MICR 270 module 2

what is the first line of defence against foreign invaders/all pathogens?

innate immunity

- operates non-specifically during early phases of IR

- operate w/o prior exposure to invading pathogen

what are the four components of innate IS?

immune barriers

pattern-recognition

inflammation

phagocytosis

key concepts of the immune barriers?

physical, soluble, cellular barriers that are scattered throughout body

key concepts of pattern recognition?

recognizes general patterns not specific for any one antigen

pattern-recognition receptors (PRR) expressed on innate immune cells

key concepts of inflammation

first IR events following breach of physical barrier by pathogen = IR

key concepts of phagocytosis

"phagocyte" cells, engulf foreign antigens

what are the 3 types of immune barriers?

physical barrier = made of every structure located at interface b/w inside and outside of body

cellular barrier = made of various cells

soluble barrier = made of macromolecules which contribute to mediation of innate IR

what makes up the physical barrier and what is its main function?

- made of physical and chemical components

- primary function = prevent or slow down invasion of pathogens

what are physical components of the physical barrier?

skin - thing layer of tissue acts as barrier to pathogens

mucous membrane - cover cavities of body (respiratory, gastrointestinal, urinary and reproductive tracts)

- membranes contain specialized structures like clia and mucous

what are chemical components of the physical barrier?

- mucous membranes secrete chemical components that attack and kill bacteria to prevent infections

- tears and saliva contain active antimicrobial substances such as lysozyme

what is lysozyme?

- enzyme that catalyzes destruction of cell walls of certain bacteria

ex. mucous membrane is gastric acid which destroys most bacteria and toxins that enter stomach

what is the cellular barrier made up of?

- made of innate immune cells which prevent/slow down invasion of pathogens that have broken through physical barrier

what are the four major cell types that play a key role in innate IR?

neutrophils

macrophages

dendritic cells

natural killer cells

which of the four cell types is NOT a phagocyte in the innate IR?

natural killer cells

neutrophils

- most common leukocyte found in blood of mammals

- phagocytes (patrol body to find, engulf and destroy pathogens)

- circulate in blood for ~12 hours before entering tissues by diapedesis

- recruited to site of infection by resident macrophages that have encountered pathogens

- lifespan = 1-3 days after entering tissues

* conduct first wave of phagocytosis but don't live long

macrophages

- phagocytes

- take up residence in specific tissue or patrol throughout larger area of tissues

- contribute to tissue repair and present antigens to other immune cells like T-cells

- activated after phagocytosising pathogens or in response to cytokine signaling

*big eaters & can phagocytose larger quantity of pathogens compared to neutrophils

which cell is a major link b/w innate and adaptive IS and arrive later in IR?

dendritic cells

dendritic cells

- phagocytes (often in contact with external enviro specifically Langerhans DC)

- engulf foreign antigens

- present antigens on cell surface through peptide MHC complexes which can be recognized by helper T cells

natural killer cells

NOT phagocytes

- recognize abnormal cells lacking antigen-specific receptors

- destroy abnormal cells of body

- bind to cell surface of target cells and release chemicals causing pores to form in cell membrane, leading to their lysis

what is soluble barrier made up of?

made of two major categories of macromolecules:

1) complement system

2) cytokines

play key role in development of IR, which is induced following penetration of an infectious agent through body's physical barriers

what is the complement system?

- made up of over 30 soluble proteins

- complement proteins circulate in blood, normally in inactive form, but directly activated in presence of extracellular pathogens

- indirectly activated by pathogen bound antibody

what does activation of complement system induces?

causes cascade of rxns b/w various complement proteins leading to form Membrane attack complex (MAC)

AND

at the same time enhance efficiency of other immune functions like inflammation and phagocytosis

what is an MAC?

membrane attack complex

- structure made of activated complement proteins

- destroy extracellular pathogens by creating holes in their membrane ** can also damage host cells tho

- ultimate complement system function of destroying invaders

- MAC structures create holes in pathogen --> lysis --> death

what are the three major pathways that activate MACs?

classical

alternative

lectin

how is inflammation enhanced through complement system activation?

- attraction of various immune cells to site of infection through release of chemotactic molecules like histamine and cytokines

- activated complement proteins bind to complement receptors on immune cells (like mast cells and basophils) to induce release of these substances, which enhance inflammatory response)

how phagocytosis is involved in complement systems?

