Anaphylaxis Cornell Video

Anaphylaxis

Serious, generalized or systemic, allergic or hypersensitivity reaction that can be life threatening or fatal

Pathways that Cause Anaphylaxis

Immunologic IgE-mediated (Classic)

• Cross linked IgE (type 1 hypersensitivity reaction)

• Insects, food, medications

• Previous antigenic stimulation

Immunologic non-IgE-mediated

• Immune-aggregates (IgG), complement

Non-immunologic

• Physical factors: hot, cold, exercise, opioids

Mediators of Anaphylaxis in the Initiation/Early Phase

Histamine

Tryptase

Heparin/chymase

Mediators of Anaphylaxis in the Mid-Phase

Platelet activating factor (PAF)

Arachidonic acid cascade

Mediators of Anaphylaxis in the Late Phase

Cytokines/inflammatory mediators

Early Phase of Anaphylaxis

• Allergen will crosslink and aggregate IgE molecules bound to high affinity receptors which will stimulate the pathway that involves tyrosine kinase

• TK will phosphorylates molecules

• When it phosphorylates phospholipase C will chew the phosphate head off of fatty acid tail

• IP3 will stimulate release of Ca from SR

• Calcium release will stimulate another protein kinase which phosphorylates myosin located on secretory granules which allows the secretory granules to move and merge with mast cell membrane and release their molecules

• This happens rapidly, within seconds

Actions of the H1 Receptor

• Skin component, dermal angioedema, itchy skin, urticaria

• See smooth muscle bronchoconstriction

• Causes rhinitis in people

• Gq pathway

• Some inotropy and chronotropy inhibition which causes the animal to not be able to respond to anaphylaxis

• Vasodilation and increased permeability

Actions of the H2 Receptor

• Gs pathway

• GI tract

  • Increase in acid production and simultaneously duodenal bicarb secretion is inhibited

  • Some heart and systemic vasodilation

Actions of the H3 Receptor

• Gi pathway

• Inhibits release of norepi from the presynaptic portion of the adrenergic system

• When we can't release norepi in response to vasodilatory changes we can't have sympathetic nervous system saving us

Actions of the H4 Receptor

• Chemotaxis and production of inflammatory cytokines

Anaphylaxis Initiation Phase Mediators

Tryptase

Heparin

Chymase

Anaphylaxis Initiation Phase Mediators - Tryptase

Serine protease, biomarker

Activates: complement, coagulation, kallikrein

Hypotension, angioedema, clot, DIC

Anaphylaxis Initiation Phase Mediators - Heparin

Modulates tryptase activity

Opposes complement, inhibits clots, plasma, kallikrein

Anaphylaxis Initiation Phase Mediators - Chymase

Stimulate AgI to AgII (ACE independent)

Decreases severity of hypotension, causes myocardia ischemia via vasoconstriction

Mid-Phase Anaphylaxis

• Same pathway that was initiated by allergen binding FC receptor and same tyrosine kinase will phosphorylate more than one second messenger

• Phospholipase A2 will be phosphorylated and stimulated

  • Will cut up a phospholipid but it cuts off a fatty acid tail and generates arachidonic acid

  • Also forms a precursor molecule lysoPAF, inactive form of PAF

• Arachidonic acid by itself isn't an inflammatory mediator but the byproducts of the arachidonic cascade are key players in inflammation in the body in general and in anaphylaxis

Prostaglandin D2

Comes from the COX pathway

Bronchodilation

Pulmonary/coronary vasoconstriction

Peripheral vasodilation

Leukotrienes

Come from the LOX pathway

1000x more potent than histamine

LTC4, LTD4, LTE4: bronchodilation, increased vascular permeability

LTB4: chemotactic agent

LYSO-PAF and PAF

• Lyso-PAF is inactivated form of PAF

• Enzyme that adds an acyl group which activates it

• Feedback mechanisms that will clear PAF as fast as it is produced as long as you have enough of an enzyme

  • Enzyme is platelet activating factor acetylhydrolase - breaks off acyl group from PAF and inactivate that

  • As long as it does that and limits this reaction you won't die from anaphylaxis

Critical Role of PAF

Key mediator of hypotension in anaphylaxis and critical illness

• Binds PAFr (GPCR)

• GPCR second messenger pathways increase calcium

• Also activates PLA2 and PI3K

  • Stimulates eNOS production of NO → vasodilation

Structurally unique

• Vasoactive phospholipid

• Target organs - heart, pulmonary vessels, and microcirculation

• Promotion of thrombi and second inflammatory mediators

  • Histamine, kinins, TXA2, leukotrienes, ROS

Nitric Oxide

Formed from L-arginine via NOS

• eNOS = endothelial = constitutive

• iNOS = inducible

• nNOS = neurogenic

NOS production/activation

• Pathway: phospholipase C, calcium, and calmodulin

Produced in septic shock states

• Due to TNF-a/inflammatory mediators inducing iNOS

• Requires hours to occur

NO induces vasodilation via sGC

• Forms cGMP that activates protein kinase G

• Produces vasodilation

Multiple new pathways discovered involving various factors

• VEGF, shear stress, estrogen

• PAF (anaphylactic, septic shock)

