Exam combination (immunology)

mucosal infections

• skin and various mucosae lining organs are in continuous contact with environmental microbes

• aberrant immune responses to harmless agents — celiac disease

  • most pathogens that cause death are either mucoasal surface pathogen or enter through mucosal surface

prevention of infection in the gut

• presence of thick mucus layer keeps most organisms in lumen away from intestinal epithelium

• antimicrobial peptides produced by intestinal epithelial cells

• IgA antibodies (produced via plasma cells in lamina propira) → transported into the lumen and neutralizes pathogens before entry into epithelium

distinctive features of mucosal immune system

• intimate interactions between mucosal polarized epithelia and lymphoid tissues

• discrete compartmants of diffuse lymphoid tissue and organized stuctures — peyer’s patch, isolated lymphoid follicales and tonsils

• specialized antigen uptake mechanisms

• broad surface area in contact with environmental agents/microbes

epithelium types

• simple columnar

• pseudostratified columnar epithelium

• non keratinized stratified squamous epithelium

• keratinized stratified squamous epithelium

effector mechanisms of the mucosal immune system

• activated/memory T cells predominate even in the absense of infection

• multiple activated ‘natural’ effector/regulatory T cells present

• production of mucins and mucus

• secretory IgA antibodies

• production of antimicrobial peptides (AMPs)

• presence of distinctive microbiota

Discrete compartments of the mucosal immune system

• cells of mucosal IS located both in anatomically defined compartments + scattered through mucosal tissue

• lymphocytes are in organised tissue (peyer’s patch), isolated lymphoid follicles forming GALT

• mesenteric lymph nodes (gut draining LN) — connected to peyer’s patch and intestinal mucosa via afferent lymphatic vessels (largest LN in body)

immunoregulatory environment of MIS

• active down regulation of immune response predominates at homeostasis (to food and other innocuous antigens)

• inhibitory macrophages and tolerance-inducing dendritic cells

• high number of FoxP3+ Treg cells and FoxP3-Tr1 cells

goblet cells

• simple columar epithelial cells that develop from stems cells in bass of crypt

• contain mucopolysaccharide in secretory granules — expand up to 500-fold after release

• act as a physical barrier to the motility and feeding of bacteria and other pathogens (intestinal parasites)

• releases Th2 cytokines including IL-5 and IL-13

gastrointestinal cells and host defence

• intestinal epithelial cells produce antimicrobial peptides that kill pathogens or reduce their entry into the epithelium

• M cells transport antigens to underlying DC

• goblet cells secrete mucus in response to microbes + metabolites (Th2 cytokines)

• Paneth cells produce antimicrobial peptides

• tuft cells detect pathogen molecules → produce IL-25 → induce ILC2 → produce IL4 and IL13

Microfold (M) cells

transport antigens from the lumen of the intestine to underlying dendritic cells for antigen delivery

Paneth cells

• in the small intestine

• produce antimicrobial peptides such as cathelicidins and defensins

tuft cells

detect pathogen molecules (chemoreceptions), and produce IL-25, which induced ILC2s to produce IL-4 and IL-13 and help initiate a Th2 type response

M cell structure

• thin glycocalyx, short microvilli and large fenestrations in htier membranes

• able to transport antigens in endocytic vesicles across the cytosol and deliver them via endocytic vesicles across the cytosol → deliver them by exocytosis at the basolateral membrane

• convoluted basal membranes in M cells form “pockets” → allows close contact with DC for antigen presentation

mucosal macrophages

• lamina propria contains largest population of macrophages in the body → highest population in small intestine

• located immediately under the epithelium, rep 75% of all mononuclear phagocytes

• highly phagocytic but with a non-inflammatory profile

major function of gastrointestinal tract humoral immunity

neutralise luminal microbes and mediated mainly by IgA produced in the lamina propria and transported across the mucosal epithelium into the lumen

MIS antibody production

• IgA is produced in higher amounts than any other antibody isotype, smaller quantaties of IgG and M are also secreted into the gut lumen

• IgA-secreting plasma cells are widely dispersed in the lamina propria of the gastrointestinal tract not just in lymphoid follicles

intrepithelial lymphocytes

long-lived resident effector cells that are interspersed between epithelial cells

instestinal conventional and unconventional T cells

• IEL

• most T cells are CD8+ in small intestine

• Mostly CD4+ in Lamina propria

• most T cells in healthy lamina propria have been activated by DC and express markers of effector or memory T cells

