TODD SINK

Ass is tant Pro fessor – Department o f Wi ld l i fe & F i sher ies Sc ience

Aquacul ture & F i sher ies Extens ion Spec ia l i s t – Texas A&M Agr iL i fe Extens ion

Serv ice

Neuroendocrine-Immune Interactions

Neuroendocrine–Immune Interaction

Physical, chemical, & biological disturbances evoke large range of endocrine & immune responses

Light penetration in water Low chloride concentrations Hunting otter or pathogen

Responses to external stimuli

Neuroendocrine & immune systems interact bi‐directionally Feedback loop Step 1 - pathogen recognition is communicated to brain by

immune system Step - immune response is influenced & partially

coordinated by physiological changes brought on by hormone release from brain

Neuroendocrine–Immune Interaction

Clear communication requires common “language” of signaling molecules & receptors

Can’t coordinate responses without means of communication

Comm. network includes corticosteroids, pituitary hormones, cytokines, neuropeptides, as well as neural pathways These “signal carriers” can be interpreted by brain,

endocrine system, & immune system, for communication Maintaining balanced internal environment

(homeostasis) based on dynamic equilibrium of bi‐directional physiological processes Fluid process of communication, hormone production,

stress, response, & immune response used to keep majority of physiological & biological processes “normal”

Neuroendocrine–Immune Interaction

Equilibrium constantly susceptible to actions of intrinsic & extrinsic stimuli or “stressors” Physical, chemical or biological nature

Examples- pathogens or threats from predator Equilibrium being constantly challenged by environment

Stress & secondary hormone hierarchy cause decreased immune functions in fish Cortisol, secondary stress hormone

Long term elevations in cortisol reduce competency of immune system

Bacterial pathogens invoke elevated immune response, but also stress response

Implies signals from endocrine system elicit effects on immune system – negative feedback loop

Neuroendocrine–Immune Interaction

Evidence some signals are variants of an evolutionary conserved communication system Blalock 1984 – evolutionarily, immune system served a

sensory role A “sixth sense” to detect pathogens & tumors body could

not hear, feel, smell or taste

Prior to ability of immune system to detect certain pathogens & tumors, so they had to be “pointed out” & communicated

To launch appropriate neuroendocrine response, pathogen detection needs to be signaled to nervous system for coordination of response among different systems

Neuroendocrine–Immune Interaction

Fish - intimate contact with aqueous environment Can harbor large quantities of pathogens

Compare with terrestrial environment

Teleost among oldest vertebrates Equipped with well‐developed immune system

Elaborate communication system of cell–cell contacts & humoral (body fluid) factors (cytokines)

Communicate pathogen recognition & coordinate response measures of different types of leukocytes (white blood cells)

Cytokines are polypeptides (small proteins) or glycoproteins that have cell signaling & localized hormone-like function

Neuroendocrine–Immune Interaction

Cytokine production is transient & actions are usually local

Signal communication & production change based on area of need (infection or wound)

Autocrine & paracrine rather than endocrine

What do these terms mean & what part of body do they effect?

Alternatively, hormones usually produced by limited cell types

Directed (transported) to restricted target cells to elicit specific responses

Cytokines produced by different cell types, elicit multiple effects in different target cells (pleiotropy) & may demonstrate functional redundancy

Neuroendocrine–Immune Interaction

Many signaling molecules referred to as either cytokine or hormone

Many of these belong to same molecular family of structurally related proteins

Members of which have evolved from a common ancestor (single protein or glycoprotein)

Compare with animal evolution based on ecosystem type

Neuroendocrine–Immune Interaction

Need for balanced communication in optimal stress coping & wound healing Keep in mind that although a powerful inflammatory

cytokine response is essential to overcome bacterial infection, but

May cut as a double‐edged sword as a too strong inflammatory response may lead to destruction of host tissues Arthritis, severe swelling of infected tissues, swelling of brain