- activated complement proteins, predrominantly C3b, opsonize pathogens thereby targeting them for destruction by phagocytes

what is the main function of the complement system in innate immunity?

opsonization -> induces phagocytosis -> chemotaxis molecues released which induces -> inflammation -> induces lysis through MAC ->pathogen death

what would happen without the complement system?

reduced lysis of microbes and less inflammation

reduced bacterial clearance therefore longer periods of infection

what are cytokines?

- small proteins secreted by immune cells

- chemical mediators, play key role in cell-to-cell communication

- large variety exists and each have strong affinity for specific type of cytokine receptor (which are expressed on cell surface of various immune cells)

function of cytokine signaling?

-regulate immune processes, such as IR, inflammation and hematopoiesis

autocrine vs paracrine vs endocrine cytokines?

these terms characterize location of action depending on site of secretion of cytokines by an immune cell

autocrine

sending and receiving cell of cytokine is the same

paracrine

sending and receiving cell of cytokine are near to each other

endocrine

sending and receiving cells of cytokine are distant from one another

what happens if pro-inflammatory cytokines are not properly controlled?

lead to complications such as tissue damage due to an excessive inflammatory state

what happens if anti-inflammatory cytokines are not controlled?

lack of immune response to pathogen may occur resulting in spread of pathogen

cytokines are...

chemical mediators

specificity

alter gene expression

pro-inflammatory

anti-inflammatory

what is inflammation

series of biological rxns in response to invasion of harmful infectious agents into body

can have local and systemic effects in body

what can inflammation be characterized as?

redness

heat

pain

swelling

what happens during inflammation?

- alteration of blood flow to injured area

- influx of phagocytic and other immune cells

- removal of foreign antigens

- healing of damaged tissue

- physical response during inflammation can result in loss of function

what is main purpose of inflammation?

- Inflammation is body's attempt at self- protection by removing harmful stimuli, including damaged cells, irritants or pathogens

- Purpose = localize and eliminate invading pathogen, in an effort to stop it from spreading, and to remove damaged tissue

what is the order of major events of inflammation in innate IS?

breach

vasodilation

permeabilization

extravasation

phagocytosis

1. breach

- physical barrier sealed, so pathogens find breach in order to enter body

ex. wound created by nail or glass

- these wounds would damage cells and given opportunity for pathogens to break through physical barrier

2. vasodilation

- first major event in inflammation

- increase in diameter of blood vessels

- increase permeabilization of capillaries near affected area -> facilitate entrance of fluids in tissues

- induced by vasoactive and chemtotactic factors (secreted by damaged tissues and activated immune cells like macrophages and mast cells)

what is a consequence of vasodilation?

redness and heat due to higher blood volume around infected tissue

3. permeabilization

- Vasodilation facilitating entrance of fluids in tissues + vasoconstriction of vessels carrying blood away = allows accumulation of excess fluid at site of infection called exudate

- swelling/edema = consequence of accumulation of fluids

what is exudate?

- fluid containing proteins that contribute to mediation of IR

- have both pro-inflammatory cytokines (specifically chemokines = complement proteins are activated in presence of extracellular pathogens)

- function of these proteins is to attract cellular barrier key players to site of infection

4. extravasation

- chemotactic factors released during vasodilation and permeabilization steps induce recruitment of more immune cells to site of infection

what are the first types of cells to arrive by chemotaxis to site of infection?

neutrophils; they adhere to endothelial cell walls via process called margination and migrate b/w capillary-endothelial cells into infected tissue by process called extravasation or diapedesis

5. phagocytosis

- at infection site, neutrophils and other phagocytes engulfs and destroys pathogens

summary of steps of inflammatory response?

- vasodilation -> increase in diameter of blood vessels

- permeabilization -> facilitation of entrance of fluids (exudate) in infected tissues

- chemotaxis ->attraction of immune cell to site of infection by specific proteins and cytokines

- margination ->adhesion of immune cells to endothelial wall of blood vessels

- extravasation -> migration of immune cells through tissues to site of inflammation

why are heat and swelling beneficial during IR?

- heat increases metabolic rates of cells allowing for fast repair

- swelling leaks proteins which helps clot blood and for scabs

- swelling also recruits local phagocytes and lymphocytes to help destroy pathogens clean up dead cells

how are innate immune cells capable of recognizing self from non-self?

major mechanism that recognizes pathogens in innate immune cells is "Pattern Recognition Receptors (PRRs)"

what is the main PRR that you learned?

