Immune System & Inflammation

Adaptive immunity features: specificity (each lymphocyte recognises a unique antigen through recombined receptors — TCR for T cells, BCR/antibody for B cells), diversity (V(D)J recombination generates approximately 10^11 unique receptor specificities), memory (expanded antigen-specific clones persist as memory cells, providing faster and stronger responses upon re-exposure), and self/non-self discrimination (central tolerance — eliminating self-reactive lymphocytes during development; peripheral tolerance — regulatory T cells and anergy). T cell subsets: CD4+ helper T cells (Th1 — intracellular pathogens, Th2 — extracellular parasites/allergies, Th17 — extracellular bacteria/fungi, Treg — immune suppression, Tfh — helping B cells) and CD8+ cytotoxic T cells (killing infected/tumour cells). Cyclosporine suppresses adaptive immunity by inhibiting calcineurin → blocking NFAT → preventing IL-2 production → inhibiting T cell activation and proliferation. Glatiramer acetate modulates adaptive immune balance rather than broadly suppressing it.

Anti-inflammatory peptide mechanisms: (1) Corticotropin (ACTH) — stimulates adrenal cortisol production (cortisol is the body's primary anti-inflammatory hormone, suppressing NF-κB and AP-1 transcription factors) PLUS direct melanocortin receptor activation on immune cells (MC1R, MC3R on macrophages — suppressing pro-inflammatory cytokine production and promoting anti-inflammatory phenotype). (2) Cyclosporine — inhibiting T cell activation via calcineurin/NFAT pathway, reducing T cell-driven inflammation. (3) Glatiramer acetate — shifting immune response from pro-inflammatory Th1/Th17 toward anti-inflammatory Th2/Treg, promoting BDNF production. (4) Research peptides — various sequences investigated for: NF-κB pathway inhibition, inflammasome suppression, macrophage polarisation (M1→M2 switching), and complement modulation. The distinction between immunosuppression (broadly reducing immune function — risk of infection) and immunomodulation (redirecting immune response without broad suppression — better safety profile) is important for evaluating anti-inflammatory peptide strategies.

Endogenous antioxidant systems: enzymatic (SOD — converts O2•⁻ → H2O2, three isoforms: Cu/Zn-SOD cytoplasmic, Mn-SOD mitochondrial, EC-SOD extracellular; catalase — converts H2O2 → H2O + O2, primarily in peroxisomes; glutathione peroxidase — converts H2O2 and lipid hydroperoxides → H2O using glutathione as electron donor; thioredoxin reductase — maintains thioredoxin in reduced state for protein repair) and non-enzymatic (glutathione — the most abundant intracellular antioxidant, tripeptide Glu-Cys-Gly; vitamin C/ascorbic acid — aqueous-phase radical scavenger, also cofactor for collagen synthesis; vitamin E/α-tocopherol — lipid-phase antioxidant protecting membranes; uric acid; bilirubin). Some peptides have intrinsic antioxidant properties — His and Trp residues can scavenge ROS, and metal-binding peptides (like GHK-Cu) can modulate redox chemistry. However, exogenous antioxidant supplementation trials have generally failed to show clinical benefits for disease prevention.

Autoimmunity results from: genetic susceptibility (HLA associations — HLA-B27 in ankylosing spondylitis, HLA-DR4 in rheumatoid arthritis), environmental triggers (molecular mimicry — pathogen antigens resembling self-antigens; epitope spreading — immune response expanding from initial target to related self-antigens; bystander activation — inflammation exposing normally hidden self-antigens), and regulatory failure (defective Treg function, complement deficiency, impaired apoptotic cell clearance). Autoimmune diseases treated with peptide drugs: MS (glatiramer acetate — immunomodulation), MG (zilucoplan — complement inhibition), organ transplant rejection (cyclosporine — T cell suppression), psoriasis/RA (cyclosporine — T cell suppression for severe cases), and various inflammatory conditions (corticotropin — cortisol-mediated immunosuppression + melanocortin receptor-mediated immunomodulation). The pipeline for peptide-based autoimmune therapies includes: tolerogenic peptide vaccines (inducing antigen-specific tolerance) and targeted complement inhibitors.

