Bacterial inflammation: The key role of interleukin-6
Recognition of a bacterial pathogen begins with the interaction of macrophages with components of the bacterial wall. In gram-negative bacteria, this is lipopolysaccharide (LPS, endotoxin); in gram-positive bacteria, it is peptidoglycan. Following phagocytosis and antigen presentation, a cytokine cascade is triggered, the key cytokine in bacterial infection being interleukin-6 (IL-6).
IL-6 binds to its receptor on hepatocytes. This activates JAK kinase, which phosphorylates the transcription factor STAT3. Phosphorylated STAT3 translocates to the nucleus and induces the expression of the CRP gene encoding C-reactive protein.
Functions of C-reactive protein
C-reactive protein (CRP) is an acute-phase hepatic protein that implements several defense mechanisms:
- Bacterial opsonization: CRP binds to phosphocholine in the bacterial membrane, marking the pathogen for phagocytosis.
- Enhanced macrophage response: Facilitates recognition and engulfment of opsonized bacteria, activates phospholipase determinants and apoptotic pathways.
- Complement activation: triggers the classical pathway via C4–C2–C3–C5 with the formation of the membrane attack complex (MAC), which perforates the bacterial cell wall.
- Reduction of NO production: Decreases nitric oxide at the site of inflammation, limiting pathological vasodilation and worsening conditions for bacterial proliferation.
- LDL binding: Promotes their hepatic uptake; therefore, with persistently high CRP levels, LDL levels may decrease.
Procalcitonin as a marker of bacterial infection
Concurrently, STAT3 induces the CALC-1 gene, which is normally expressed in parafollicular cells of the thyroid and parathyroid glands. The gene product is procalcitonin.
Normally, procalcitonin is cleaved by the enzyme peptidyl-glycine alpha-amidating monooxygenase / convertase 1/3 into calcitonin and two other peptides. This enzyme is present exclusively in endocrine glands. In systemic bacterial infection, CALC-1 is activated in the lungs, liver, kidneys, and other tissues lacking the convertase, causing procalcitonin to accumulate in the blood without conversion into calcitonin. This is precisely what makes procalcitonin a specific marker of bacterial infection and sepsis.
Viral inflammation: Why CRP and procalcitonin remain low
The antiviral response is mediated predominantly by interferons, specifically interferon-gamma and type I interferons. They also activate JAK kinases, but phosphorylate a different transcription factor — STAT1.
STAT1 exerts an opposite effect relative to STAT3: it suppresses the expression of the CRP and CALC-1 genes. Therefore, in viral infection, CRP and procalcitonin levels typically remain low or moderately elevated. This fundamental difference underlies the laboratory differentiation between viral and bacterial inflammation.
Clinical criteria for prescribing antibiotics
Laboratory markers (CRP, procalcitonin, leukocytosis) guide the physician but are not independent grounds for diagnosis. In the Russian Federation, the prescription of antibacterial therapy is regulated by clinical guidelines and the document AMMI 2.0 (Antimicrobial Stewardship Strategy, 2022).
Empiric antibiotic therapy is indicated when clinical and laboratory signs are combined:
- Leukocytosis with a neutrophil shift.
- Fever above 38.5 °C, especially with chills.
- Focal signs of bacterial infection: purulent discharge, local infiltrates, bacterial isolates.
- High procalcitonin level and a significant elevation in CRP.
- Conditions that preclude waiting for pathogen verification: sepsis, septic shock, severe pneumonia, life-threatening infections.
The AMMI guidelines are advisory; for specific nosologies, one should rely on disease-specific clinical guidelines (e.g., for community-acquired pneumonia), which are mandatory.
Common mistakes in prescribing antibiotics
- Prescribing for isolated fever without signs of a bacterial focus and with normal CRP/procalcitonin.
- Ignoring the viral etiology of the disease when clinical presentation is characteristic and acute-phase markers are low.
- Relying solely on leukocytosis without considering the differential white blood cell count and dynamics.
- Initiating antibiotic therapy without regard to AMMI recommendations and disease-specific clinical guidelines.
- Failing to reassess therapy after receiving microbiological test results.
Key points
- Bacterial inflammation is predominantly mediated by IL-6 → JAK → STAT3, whereas viral inflammation is mediated by interferons → STAT1.
- CRP opsonizes bacteria, activates complement and the MAC, enhances phagocytosis, and reduces NO production.
- Procalcitonin increases in bacterial infection because the enzyme required for its cleavage is absent outside of endocrine glands.
- During an antiviral response, STAT1 suppresses the CRP and CALC-1 genes — hence the low markers in viral infections.
- Laboratory markers are guides, not a diagnosis on their own.
- Indications for empiric antibiotic therapy: leukocytosis, fever >38.5 °C, purulent focus, sepsis.
- The AMMI 2.0 document is advisory; disease-specific clinical guidelines are mandatory.
- Persistently high CRP may be accompanied by decreased LDL levels due to hepatic uptake.