Toll-like Receptors (TLRs)

where do PRRs come from?

- Innate IS developed with a focus on pattern recognition sequences that are unique to the microorganisms and necessary for their survival

-Biological patterns allow the PRRs of the innate IS to target the specific pathogens quickly/effectively and avoid attacking self-cells

are there PRRs in adaptive immune cells too?

- yes; PRRs can be expressed by both innate and adaptive immune cells

*** PRRs are an integral signaling component of innate IS

what are molecular patterns?

repeated molecular patterns are conserved motifs and are recognized by PRRs

certain subsets of molecular patterns found in various groups of pathogens are called...

pathogen associated molecular patterns (PAMPs)

ex. flagellin

viral glycoprotein

lipopolysaccharide

How PAMPs help distinguish self from nonself

molecular structures either expressed on the surface of or found inside pathogens

- lipopolysaccharides (LPS) found on cell surface of gram-negative bacteria

- double stranded RNA found inside dsRNA viruses

these patterns are specific for pathogens and not found in host cells *** which gives ability to innate cells to distinguish nonself from self

what are two major categories of molecular patterns that can be recognized by PRRs?

1) pathogen-associated molecular patterns (PAMPs)

2) danger-associated molecular patterns (DAMPs) - also called "damage" associated molecular patterns

what are PAMPs?

molecules associated with groups of pathogens that are recognized by immune cells which include:

- functional structures of a pathogen

- repeated sequences of protein, glycoprotein, lipoprotein, amino acids, etc, that are conserved across specific groups of microbes

- this allows quick response to infection by inducing innate IR

what are DAMPs

molecules released by stressed cells undergoing necrosis (cell death)

- DAMPs are host biomolecules, indicate damage to body, initiate IR

- ex. abnormal location of a cell structure = DNA found outside of mito or nucleus

- ex. cell stress indicator molecules = heat-shock proteins

what are Toll-like receptors

- class of PRRs whose signaling plays essential role in innate IR

- depending on type of PAMP or DAMP it recognizes, TLRs are either expressed on plasma membrane or endosomal/lysosomal membranes of mammalian cells

what happens when TLRs are activated?

initiate transcription of genes encoding for:

- inflammatory cytokines

- chemokines

- costimulatory molecules

these molecules contribute to activation of innate immune cells (ex. increase phagocytosis and enhance their ability to present antigens to adaptive IS)

what are the two major roles of TLRs?

1) recognize PAMPs and/or DAMPs

2) induce expression of signaling to activate T cells

what is the TLR signaling process?

TLRs recognizes PAMPs and/or DAMPs in body and in response, activate T cells to initiate adaptive IR

step 1 of TLR signaling process

- TLRs sense presence of infection through recognition of PAMPs and/or DAMPs

- bacterium will be engulfed though phagocytosis by phagocyte cell (ex. dendritic)

step 2 of TLR signaling process

- after engulfing bacterium, the immune cell/antigen presenting cell, will present pieces of the pathogen (antigens) on its cell surface through the peptide: MHC complex

step 3 of TLR signaling process

the antigen presenting cell will also increase its production of costimulatory molecules, which are involved in strength and stability of the antigen presenting process

step 4 of TLR signaling process

- an immunoreceptor naive T cell specific for the antigen presented by the dendritic cell will bind to peptide: MHC complex through its TCR -> this interaction activates the t-cell and initiates an adaptive IR

essential for IR, _______ such as ______, are receptors which recognize structures from the microbes, such as ______ or molecules released by dying cells, such as ______.

PRRs

TLRs

PAMPs

DAMPs

when pathogens penetrate barriers, what is the innate IS's next line of defence?

phagocytosis

what is phagocytosis?

- type of endocytosis = cell takes up particulate material like bacteria from its enviro by invaginating its membrane to form a vacuole

how is phagocytosis induced?

- phagocyte's PRR recognizes PAMP

- through pathogen opsonization

what are the major innate immune cells that are involved in maintaining cellular barrier and have phagocytic functions?

- neutrophils

- macrophages

- dendritic cells

RECALL roles of neutrophils, macrophages and dendritic cells...

neutrophills

- 1st cells to arrive to infection

- early phagocytosis, eliminate pathogen quickly

- initiate IR

macrophages

- monocytes migrate from blood to tissues to become macrophages

-most efficient at phagocytosis (big eaters)

- release cytokines that stimulate inflammation and recruit other immune cells

dendritic cells

- recognize microbes & initiate phagoyctosis

- most efficient antigen presenting cell

- major role in initiation of adaptive IR

what are the five stages of phagocytosis?

