Neuronal Control of Immunity Flashcards
What are the 3 ways ‘neuroimmunology’ can be considered.
• One area concerns inappropriate immune responses , eg) autoimmune reactions such as MS, as well sa cytokine mediation of inflammation injury
− Highlights a few problems regarding peptide mediators
− The terms cytokine and neuropeptide need careful use, as neither is limited to immune or nervous origin
− Eg, catecholamines primarily sourced from sympathetic nerves can be sourced from immunocytes
• A second category includes the study of homeostatic neuroendocrine mechanisms → the core of which is the adaptive effect of acute activation of the HPA axis that is sufficient to terminate inflammatory responses and steer Th cell cytokine balance → Lecture 5!
− Immune status coupled to neuroendocrine control via cytokine levels including IL-1
• A third category involves homeostatic activity mediated by direct, hardwired interactions between the nervous and immune systems
What do immunologists belief regulates homeostatic control of the immune system?
• Effective immunity requires the elimination of pathogens without host injury
• The immune system has long been considered a self-regulating autonomous system in the body → but it does need control to prevent oversuppression and infection/excessive activation and autoimmunity
• We can regard the immune system as a homeostatically controlled system, much the same as temperature regulation
• Immunologists believe this homeostatic control is regulated by immune cells themselves, with no outside influence.
• Therefore, immunology has produced theories of self-regulation in which:
− The immune system has built-in mechanisms of control and coordination
− Millions of different cells doing every kind of job for the self regulating system
− Immune sense, sentry/surveillance cells – DCs delivering parts of host challenge to lymph nodes to activate adaptive responses
− Cytokine network to inform different parts of the immune system what is going on
What is the problem with the immunologists view of immune regulation?
• If you get an infection in your thumb, how does the bone marrow know what it is order to produce the right cells in the right number to fight it?
• No information going to the bone marrow from the thumb apart from panic signals (IL-1, IL-6, TNFa) and they don’t say what the specific infection is
• Yet, the bone marrow produces exactly the right cells in exactly the right number
− How long would that take? Immunologists say it varies, but the immune system will always respond in the correct time frame
− How quickly does it respond? The response is always quicker the second time round, even though immunologists say the processes occurring are the same
- Can you explain this on immune theory alone?
- At the cellular level, little doubt that immune cells can regulate each other, but evidence suggests that neural pathways are essential for the co-ordination of immune activity.
What are the proposed functions of C-fibres?
• Smallest diameter, unmyelinated nerve fibres
• Lowest threshold of any nerve → above this threshold you feel pain
• Pain is thought to be their secondary function – because their have the lowest threshold, their primary function is thought to be the most important function in the body
• So, their function could be nociception OR
• Host monitoring/host defence:
− Includes nociception PLUS
− Monitoring and structural integrity of outer defences (skin/mucous membrane)
− Monitoring status of all organs
− Monitoring and registering host challenge
− Activating/controlling responses
− Remembering host challenges
• Another thing to note is that if all the fibres were merely pain fibres – there should be a large pain centre in the brain, and there isn’t.
What are the 3 requirements that must be met in order for C-fibres to have control over the immune system?
− They must be present and functional throughout the host defense system, inc. BM and lymphoid organs and sites of antigen exposure, so the static nervous system has as many contacts as possible with the mobile immune system.
− Immune cells must be capable of responding to neurotransmitters released by this fibres
− There must be sensory monitoring of host challenge and immune activity
What are the 3 neuro-immune components we need to identify in order to demonstrate neuronal control of immunity?
− Nerve connections to host defence systems
− Sensory system collecting host status information
− Central control centre integrating information and coordinating functions
What is the evidence of neuronal control of the spleen?
• Nerve fibres in the spleen thought to be vasomotor → but they are not associating with blood vessels, they use the vessels to find their way to the organ, but then they fan out to the T cell region. The B cell region is not innervated.
What is the evidence for neural innervation of bone marrow haematopoetic cells?
• Bone marrow transplant is associated with being one of the most painful procedures in man – why are there pain fibres in the BM?
• Nerve fibres are associating with CD11B positive cells –stromal cells
− Stromal cells are involved in the regulation of HSC function
• The presence of neuropeptide Y, substance P and other neurotransmitters has been noted
• The marrow is the only site where a ‘true’ synaptic interaction between nerve and haematopoietic components has been identified (Yamazaki & Allen)
• Perivascular cells completely ensheath blood vessels in the marrow, forming a barrier to cell movement → perivascular cells are targeted by nerve fibres.