Stress induced energy redistribution as result of enhanced cortisol activity

Pathogen infection is crucial extrinsic stressor May result in suppression of immune functions when

survival is at stake

Neuroendocrine–Immune Interaction

Finally, immune responses may be related to: Smolting

Temperature or season

Gender

Circadian rhythm

Underlines physiological importance of interaction & communication between neuroendocrine & immune systems Neuroendocrine system signals if an immune system

response is needed or not due to these external stimuli

Immunity in Fish

Fish have developed effective protective immune system

Level 1 prevention

Produce biochemical barrier to prevent pathogen invasion

Integumental (natural covering) mucus layer contains many anti‐bacterial peptides: lysozyme, lectins & proteases

Level 2 invasion response

When pathogens pass mucus barrier

An array of soluble (biochemical) & cellular defense (biochemical & physical) mechanisms is activated

Immunity in Fish

Fish earliest vertebrates that developed both arms of immune system Innate & adaptive immune response

Innate immune system functions in “generic” recognition of pathogens & prevention of pathogen dispersal Generalists - Basically recognize & attack any pathogen

without any specificity for a particular pathogen • Napalm pray & pray defense

Adaptive immune system specifically recognizes pathogens on basis of specific surface antigens Specialists - Clears an infection via production of

antibodies & cytotoxic lymphocytes specific to pathogen• Smart bomb pinpoint offense

Immunity in Fish

Most importantly, adaptive immune system will generate a memory If fish encounters that specific pathogen again, it can

be immediately recognized by adaptive immune system

Immune response generated much more quickly & specific to that pathogen Why is this important?

Think temperature & energy

The combined immune responses regulated to be rapidly terminated after pathogen clearance Prevent collateral damage to host

Immune Organs & Communication

Fish lack bone marrow as a primary immune organ Instead, hematopoietic (blood cell forming) cells reside in

head kidney Produce both

myeloid (granulocytes) –many are phagocytes • Secret substances that stimulate monocytes & macrophages

(battleships – engulf & digest pathogens)• Reach infection site within 30 minutes of signaling, turn to pus

cells & die – granules within are potent protein destroyers• Chemical defense- ingest & digest, antimicrobials (granule

proteins), extracellular traps lymphoid immune cells (lymphocytes)

• Natural killer cells, T (thymus) cells, B (bursa-derived) cellsCytotoxic innate & adaptive immune response

Immune Organs & Communication

Fish also lack lymph nodes as secondary immune organs Spleen & head kidney are sites for interaction of

immune system with antigens Contain antibody‐producing lymphocytes (B cells)

Thymus (anterior to heart) is center of T‐lymphocyte maturation

Mucosal immune system associated with epithelial surfaces of gut, gills & skin forms first line of defense Connective associated tissues densely populated with

immune cells to attack any penetrating infectious agent

Immune Organs & Communication

Immune cells communicate via cytokines & associated receptors Critically important for fast & effective attack on pathogens Minute amounts of cytokines generate strong inflammatory responses

Tight control over cytokine production required to prevent cell damage

Immune cells produce different types of cytokines dependent on stage of infection & function of particular cell Among cytokines - distinguish interleukins (ILs) & chemokines

ILs communicate between leukocytes Chemokines, an acronym for chemo‐attractant cytokines, supervise cell

migration (chemotaxis)

Cytokines elicit broad range of actions on cell growth & differentiation Cytokine expression after detection of pathogen ensures effective

clearing of pathogen while minimizing damage to host

Prerequisites for Interaction with Neuroendocrine System

Widespread presence of hormone receptors in/on immune cells E. Receptors hypothesized to

Create delicate balance between immune stimulation & suppression

Contribute to normalized neuroendocrine functioning

For neuroendocrine system to reciprocally receive signals from immune system Endocrine cells need specific cytokine receptors &

Cytokines need to be produced close to endocrine cells, or

Should be released into circulation

Prerequisites for Interaction with Neuroendocrine System

Direct (para) sympathetic innervation of teleost immune system is possible

Catecholamine receptors of sympatho‐adrenomedullary system are present on immune cells

Hormone receptors found on/in immune cells of higher & lower vertebrates, or

Presence corroborated by a functional response after hormone stimulation

Cytokines that affect neuroendocrine functions are produced by immune‐related cells in brain