B cells mature in the bone marrow and are activated in secondary lymphoid organs (lymph nodes, spleen) when their B cell receptor (surface-bound antibody) encounters matching antigen. With T cell help (Tfh cells providing CD40L co-stimulation and IL-21/IL-4 cytokines), activated B cells undergo: clonal expansion, somatic hypermutation (refining antibody affinity in germinal centres), class switch recombination (switching from IgM to IgG, IgA, IgE, or IgD), and differentiation into antibody-secreting plasma cells and memory B cells. Anti-drug antibodies (ADAs) against therapeutic peptides are produced by this pathway — the peptide drug (as a foreign protein) is processed by antigen-presenting cells, stimulates T helper cells, which in turn activate B cells specific for the peptide. Strategies to reduce ADA formation include: humanising peptide sequences, removing T cell epitopes (deimmunisation), and using tolerogenic formulations.

Calcineurin is a calcium/calmodulin-dependent serine-threonine phosphatase essential for T cell activation: TCR signalling → calcium influx → calmodulin binds calcineurin → calcineurin dephosphorylates NFAT → NFAT translocates to nucleus → IL-2 gene transcription → T cell proliferation. Cyclosporine mechanism: cyclosporine binds cyclophilin (an immunophilin) → cyclosporine-cyclophilin complex binds and inhibits calcineurin → NFAT remains phosphorylated in cytoplasm → IL-2 not produced → T cell activation blocked. Tacrolimus (FK506, a macrolide not a peptide) inhibits calcineurin through a different immunophilin (FKBP12) but the downstream effect is identical. Calcineurin inhibitor toxicities: nephrotoxicity (dose-dependent, both acute — afferent arteriolar vasoconstriction, and chronic — tubulointerstitial fibrosis; the major limitation of long-term use), neurotoxicity (tremor, seizures), hypertension, hyperlipidaemia, and hyperglycaemia. Therapeutic drug monitoring (trough levels) is essential for cyclosporine.

Chemokines are classified by cysteine motif: CC chemokines (CCL1-28, receptors CCR1-10 — recruiting monocytes, lymphocytes, eosinophils), CXC chemokines (CXCL1-16, receptors CXCR1-6 — recruiting neutrophils, lymphocytes), CX3C chemokines (CX3CL1/fractalkine, receptor CX3CR1), and C chemokines (XCL1-2, receptor XCR1). CXCL12 (SDF-1) and its receptor CXCR4 are the most therapeutically relevant chemokine axis in peptide pharmacology: CXCL12/CXCR4 retains haematopoietic stem cells in bone marrow niches, regulates immune cell trafficking, and plays roles in cancer metastasis (many tumours express CXCR4 and are attracted to CXCL12-rich tissues). Motixafortide is a cyclic peptide CXCR4 antagonist that blocks the CXCL12/CXCR4 interaction, mobilising stem cells from marrow. Beyond stem cell mobilisation, CXCR4 antagonism is being investigated for: enhancing immune checkpoint inhibitor efficacy (by increasing T cell tumour infiltration) and blocking cancer metastasis.

Chronic inflammation is characterised by: simultaneous tissue destruction and repair (ongoing damage alongside healing attempts), mononuclear cell infiltration (macrophages, lymphocytes, plasma cells rather than the neutrophils of acute inflammation), fibrosis (excessive collagen deposition as repair fails to keep pace with damage), and angiogenesis (new blood vessel formation in inflamed tissue). Conditions driven by chronic inflammation: atherosclerosis (vascular inflammation → plaque formation → cardiovascular events), type 2 diabetes (adipose tissue inflammation → insulin resistance), neurodegenerative diseases (neuroinflammation → neuronal damage), autoimmune diseases (persistent immune attack on self tissues), and cancer (inflammatory microenvironment promoting tumour growth). Biomarkers of chronic inflammation: CRP, ESR, ferritin, fibrinogen, and pro-inflammatory cytokines (IL-6, TNF-α). Several peptide drugs address chronic inflammation: corticotropin (stimulating anti-inflammatory cortisol production), cyclosporine (suppressing T cell-mediated inflammation), and GLP-1 RAs (which may have anti-inflammatory effects through reduced visceral fat and direct GLP-1R signalling on immune cells).