• Inflammatory cytokines (via NFkB pathway)

• P53 (tumor cells)

• Hypoxemia (HIF-1a inducible factor)

Nitric Oxide Pathway

• Binding of PAF to its GPCR via PI3 pathway upregulates the amount of eNOS available

• L arginine is converted to NO which diffuses into the adjacent vascular smooth muscle

• NO will stimulate sGC which increases cGMP which leads to vasodilation of vascular smooth muscle

Anaphylaxis Mediators of Vascular Permeability

PAF

Histamine

Prostaglandins

Leukotrienes

Vascular Permeability - Capillary Leak Syndrome

Massive fluid shifts

• 35% of IV volume into extravascular space in 10 mins

• Rapid hemodynamic collapse

Capillary leak - endothelial cell contraction via the actin-myosin light chain complex

• Ca2+-calmodulin and myosin kinase = pores

• Ongoing Ca2+ influx prevents conversion of ATP to cAMP favoring contraction = bigger pores

• Reversal requires epinephrine bindings to its B2 receptor (Gs protein) mediated cAMP formation from ATP

  • Or inhibition of cAMP phosphodiesterase

Histamine/PAF Induced Changes in Vascular Permeability

• Endothelial barrier integrity is maintained by cadherin molecules

• When histamine and PAF bind their receptors the phospholipase C, DAG, IP3 pathway releases Ca

• Ca can modulate myosin light chain kinase

• Activation of actin and myosin turns on contractile units which cause the endothelial cells to contract and pull away from each other and causes a disruption in the endothelial barrier

PAF Summary

PAF generated from mast cells

PAF binds its receptor PAFr

• Initiates and amplifies inflammation and thrombosis

• Leads to marked vasodilation and hypotension

• Increases vascular permeability contributing to massive fluid shifts

PAF/PI3K pathway increases eNOS and thus increases NO

NO activates soluble guanylate cyclase (sGC)

• Leads to marked vasodilation and hypotension

PAF bindings PAFr on systemic mast cells likely responsible for amplification of anaphylaxis

Late Phase Anaphylaxis

• 2-24 hours after allergen has bound to mast cells

• Takes so long because tyrosine kinase pathway has to activate factors within the nucleus of the mast cells, when these factors are activated have to have further transcription and translation of new proteins which takes time

• This is the stage that gets manipulated by steroids

Cytokines and Anaphylaxis

TNFa (late phase)

• Activates neutrophils

• Recruits effector cells

• Increases chemokines

Interleukins

• Inflammatory mediators

• IL13 antibody generation

• IL4 increases cellular responses (3-6x) to mediators

Primary Mediators of Anaphylaxis from Mast Cells

Histamine

Proteases

Heparin

NCFTA

ECTFA

Secondary Mediators of Anaphylaxis from the Arachidonic Acid Cascade

PGE2/PGD2

Prostacyclin

Leukotrienes

Thromboxane A2

PAF

Effects of Histamine in Anaphylaxis

Increased vascular permeability

Increased vasodilation

Increased bronchoconstriction

Increased gastric acid

Effects of Proteases in Anaphylaxis

Increased kinin, activate complement, DIC

Effects of Heparin in Anaphylaxis

Anticoagulant

Urticaria

Immune

Effects of NCFTA in Anaphylaxis

Neutrophil chemotaxis

Effects of ECTFA in Anaphylaxis

Eosinophil chemotaxis

Effects of PGE2/PGD2 in Anaphylaxis

Increased vascular permeability

Increased vasodilation

Increased bronchoconstriction

Effects of Prostacyclin in Anaphylaxis

Increased vasodilation

Platelet aggregation

Effects of Leukotrienes in Anaphylaxis

Increased vascular permeability

Increased vasodilation

Increased bronchoconstriction

Increased WBC

Effects of Thromboxane A2 in Anaphylaxis

Platelet aggregation

SM contraction

Therapeutic Options for Anaphylaxis

Epinephrine (IM then CRI)

Vascular access (IV vs EzIO)

Fluids (colloid/crystalloid)

Vasopressin/vasopressors (Bolus/CRI)

Airway (endotracheal/tracheostomy)

Anti-histamines (H1/H2)

Bronchodilators (Aminophylline)

Steroids - the after thought

Role of Epinephrine in Anaphylaxis

Signaling thorugh the PAFr can be downregulated agents that increase intracellular cAMP levels

Epinephrine acts in part by phosphorylation and inactivation of the PAFr

Early administration

• Reduces morbidity and mortality in human anaphylaxis

Delayed administration

• Associated with increased mortality

• Progressively less effective with time