Innate Immunity

Question 1

Give a broad overview of the innate system. Adaptive/non-adaptive? Do they undergo gene rearrangements? Somatic hypermutation?

How do non-immune type cells provide a dense system? Give an example involving epithelial cells.

Answer 1

The innate system is our non-adaptive host defense against pathogens. This
system is very old, conserved throughout eukaryotic life. Being innate implies
that it is already part of an individual. The genes encoding molecules involved in
innate immunity are in the germ line and they do not undergo gene rearrangments
or somatic hypermutation like many genes of the adaptive immune system.

The innate immune system has many functions which overlap with those of the
adaptive immune system and many that are not possessed by the adaptive immune
system. This makes the innate immune system extremely important to host
defense from infection.

Non-immune type cells have the ability to provide dense system. For example,
epitherial cells produce anti-microbial peptide called defensin, which get
embedded in the microbial cell membrane and creates pores.

Question 2

Describe phagocytes role in the innate immune response. What do they do?

Answer 2

Phagocytes. These are one set of cells that distinguish the innate immune system from the adaptive immune system.

Leukocytes that recognize, ingest, and
kill invading microbes are called phagocytes.

Question 3

There are two major types of

phagocytes. Describe each.

Answer 3

Macrophages/Monocytes

• are long-lived leukocytes.
• are widely distributed in normal tissues.
• are often the first cell to encounter a pathogen.
• represent the mature form of circulating monocytes.
• increase in number at sites of injury or infection.

Neutrophils

• are short-lived circulating leukocytes.
• are the most abundant type of white cell in the circulation.
• are rarely found in normal tissues.
• can be quickly recruited to sites of injury or infection.

Question 4

Macrophages and neutrophils both perform phagocytosis. Describe this process. What is it?

Where are professional and tissue bound phagocytes found?

What is opsonization?

Answer 4

The process by which particulate materials are engulfed by a cell
and delivered to a digestive compartment within the cell is called phagocytosis.
Professional phagocytes are found in large numbers in the peripheral circulation.
Tissue bound phagocytes are found in most tissues, but are prominent in the lung,
liver, spleen, and skin. Several types of receptors can mediate recognition of foreign
particles. In some circumstances, pathogens undergo opsonization.

Opsonization is
the coating of particles by molecules that enhance recognition by phagocytes.
Adaptive immunity can opsonize pathogens with antibodies. The innate immune
system can opsonize pathogens with proteins of the complement system.

Question 5

Both macrophages and neutrophils perform mediator production. Describe this process.

Upon activation, what do phagocytes produce? What is the effect?

Answer 5

Upon activation by an appropriate stimulus, phagocytes,
particularly macrophages, produce a large variety of cytokines (which act on cells to program them for microbial combat) and chemokines (e.g. with chemoattractant
properties to recruit leukocytes to sites of infection) and lipid mediators (e.g. prostaglandins) which act similarly cytokines in that they affect how cells behave.

They also produce hydrolytic enzymes and antimicrobial peptides that assist in
clearance of pathogens. This can occur solely based on innate responses and can be
greatly amplified by an adaptive response.

Question 6

Both macrophages and neutrophils participate in pathogen associated molecular pattern (PAMP) recognition. What is this? Describe.

Where are its receptors located?

Answer 6

Germline
encoded intracellular and cell surface PAMP recognition receptors (PRRs) present in
many cells but highly expressed in macrophages and dendritic cells. PAMPs are
molecules that are highly concerved (e.g. DNA or RNA structures, flagellin, bacterial
cell wall components) that are present in most pathogens. Because PRRs can easily
adapt to recognize pathogens they must recognize these conserved structures. These PRRs can recognize bacterial, viral and fungal components and initiate signals
leading to the recruitment and direction of B and T-cells. These receptors include the
Toll like receptors (TLRs), NOD-like receptors (NLRs), RIG-I helicase-like receptors (RLRs) and, C-type lectin receptors (CLRs).

Question 7

What is inflammation? Is initiation of inflammation part of the innate immune response or adaptive?

When is inflammation triggered?
What is its intended purpose? How does it relate to tissue repair? When might it be destructive?

Answer 7

Inflammation is a general term for the accumulation of fluid, plasma proteins and white blood cells that occurs in tissue subjected to injury, infectious agents, or immune responses.

The initiation of inflammation is part of the innate immune response, and the inflammation that results from an innate response to a threat is termed acute inflammation.