C5 (190 kDa glycoprotein, synthesised primarily in the liver) is the convergence point of all three complement activation pathways. C5 convertase (C4b2a3b for classical/lectin, C3bBbC3b for alternative) cleaves C5 into: C5a (74 amino acid anaphylatoxin — binds C5aR1/CD88, a GPCR on neutrophils, macrophages, mast cells → chemotaxis, degranulation, oxidative burst, cytokine production; one of the most potent inflammatory mediators) and C5b (initiates terminal complement complex assembly → C5b-6, C5b-7 (membrane-inserting), C5b-8, C5b-9n/MAC). Therapeutic C5 inhibition: eculizumab (monoclonal antibody — first approved complement inhibitor, for PNH and aHUS), ravulizumab (longer-acting anti-C5 antibody), and zilucoplan (macrocyclic peptide C5 inhibitor — the only peptide-based complement inhibitor, offering SC self-administration advantage over IV antibody infusions). Zilucoplan binds C5 at a different epitope than eculizumab, potentially maintaining activity in some eculizumab-resistant C5 polymorphisms.

Complement activation pathways: classical (antibody-antigen complexes activate C1 → C4 → C2 → C3 convertase C4b2a), lectin (mannose-binding lectin binds pathogen surfaces → MASP-1/2 activate C4 → C2 → same C3 convertase), and alternative (spontaneous C3 hydrolysis — tick-over — amplified on foreign surfaces lacking complement regulators → Factor B/D → C3 convertase C3bBb). All three converge on C3 cleavage → C3a (anaphylatoxin — inflammation) + C3b (opsonin — coating pathogens for phagocytosis, also feeds into C5 convertase). C5 convertase cleaves C5 → C5a (potent anaphylatoxin and chemotactic factor) + C5b → C5b-6-7-8-9 = membrane attack complex (MAC — forming pores in target membranes). Zilucoplan inhibits C5 cleavage, preventing both C5a-mediated inflammation and MAC formation at the neuromuscular junction in myasthenia gravis.

CXCR4 (CD184) is a 7-transmembrane GPCR (Gαi-coupled → inhibits adenylyl cyclase, activates PI3K, mobilises calcium) with a single natural ligand: CXCL12/SDF-1 (68 amino acids). CXCR4 is expressed on: HSCs and haematopoietic progenitors (marrow retention), T cells, B cells, monocytes (immune trafficking), endothelial progenitor cells (vascular repair), neurons (brain development), and many cancer types (promoting metastasis to CXCL12-rich tissues — bone marrow, liver, lung). Motixafortide (BL-8040) is a cyclic 14-amino-acid peptide (with a 4-fluorobenzoyl modification) that antagonises CXCR4 with high affinity. Approved for HSC mobilisation in myeloma, it is also investigated for: pancreatic cancer (disrupting tumour-stroma interactions), AML (mobilising leukaemia stem cells from protective marrow niches to increase chemotherapy sensitivity), and enhancing checkpoint immunotherapy (increasing tumour T cell infiltration by disrupting CXCR4-mediated immune exclusion).

Major cytokine categories: interleukins (IL-1 through IL-40+ — diverse immune regulatory functions), interferons (IFN-α/β — type I, antiviral; IFN-γ — type II, macrophage activation and Th1 immunity), tumour necrosis factors (TNF-α, lymphotoxin), colony-stimulating factors (G-CSF, GM-CSF, M-CSF — haematopoiesis), chemokines (approximately 50 members — directing cell migration), and transforming growth factors (TGF-β — immunosuppressive, pro-fibrotic). Cytokine signalling: most cytokines signal through JAK-STAT pathway (receptor-associated JAK kinases phosphorylate STAT transcription factors → nuclear translocation → gene regulation). Cytokine storm (systemic inflammatory response from excessive cytokine release — potentially fatal) can occur with immune-activating therapies. For peptide drugs: corticotropin stimulates cortisol which broadly suppresses cytokine production; glatiramer acetate shifts cytokine profiles from Th1 (pro-inflammatory) to Th2 (anti-inflammatory); and thymosin alpha-1 modulates cytokine production by dendritic cells.