• Stromal cells have receptors for neurotransmitters – CGRP, neuropeptide Y, substance P and somatostatin
• As stem cells are also static, there may be direct input to these for more rapid responses, and stem cells do indeed possess some neuropeptide receptors
What is the evidence for neural involvement in leukocyte release from the bone marrow?
• There is a dose response for neurotransmitters on whole bone marrow CFU-GM colony forming ability
− Tachykinins are positive
− CGRP has a bimodal response
− NE is inhibitory
• This is on whole bone marrow – we know there are abundant receptors on stromal cells. Do the nerves affect the stem cells?
− Do the same dose response experiment
− Only CGRP has a direct effect (bimodal)
− Therefore, apart from CGRP – neural control operates through stromal cells.
Loss of nerve control results in the release of CFU into the circulation = MDS/Leukemia:
• Cutting through the femoral nerve results in a sustained outflow of marrow cells → looks like full blown leukemia
• Elevated WBCs for as long as you keep the animal alive
− Suggests activation of haemopoeisis as well as opening of the perivascular barrier
• Stem cells increase over 1000%
• So from this single haematopoietic site losing innervation, you have uncontrollable release of WBCs and HSCs.
• These cells are not genetically abnormal like you would see in leukemia – they have just loss control
− For every genetic based blood disorder, there are over 300 similar ones that do not have a genetic defect
− Likely to be because of a control system fault
• See the same in spinal injury patients – look at the bone marrow below the injury – it has stopped working.
− Immune system below the site of injury isn’t working, you get poor wound healing etc…
→ This tells us nerve fibres are important for bone marrow function and immune response
Describe experiments using capsaicin to demonstrate a sensory system for immune responses
• Using capsaicin – TRPV1 agonist but toxic to sensory terminals at high doses – use this to specifically switch off C-fibres
• Effect of capsaicin treatment:
− Initially increases marrow and blood cellularity and bone marrow CFU-GM activity, but then decreases → by removing the C-fibres, you remove the bone marrow response
− So removing the challenge turns the response off (just like any normal sensory system)
− Interestingly → there is no effect on blood CFU-GM levels, indicating the response is due to a direct neural effect, not circulating factors.
• Not only is there a 50% reduction in output from the bone marrow, but the output seems largely restricted to macrophages
− No signals going to the marrow to tell it that it needs to produce different cell types
Describe the evidence for neurostransmitter control of leukocytosis in acute stress.
- Studies have shown that acute physiological stress induces the mobilization of lymphocytes into the blood, and that catecholamines (adrenaline) play a critical role
- Catecholamine lymphocytosis characterized by a rapid increase in lymphocyte numbers in the circulation, and NK cell numbers increase by 400%
- Effect is too rapid to be explained by proliferation so cells loosely adherent to venule walls, lungs, spleen, BM and lymph nodes are all possible reservoirs, and are all served by adrenergic nerve fibres.
Describe the evidence for an immune sensory system in the skin
- Every 3rd or 4th cell in the skin is an APC DC
- We know we have ‘free nerve endings’ in the skin
- If we stain for the presence of both, we can see that every one of the DCs is sitting on a C-fibre - so although widely considered to be free nerve endings, doesn’t appear to be the case – associated with cells of the immune system.
- Suggests an immune sensory system
Beresford et al,: Nerve fibres required for contact hypersensitivity in mice
• Long been accepted that peptide mediators from cutaneous C-fibres are potent inflammatory agents, giving rise to neurogenic inflammation and underlying a range of conditions including dermatitis, psoriasis, eczema, Crohns and colitis.
• Collateral c-fibre axons provide an efferent route for inflammatory cues to spread laterally
• A major component of allergic reactions both in the airways and skin, neurogenic inflammation may be intiated by the release of substance P and CGRP
• nM concentrations of SP induce mast cell degranulation and the release of histamine, as well as the release of TNFa
• Experiments have shown that neurons growing in tissue culture are attracted towards a mast cell-like line and can form sustained contacts, followed by increase in the number of granules they harbor
• This only occurs in cells directly contacting a nerve membrane
• Nerve fibres also release potent pro-inflammatory mediators themselves (noradrenaline) – important in promoting vasodilation and vascular permeability
Experiment:
• Model whereby DNCB hapten given to abdnomen to sensitized, then applied 4 days later to the ear to elicit a response
• Chemical deletion of nerve fibres at the site of sensitisation with capsaicin abolished the contact hypersensitivity response of DNCB at the challenge site, providing denervation was carried about before initial sensititisation (remember – CHS involves sensitization phase and elicitation phase)
• Same outcome was observed following denervation at both site of sensitization and challenge (abdomen and ear)
→ Dermatologists could use this to turn off skin reactions like eczema and psoriasis! Use capsaicin cream.
