Kuby Immunology Ch. 5

7/25/2019 Kuby Immunology Ch. 5

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1.1: Intro

The innate immune system includes anatomical barriersagainst infection both physical

and chemical as well as cellular responses.

o Anatomical Barriers: The main physical barriersare the epithelial layers of the skin and the

epithelial layers of the mucosal and glandular tissue surfaces. pithelial

barriers pre!ent infection by blocking pathogens from entering the body.

The main chemical barriersat these surfaces are speciali"ed soluble

substances that possess antimicrobial acti!ity A#$ acid p%.

o &ellular 'esponses:

The cellular innate immune response occurs after an infectious agent

o!ercomes the initially epithelial barriers.

This response is rapid.

It is triggered by cell surface or intracellular receptors that recogni"econser!ed molecular components of pathogens.

(hat does it constitute) *acrophages and neutrophils +white blood cells, are

acti!ated to engulf and destroy microbes !ia phagocytosis. -ther receptors

induce production of proteins and other substances that ha!e !arious effects

such are direct antimicrobial acti!ity or recruitment of fluid cells and

molecules.

The influ/ of fluid cells and molecules causes swellinginflammation.

'0' T- 0I2' 341

1.5: Anatomical Barriers to Infection

'0' T- 0I2' 345

6ets go into more detail about these barriers to infection. The epithelial barriers include the skin

and the tissue surfaces connected to the body7s opening: the mucous epithelial layers that line the

respiratory gastrointestinal and urogenital tracts and the ducts of the secretory glands such as

the sali!ary lacrimal and mammary glands


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A. pithelial Barriers 8re!ent 8athogen ntry into the Body7s Interior

pithelial Barrier 91: kin

kin has two layers: thin outer layer +epidermis, and thick inner layer +dermis,

Epidermis:

o &ontains tiers of epithelial cells

o &onsists of mostly dead cells filled with a waterproofing protein called keratin.

Dermis:

o &omposed of connecti!e tissues and contains blood !essels hair follicles sebaceous

glands sweat glands and scattered myeloid leukocytes such as dendritic cells

macrophages and mast cells.

pithelial Barrier 95: Tissue urfaces

Instead of skin the respiratory gastrointestinal and urogenital tracts as well as the ducts of

the sali!ary lacrimal and mammary glands are lined by strong barrier layers of epithelial

cells stitched together by tight junctionsthat pre!ent pathogens from s;uee"ing between

them to enter the body.

-ther Barriers

ecretions of secretory tissue +mucus urine sali!a tears and milk, wash away potentialin!aders and also contain antibacterial and anti!iral substances.

Mucusfrom mucosal epithelial layers entraps foreign microorganisms.

Ciliain the respiratory tract are hairlike protrusions of the cell membrane which co!er

epithelial cells. The mo!ement of cilia propers mucus4entrapped microorganisms from the

respiratory tract.

The flow of urinesweeps bacteria from the urinary tract.

Salivahas antimicrobial compounds that attack microbes the moment we ingest food.

The !agina has !aginal secretions with an acidic pH.

ome organisms e!ol!ed to e!ade these barriers. 0or instance influen"a !irus has a surface

molecule that enables it to attach firmly to cells in mucous membranes of the respiratory

tract pre!enting the !irus from being swept out by the ciliated epithelial cells.


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B. Antimicrobial 8roteins and 8eptides >4amino acids linked together. A protein isanything with more than 1>>4amino acids linked together. They are basically the same thing.

pithelial cells secrete a broad spectrum of proteins and peptides that pro!ide protection

against pathogens.

Antimicrobial 8roteins

Among the antimicrobial proteins produced by the skin and other epithelia in humans +Table

345, se!eral are en"ymes and binding proteins that kill or inhibit the growth of bacterial and

fungal cells.o Lysozymeis an en"yme found in sali!a tears and fluids of the respiratory tract that

clea!es the peptidoglycan components of bacterial cells walls.

o Lactoerrinand calprotectinare two proteins that bind and se;uester metal ions

needed by bacteria and fungi limiting their growth.

o !soriasinis an antimicrobial protein that kills .coli on the skin. %owe!er it cannot

kill .auerus. This highlights the fact that antimicrobial proteins show some

specificitytoward particular pathogens.

Antimicrobial 8eptides

These are also antimicrobial components secreted by skin and other epithelial layers.

Antimicrobial peptides generally are cysteine4rich cationic and amphipathic +containing

both hydrophilic and hydrophobic regions,.

Because of their positi!e charge and amphipathic nature they interact with acidic

phospholipids in lipid bilayers disrupting the membranes of bacteria fungi parasites and

!iruses.

They then can enter the microbes where they ha!e other to/ic effects such as inhibiting the

synthesis of $#A '#A or proteins and acti!ating antimicrobial en"ymes resulting in

death.