• other unconventional T cells reside in intestinal epithelium — TCRγδ and TCRαβ

IEL function

• recognise and destroy intestinal epithelial cells that display properties of infection, damage or stress

• highly motile within the intestinal epithelium and present a fully mature effector status

microbiome

• community of microorganisms that naturally that exists in a particulr environment

• bacteriome — major component of human microbiota

• virome — primarily composed of bacteriophages

• archaeome — methanogens i.e Methanobrevibacter smithii

• mycobiome — Saccaromyces, Malassezia and Candida genera

• parasitome — Blastocystic spp.

gut microbiota

• one of the largest components of our body

• composed of mostly of bacteria, also arachaea, viruses, fungi and protozoa

• symbiotic relationship with human body — protect and support the structure of intestinal mucosa

• may be influenced by a complex combination of environmental, genetic and lifestyle factors

dysbiosis

observed in metabolic disorders i.e diabetes, obesity, in cancer, autoimmune disorders, and stress related neuropsychiatric disorders i.e depression and anxiety

probiotic organisms

non-pathogenic, living microbes that provide a benefit to the host

probiotics

specific nutrients (usually complex carbohydrates) that support and encourage the growth of beneficial commensal microbes

postbiotics

referes to the waster left behind after digestion of prebiotics and probiotics and may include nutrients ie vitamins B and K, amino acids, and antimicrobial peptides

immune privilege

• coined by Peter Medawar in 1948 to describe the absence of an immune response to allografts placed into the anterior chamer of the eye or brain

immune privilege sites

• brain, eye, testes, placenta, and foetus

• have mechanisms that suppress inflammation and promote immune tolerance

• lack of classical lymphatic drainage system for antigen-presenting cells that reside in the parenchyma and perivascular spaces

• lack of cell-mediated response to instilled antigens

• prolonged survival of tissues when grafted into the brain, eyes, testicles, compared to other sites

parenchymal microglial cell

• brain resident macrophages

• enters brain very early in embryogenic phase — derived from yolk-sac precursores

• functions as immune sentinels and contributes for maintenance of brain homeostasis — synaptic remodeling, neurogenesis, and the routine clearance of debris and dead cell

choroid plexus

• modified ependymal cells that produce CSF

• acts as diffusion barrier between blood and csf and it is home to various immune cells

• choroid plexus is key point of entry for peripheral immune cells into csf space

epiplexus cells

• innate immune cells located in the choroid plexus of brain ventricles

• express markers of macrophaes, dendritic cells and microglia

• function includes phagocytosis, antigen presentation, iron accumulation and NO production

meningeal, perivascular, and ventricular macrophages

• re-stimulation of lymphocytes with peptide-MHC complexes

• allow primed lymphocytes to enter the CNS parenchyma

• derived from blood-borne monocytes (population homeostasis)

• neutrophiles and other granulocytes are absent from the health CNS

Cerebrospinal fluid

• CSF surrounds the brain and spinal cord, both internally and externally

• mainly produced by the choroid plexuses — highly vascularised tissues located within each ventricle of the brain

• reabsorption into blood via arachnoid villi

immune cell content of CSF

• 90% T cells

• 5% B cells

• 5% monocytes

• <1% dendritic cell

• T cells constitutively monitor the CNS by trafficking through CSF

CSF cytology

• Colourless and clear

• antibodies and complement is normally absent

• up 5 WBCs per mm³ in adults and 20 WBCs per mm³ in newborns

blood brain barrier

• depends on stringent ionic homeostasis

• strictly controls the movement of solutes across the CNS vasculature

• direct membrane-membrane contact between endothelial cells — tight and adherens junctions seal capillary endothelium

• pericytes aid in regulation of transcellular barrier and maintenance of vessel function

glia limitans perivascularis

inner most layer of BBB formed by astroglial endfeet surrounding blood vessels

BBB disruption

• vasogenic agents — histamie, thrombin, proinflammatory cytokines

• infectious agents — bacteria, bacterial toxins, viruses, parasites, and fungal pathogens

T cell cross reactivity

reaction of T cells to more than one distinct peptide - MHC ligand

molecular mimicry

resemblance between epitopes contained in microbial and host proteins, leading to cross-reacitivity of T cells in the host

TCR binding degeneracy

refer to the promiscuity of T cell receptor engagement that allows a single TCR to bind to different peptide-MHC complexes

original antigenic sin

• A footprint of immne response is established during the first exposure to a pathogen.