-Inflammation is fundamentally protective.
-Is triggered when pro-inflammatory mediators are released from stores
or quickly produced in response to either a infectious or traumatic event.
- Is intended to destroy or wall off the offending agent.
- Is interwoven with tissue repair; as the inflammatory process resolves,
tissue regeneration or scar formation occurs.
- Can be destructive, particularly if prolonged.

Question 8

What are the 3 key events in the inflammatory response. Describe.

Answer 8

a. Alteration in blood flow (calor, rubor, dolor)
Vasodilatation is an early response, and leads to increased blood flow. Increased blood
flow facilitates the movement of additional serum mediators and white cells into the area
of insult.

b. Increased vascular permeability (tumor)
Endothelial cells contract, leading to widened intracellular junctions. This is an
immediate response and occurs primarily in venules. Later, direct endothelial injury
can also occur, causing cell necrosis and detachment. The result is increased vascular
permeability and the leakage of serum components into the tissue space.

c. Infiltration of white blood cells into the affected area (tumor)
The time course for the movement of leukocytes into sites of inflammation can be within
from hours after injury. The early inflammatory lesion is marked by a preponderance of
neutrophils. Later, macrophages (derived from monocytes) predominate. Lymphocytes
are generally the last cell type to arrive. In chronic inflammation, lymphocytes may
eventually become the predominant cell type.

Question 9

How does the innate system recognize danger?

How quickly can it be activated?

Answer 9

The innate system has the ability to recognize
danger by microbial pattern recognition. A key advantage of this system is that it can be
activated in minutes. Understanding how pattern recognition shapes the subsequent nature
of an immune response is the key to understanding an optimal immune response works and,
even more importantly, understanding how an immune response can go wrong and cause
disease.

Question 10

What turns on the innate system?/What activates an immune response?

Answer 10

Pattern recognition receptors (PRRs) on DC or macrophages activate an immune response. Innate immunity likely represents the first version of an immune system where
pathogens were sensed by PRRs. Eons ago, there was probably one type of PRR because there
were simple threats to an organism. Thus, before the adaptive immune system came to be,
organisms evolved to possess numerous PRRs to maximize coverage of all possible pathogens.

Question 11

What are PRRs?

Answer 11

PRRs are transmembrane (TLRs and CLRs) or soluble intracytoplasmic (NLRs and RLRs) receptors found on many different cell lineages but especially on dendritic cells, macrophages
and monocytes.

TLR- Toll-like receptor
NLR- NOD-like receptor
RLR- RIG-I helicase-like receptor
CLR- C-type lectin receptor

Question 12

What do PRRs bind to?

Answer 12

They can bind to a wide array of bacterial, fungal, viral and parasitic molecular patterns. The patterns are called Pathogen Associated Molecular Patterns (PAMPs)

There are at least 11 human TLRs, 22 NLRs, 3 RLRs and 15 CLRs (DON’T MEMORIZE).
They have arisen in an evolutionary response to increasingly complex forms of life that pose a
threat to humans.

Question 13

What do typical PAMPs include?

Answer 13

Typical PAMPs include mannose containing structures, lipids in complex microbial
lipopolysaccharides and viral and bacterial nucleic acids.

Question 14

How are PAMPs recognized? (Give ex. using DNA)

Answer 14

While some of these PAMPs are not present in eukaryotes possessing PRRs making them
easily distinguished from “self”, other PAMPs such as DNA are recognized by PRRs by virtue of their improper location (e.g. DNA is not normally found in the cytoplasm or lysosomes of
healthy cells unless they are infected with DNA viruses or intracellular bacteria).
PRRs confer a limited specificity to the innate response – nothing like the exquisite specificity
of the adaptive response

Question 15

Natural selection has dictated that PRRs will recognize a specific PAMP and activate an
immune response best suited to eliminate the type of infecting organism that bears the PAMP. Give an example.

Answer 15

A prime example is the TLR that specifically recognizes viral nucleic acids. Activation
of this TLR will then initiate specific anti-viral responses by signaling for the production of specific antiviral mediators.

In some cases multiple PRRs may recognize a pathogen and their integrated signal will lead to an appropriate response.

Question 16

What are some common characteristics of TLR activation?

What is the final common path for pro-inflammatory activation?
What results?

What is “inflamemasome” and what does it activate? What is it a precursor of?