Local neuroendocrine modulation of skin function:
• Cells within the skin are capable of not only responding to neuroendocrine mediators, but can synthesise and process the same mediators themselves
• This generates an additional local level of fine-tuning of immune responses
• CRH, alpha-MSH and ACTH can all be produced by cells in cutaneous compartments
• Along with resident macrophages, keratinocytes harbor b-adrenoreceptors and can also produce catecholamines
• Defects in catecholamine responsiveness have been implicated in vitiligo and atopic eczema
• Treatment of LCs with adrenaline inhibits their antigen presenting capacity and diminishes contact sensitivity responses
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Describe the evidence for an immune sensory system in the gut
- Known that free-nerve endings act as nociceptors, but appears that free-nerve endings also have TLRS for direct antigen challenge receiving
- Every microvillous in the intestine is filled with nerve fibres, associated with epithelium → some are in direct contact with the gut contents
- More nerve fibres in the gut than in the spinal cord – known as the second brain!
- Thought they were always associated with muscles to control peristalsis or secretory cells, but here they are associating directly with gut contents
- May be influencing microbiome → neurotransmitter release may attract certain microbiota over others
What cell types are typically associated with free nerve endings?
• Endothelial cells → associations involved in neuroinflammatory responses including oedema, opening up the vessels and allowing fluid accumulation
• DCs →
− Interesting observation that young children can get tolerised to allergens upon exposure and not have a response
− However, older people, upon travelling to different countries, can have non-tolerogenic responses due to encountering antigen they were not tolerised against as a child
− What causes the difference?
− In children, nerves are not hard-wired → can decided not to elicit a response
− In adults, they are hard wired → you are forced to respond
− If you ‘take the nerves back to childhood’ eg, disconnect the wirings – can remove the response, and get tolerance. If you re-connect the nerves, the allergy has disappeared!
− Has been shown experimentally
• Reticular cells
• Mast cells → oldest cells we know to be connected to nerve fibres. Mast cells degranulate in response to stimulation
• T cells
• Stromal cells and HSCs
Describe the experiments that suggest a higher control centre for host defence and neuronal memory
- Using viral trans-synaptic tracing mathods, a pathway from bone marrow, spleen and lymph nodes to the spinal cord, medulla, hypothalamus and insular cortex has been described
- This reinforces the hypothesis that the BM and immune system function can be brought under the control of the autonomic nervous system
- Regulatory loops could be local, depending only on nerve fibres in the periphery, or decisions could be made at the level of the sympathetic ganglia, spinal cord, or brain itself
Time course of PRV following inoculation to the BM:
• Injection of PRV into the BM followed by it jumping synapses to the root ganglia → spinal cord → medulla → hypothalamus → ends in the cortex
• Injection into liver, spleen and lymph nodes has same path
→ Seems to be a cortical centre for host defence and immune function
• Recordings from the PVN and preoptic nucleus of the hypothalamus to infection shoes an immediate response of neurons.
• Exposure of a rat sitting in a functional imaging system to a contact allergen on day one shows mild cortical neuronal activity – exposure to the same allergen on day 3 gives a much greater response → seems to be a form of neural memory.
− Suggests two memory forms, classical immune memory and also neural memory
− Evidence for this is that you can elicit an antibody response without antigen via classical conditioning → if you associated the antigenic challenge with a smell, years later you can induce an antibody response if you stimulate with the same smell
− This is the basis of sick building syndrome – smell of a building can elicit a similar response! These kind of things have been treated by changing the smell of buildings every few months!
• Can show this is mediated by C-fibres by killing them with capsaicin
− Cortical response to DNCB contact allergen almost extinguished by disabling C-fibres
− Residual activity to DNCB may be mediated by non-TRPV1 pathways, eg) TLRs on c-fibres
So, there seems to be a central control system for coordinated host defence, and there is an immediate brain awareness of host challenge.