1) attachment

2) ingestion

3) fusion

4) digestion

5) release

1st step of phagocytosis

attachment

- pathogen becomes attached to membrane evaginations called pseudopodia

- evaginations = protruding structure produced by turning membrane outward

***bacterium recognized by phagocytes and binds to cell membrane

2nd step of phagocytosis

ingestion

- pathogen is ingested -> forms vacuole called phagosome within cell

- Phagosome = vesicle with phagocyte containing phagocytosed material

*** pseudopedia encircle bacteria and from vacuole called phagosome

third step of phagocytosis

fusion

- phagosome fuses with lysosome = phagolysosome, which releases lysosomal enzymes that degrade macromolecules and other materials like bacteria

*** phagosome fuses with lysosomes to form phagolysosome

fourth step of phagocytosis

digestion

- pathogen destroyed and digested by lysosomal enzymes

***digestive enzymes released and destroy pathogens inside phagolysosome

5th step of phagocytosis

release

- digested products released from cell via exocytosis (process in which vacuole membrane fuses with cell membrane)

*** phagolysosome fuses with cell membrane and release digested bacterial products from phagocyte

what is second line of defence against foreign invaders if innate IS fails?

adaptive IS

what are the main characteristics of adaptive immunity which differ from innate IS?

specificity and diversity

- longer time to initiate, creates specific response against pathogen + produces memory cells

what does specificity mean?

- each cell of adaptive IS recognizes on specific epitope of a pathogen (epitope = antigenic determinant, part of an antigen recognized by adaptive immunity)

- for each pathogen that the adaptive IS encounters, it creates a unique immune reaction to eliminate infectious agent

what does diversity mean?

because each cell is so specific, the adaptive IS is composed of many cells able to fight any pathogen encountered

what are the major components of the adaptive IS and what cells do they consist of?

humoral immunity -> B cells and antibodies

cell mediated immunity -> T cells

what can B and T cells do?

both differentiate into specialized subsets following their activation

characteristics of B cells

- key component of humoral response

- mature in bone marrow

- surface receptor = B cell receptor (BCR)

- function = antibody factory

what are specialized subsets of B cells?

plasmocytes

- effector cell

- produce lots of antibodies

memory B cell

- memory cell

- expresses BCR on their cell surface

specifity of T cells

- key component of cell mediated response

- mature in thymus

- surface receptor = T cell receptor (TCR)

- Function = cytotoxic activity or help activation of immune response

T cell diversity

CD4 helper T cell

- effector cell

- help activate adaptive IR

CD8 cytotoxic T cell

- effector cell

- kill infected cells

memory T cells

- memory cell

- expresses TCR and CD4 and CD8 on their cell surface

if innate immune system fails, how is adaptive immunity activated?

antigen presenting cells (APCs), like dendritic cells, that have engulfed pathogens by phagocytosis can present antigens to naive CD4 helper T cells (TH)

upon antigens being presented to naive CD4 helper T cells (TH), what occurs next?

depending on type of antigen TH cell encounters, TH cells can differentiate into two subsets

1) TH1

2) TH2

what does TH1 do?

induces CELL-MEDIATED immunity, activated TH cells will differentiate into subset called TH1

what does TH2 do?

induces HUMORAL immunity, activated TH cells will differentiate into subset called TH2

humoral immunity and TH2 cells

activated and differentiated TH2 cells activate B-cells and induce their differentiation into plasmocytes

plasmocytes then produce antibodies specific for invading antigen

cell-mediated immunity and TH1 cells

activated and differentiated TH1 cells activate CD3*+ cytotoxic T cells and induce their differentiation into CTL (cytotoxic T lymphocytes)

CTL recognizes and eliminates cells displaying specific antigen presented at their cell surface by MHC class I complex

_____ produce antibodies and are key cell of _____ immunity, while _____ have cytotoxic activity and are key role of _____ immunity

B cells

humoral

T cells

cell-mediated

what are antibodies and what are they also called?

-essential part of humoral immunity

- large Y-shaped protein

- each antibody is highly specific and recognizes one epitope

- also called immunoglobulin or Ab