Types of antimicrobial peptides: alpha4defensins beta4defensins and cathelicidin.

Antimicrobial peptides also attack the lipoprotein en!elope of en!eloped !iruses.

$efensins and cathelicidin 664?@ +the only cathelicidin e/pressed in humans, are secreted

constituti!ely by epithelial cells in many tissues as well as stored in neutrophil granules

where they contribute to killing phagocytosed microbes.

As we will see later production of antimicrobial peptides can be induced in many epithelial

and other cell types by the binding of microbial components to cellular receptors.


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urfacants

The epithelium of the respiratory tract secretes a !ariety of lubricating lipids and proteinscalled suracants.

There are two: 84A and 84$. They are members of a class of microbe4binding proteins

called collectins.

84A and 84$ bind differentially to sets of carbohydrate lipid and protein components

of microbial surfaces and help to pre!ent infection by blocking and modifying surface

components and promoting pathogen clearance.

Now that weve covered the physical and chemical anatomical barriers, lets discuss the cellular

response that results when anatomic barriers fail.

1.?: 8hagocytosis

A. -!er!iew

-nce pathogens penetrate through the epithelial barrier layers into the tissue spaces of the

body an array of cellular membrane receptorsand soluble proteinsthat recogni"e

microbial components play the essential roles of detectingthe pathogen and triggering

effecti!e deensesagainst it.

The ne/t line of defense after the anatomic barriers are the phagocytic cells. 8hagocytosis isthe cellular uptake +eating, of particulate materials such as bacteria. 8hagocytes are non4

specific.

o In the tissues the phagocytic cells are macrophages neutrophils and dendritic cells.

o In the blood the phagocytic cells are the monocytes.

%ow $o *acrophages (ork):

o *ost tissue contain macrophages that are on standby for the innate response.

o Through !arious cell surface receptor the macrophages recogni"e microbes such as

bacteria e/tend their plasma membrane to engulf them and internali"e them inphagosomes +endosomes resulting from phagocytosis,

o 6yso"omes then fuse with the phagosomes deli!ering agents +proteases

glycopeptidases primarily acidic protein eny"mes, that kill and degrade the microbes.

These agents are produced in the golgi of the macrophage. The lysosome becomes

acidic when it fuses with the phagosome because of a proton flow into the lysosome

which lowers the p%. This change in acidity makes the digesti!e agents acidic as

well.


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#eutrophils are a second maor phagocyte.

$endritic cells are the third phagocyte. 2ptake a degradation of microbes by dendritic cells

play key roles in the initiation of adapti!e immune response.

In addition to triggering phagocytosis !arious receptors on phagocytes recogni"e microbes

and acti!ate the production of a !ariety of molecules that contribute in other way to

eliminating infection as will be described later.

B. *icrobes are 'ecogni"e by 'eceptors on 8hagocytic &ells

How does a phagocytic cell recognize microbes, triggering their phagocytosis?

8hagocytes ha!e a !ariety of surface receptors. (e can organi"e them into two categories +1,

8attern recognition receptors -' +5, -psonin receptors.

'0' T- TAB6 34?

'0' T- 0I2' 34

8attern 'ecognition 'eceptors

o *icrobes ha!e specific conser!ed molecular components on their surfaces.

o These conser!ed molecular components +or motifs, are known as pathogen"

associated molecular patterns #!$M!s%

o 8hagocytes ha!e receptors that recogni"e the 8A*8s. These receptors are called

pattern recognition receptors #!&&s,.


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o 8''s can operate in two ways:

+1, They can bind the microbes +8A*8 C 8'', and trigger phagocytosis of

the bound microbe. The 8A*8s that cause this response are usually cell4wall

components of the microbes +comple/ carbs such as mannans and beta4

glucans lipopolysaccharides peptidoglycans surface proteins,

+5, They can bind the microbes and instead of triggering phagocytoses theytrigger a different type of response.

-psonin 'eceptors

o Instead of directly acti!ating phagocytosis +the receptor on the phagocyte +the 8'',

directly binds to the 8A*8s on the microbe and results in phagocytosis, we can

acti!ate phagocytosis indirectly.

o There are soluble proteins that can bind to microbial surface and can be easily

recogni"ed by phagocytes. These soluble phagocytosis4enhancing proteins +called

opsonins, will bind to carbohydrate structures lipopolysaccharides and !iral proteinson the microbe.

o -nce bound to microbe surfaces opsonins are recogni"ed by membrane opsonin

receptors on phagocytes thereby acti!ating phagocytosis.

-psonins

o A !ariety of soluble proteins function as opsoninsD many play other roles as well in

innate immunity. 8roduced elsewhere and found floating around.

o 6et7s look at an e/ample: S!"$ and S!"D.

(e said earlier that 84A and 84$ are anatomic barriers specificallysurfacants that lubricate.

