosis, Crohn disease, beryllium.	Exclude infection and foreign body; assess organ pattern.
Foreign-body	Suture, talc, splinter, keratin or crystals.	Polarised light and search for material.

Granulomas can cause fibrosis, obstruction, vascular compromise and organ dysfunction. A biopsy pattern is not a complete diagnosis without microbiology and clinical correlation.

6. Tissue destruction and repair

• Macrophage proteases and ROS destroy extracellular matrix and parenchymal cells.
• TNF and IL-1 activate endothelium and sustain leukocyte recruitment.
• VEGF promotes angiogenesis, while TGF-β activates fibroblasts and collagen deposition.
• Matrix metalloproteinases degrade tissue; tissue inhibitors control remodelling.
• Repeated injury and repair produce architectural distortion, strictures and loss of specialised function.

7. Fibrosis and organ consequences

Organ/condition	Chronic injury and consequence
Liver	Chronic hepatitis/steatohepatitis → bridging fibrosis, cirrhosis, portal hypertension.
Lung	Interstitial inflammation → collagen deposition, restrictive disease and hypoxia.
Kidney	Interstitial inflammation → nephron loss, fibrosis and chronic kidney disease.
Heart/vessels	Atherosclerotic inflammation → plaque rupture, infarction and stroke.
Gut	Crohn disease/ulcerative colitis → strictures, fistulae, bleeding and malabsorption.
Joints	Rheumatoid synovitis → pannus, cartilage destruction and deformity.

8. Systemic effects

• Persistent IL-6 and TNF cause anaemia of inflammation, thrombocytosis, fatigue and cachexia.
• Chronic inflammation increases cardiovascular risk through endothelial activation and plaque instability.
• Immune dysregulation produces fever, night sweats, weight loss and recurrent flares.
• Chronic infection can produce amyloidosis through sustained serum amyloid A.
• Inflammatory cytokines and oxidative stress promote DNA damage and cancer development.

9. Chronic inflammation and cancer

Persistent inflammatory signalling promotes angiogenesis, cell proliferation, DNA damage, matrix remodelling and immune evasion. Examples include H. pylori-associated gastric cancer, chronic hepatitis and hepatocellular carcinoma, inflammatory bowel disease and colorectal cancer, and chronic pancreatitis with pancreatic cancer risk.

10. Diagnosis

1. History: duration, infection exposure, autoimmune symptoms, occupation, smoking, drugs and foreign material.
2. Examination: organ-specific signs, lymphadenopathy, hepatosplenomegaly, joint changes, fibrosis and nutritional status.
3. Laboratory: FBC, CRP/ESR, renal/liver function, autoantibodies, cultures, viral tests and targeted metabolic markers.
4. Imaging: ultrasound, CT, MRI, endoscopy, echocardiography or pulmonary function tests for remodelling and fibrosis.
5. Biopsy: identifies cell type, granuloma, fibrosis, organisms, vasculitis and neoplasia.

11. Management principles

• Eradicate persistent infection with appropriate antimicrobials and source control.
• Remove foreign bodies and stop toxin exposure, including smoking.
• Use immunomodulation for proven autoimmune disease after excluding infection.
• Control metabolic drivers such as obesity, diabetes, dyslipidaemia and hyperuricaemia.
• Prevent and treat fibrosis complications; monitor organ function and cancer risk.
• Rehabilitate with nutrition, exercise, vaccination and functional support.

12. Emergency flares

• Sepsis on chronic liver, lung or kidney disease can cause rapid multi-organ failure.
• Acute asthma or COPD exacerbation adds neutrophilic/eosinophilic inflammation to chronically remodelled airways.
• Rheumatoid or inflammatory bowel flares may mimic infection; severe pain, bleeding or obstruction requires urgent review.
• Decompensated cirrhosis, pulmonary fibrosis and chronic kidney disease have low reserve during acute injury.

13. Quick self-test

1. What three features define chronic inflammation?
   Answer: Mononuclear infiltrate, ongoing tissue destruction and simultaneous repair/fibrosis.
2. Name three causes of chronic inflammation.
   Answer: Persistent infection, autoimmunity, toxin/foreign body, metabolic stress or unresolved acute inflammation.
3. What is the main role of M1-like macrophages?
   Answer: Microbial killing and inflammatory cytokine production.
4. What is the risk of M2-like activation?
   Answer: Excessive fibrosis, remodelling and tumour-supporting angiogenesis.
5. What is a granuloma?
   Answer: An organised collection of activated macrophages, usually with lymphocytes and giant cells.
6. Why must infection be excluded before immunosuppression?
   Answer: Suppression can allow an occult infection to disseminate and cause organ failure.
7. How does chronic inflammation promote cancer?
   Answer: Cytokines, ROS, proliferation, angiogenesis and matrix remodelling create a mutagenic, growth-supportive environment.

14. Take-home summary

Selected references

• NCBI Bookshelf: Chronic Inflammation
• NCBI Bookshelf: Inflammation and Repair
• NCBI Bookshelf: Sepsis and Septic Shock
• NCBI Bookshelf: Granulomatous Diseases

Educational note: This resource supports learning and clinical reasoning. Current local protocols, senior supervision and national guidelines take precedence in patient care.