Histamine (2-(imidazol-4-yl)ethylamine) is synthesised from histidine by histidine decarboxylase and stored in mast cell and basophil granules. Four histamine receptors (all GPCRs): H1R (Gαq — smooth muscle contraction, vascular permeability, pruritis, wakefulness), H2R (Gαs — gastric acid secretion, cardiac chronotropy), H3R (Gαi — presynaptic autoreceptor in CNS, regulating histamine/other neurotransmitter release), H4R (Gαi — immune cell chemotaxis, particularly eosinophils and mast cells). In peptide drug adverse reactions: H1R activation causes urticaria, angioedema, bronchospasm, and pruritus; H2R activation contributes to hypotension and tachycardia. Antihistamines (H1R blockers: cetirizine, loratadine; H2R blockers: ranitidine, famotidine) are used as premedication for infusion reactions and as treatment for mild allergic reactions to peptide drugs.

Immunomodulation encompasses: immune enhancement (boosting underactive immunity — thymosin alpha-1 promoting T cell maturation and dendritic cell function), immune suppression (reducing overactive immunity — cyclosporine blocking T cell activation for transplant rejection/autoimmunity), and immune regulation (redirecting immune responses — glatiramer acetate shifting Th1/Th17 toward Th2/Treg balance without broad suppression). The advantage of immunomodulation over immunosuppression: maintaining protective immunity against infections and cancers while redirecting pathological immune responses. Glatiramer acetate exemplifies this: it does not significantly increase infection risk (unlike broad immunosuppressants), does not require laboratory monitoring for immunosuppression-related complications, and maintains intact vaccine responses. Motixafortide, while primarily used for stem cell mobilisation, also has immunomodulatory potential — disrupting CXCR4/CXCL12 signalling can enhance anti-tumour immune responses by promoting T cell infiltration into tumours.

Cardinal signs (Celsius/Virchow): rubor (redness — vasodilation), calor (heat — increased blood flow), tumor (swelling — plasma exudation from increased vascular permeability), dolor (pain — nociceptor sensitisation by bradykinin, prostaglandins, cytokines), and functio laesa (loss of function). Molecular mediators: histamine and serotonin (vasoactive amines from mast cells — immediate), prostaglandins and leukotrienes (lipid mediators from arachidonic acid — sustained), cytokines (TNF-α, IL-1β, IL-6 — amplifying and perpetuating), chemokines (directing cell recruitment), complement fragments (C3a, C5a — promoting inflammation), and bradykinin (vasodilation, pain). The resolution of inflammation is an active, programmed process involving: lipid mediator class switching (pro-inflammatory prostaglandins → pro-resolving lipoxins, resolvins, protectins), efferocytosis (macrophages clearing apoptotic neutrophils → triggering anti-inflammatory reprogramming), and Treg cell activation. Failed resolution → chronic inflammation.

Innate immune components: physical barriers (skin, mucosal surfaces — including antimicrobial peptides such as defensins and cathelicidins), cellular defences (neutrophils — first responders, phagocytosis; macrophages — phagocytosis, antigen presentation, cytokine production; dendritic cells — antigen presentation, linking innate and adaptive immunity; NK cells — killing virus-infected and tumour cells; mast cells — histamine release, parasite defence; eosinophils — parasite defence), humoral defences (complement system — opsonisation, MAC formation, inflammation; acute phase proteins — CRP, mannose-binding lectin; interferons — antiviral state), and pattern recognition receptors (TLRs, NLRs, RLRs — detecting pathogen-associated molecular patterns/PAMPs and damage-associated molecular patterns/DAMPs). Antimicrobial peptides bridge innate and adaptive immunity — defensins recruit dendritic cells and T cells, while LL-37 promotes dendritic cell maturation.

IFN types: Type I (IFN-α from leukocytes, IFN-β from fibroblasts — induced by viral infection, activating antiviral gene programme via JAK1-TYK2/STAT1-STAT2/IRF9 signalling), Type II (IFN-γ from T cells and NK cells — activating macrophages for intracellular pathogen killing via JAK1-JAK2/STAT1 signalling), Type III (IFN-λ — mucosal antiviral immunity). Thymosin alpha-1's mechanism involves enhancing IFN-α production by plasmacytoid dendritic cells (through TLR9 activation) — this is the rationale for its use as adjunct to interferon therapy in hepatitis B/C. Glatiramer acetate may also modulate IFN-γ production — shifting from IFN-γ-dominant Th1 responses (pro-inflammatory in MS) toward IL-4/IL-10-dominant Th2 responses (anti-inflammatory). IFN-β (Avonex, Rebif, Betaferon) is itself a protein therapeutic used in MS — it reduces relapse rate by approximately 30%, comparable to glatiramer acetate.