Answer 16

Common characteristics of TLR activation are downstream signaling pathways in
phagocytes/DC (see above figure). The final common path for pro-inflammatory activation is NF-kB. Activation of transcription factor NF-kB activates genes encoding pro-inflammatory
cytokines.

One of the most important factors is IL-1 beta.
Generation of IL-1beta also
requires activation of a protein complex called “inflamemasome”, whose main function is to activate a protease called caspase I, which cleaves precursor of IL-1beta to the mature form.

Question 17

How are simple threats resolved?

Answer 17

On many occasions, PRR activation on a phagocyte (macrophage or neutrophil) is all that is required to resolve a simple threat by phagocytosis and intracellular destruction of foreign particles and “simple” bacteria.

Question 18

Describe PRRs and the downstream effect.

Describe mannose binding lectin. What type of receptor/what does it activate? What is the result?

Answer 18

Some PRRs are secreted. One of the most important factor is called Mannose binding lectin
(MBL). This protein, as name suggest, binds mannose present often on the surface of
microbes. Binding of MBL will –another mannose receptor. Ligand-bound MBL activates
complement cascade, which immediately lysis target cells.

Question 19

What is the complement system. How do complement proteins occur in serum?

Describe three ways the complement system bridges innate and acquired immunity.

Answer 19

The complement system is an enzyme cascade that helps defend against Infection. Many complement proteins occur in serum as inactive enzyme precursors (zymogens); others reside
on cell surfaces.

The complement system bridges innate and acquired immunity by

• Augmenting antibody (Ab) responses and immunologic memory
• Lysing foreign cells
• Clearing immune complexes and apoptotic cells

Question 20

What are the three pathways that activate the complement system?

When do the three activation pathways converge into a final common pathway?

Answer 20

classical pathway
lectin pathway
alternate pathway

The 3 activation pathways converge into a final common pathway when C3 convertase
cleaves C3 into C3a and C3b.

Question 21

Describe the classical pathway.

Answer 21

Classical pathway activation is Ab-dependent, occurring when C1 interacts with Ag-IgM or
aggregated Ag-IgG complexes, or Ab-independent, occurring when polyanions
(eg, heparin, protamine, DNA and RNA from apoptotic cells), gram-negative bacteria, or
bound C-reactive protein reacts directly with C1.

Question 22

Describe the lectin pathway.

Answer 22

Lectin pathway activation is Ab-independent; it occurs when mannose-binding lectin
(MBL), a serum protein, binds to mannose, fructose, or N -acetylglucosamine groups on bacterial cell walls, yeast walls, or viruses. This pathway otherwise resembles the classical
pathway structurally and functionally.

Question 23

Describe the alternative pathway.

Answer 23

Alternate pathway activation occurs when components of microbial cell surfaces (eg, yeast
walls, bacterial cell wall lipopolysaccharide [endotoxin]) or Ig (eg, nephritic factor,
aggregated IgA) cleave small amounts of C3. This pathway is regulated by properdin, factor H, and decay-accelerating factor (CD55).

Question 24

What receptor do phagocytes express on their cell surface? What does binding of this receptor initiate?

Answer 24

Phagocytes express C3b receptor on the cell surface. Binding of C3b to microbe helps
phagocytes to engulf the target and then destroys them by superoxide generate in that
organelle called phagosomes.

Question 25

Describe the characteristics of cells of the innate immune response. Describe roles of dendritic cells, NK, NKT and gamma-delta T cells.

Answer 25

These cells are not only responsible for initiating
and maintaining an innate immune response but also provide the transition to an
adaptive immune response and determine its character, intensity and type. Dendritic
cells quarterback the adaptive immune response by presenting antigens to T-cells and
B-cells. Innate lymphoid cells like NK, NKT an gamma-delta T-cells possess similar
effector functions as T-cells of the adaptive response but recognize molecules common to many bacterial, fungal or viral infections.

Question 26

Describe the role of the dendritic cell. What are four main characteristics?

Answer 26

In many ways, DC are the most important cell in the immune system.
DC are key sentinel cells that act as the gateway to the adaptive response. DC determine
what type of adaptive response will occur.

a. Roam freely throughout most, if not all, tissues and organs.
b. Display multiple types of PRR
c. Phagocytize pathogens (antigens)
d. Become effective antigen presenting cells. These cells are most potent antigen presenting cells
in the immune system.

Question 27

Which are the major antigen presenting cells to T cells? What does this initiate?

