Dectin-1 Signalling and Trained Immunity Mechanisms
Dectin-1 (CLEC7A; C-type lectin receptor; pattern recognition receptor; expressed on: monocytes, macrophages, DCs, neutrophils, NK cells; recognises β-1,3-glucan (cell wall of fungi, yeast, some bacteria; major Dectin-1 ligand; particulate > soluble); structure: single TM; short cytoplasmic tail with hemITAM (hemi-ITAM; single pY residue; Tyr238 phosphorylation by Syk); signalling: ligand binding → Dectin-1 clustering → Src kinase (Fgr/Hck/Lyn) → Tyr238 phosphorylation → Syk (spleen tyrosine kinase; ITAM-mode; Syk SH2 domain → PLCgamma2 (DAG/IP3) → PKCδ → CARD9/BCL10/MALT1 (CBM complex; CARD9 is myeloid-specific; CARD9 → TAK1/TAB1/2 → IKKβ → NF-κB; also CARD9 → p38 MAPK → MKK3/6 → p38); Syk also → NLRP3 priming (first signal: NF-κB → pro-IL-1β/NLRP3 transcription; second signal: DAG/K+ efflux/mtROS → NLRP3 assembly → IL-1β/IL-18); mTOR (Dectin-1 → Akt → mTORC1 → HIF-1α → Warburg shift in trained macrophages)); Dectin-2 (CLEC6A; recognises α-mannans; Syk via FcRγ; downstream similar to Dectin-1); Mincle (CLEC4E; trehalose dimycolate/SAP130; FcRγ-Syk-CARD9); trained immunity (a.k.a. innate immune memory; BCG/β-glucan exposure → monocyte/macrophage epigenetic reprogramming → H3K4me3 (active; at TNF/IL-6/IL-12 promoters) + H3K27ac (enhancer) by KMT2A/MLL/SETD7; mediated by: mTORC1-HIF-1α (Warburg glycolysis → acetyl-CoA → H3K27ac by ACLY/CBP/p300); fumarate accumulation (SDH inhibition in trained monocytes → TET enzyme inhibition → DNA methylation ↓ at trained loci); JMJD3 → H3K27me3 demethylation); trained immunity lasts months (monocyte lifespan; ∼1–3 days, but progenitors in BM also trained by β-glucan → epigenetic in HSC → myeloid progenitors → functional training maintained); contrast: tolerance (LPS repeated exposure → H3K27me3 (repression) → hyporesponsiveness vs training → H3K4me3 → hyperresponsiveness).
Spirulina Mechanisms in Dectin-1 and Trained Immunity
Spirulina Polysaccharide Dectin-1 Engagement
Spirulina polysaccharides (primary immune-active fractions: calcium spirulan (Ca-SP; sulphated polysaccharide; L-rhamnose + 3-O-methylrhamnose backbone; ~70 kDa; antiviral/anticoagulant; HSV-1/HIV-1 inhibition); spirulan (SPPS; similar to Ca-SP); immulan (high molecular weight; ~50 kDa; spiro-polysaccharide); β-glucan-like structures in spirulina cell wall (spirulina lacks classic fungal β-glucan; but contains β-1,3-glucan-like linkages in extracellular polysaccharide; Dectin-1 recognition of spirulina at IC50 ~50–200 μg/mL in THP-1 monocyte models); TLR2 and TLR4 (additional PRRs recognising spirulina polysaccharides; TLR2 + Dectin-1 co-signalling amplifies NF-κB via MyD88-TRIF; “outside-in” signalling)); spirulina polysaccharides at 10g/day intake → intestinal macrophage/DC Dectin-1 engagement → Syk Tyr238 → CARD9/NF-κB → IL-6/IL-12p70/TNF-α secretion +15–30%; splenic NK cell activation (Dectin-1 on NK → IFN-γ +10–20%); importantly: spirulina polysaccharides are at physiological sub-maximal concentrations → activation without pathological NLRP3 over-amplification.
Syk/CARD9/NF-κB Innate Activation
Syk (spleen tyrosine kinase; 72 kDa; tandem SH2 domains (N-SH2 + C-SH2) bind diphospho-ITAM or hemITAM; dual Tyr315/Tyr319 + Tyr525/Tyr526 phosphorylation for full activation; Syk downstream effectors: PLCgamma2 (IP3 → Ca2+/ER → calcineurin-NFAT; DAG → PKCδ/θ); PKCdelta → CARD9 (caspase recruitment domain 9; CARD9-BCL10-MALT1 signalling complex; CBM; CARD9 filament polymerisation via CARD–CARD interactions → BCL10 activation → MALT1 (paracaspase; cleaves A20/CYLD → NF-κB disinhibition) → IKKβ → NF-κB; CARD9 also → MAPK: MKK4/7 → JNK; MKK3/6 → p38); PI3K (Syk → PI3K p85/p110 → PIP3 → Akt → mTORC1 → HIF-1α (trained immunity metabolic switch))); spirulina CARD9 pathway: spirulina polysaccharide-Dectin-1 → Syk → CARD9 → NF-κB → cytokine production; phycocyanin paradox: PC suppresses NF-κB (−30–45% in inflammatory contexts); but spirulina polysaccharide-mediated CARD9 NF-κB activation is “physiological first-signal” PRR-driven (moderate); resolution: net spirulina effect: moderate innate priming (CARD9 +moderate) balanced by phycocyanin anti-inflammatory (chronic inflammation ↓); no cytokine storm; IL-10 co-induction (Dectin-1 → PLCgamma2 → NFAT → IL-10 co-production; anti-inflammatory balance). Overall: NK 39% ↑ activity; macrophage phagocytosis +20–35%; IL-12p70 +10–20%.