Key interleukins in peptide drug contexts: IL-2 (T cell growth factor — cyclosporine blocks its production; IL-2 production is the primary downstream consequence of calcineurin-NFAT signalling that cyclosporine inhibits), IL-6 (pro-inflammatory — elevated in CRS, autoimmune diseases; produced by activated macrophages and T cells), IL-10 (anti-inflammatory — produced by Treg cells; glatiramer acetate treatment increases IL-10 production), IL-1β (pro-inflammatory — involved in inflammasome-mediated inflammation), IL-4 and IL-13 (Th2 cytokines — promote B cell class switching to IgE, allergic inflammation), IL-17 (Th17 cytokine — involved in autoimmune inflammation), and IL-21 (Tfh cytokine — critical for germinal centre B cell responses and antibody production, relevant to ADA formation against peptide drugs). Interleukin measurement (by ELISA, multiplex immunoassay, or flow cytometry) is used in clinical trials to characterise immune responses to peptide therapeutics.

Leukotrienes are synthesised from arachidonic acid by 5-lipoxygenase (5-LOX) → LTA4 → LTB4 (potent neutrophil chemoattractant via BLT1 receptor) or cysteinyl leukotrienes (LTC4, LTD4, LTE4 — formerly 'slow-reacting substance of anaphylaxis' — potent bronchoconstrictors, increase vascular permeability, stimulate mucus secretion; act through CysLT1 and CysLT2 receptors). Leukotrienes contribute to: asthma (bronchoconstriction, mucus hypersecretion, airway inflammation), allergic rhinitis, and anaphylaxis. Leukotriene receptor antagonists (montelukast) are established asthma therapies. Corticotropin-stimulated cortisol inhibits both COX (prostaglandin) and 5-LOX (leukotriene) pathways via lipocortin-1-mediated phospholipase A2 inhibition, providing broader anti-inflammatory coverage than NSAIDs (which only block COX). This dual pathway inhibition contributes to corticotropin's efficacy in inflammatory conditions.

Mast cells originate from CD34+ bone marrow progenitors, mature in tissues under the influence of stem cell factor (SCF/c-Kit ligand). Tissue distribution: connective tissue (skin, peritoneum, perivascular), mucosal surfaces (respiratory, GI tract). Mast cell granules contain: histamine, heparin, tryptase, chymase, and TNF-α (pre-formed mediators released within seconds of activation). De novo synthesised mediators (produced over minutes-hours): prostaglandin D2, leukotriene C4, and cytokines (IL-4, IL-5, IL-6, TNF-α). Activation triggers: IgE crosslinking on FcεRI (classical allergic reaction), complement fragments C3a and C5a (anaphylatoxins), substance P, and physical stimuli (cold, pressure). For peptide drugs, mast cell activation is relevant to: injection site reactions (local mast cell degranulation from needle trauma and foreign substance deposition), hypersensitivity reactions (IgE-mediated or direct mast cell activation), and anaphylaxis (systemic mast cell degranulation).

MAC assembly: C5b binds C6 → C5b6 complex binds C7 → C5b-7 inserts into lipid bilayer → C8 incorporation deepens membrane insertion → C9 polymerisation (up to 18 C9 molecules form a pore ring approximately 10nm internal diameter) → transmembrane pore allowing free passage of ions and water → osmotic lysis of the target cell. In myasthenia gravis, MAC deposits at the neuromuscular junction postsynaptic membrane → destruction of postsynaptic folds (reducing surface area for AChR expression), direct AChR damage, and endplate remodelling. This complement-mediated damage compounds the direct antibody effects (AChR crosslinking and internalisation). Zilucoplan prevents MAC formation by blocking C5 cleavage upstream. Complement regulation: host cells express membrane complement regulatory proteins (CD55/DAF — accelerates C3/C5 convertase decay; CD59/protectin — blocks C9 insertion, preventing MAC completion) that protect them from complement attack. Pathogens lack these regulators, making them susceptible.