They are A6- found in the blood in addition to the mucosal secretions

throughout the body. In the blood they can function as opsonins.

After binding to microbes they are recogni"ed by the &$E1 opsonin receptor

on al!eolar and other phagocyte +in this case macrophages, populations.

Binding acti!ates phagocytosis. This function of 84A and 84$ contributes to the clearance of the fungal

respiratory pathogen 8neumocystis carinii a maor cause of pneumonia.

o 6et7s look at another e/ample: Mannose"binding lectin #M'L%

This is a third collectin with opsoni"ing acti!ity.

It is found in the blood and respiratory fluids.

o 6et7s look at another e/ample: L"ociolin

A member of the ficolin family that is related to *B6 and other collectins.

It is found in the blood where it binds to acetylated sugars on microbes

including some streptococcal bacteria.

o 6et7s look at another e/ample:C()


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The complement component &1; also functions as an opsonin binding

bacterial cell wall components such as lipopolysaccharides and some !iral

proteins.

o *B6 +and other collectins, ficolins and &1; share structural features including

similar polymeric structures with collagen4like shafts but ha!e recognition regions

with different binding specificities

Because of their structural similarities they are all recei!ed by the same

receptor on a phagocyte +the &$E1 opsonin receptor,:

I#'T 0I2' 34@

o Another opsonin C"reactive protein #C&!% regno"ies phosphorylcholine and

carbohydrates on bacteria fungi and parasites and is then bound by 0c receptors

+0c's, for Ig found on most phagocytes. 0c's are also important for the opsoni"ing acti!ity of IgA antibodies and

some Ig antibody subclasses. After binding specifically to antigens on microbe surfaces the 0c regions of

these antibodies can be recogni"ed by specific 0c's triggering phagocytosis.

Brief *entioning of the &omplement ystem

o Among the most effecti!e opsonins are se!eral components of the complement

system.

o In !ertebrates complement consists of more than ?> binding proteins and en"ymes

that function in a cascade of se;uential acti!ation steps.o It can be triggered by se!eral innate soluble pattern4recognition proteins +including

the first complement component &1; and the structurally related lectins *B6 and

ficolins &4reacti!e protein and properdin, as well as by microbe4bound antibodies

generated by the adapti!e immune response.

o 8hagocytosis is one of the many important antimicrobial effects resulting from

complement acti!ation.

o *B6 is important in that it is both an opsoni"er and an inducer of complement

acti!ation.


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&. 8hagocytosed *icrobes are


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o

-/idati!e attack on the phagocytosed microbes employs highly to/ic reactiveo*ygen species #&+S%and reactive nitrogen species #&,S, which damage

intracellular components.

o The reacti!e o/ygen species are generated by the phagocytes7 ,$D!H o*idase

enzyme comple*+aka phagosome o*idase,. This comple/ is acti!ated when the

microbes bind to the phagocyte receptors. #A$8% o/idase con!erts o/ygen +which is supplied !ia a metabolic process

known as respiratory burst, con!erts o/ygen to supero/ide ion +G-54,. Hou

also get other en"ymes producing %5-5 and %&l-.

o The reacti!e nitrogen species are generated in a different way. eneration of '#

re;uires the transcriptional acti!ation of thegenefor an en"yme known as inducible

nitric o*ide synthase #i,+Sor ,+S-,.

o &ollecti!ely the '- and '# are highly to/ic to phagocytosed microbes due to the

alteration of microbial molecules through o/idation hydro/ylation chlorination

nitration and 4nitrosylation along with formation of sulfonic acids and destruction

of iron4sulfur clusters in proteins.

o 0or e/ample o/idation by '- of cysteine sulfhydryls that are present in the acti!e

site of many en"ymes in pathogens results in the inacti!ation of those en"ymes.

$. 8hagocytosis &ontributes to &ell Turno!er and the &learance of $ead &ells

!s the body"s main scavenger cells, macrophages also utilize their phagocytic receptors to take

up and clear cellular debris, cells that have died from damage or toic stimuli #necrotic cell

death$ or from apoptosis #programmed cell death$, and aging red blood cells.

&ollecti!ely the components of deadFdying cells and damaged tissues that are recogni"ed by

8''s leading to their clearance are referred to as damage"associated molecular patterns

#D$M!s%. 8hagocytosis is the maor mode of clearance of cells that ha!e undergone apoptosis as part of

de!elopmental remodeling of tissues normal cell turno!er or killing of pathogen4infected or

tumor cells by innate or adapti!e immune responses.

Apoptic cells attract phagocytes by releasing the lipid mediator lysophosphatidic acid which

functions as a chemoattractant.

o These dying cells facilitate their own phagocytosis by e/pressing on their surfaces an

array of molecules not e/pressed on healthy cells including phospholipids +such as


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phosphatidyl serine and lysophosphatidyl choline, proteins +anne/in I, and altered

carbohydrates.