Prerequisites for Interaction with Neuroendocrine System

In addition to endocrine effects Paracrine & autocrine effects within immune system

possible if immune cells are capable of producing hormones Traditional hypothalamic & pituitary hormones modestly but

widely expressed in fish immune cells Creates shortcut bypassing brain

Corticotropin‐releasing factor (CRF) immunoreactivity found in macrophage‐like cells of gill & skin

Pro‐opiomelanocortin (POMC)‐derived peptides Adrenocorticotropin (ACTH), β‐endorphin & alpha‐

melanocyte‐stimulating hormone (α‐MSH) Demonstrated in thymus of fish

In leukocytes, growth hormone & prolactin expression has been confirmed

Other classical hormones such as leptin I & II were found in thymus & spleen

Prerequisites for Interaction with Neuroendocrine System

Teleost head kidney is immune & endocrine organ: Hematopoietic tissue situated adjacent to endocrine tissue

Textbook location for neuroendocrine–immune interaction

Chromaffin cells of head kidney produce catecholamines (epinephrine/norepinephrine)

Interrenal cells of head kidney produce cortisol These make head kidney functional analogue of mammalian

adrenal gland

At same time, both lymphoid (B‐ & T‐lymphocytes) & myeloid (phagocytic) cells are produced in head kidney hematopoietic tissue Makes it functional analogue of mammalian bone marrow

Cytokine Families & Phylogenetic Relationship between Cytokines & Hormones

Separation of hormones & cytokines convenient to define particular field of research However, separation of hormones & cytokines is arbitrary

Cytokines also released into circulation & signal in an autocrine, paracrine & even endocrine fashion

Availability of whole genome databases of several teleost Make possible to analyze phylogenetic relationships of gene

families

Analyses show many signaling molecules referred to as cytokines, hormones or growth factors All belong to same family of structurally related proteins

All members of family probably evolved from common ancestor through series of successive duplication events

Although, level of sequence conservation varies greatly

Classification of Cytokines & their Receptors

Many cytokines & cytokine receptors grouped as families Cytokine families have a high degree of sequence homology (same basic

structure)

However, tertiary structure (side structures that make cytokine unique & these structures dictate reception & function) & presence of distinct family themes also important for classification Most important cytokine families are

Type I cytokine family

• Enhance cellular immune responses - antigen-stimulated lymphocytes production, leukocytes stimulation, erythrocyte (red blood cell) production stimulation

Type II helical cytokines

• Favor antibody responses - host defense, immune regulation, somatic growth, reproduction, food intake & energy metabolism, regulation of neural growth

IL (interleukin) ‐1 family

• IL-1 family is group of 11 cytokines, which play central role in regulation of immune & inflammatory responses to infections

Classification of Cytokines & their Receptors

Among families of cytokines, type I cytokine family of ligands & receptors is most extensive

Ligand is substance (hormone or cytokine) that forms complex with a biomolecule (cell receptor) to serve a biological purpose

Ligand usually signal-triggering molecule binding to site on target protein on target cells

Binding results in a change of conformation of target protein – on/off switch

Type I cytokine receptor family, comprises receptors for many:

Crucial hematopoietic (blood cell formation-related) cytokines (e.g., IL‐2, IL‐3, IL‐4, IL‐5, IL‐6, IL‐7, IL‐12 & IL‐13)

Hematopoietic growth factors e.g., colony‐stimulating factors (CSFs) Growth factors e.g., growth hormone (GH) & prolactin (PRL)], & satiety

factor leptin Family of ligands (with four tightly packed α‐helices) & receptors

comprises array of “endocrine” & “immune” signals that co‐evolved from same ancestral precursor molecule

Classification of Cytokines & their Receptors

Type II helical cytokines (HCII) IL‐10, IL‐26, IL‐22, & interferons (signaling proteins)

Composed of two cytokine molecules that contain six α‐helices each.

Evolution of HCII & receptors suggests diversification of ancestral mechanism necessary for host defense against infections

IL‐1 family Pro‐inflammatory cytokines (IL‐1β & IL‐18), but also

receptor antagonists Characterized by three‐dimensional structure with fold rich in

beta strands (chain of continuous amino acids) Few IL‐1 family members & their receptors have unambiguous

orthologues (distinctive common ancestor) in fish However, phylogenetic analyses demonstrates shared signaling

mechanisms in neuroendocrine & immune systems