• specific memory T populations are preferentially re-expanded when reexposed to the same antigen or one that is similar

• limits the clonal expansion of new antigen specific T cells

trained immunity

long term functional reprogramming of innate immune cells by exogenous or endogenous activation events → altered response to a secondary challenge after the return to a non-activated state

immune surveillance

physiologic function of the immune system is to recognize and destroy clones of transformed cells before they grow into tumours and to kill tumours after they are formed

tumour specific antigens

antigens that are expressed on tumours cells but not on normal cells

• some are unique to individual tumours, whereas others are shared among tumours of the same

tumour associated antigens

tumour antigens that are also expressed on normal cells.

• these antigens are normal cellular constituents whose expression is aberrant or dysregulated in tumours

Products of mutated genes

• oncegenes and mutated tumour suppressor genes produce proteins that differ from normal cellular proteins → can induce immune responses

• tumour antigens may be produced by randomly mutated genes whose products are not related to the malignant phenotype

antigens of oncegenic viruses

products of oncogenic viruses function as tumour antigens and elicit specific T cell responses that may serve to eradicate the tumours

Antigens of oncogenic viruses

• viral peptides are foreign antigens — virus induced tumours are among the most immunogenic tumours known

• virus encoded tumour antigens are not unique for each tumour but are shared by all tumours induced by same type of virus

oncofetal antigens

proteins that are expressed at a high level in cancer cells and in normal developing foetal but not adult tissues

• encoding genes are silenced during development — de-repressed with malignant transformation

tissue-specific differentiation antigens

tumours may express molecules that are normally expressed only on the cells of origin of the tumours and not on cells from other tissues

principal mechanism of adaptive immune protection against tumours

• Killing of tumour cells by CD8+ CTLs

• surveillance function by recognizing and killing potentially malignant cells that express peptides from tumour antigens

cross presentation of tumour antigens by dendritic cells

• most tumour cells not derived from APC → do not express co-stimulators needed to initiate T cell responses/class II MHC needed.

• cross presentation by DC needed sometimes

Escaping immune recognition by loss of antigen expression

• Immune responses to tumour cells impart selective pressures that result in survival and outgrowth of variant tumour cells with reduced immunogenicity

• process is called tumour immunoediting

• tumours developing in setting of normal immune system become less immunogenic over time

immune evasion by tumours

• Tumour immunoediting

• Class I MHC expression down regulation → not recognised by CTL

• secreted products of tumour cells may suppress anti-tumour immune responses → TGF-B

• Treg may supress T cell response to tumours

phases of tumour immunity

• elimation phase — number of immune cells can recognise and eliminate tumour cells

• equilibrium phase — variant tumour cells arise that are more resistant to elimination

• escape phase — one variant may escape killing mechanism/recruit regulatory cells to protect it → spreads unchallenged

Tumour vaccines

• id of peptides recognized by tumour-specific CTL and cloning of tumour-specific antigen genes provides candidate antigens for vaccines

• most are therapeutic vaccines — have to be given to host after encountering the tumour → has to overcome immune regulation that cancers establish

immune checkpoints/checkpoint inhibitors

• novel group of monoclonal antibodies with proven effectiveness in a wide range of malignancies

• promote anti-tumour immune response by blocking signalling via CTLA4 or PD1 pathway

CTLA 4 mechanisms of action

• blocking CTLA-4 with antibodies results in increased immune responses

• CTLA-4 expression is low on most T cells until the cells are activated via antigen → once expressed it terminates continuing activation of responding T cells

• expressed on Treg and mediates suppresive function of cell by inhibiting activation of naive T cells

PD-1 check point protein

• ligated PD-L1 expressed on APC and many other cells

• ligand leads to inactivaion of T cells or inhibiting signal transduction from TCR

• terminates peripheral response of effector T cells

• antibodies that block PD-1 and PD-L1 allows T cells to kill tumour cells

CAR T cells

• Chimeric antigen receptors in T cells

• patient peripheral blood T cells are isolated — stimulated with anti-CD3 or anti-CD28 and subjected to gene transduction with car encoding vectors

• expanded in vitro and injected into patient

• undergo further robust proliferation in patient → tumour killing achieve via cytotoxic and cytokkine mediated mechanisms

