IL-6

**Source/Stimulus:** Produced by macrophages (in response to TLR signaling by PAMPs such as LPS), T cells, endothelial cells, fibroblasts, and adipocytes. Also released by muscle during exercise (myokine).

**Signaling:** Binds IL-6 receptor (membrane-bound or soluble) plus gp130, activating JAK/STAT3 (also MAPK and PI3K pathways). Trans-signaling via soluble IL-6R is largely pro-inflammatory.

**Key functions (high-yield):** - **Fever:** Acts with IL-1 and TNF-α as an endogenous pyrogen. These raise PGE2 in the hypothalamic OVLT/preoptic area, resetting the thermoregulatory set point. - **Acute-phase response:** Induces hepatocytes to make CRP, fibrinogen, serum amyloid A, and hepcidin, and decreases albumin and transferrin (negative acute-phase reactants). Fibrinogen coats RBCs and causes rouleaux formation, which raises ESR. - **Hepcidin:** Hepcidin blocks ferroportin, trapping iron in macrophages and enterocytes. This causes **anemia of chronic disease** (low serum iron, low TIBC, high ferritin). - **Adaptive immunity:** Promotes B-cell differentiation into plasma cells and antibody secretion, supports Th17 differentiation (with TGF-β), and promotes T follicular helper cell function. It also inhibits Treg differentiation. - **Hematopoiesis/bone:** Stimulates thrombopoietin (reactive thrombocytosis), and promotes osteoclast activation (contributes to bone loss in inflammation and multiple myeloma).

**Clinical associations:** - **Cytokine release syndrome (CRS)/CAR-T therapy:** IL-6 is the central mediator (fever, hypotension, hypoxia). Treat with **tocilizumab**. - **Sepsis/cytokine storm and severe COVID-19:** Elevated IL-6 correlates with severity. Tocilizumab is used in selected hospitalized patients. - **Rheumatoid arthritis:** Tocilizumab and sarilumab (anti-IL-6R) are used when TNF inhibitors fail or are not tolerated. - **Giant cell arteritis:** Tocilizumab is FDA-approved. - **Multiple myeloma:** IL-6 is a growth/survival factor for myeloma cells. - **Castleman disease:** Excess IL-6 (often HHV-8–associated) causes systemic symptoms; siltuximab (anti-IL-6) is used. - **Other conditions with elevated IL-6:** Systemic JIA, Still disease, Crohn disease, and chronic inflammation-related cachexia.

**Anti-IL-6 therapy adverse effects:** Increased risk of infections (including TB reactivation), neutropenia, elevated LFTs, dyslipidemia, and GI perforation (particularly with diverticulitis). Note that CRP is suppressed during therapy and can mask infection.

**Comparison for boards:** - **IL-1:** Fever, endothelial activation, and leukocyte recruitment ("osteoclast-activating factor"). Blocked by anakinra. - **TNF-α:** Septic shock, granuloma maintenance, and cachexia. Blocked by infliximab and etanercept. - **IL-6:** Acute-phase proteins and B-cell differentiation. Blocked by tocilizumab.

**Mnemonic:** IL-1 = "hot" (fever); IL-6 = "sticks" (acute-phase proteins, fibrinogen sticks RBCs → ↑ESR). IL-1, IL-6, and TNF-α are the three main pro-inflammatory cytokines of the acute-phase response.