An important additional acti!ity of macrophages in the spleen and those in the li!er +


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By ha!ing so many different 8''s in two locaations it ensure that the cell can recogni"e

8A*8s on both e/tracellular and intracellular pathogens.

Also its not ust 8A*8s that are recogni"ed. Hou can ha!e $A*8s that are released by

damaged cells or tissue which can be recogni"ed.

There are multiple cells that e/press these intracellular and e/tracellular 8''s:

o The myeloid white blood cells:

*onocytes

*acrophages

#eutrophils

osinophils

*ast &ells

Basophils

$endritic &ells

o And ubsets of the tree types of lymphocytes: B cells

T cells

#< cells

o As well as other cell types especially those e/posed to infectious agents:

kin cells

*ucosal cells

landular epithelial cells

Lascular endothelial cells that line blood !essels

0ibroblasts and stromal support cells in !arious tissues.

&e will now look at one of four main families of mammalian '((s. &e wont look at the

signaling pathways that these '((s activate and ultimate cause a protective response.

B. Toll46ike 'eceptors 'ecogni"e *any Types of 8athogen *olecules

Toll4like receptors +T6's, were the first family of 8''s to be disco!ered.

o 0irst it was disco!ered that mutations of the tollgene +a gene that coded for the Toll

membrane protein, in $rosphila made the flies highly susceptible to lethal infection

with!spergillus fumigatus a fungus to which wild4type flies were usually immune.o This obser!ation let to other studies showing that Toll and related proteins are

in!ol!ed in the acti!ation of innate immune responses in in!ertebrates.

o Then scientists disco!ered a human gene for a protein similar to Toll that acti!ated

the e/pression of innate immunity genes in human cells. The proteins came to be

known as Toll4like receptors.

o A scientists Beutler obtained the important proof that T6's contribute to normal

immune function in mammals.


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*ice homo"ygous for a mutant form of a gene called lpswere resistant to the

harmful responses induced by lipopolysaccharide +aka endoto/in, a maor

component of the cell walls of )ram*negative bacteria.

In humans a buildup of endoto/in from se!ere bacterial infection can induce

too strong of an innate immune response causing septic shoc a life4

threatening condition in which !ital organs such as the brain heart kidneyand li!er may fail.

Beutler found that the defecti!e mouse lpsgene encodaed a mutant form of

one T6' T6'K which differed from the normal formb y a single amino acid

so that it was no longer acti!ated by 68. Thus he showed that T6'K is the cellular innate pattern recognition receptor

that recogni"es 68.

&. /pression of Innate Immunity 8roteins is Induced by 8'' ignaling

#-T 2' I0 ( #$ T-


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o (ithin a few hours leukocytes also enter the tissue from the local blood !essels.

These leukocytes area acti!ated to phagocytose bacteria and debris and to amplify the

response by producing additional mediators.

%ow does the inflammatory response end)

o 'esolution of acute inflammatory response includes the clearance of in!ading

pathogens dead cells and damaged tissueD the acti!ation of the systemic acute phaseresponse and additional physiological responses including the initiation of wound

healingD and the induction of adapti!e immune responses.

o If the infection or tissue damage is not resol!ed it can lead to a chronic inflammatory

state that can cause more local tissue damage and potentially ha!e systemic

conse;uences for the affected indi!idual.

B. Inflammation 'esults from Innate 'esponses Triggered by Infection Tissue $amage or

%armful ubstances

(hen there is local infection tissue damage or e/posure to some harmful substances

sentinel cells residing in the epithelial layer +basically macrophages mast cells and dendritic

cells, are acti!ated by 8A*8s $A*8s crystals and so on to starts phagocytosing the

offending in!aders.

o The cells are also acti!ated to release innate immunity mediators that trigger a series

of processes that collecti!ely constitute the inflammatory response.

The recruitment of !arious leukocyte populations to the site of infection or damage is a

critical early component of inflammatory responses.

o 8'' signaling acti!ates resident macrophages dendritic cells and mast cells to

release the initial components of cellular innate immune responses.

o These components act on the !ascular endothelial cells of local blood !essels

increasing the !ascular permeability and the e/pression of cell adhesion molecules

#C$Ms%and chemokines. This epithelium is said to be inflamed or acti!ated.

o &ells flowing through local capillaries are induced by chemoattractants and adhesion

molecule interaction to adhere to !ascular endothelial cells in the inflamed region and

pass through walls of capillaries and into the tissue spaces a process called

e*travasation.

o #eutrophils are the first to be recruited to a site of infection where they enhance local

innate responses followed by monocytes that differentiate into macrophages that

participate in pathogen clearance and help initiate wound healing.


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Kuby Immunology Ch. 5