CD19 CAR

single chimeric protein that is the recognition domain of a specific antibody against a tumour associated antigen and intracellular signalling domain capable of activating T cells

anti-tumour antibodies

• may eradicate tumours by the same effector mechanisms that are used to eliminate microbes — opsonization and phagocytosis, complement system etc

• outgrowth of antigen loss variants no longer express antigens that the antibodies recognize

cytokine

general term for any soluble protein secreted by immune cells that affects the behavior of cells bearing appropriate receptors

biological activity of cytokines

• pleiotropic activity

• redundant activity

• synergist activity

• antagonist effect

• cascade induction

pleiotropic activity

different biological effects depending on the nature of the target cell

redundant activity

two or more cytokines can mediate similar functions

synergist activity

combines effects of two cytokines on cellular activity is greater than the additive effects of individual cytokines

antagonist effect

the effect of one cytokine inhibits the effects of another

cascade induction

the action of one cytokine on one target cell induces that cell to produce one or more additional cytokines

chemokines

small cytokines that bind to cell surface receptors and induce cell movement towards the chemokine source. Low molecular weight and structurally homologous, with highly conserved disulphide bonds

roles of chemokines

roles in inflammation, development of lymphoid organs, regulation of lymphocyte trafficking in LN, migration of DCs from site of infection into draining lymph nodes

Chemokines and chemotaxis of inflammatory cells

• increase adhesion of leukocytes to endothelium

• induce migration of leukocytes toward site of infection of tissue damage

• chemokine gradients strongly bias the actin assembly to the cell’s leading edge and in the direction of cell movement

Functions of the immune system

• immunological response

• immune effector functions

• immune regulation

• immunological memory

myeloid lineage

red blood cells, megakaryocytes, granulocytes, monocytes, macrophages and dendritic cells

lymphoid lineage

B lymphocytes, T lymphocytes, innate lymphoid cells, and natural killer cells

defensins

Disrupt bacteria, fungi, and membrane envelopes of some viruses

cathelicidins

secreted constitutively by epithelial cells, and in addition to antimicrobial activities, also modulate the immune response

histatins

potent antifungal

complement system

several plasma proteins that work together to opsonise microbes, promote phagocyte recruitment, and directly kill microbes

acute phase proteins

biologically active molecules, secreted mainly by hepatocytes, that assist the host in eliminating bacteria

C-reactive protein and Mannose-binding protein

activate complement and act as opsonins; CRP recognises altered self and non-self molecules

serum amyloid proteins

induce production of proteins tha degrade the extracellular matrix; recruit immune cells to inflammatory sites

phagocytes

internalise opsonised microbes into phagosomes where they are digested

NK cells

kill targets, without killing themselves, by releasing lytic granules perforin and granzymes

humoral immunity

B lymphocytes secrete antibodies that eliminate extracellular microbes

Cell mediated immunity

T lymphocytes either activate macrophages to destroy phagocytosed microbes or kill infected cells directly.

Functional significance of specificity

ensures that immune responses are precisely targeted to microbial pathogens

Functional significance of diversity

enables immune system to respond to a large variety of antigens

Functional significance of memory

leads to enhanced responses to repeated exposures to the same antigens

Functional significance of clonal expansion

increases number of antigen-specific lymphocytes from a small number of naive lymphocytes

Functional significance of specialization

generates responses that are optimal for defence against different types of microbes

Functional significance of Contraction and homeostasis

allows immune system to respond to newly encountered antigens

Functional significance of nonreactivity to self

prevents injury to the host during responses to foreign antigens

Antibody function

• recognises epitopes on surface of antigens

• antigens may be recognise by more than one antibody when more than one epitope exists

• the antigen-binidng site can accommodate soluble macromolecles in their native state

neutralisation

• neutralises diptheria toxin by blocking attachment to target cells

• blocks locally acting toxins or extracellular matrix-degrading enzymes

• bind to flagellum and interfere with motility

• prevent bacteria binding to epithelial cells

haematopoietic stem cells

gives rise to all red and white blood cells

macrophage

phagocytosis and activation of bactericidal mechanisms, antigen presentation and cytokine production

neutrophil

phagocytosis and activation of bactericidal mechanisms. Degranulation and release of neutrophil extracellular traps (NETs)

• most abundant population of circulating white blood cell and acute inflammatory reactions

• cytoplasmic granules have lysozyme, collagenase, elastase, defensins and cathelicidins

basophil

promotion of allergic response and augmentation of anti-parasitic immunity