BCG (Bacillus Calmette-Guérin)

Overview

BCG was developed by Albert Calmette and Camille Guérin at the Institut Pasteur in Paris over 13 years of serial passage (1908–1921), during which a virulent strain of Mycobacterium bovis was grown through more than 230 passages on a medium of potato bile glycerine — progressively selecting for attenuated variants that retained immunogenicity while losing virulence. The first human dose was administered on July 18, 1921, to a newborn in Paris whose mother had died of tuberculosis.

BCG is the most widely used vaccine in human history — approximately 100 million doses are given annually. It is a cornerstone of every national tuberculosis control program in endemic countries, covering ~157 WHO-reporting countries. The attenuation is stable and irreversible: the critical event was progressive loss of the RD1 genomic region (encoding the ESX-1 secretion system, responsible for phagolysosomal membrane lysis in virulent mycobacteria), which cannot revert across independent BCG sub-strains.

BCG's scientific relevance extends far beyond TB. It is the founding example of trained innate immunity — a recently characterized epigenetic phenomenon whereby BCG vaccination reprograms monocytes and NK cells (and possibly myeloid progenitors in bone marrow) to mount enhanced responses to heterologous pathogens for months to years. This mechanism explains BCG's ~38% reduction in all-cause neonatal mortality, far exceeding what TB prevention alone could account for at this age.

Platform & Antigen Design

Live Attenuated Mycobacterium bovis

  Intradermal injection (left upper arm)
        │
        ▼
  BCG taken up by macrophages + dendritic cells at injection site
        │  RD1 deletion → cannot lyse phagosome → survives intracellularly
        │  Replicates in phagosome; prevents acidification
        │
        ├─ INNATE ACTIVATION
        │    TLR2 (lipoproteins) + TLR4 (LPS-like) + TLR9 (mycobacterial DNA)
        │    NOD2 (muramyl dipeptide) + NLRP3 inflammasome (LAM)
        │    → IL-12, IL-18, TNF-α, IFN-β production
        │
        ├─ ADAPTIVE RESPONSE
        │    MHC II presentation → CD4⁺ Th1 priming
        │         IL-12 → Th1 → IFN-γ + TNF-α → macrophage activation
        │         Granuloma formation (histological scar)
        │    MHC I cross-presentation → CD8⁺ CTL priming
        │    Anti-mycobacterial IgG (secondary arm)
        │
        └─ TRAINED IMMUNITY (epigenetic, non-specific)
             NOD2/dectin-1 → PI3K/Akt → H3K4me3 at cytokine promoters
             Monocytes/NK cells: enhanced IL-6, TNF, IL-12 on re-challenge
             Duration: months to years; non-antigen-specific

1. Intracellular niche: BCG replicates inside macrophage phagosomes, preventing phagolysosomal fusion — the same evasion mechanism as virulent M. tuberculosis — enabling sustained antigen presentation.
2. Th1 polarization: IL-12 from infected macrophages and DCs drives naive CD4&sup+ cells to a Th1 phenotype; IFN-γ and TNF-α are the critical effector cytokines for macrophage activation and mycobacterial killing.
3. Granuloma and scar: Local granuloma forms 2–12 weeks post-injection (papule → pustule → ulcer → scar) — the histological footprint of Th1 immunity, confirming successful vaccination.
4. Trained immunity: Epigenetic H3K4me3 marks accumulate at promoters of proinflammatory genes in monocytes and NK cells, conferring heterologous protection against diverse future pathogens.
5. No exogenous adjuvant: The live organism's own PAMPs (lipoarabinomannan, peptidoglycan, mycobacterial DNA) activate TLR2/4/9, NOD2, and NLRP3 — self-adjuvanting with no added components needed.

Immunogenicity

Clinical Efficacy

Safety Profile

• Expected Local reaction sequence — predictable papule (wk 1–2) → pustule/ulceration (wk 2–4) → scar (2–10 mm, permanent). This scar is the clinical marker of successful vaccination. Hypertrophic scarring or keloid in ~1–3% (more common in darker-skinned individuals).
• Uncommon Regional lymphadenopathy (BCG-itis) — ipsilateral axillary/supraclavicular lymphadenopathy in ~1–2%; usually resolves spontaneously within 2–6 months. Suppurative lymphadenitis (abscess formation) ~0.1–0.4%; may require aspiration or isoniazid therapy.
• Rare / Serious Disseminated BCG disease — occurs in children with SCID, IFN-γ receptor deficiency, IL-12/IL-12R pathway defects, or advanced HIV. BCG replicates uncontrolled when cell-mediated immunity is absent. Fatal without multi-drug anti-mycobacterial therapy. Incidence: 0.06–1.56 per 100,000 vaccinees.
• Very Rare BCG osteitis — rare bone infection, predominantly reported with some BCG strains (BCG-Japan 172 historically). More common than disseminated disease; usually mono-articular; responds to anti-TB therapy.
• Context No association with autoimmunity, pulmonary TB, or disseminated disease in immunocompetent individuals. The attenuated organism cannot cause pulmonary TB.

Administration

Connections

References

• Colditz GA, Brewer TF, Berkey CS, et al. Efficacy of BCG Vaccine in the Prevention of Tuberculosis: Meta-analysis of the Published Literature. JAMA. 1994;271(9):698-702. doi:10.1001/jama.271.9.698 · PMID 8309034
• Moorlag SJCFM, Arts RJW, van Crevel R, Netea MG. Non-specific effects of BCG vaccine on viral infections. Cell Host Microbe. 2020;27(6):931-941. doi:10.1016/j.chom.2020.05.013 · PMID 32497527
• Calmette A, Guérin C. Nouvelles recherches expérimentales sur la vaccination des bovins contre la tuberculose. Ann Inst Pasteur. 1920;34:553-560. Historical — first development record of BCG at Institut Pasteur; first human dose administered 1921.