TNF-α
Summary
TNF-α (Tumor Necrosis Factor-alpha) is a pro-inflammatory cytokine produced mainly by activated macrophages, playing a key role in systemic inflammation, immune regulation, and apoptosis. It is central to septic shock pathophysiology and autoimmune disease pathogenesis, particularly rheumatoid arthritis, psoriasis, and inflammatory bowel disease. It is also a major target of biologic therapies (e.g., infliximab, etanercept, adalimumab).
Detail
TNF-α is a cytokine secreted primarily by macrophages (and also by T cells, NK cells) in response to bacterial products like LPS (via TLR4 signaling), as well as other inflammatory stimuli. It acts on TNF receptors (TNFR1/TNFR2) present on nearly all nucleated cells.
Physiologic/Pathologic Roles: - Mediates fever by stimulating hypothalamic prostaglandin production (acts with IL-1 as an endogenous pyrogen). - Activates vascular endothelium, increasing expression of adhesion molecules (promotes neutrophil recruitment) and increasing vascular permeability, contributing to septic shock (hypotension, DIC, multiorgan failure). - Stimulates production of other pro-inflammatory cytokines (IL-1, IL-6) and acute phase reactants. - Induces cachexia in chronic disease/cancer (hence historically called "cachectin") by suppressing appetite and promoting lipolysis/proteolysis. - Promotes apoptosis via death receptor pathway (TNFR1-associated FADD/caspase-8 activation). - Central mediator in granuloma formation (important in TB reactivation risk with anti-TNF therapy). - Key driver of chronic inflammatory/autoimmune diseases: rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, inflammatory bowel disease (Crohn's disease, ulcerative colitis).
Clinical Relevance: - Anti-TNF biologics (infliximab, adalimumab, etanercept, certolizumab, golimumab) are used to treat these autoimmune conditions by blocking TNF-α activity. - Side effects of anti-TNF therapy include reactivation of latent TB (due to granuloma breakdown) and increased risk of infections, so patients require PPD/IGRA screening before starting. - Elevated TNF-α is implicated in the pathogenesis of septic shock alongside IL-1 and IL-6. - Both TNF-α and IL-1 share overlapping actions: fever, endothelial activation, acute phase response.
High-Yield Associations: - Septic shock cascade: LPS → macrophage activation → TNF-α/IL-1 → systemic inflammation, DIC, hypotension. - "Cachectin" nickname reflects role in cancer/chronic disease-associated wasting. - Screen for latent TB before starting anti-TNF agents.
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- Kaplan USMLE Step 1 Immunology Lecture Notes
- Janeway's Immunobiology
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