What is the difference between presenting antigen peptide via class I or class II MHC molecules?

Answer 27

DCs and macrophages are major antigen presenting cells to T cells. This will start the adaptive
immune responses. After engulfment of microbes, these cells can present antigen peptide via both
class I and class II MHC molecules. Class II MHC is primarily used to present exogenous
antigens while Class I MHC molecules are major presenter of peptide generated within the cells.

Question 28

Describe the subsets of innate lymphoid cells. Type I, II, III. What do they produce?

Answer 28

Several minor subsets of lymphocytes express no diversity or only a limited diversity of
receptors, and are thus termed innate-like lymphocytes.

The functions of these subsets are diverse. There are three types of ILCs. Type I (such as NK cells and NKT cells) produce pro - inflammatory cytokines such as IFN gamma and activates DCs. Type II cells produce IL-4 and
help maturation of DCs. Type III cells produce IL-17 which helps recruitment of neutrons and
production of anti-microbial peptides.

Question 29

What are natural killer cells? From what are they derived?

Describe the receptors. How does it compare to T/B cells?

What is their function?

Answer 29

a. A small proportion of lymphocytes circulating in the blood are called natural killer
(NK) cells because they are can directly kill specific target cells, including virally
infected cells and some cancer cells.

b. Natural killer cells are derived from lymphoid progenitors.

c. The specificity of the receptors that NK cells use to recognize targets is not diversified like T and B cells. Because NK cells are pre-programmed to respond to
their targets, and do not diversify in response to antigens, they are often considered a part of the innate immune response.

d. NK continuously sample the environment by looking for cells with altered self
expression, such as virally infected cells and some cancer cells.

e. There is increasing evidence that there are interactions between activated DC and
NK cells that are critical for determining the type of adaptive response.

Question 30

Where is innate immunity activated? What is it characterized by?

Answer 30

The very short geographic course on innate immunity is that it will activated wherever there
is a breach of defense and will be characterized by recruitment of inflammatory cells

B. The very short anatomic/geographic course on adaptive immunology is then:
1.Antigens (immune stimulatory molecules) derived from pathogens are either delivered to
the Lymph Node/Spleen system by dendritic cells or via the blood.

2.Once there, the antigens activate the adaptive system of B and T cells residing in the
lymphoid tissue

3.The adaptive response culminates in activated T cells and antibodies

4.The activated T cells and antibodies exit the Node/Spleen and migrate to area of infection and
eliminate the infection.

Question 31

Describe the major difference between innate and adaptive immune systems.

Answer 31

A major difference between the Innate and Adaptive immune systems is that the innate system
has a relatively simplistic way of identifying microorganisms by sensing foreign sugars, nucleic acids and conserved lipidic and protein structures but the adaptive system recognizes microbial
peptide derivatives by virtue of specific cell receptors on lymphocytes.

Lymphocytes in the adaptive system can generate large populations of antigen specific cells by
clonal expansion after they recognize “their” antigen but there is no such parallel in the innate
system. The innate system modified its danger sensing mechanisms to include ways of showing the adaptive system what the precise cause of danger was. The adaptive system in parallel developed ways to specifically attack the pathogen after being alerted to it by the innate system to and uses components of the innate system as the end solution to the threat.

Question 32

How can the innate system be trained to remember an infection?

Answer 32

The innate system can be trained to remember an infection so that it can respond more strongly
and quickly during secondary infections. This occurs via epigenetic changes that result in perpetual activation of genes necessary to support the increased metabolic activity of these
trained cells. These trained cells do not adapt to respond specifically to a single organism during
subsequent infections. Rather, they may respond to similar infections (e.g. to cutaneous infections by a variety of organisms).

Question 33

What is cellular immunity? Humoral immunity?

Answer 33

The 2 systems have joined forces to provide the host with a coordinated, highly efficient, focused
response to external threats characterized by cellular immunity, a process during which host
effector cells are induced to kill pathogens directly, and humoral immunity, a process that
utilizes the production of antigen specific antibody immunoglobulins to neutralize or target
pathogens for elimination.

(see cartoon on h/o)

Question 34

Study questions:

1. Describe three types of myeloid cells important for innate responses. How do they
   work?
2. How can Pattern recognition receptors control the type of immune response?
3. What is complement cascade and how does it work to enhance the innate response?
4. Which innate cells are critical links to the adaptive system and what type of
   link do they provide?

Answer 34

?