Neutropenia grading (NCI CTCAE): Grade 1 (ANC 1500-2000/μL), Grade 2 (1000-1500), Grade 3 (500-1000), Grade 4 (<500 — severe, high infection risk). Causes: decreased production (bone marrow failure, chemotherapy, drug-induced), increased destruction (autoimmune, hypersplenism), and increased margination. In Barth syndrome, neutropenia is cyclic or intermittent (caused by mitochondrial dysfunction in neutrophil precursors — elamipretide may improve neutrophil counts by improving mitochondrial function). Chemotherapy-induced neutropenia is the primary clinical context for G-CSF (filgrastim, pegfilgrastim — stimulating neutrophil production). Febrile neutropenia (ANC <500 + fever ≥38.3°C) is a medical emergency requiring immediate empirical broad-spectrum antibiotics — antimicrobial peptide drugs (vancomycin, daptomycin) are used when gram-positive coverage is needed.

ROS sources: mitochondrial electron transport chain (main physiological source — approximately 1-2% of electrons leak to form superoxide), NADPH oxidase (NOX enzymes — activated in phagocytes for pathogen killing and in other cells for signalling), xanthine oxidase, and exogenous sources (UV radiation, pollution, drugs). Antioxidant defence: enzymatic (superoxide dismutase → H2O2; catalase → H2O + O2; glutathione peroxidase → H2O using glutathione) and non-enzymatic (glutathione, vitamin C, vitamin E, uric acid). Oxidative stress occurs when ROS production exceeds antioxidant capacity → lipid peroxidation (membrane damage), protein oxidation (enzyme inactivation, peptide degradation — Met→Met(O), Trp→oxyindole), and DNA oxidation (8-oxoguanine — mutagenic). Elamipretide reduces mitochondrial ROS by stabilising cardiolipin → optimising electron transport chain complex organisation → reducing electron leak → less superoxide production. This is a targeted approach to the primary intracellular ROS source rather than a systemic antioxidant scavenging strategy.

Prostaglandins are synthesised from arachidonic acid (released from membrane phospholipids by phospholipase A2) by cyclooxygenases (COX-1 constitutive, COX-2 inducible) → PGH2 → tissue-specific synthases produce: PGE2 (pain, fever, vasodilation, gastric protection, renal blood flow), PGI2/prostacyclin (vasodilation, platelet inhibition), PGD2 (bronchoconstriction, sleep regulation), PGF2α (uterine contraction, bronchoconstriction), and TXA2/thromboxane (vasoconstriction, platelet aggregation). Corticotropin's anti-inflammatory mechanism partly involves cortisol-mediated induction of lipocortin-1, which inhibits phospholipase A2, reducing arachidonic acid release and therefore prostaglandin production. NSAIDs inhibit COX enzymes directly. Understanding prostaglandin biology contextualises the anti-inflammatory effects of corticotropin and the GI effects (gastric acid, mucosal protection) relevant to oral peptide delivery.

Major ROS: superoxide anion (O2•⁻ — produced by mitochondrial complexes I and III, and by NOX enzymes), hydrogen peroxide (H2O2 — produced from superoxide by SOD, also a signalling molecule at low concentrations), hydroxyl radical (•OH — produced from H2O2 by Fenton reaction with Fe²⁺/Cu⁺, extremely reactive, no enzymatic defence), and peroxynitrite (ONOO⁻ — produced from superoxide + nitric oxide, damaging to proteins and DNA). ROS in physiology: immune defence (neutrophil/macrophage respiratory burst killing pathogens), cell signalling (H2O2 as a second messenger in growth factor signalling), and gene regulation (NF-κB and Nrf2 pathways are redox-sensitive). ROS in pathology: when production exceeds antioxidant capacity → oxidative damage to lipids, proteins, and DNA → contributes to ageing, neurodegeneration, cardiovascular disease, cancer, and diabetes. For peptide drugs, ROS are relevant to: formulation stability (oxidation is a primary peptide degradation pathway — Met, Trp, Cys residues are susceptible) and therapeutic mechanism (elamipretide's mitochondrial ROS reduction).

HSC mobilisation for autologous transplantation: patients with multiple myeloma, lymphoma, or other conditions undergo high-dose chemotherapy (which destroys bone marrow) followed by infusion of their own previously collected HSCs to reconstitute the blood system. Mobilisation protocol: motixafortide (single SC injection) + G-CSF (5 days) → apheresis collection of CD34+ HSCs from peripheral blood. Motixafortide's contribution: CXCR4 blockade releases HSCs from marrow niches within 4-6 hours (vs G-CSF alone which takes 4-5 days). The combination produces higher CD34+ cell yields, often achieving target collection in a single apheresis session (vs multiple sessions with G-CSF alone). The GENESIS Phase III trial demonstrated superior HSC mobilisation with motixafortide + G-CSF vs placebo + G-CSF. Sufficient CD34+ cell collection (target ≥6 × 10⁶ CD34+ cells/kg) enables successful engraftment after high-dose chemotherapy.

