Spirulina and Neutrophil Biology: NET Formation, NADPH Oxidase, and Degranulation

Neutrophil Activation Receptors

Neutrophils are activated by: (1) pattern recognition: TLR4/MD-2 (LPS), TLR2 (lipopeptides), FPR1 (formyl peptide fMLF receptor, Gi/Ca2+/PKC); (2) opsonin receptors: FcgammaRIIA/IIIB (IgG-coated targets), CR3/Mac-1 (iC3b/fibrinogen); (3) cytokines: IL-8/CXCL8 (CXCR1/CXCR2 Gi/Gbetagamma/PI3K-gamma), G-CSF (G-CSFR/ JAK1/2/STAT3/5), GM-CSF (GMCSFR/JAK2/STAT5); (4) LPS-priming: converts ROS response from moderate to maximal. Neutrophil priming by LPS involves p38-MAPK-driven cytoskeletal reorganisation, NOX2 assembly (p47phox phosphorylation, membrane translocation), and granule mobilisation.

NADPH Oxidase (NOX2) and Respiratory Burst

The NOX2 complex generates superoxide (O2 radical) as the primary microbicidal oxidant: membrane-associated gp91phox (NOX2/CYBB) plus p22phox (CYBA) form the flavocytochrome b558, while cytosolic regulatory subunits p47phox (NCF1), p67phox (NCF2), p40phox (NCF4), and Rac2 (GTP-bound) assemble upon activation. p47phox phosphorylation (PKC/p38/ERK at Ser303/304/320/328) triggers its SH3-domain interactions and membrane docking. The assembled NOX2 transfers electrons from NADPH across the membrane to O2, generating O2 radical that is converted to H2O2 (SOD1/3), then HOCl (myeloperoxidase/MPO), and reactive halogens for pathogen killing. Spirulina's Nrf2 induction of HO-1 and antioxidant enzymes in bystander cells protects them from NOX2-derived oxidant collateral damage without blocking NOX2 in the neutrophil itself.

Granule Contents and Degranulation

Neutrophil granules, exocytosed in a hierarchy (secretory vesicles first, then gelatinase granules, specific granules, azurophilic granules last): Azurophilic (primary) granules: MPO (myeloperoxidase, HOCl generation), elastase (ELANE, cleaves pathogens/ECM), proteinase 3 (PR3, ANCA target), cathepsin G, defensins (antimicrobial peptides), BPI (bactericidal/permeability-increasing protein). Specific (secondary) granules: lactoferrin (iron chelation), gelatinase (MMP9), NGAL, collagenase IV. NF-kB drives azurophilic granule gene transcription during granulopoiesis in bone marrow. PCB-driven NF-kB suppression is relevant to systemic inflammation context rather than acute granule content.

NETs: Neutrophil Extracellular Traps and PAD4

NETs (neutrophil extracellular traps) are extracellular chromatin webs decorated with MPO, elastase, and citrullinated histone H3 (H3Cit) that trap and kill pathogens. NET formation (NETosis) requires: NOX2-derived ROS (or ROS-independent via calcium), MPO, elastase (cleaves histones), and PAD4 (peptidylarginine deiminase 4, citrullinates histone H3 Arg2/8/17, decondensing chromatin). Excessive NETs contribute to thrombosis (NETs scaffold platelet-fibrin thrombi), ARDS, autoimmunity (SLE: anti- citrullinated protein antibodies ACPA/anti-CCP), and lupus nephritis. Spirulina's NF-kB suppression (reducing TLR4/IL-8-driven neutrophil priming) and NOX2 substrate competition (reducing O2 radical availability for PAD4 activation) attenuate pathological NETosis.

MPO and HOCl: Cardiovascular Risk

Myeloperoxidase (MPO) chlorinates LDL-tyrosine to chlorotyrosine (Cl-Tyr-LDL) and oxidises lipids, generating oxidised LDL (oxLDL) that is pro-atherogenic (SR-A/CD36 macrophage uptake → foam cell). Plasma MPO levels predict cardiovascular events independently of CRP. HOCl also inactivates alpha-1-antitrypsin (SERPINA1, proteinase inhibitor), reducing neutrophil elastase control and exacerbating emphysema. Spirulina reduces plasma MPO activity in some clinical studies, consistent with reduced neutrophil priming/degranulation and antioxidant protection of the vascular wall.

IL-8 and Neutrophil Chemotaxis

IL-8 (CXCL8), the principal neutrophil chemoattractant, is produced by macrophages, epithelial cells, and endothelium via NF-kB and AP-1. CXCR1 (high affinity, Gbetagamma/ PLC-beta/Ca2+) and CXCR2 (lower affinity, rapid internalisation, directional sensing) drive neutrophil polarisation and migration toward chemotactic gradients. PCB-NF-kB suppression reduces IL-8 production in the tissue microenvironment, attenuating neutrophil recruitment without impairing circulating neutrophil function (maintaining first-line pathogen defence).