Spirulina and T Cell Exhaustion: PD-1/PD-L1, TIM-3, and Checkpoint Pathways

T Cell Exhaustion: Definition and Context

T cell exhaustion (Tex) is a distinct differentiation state arising from chronic antigen exposure (chronic viral infection, tumour antigen stimulation) characterised by: progressive loss of cytokine production (IL-2 first, then IFN-gamma, TNF-alpha), upregulation of multiple inhibitory receptors (PD-1, TIM-3, LAG-3, CTLA-4, TIGIT, CD244/2B4), impaired proliferative capacity, and decreased cytotoxic granule release. Tex cells express the transcription factors TOX and TOX2 (essential for exhaustion programme establishment) and have a distinct epigenetic state (ATAC-seq-accessible exhaustion-specific loci) that limits reinvigoration by anti-PD-1 therapy alone.

PD-1/PD-L1 Checkpoint Axis

PD-1 (PDCD1/CD279) is upregulated on activated/exhausted T cells; its ligands PD-L1 (CD274/B7-H1) and PD-L2 are induced by IFN-gamma on tumour cells, stroma, and APCs. PD-1 signalling: PD-L1 ligation recruits SHP-2 (PTPN11) to PD-1 ITSM motif, and SHP-1/SHP-2 dephosphorylate CD3zeta ITAMs and ZAP-70, reducing TCR signal amplitude. PD-1 also activates PTEN (via SHP-2-mediated dephosphorylation of SHIP1), reducing PIP3 and Akt activity, suppressing metabolic activation, and impairing mTORC1. NF-kB induces PD-L1 (ARE-like NF-kB binding site in CD274 promoter); PCB- driven NF-kB suppression reduces tumour/stromal PD-L1 induction, potentially reducing the PD-L1 load on exhausted T cells in inflammatory tumour contexts.

TIM-3 and the Galectin-9 Ligand

T cell immunoglobulin and mucin domain-3 (TIM-3/HAVCR2) is a co-inhibitory receptor whose ligands include galectin-9 (GAL-9), HMGB1 (from necrotic cells), PtdSer (phosphatidylserine on apoptotic bodies), and CEACAM1. TIM-3 signalling recruits BAT3/BAG6 to its cytoplasmic tail under low-stimulation conditions (protective) and loses BAT3 under high-stimulation (permitting exhaustion). TIM-3+PD-1 co-expression marks the most deeply exhausted T cell population in tumours. Galectin-9 is induced by NF-kB and IFN-gamma in tumour stroma; PCB-driven NF-kB attenuation reduces galectin-9 production, potentially reducing TIM-3 ligation.

TOX: Epigenetic Enforcer of Exhaustion

TOX (thymocyte selection-associated HMG box protein) is induced by NFAT1 under chronic TCR stimulation and is required for exhaustion programme establishment. TOX interacts with NuRD (nucleosome remodelling deacetylase) complex to remodel chromatin at exhaustion-specific loci, creating stable epigenetic marks (H3K27me3 at effector loci, H3K4me3 at exhaustion loci). TOX drives expression of PD-1, TIM-3, LAG-3, and EOMES (at the expense of T-bet, which drives effector function). This epigenetic lockdown limits the depth of T cell reinvigoration by single checkpoint blockade.

AMPK and T Cell Metabolic Fitness Against Exhaustion

Exhausted T cells show impaired mitochondrial mass and function (low PGC-1alpha, fragmented mitochondria, high ROS). Restoring mitochondrial biogenesis via AMPK or PGC-1alpha overexpression revitalises exhausted T cells in preclinical models. AMPK activation also increases fatty acid oxidation, shifting fuel use from glycolysis (tumour-competitive) to lipids (available in the tumour microenvironment). Spirulina's AMPK and PGC-1alpha induction could support metabolic fitness of tumour-infiltrating T cells, complementing checkpoint inhibitor reinvigoration of the PD-1/TIM-3 signalling axis.

LAG-3 and TIGIT: Emerging Checkpoints

Lymphocyte activation gene-3 (LAG-3/CD223) binds MHC class II (high affinity, Kd ~500 nM) on APCs, inhibiting T cell activation via KIEELE motif-FYN-LCK suppression. TIGIT (T cell immunoreceptor with Ig and ITIM domains) competes with CD226 (DNAM-1) for CD155/PVR on APCs/tumour cells; CD155-TIGIT delivers inhibitory signals (SHIP1/ PTEN activation), while CD155-CD226 delivers activating signals. NF-kB drives CD155 on tumour cells; spirulina's NF-kB suppression could reduce CD155 expression, tipping CD226 competitive advantage over TIGIT.