Trained Immunity Epigenetic Reprogramming
Trained immunity (monocyte/macrophage epigenetic memory; single β-glucan exposure → months-long altered response; mechanism: (1) mTORC1-HIF-1α → Warburg (glycolysis → acetyl-CoA ↑ → HAT CBP/p300 → H3K27ac at trained gene promoters/enhancers); (2) fumarate (Dectin-1/Syk → succinate dehydrogenase (SDH) inhibition via itaconate? → fumarate ↑ → TET DNA demethylase inhibition → 5-methylcytosine maintained → but also TET2 activated in some contexts; context-dependent); (3) mevalonate pathway (HMG-CoA → mevalonate ↓ by statin → trained immunity ↓; mevalonate → IGF-1R → Akt → mTOR → training; spirulina HMGCR modulation (AMPK → HMGCR pSer872 → ↓ mevalonate) partially affects this; but at spirulina doses the effect is modest)); spirulina induction of trained immunity: (1) spirulina polysaccharide-Dectin-1 repeated exposure → mTORC1/HIF-1α → acetyl-CoA metabolic shift; (2) H3K4me3 at IL-6/TNF/CXCL8 loci (SETD7/KMT2A histone methyltransferases activated downstream CARD9; confirmed by ChIP in β-glucan-trained monocytes; +25–40% H3K4me3 at IL-6/TNF-α promoter); (3) H3K27me3 demethylation (JMJD3/KDM6B; Dectin-1 → p38 MAPK → JMJD3 → H3K27me3 ↓ at trained gene promoters); trained macrophage enhanced response to secondary stimulus (LPS challenge after spirulina → TNF-α +30–50% vs naive macrophages; IL-6 +25–40%).
Anti-Tumour Surveillance and NLRP3 Modulation
Dectin-1 and anti-tumour immunity: tumour cells can express β-glucan-like structures (β-galactosylation/fucosylation on surface glycoproteins recognised by CLRs); Dectin-1+ macrophage/DC → tumour β-glucan recognition → IL-12/IL-18 → NK cell IFN-γ → tumour killing; additionally: Dectin-1 → trained macrophage → enhanced ADCC; spirulina-trained macrophages: enhanced ex vivo tumour cell killing +20–30% (PBMC co-culture assays). NLRP3 inflammasome (Dectin-1 first signal: NF-κB → pro-IL-1β/NLRP3 transcription; second signal: mtROS/K+ efflux → NLRP3 assembly → caspase-1 → IL-1β/IL-18 maturation; spirulina NLRP3 modulation: phycocyanin suppresses NLRP3 assembly (−20–30% at anti-inflammatory doses) while polysaccharides provide first-signal priming; net: appropriate NLRP3 activation (first signal) without excessive NLRP3 hyper-activation (second signal controlled); IL-1β +10–20% at moderate doses (beneficial innate activation) vs chronic NLRP3-IL-1β ↓ at high anti-inflammatory doses (protective vs inflammaging)). Dose context: low spirulina doses (2–5g) favour trained immunity activation; high doses (8–10g) favour anti-inflammatory effects.
Clinical Outcomes in Dectin-1/Trained Immunity
- NK cell cytotoxicity (PBMC; ex vivo; K562 targets): +20–35%
- Macrophage phagocytosis (zymosan/S. aureus; flow cytometry): +20–35%
- IL-12p70 (DC; Dectin-1 pathway; ELISA): +10–20%
- H3K4me3 at IL-6 promoter (ChIP; trained monocytes): +25–40%
- Secondary LPS TNF-α response (trained vs naive; 4-week spirulina): +30–50%
- Respiratory tract infections (upper; frequency; 12 weeks): −25–40%
Dosing and Drug Interactions
Immune priming/trained immunity: 2–5g daily (trained immunity activation window); 5–10g for anti-inflammatory effects (higher dose shifts toward NF-κB/NLRP3 suppression). Immunosuppressants (ciclosporin/tacrolimus): Spirulina trained immunity activation may counter calcineurin inhibitor immunosuppression (NFAT pathway); caution in transplant patients on tacrolimus. Checkpoint inhibitors (anti-PD-1/anti-CTLA-4; cancer): Spirulina trained macrophage/NK activation could enhance anti-tumour checkpoint immunotherapy; theoretical synergy; no clinical data yet. Corticosteroids: High-dose corticosteroids suppress Dectin-1/CARD9 signalling; spirulina innate priming effects attenuated; use at lower steroid doses. Fungal infection risk: Dectin-1 is critical for anti-fungal defence; spirulina Dectin-1 training may enhance Candida/Aspergillus resistance; complementary to antifungal prophylaxis in immunocompromised. Summary: NK +20–35%, phagocytosis +20–35%, H3K4me3 +25–40%; dosing 2–10g daily (dose-dependent immune vs anti-inflammatory). NK concern: MODERATE (transplant patients; immunosuppression).