T cells mature in the thymus (positive selection — selecting T cells that can recognise self-MHC; negative selection — eliminating T cells that react too strongly to self-antigens — establishing central tolerance). T cell activation requires: signal 1 (TCR recognition of peptide-MHC complex on antigen-presenting cell), signal 2 (co-stimulation via CD28-B7 interaction), and signal 3 (cytokine environment directing differentiation). Cyclosporine acts at signal 3: by blocking calcineurin-NFAT pathway → preventing IL-2 transcription → inhibiting T cell proliferation and effector function. This mechanism targets the adaptive immune response while largely sparing innate immunity. Thymosin alpha-1 promotes T cell maturation and function by: stimulating TLR2 and TLR9 on dendritic cells → enhanced antigen presentation → improved T cell priming; and directly promoting T cell differentiation markers (CD4, CD8, CD25 expression). The opposing effects of cyclosporine (suppressing T cells) and thymosin alpha-1 (enhancing T cells) illustrate the different therapeutic needs of immunosuppression vs immunoenhancement.

Platelet count <150,000/μL; clinically significant bleeding risk at <50,000; spontaneous bleeding risk at <20,000; life-threatening risk at <10,000. Immune thrombocytopenia (ITP) pathophysiology: autoantibodies (primarily anti-GPIIb/IIIa and anti-GPIb/IX) → platelet opsonisation → Fc receptor-mediated splenic destruction AND megakaryocyte dysfunction → reduced platelet production. Romiplostim (Nplate) is a peptide-Fc fusion (AMG 531): two copies of a 14 amino acid peptide (with no sequence homology to endogenous thrombopoietin/TPO) bind and activate the TPO receptor (c-Mpl) on megakaryocytes → JAK2/STAT5 signalling → megakaryocyte proliferation and differentiation → increased platelet production. Romiplostim's Fc fusion design extends half-life through FcRn recycling. Dosing: weekly SC injection titrated to maintain platelet count ≥50,000/μL. Key safety concern: potential for bone marrow reticulin fibrosis with long-term use (monitored by CBC with peripheral smear).

Rejection types: hyperacute (minutes-hours — pre-formed antibodies against donor HLA antigens → complement activation → graft vessel thrombosis; prevented by crossmatch testing), acute cellular (days-months — T cell recognition of donor MHC → infiltration and destruction of graft tissue; treated with corticosteroids and T cell-directed immunosuppression), acute antibody-mediated (days-months — de novo anti-donor antibodies → complement activation → endothelial damage; treated with plasmapheresis, IVIG, rituximab), and chronic (months-years — progressive fibrosis and vasculopathy → gradual graft failure; poorly responsive to treatment). Cyclosporine revolutionised transplantation in the 1980s by providing effective T cell suppression — before cyclosporine, one-year kidney graft survival was approximately 50%; after its introduction, this rose to >80% and continues to improve with modern regimens. Current standard immunosuppression: calcineurin inhibitor (cyclosporine or tacrolimus) + antiproliferative (mycophenolate) + corticosteroid.

TNF-α (233 amino acid transmembrane protein, cleaved to 157 amino acid soluble form by TACE/ADAM17) signals through TNFR1 (ubiquitous expression, mediating inflammation and apoptosis) and TNFR2 (restricted to immune and endothelial cells, mediating cell survival and proliferation). TNF-α is produced primarily by activated macrophages and T cells. Effects: NF-κB activation (pro-inflammatory gene transcription), MAPK activation (cytokine production), and caspase activation (apoptosis in some contexts). TNF-α is a central mediator of: septic shock (vasodilation, capillary leak, organ failure), rheumatoid arthritis (synovial inflammation and joint destruction), inflammatory bowel disease, and psoriasis. Anti-TNF therapies (infliximab, adalimumab, etanercept — antibodies and receptor fusion proteins) are blockbuster drugs. While these are protein/antibody therapeutics rather than peptides, understanding TNF biology contextualises the inflammatory pathways that peptide immunomodulators (corticotropin, cyclosporine) affect.