Dendritic Cell Subsets and Pattern Recognition
Conventional dendritic cells (cDC1: CLEC9A+, XCR1+, specialised in cross-presentation; cDC2: CD1c+, broad antigen presentation) and plasmacytoid DCs (pDC: BDCA-2/CD303+, type I IFN producers) each express distinct pattern recognition receptors. TLR4 (LPS/MD-2 co-receptor) and TLR2 (lipopeptides/peptidoglycan, requiring TLR1 or TLR6 heterodimers) signal through MyD88 adaptor to IRAK4/IRAK1/TRAF6/TAK1/NF-kB and MAPK. TLR3 (dsRNA) signals through TRIF to IRF3/IFN-beta. Spirulina polysaccharides (beta-1,3-glucan-like structures and sulfated polysaccharides) are TLR4 and Dectin-1 agonists, stimulating innate DC activation.
DC Maturation: Co-stimulatory Molecule Upregulation
Immature DCs capture antigen but present it poorly due to low MHC class II, CD80, CD86, and CD40 expression. TLR stimulation drives DC maturation: NF-kB induces CD80/CD86/CD40 surface expression, CCR7 upregulation (lymph node homing), IL-12p70 (Th1 inducing), and TNF-alpha production. Spirulina polysaccharide fractions (water-soluble, MW >10 kDa) activate murine bone marrow-derived DCs, increasing CD40/CD80/CD86 and IL-12 production in vitro, consistent with Th1 polarisation capacity. PCB, in contrast, suppresses NF-kB in LPS-stimulated DCs, creating a counterbalancing Th1-attenuating signal.
Cytokine Profile and Th Polarisation
The cytokines secreted by DCs during T cell priming determine helper T cell fate: IL-12p70 drives Th1 (IFN-gamma, cellular immunity); IL-4 (from basophils/mast cells acting on DCs) drives Th2 (IgE, allergy); IL-6 plus TGF-beta drives Th17 (IL-17, mucosal/autoimmune); TGF-beta alone with retinoic acid drives Treg (FoxP3+, tolerance). Spirulina's bidirectional DC effects, activating with polysaccharides and suppressing NF-kB with PCB, result in an overall tolerogenic shift: reduced IL-6/TNF-alpha production (attenuating Th17/Th1 inflammatory extremes) while maintaining IL-12 for basal Th1 surveillance. This is consistent with clinical data showing spirulina reduces allergic rhinitis (Th2 attenuation) without causing immunosuppression.
Tolerogenic DCs and FoxP3+ Treg Induction
Tolerogenic DCs (tDCs) express low co-stimulatory molecules, high PD-L1 (CD274), and IL-10, instructing naive T cells toward FoxP3+ regulatory T cells (Tregs). tDC induction requires IDO1 (indoleamine 2,3-dioxygenase, kynurenine pathway), ALDH1A (retinoic acid synthesis), and IL-10 autocrine signalling. Nrf2 activation in DCs promotes the tDC phenotype: Nrf2-induced HO-1 generates CO and biliverdin/bilirubin, which induce IDO1 and suppress inflammatory DC maturation. Spirulina's PCB-driven DC Nrf2 activation provides a mechanism for allergy and autoimmune attenuation.
Cross-Presentation and cDC1
cDC1s uniquely cross-present exogenous antigens on MHC class I via the cytosolic pathway (antigen escape from endosome to cytosol, proteasomal processing, TAP1/2 transport to ER). This requires NOX2 phagosomal pH alkalinisation (preserving antigen intact) and SEC61-mediated retrograde translocation. Spirulina-derived beta-glucan structures activate Dectin-1 on cDC1s, and Dectin-1 signalling (Syk/CARD9/NF-kB) enhances cDC1 cross-presentation capacity, potentially improving anti-tumour and anti-viral CTL responses.
DC Migration and CCR7 Signalling
Upon antigen capture and maturation, DCs upregulate CCR7 (receptor for CCL19/CCL21) and migrate to T cell zones of draining lymph nodes. S1P1 (SIPR1) controls DC egress from lymph nodes. NF-kB activation directly induces CCR7 in DCs. Spirulina's partial NF-kB attenuation reduces CCR7 induction in LPS-stimulated DCs, potentially reducing the velocity of DC-driven adaptive immune responses, which may be beneficial in autoimmune contexts where DC hyperactivation drives tissue damage.
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