Immune Response to Infectious Agents

Question 1

Discuss the four categories of locations where pathogens can be and how are body responds based on these locations

Answer 1

Exracellular - Interstitial blood, lymph - We use antibodies, complement, phagocytosis, neutralization, basically anything we want

Extracellular - Epithelial surfaces - We use antibodies, especially IgA, and antimicrobial peptides (can’t use much more than this since it’s outside the skin barrier.

Intracellular - Cytoplasmic - Cytotoxic T cells and NK cells love getting these guys.

Intracellular - Vesicular (in vesicles within the cytoplasm) - T Cell and NKcell dependent macrophage activation

Question 2

Discuss how are innate immune system fights intracellular viruses

Answer 2

• Stops them from getting in via type I interferons

- If they do infect, we kill those cells with NK cells

Question 3

Discuss interferon production in cells

Answer 3

Type I inteferon (alpha and beta types, you won’t be asked to differentiate these) are produced when Viral PAMPs (ssRNA, dsRNA, uncapped RNA, or anything else that says “Hey immune system! I’m not supposed to be here!”) binds to PRRs (pattern recognition receptors)(TLR3, TLR7, TLR8, RIG-1) on cell surfaces.

The inteferon response then begins

Question 4

Discuss the interferon response

Answer 4

3 main functions

1. Induce resistance to viral replication in all cells)
2. Increase expression of ligands for receptors on NK cells
3. Activate NK cells to kill virus-infected cells

Question 5

For the first interferon response, inducingresistance to viral replication, how does this occur?

Answer 5

So the virus infects the cell, releases PAMPs, which are recognized by the cell’s own PRRs, causing production of Type 1 Interferons.

These interferons do 3 things

1. Autocrine stimulation - Cell keeps telling itself to sequester virus resources
2. Paracrine - Tells other cells nearby “Hey! Shit is going down! Hide your shit!”
3. Hides its shit - Sequesters nucleic acids, TFs, etc so that the virus can’t replicate properly.

Question 6

Discuss the mechanisms behind the second and third interferon functions:

2. Increase expression of ligands for receptors on NK cells
3. Activate NK cells to kill virus-infected cells

Answer 6

Type 1 interferons bind NK cell receptors to cause NK cells to proliferate and then differentiate into NK effector cells that then kill virus infected cells by inducing them to apoptose.

Usually NK cells do not kill healthy cells, because the inhibitory receptor of the NK cell binds to the MHC Class I on the healthy cell (basically, the NK cell gets the inhibitory signal from normal cells). But in virus infected cells, there is no longer an MHC Class I receptor, so NK cell gets no inhibition and releases granules to cause apoptosis of that cell.

Question 7

How do the NK granules kill the cell?

Answer 7

2 parts to the granules: Perforin and Granzymes.

Perforin - Makes a barrel shaped opening in the cell membrane to make a pore

Granzymes - Come through the pores to initiate a signaling pathway for apoptosis

Question 8

Alright so these mechanisms for innate immunity against cytoplasmic viruses are great, but how do we adapt to be more swift?

Answer 8

Main goal is for infected cells to be killed by cytolytic (cytotoxic) T lymphocytes (CTLs)
(Angry activated CD8 T cells)

The next main goal is the production of neutralizing antibodies that block virus binding and entry into host cells

Question 9

Let’s back up one second. We stated in a previous card that MHC Class I receptors are not present on viral infected cells, thus the NK cells won’t be inhibited and will degranulate.

Why the hell would viral infected cell do this?

Answer 9

Because keeping the MHC receptors would be more dangerous.

When a virus infects the cell, proteins are made in the cytosol by its material. Fragments get bound by MHC Class I in the ER and then get transported to the cell surface.

The virus inhibits this process because this is what activates the adaptive immunity which creates an insanely better response system than the local random NK cell death.

Question 10

So the MHC Class I + peptide binds with TCR on a CD8 cell as well as the CD8 cell receptor to cause lysis.

How does this lysis occur?

Answer 10

CD8 cells do the same thing NK cells do with the perforin and granzymes.

BUT they also do a Fas ligand pathway (Fas ligand on the CTL and Fas on the cell target) which causes another apoptotic pathway

Question 11

How do antibodies in the adaptive response neutralize virus entry?

Answer 11

For a virus to enter the cell, it expresses a bunch of different proteins on its surface, in the hopes that it will be recognized by some receptor on its target and be absorbed for endocytosis.

Antibodies gum up the virus proteins so binding can not occur, thus neutralizing the virus.

Question 12

What antibodies are great at doing neutralization?

Answer 12

IgG and IgA.

IgM a little, but not really all that much

Question 13

What antibodies are good at opsonizing?

Answer 13

IgG1>IgG3>IgG4 = IgA>IgG2

Question 14

What antibodies are good at sensitizing cells for death by NK cells

Answer 14

IgG1 and IgG3

Question 15

What antibodies are good at sensitizing cell for mast cell attack?

Answer 15

IgE is the most potent

IgG1 and IgG3 also ok at this

Question 16

What antibodies activate complement?

Answer 16

IgM and IgG3 are the best at this, IgG1 is great too.

IgG2 and IgA are decent at this

Question 17

So in summary, when do we see these different events occurring, and what happens to the virus?

Answer 17

Innate (type 1 interferon, TNF-a, etc) happens first, causing NK cell use. All in the first 2-5 days.

Adaptive kicks in after about a week. This is when the virus really starts to die off, with T cell mediated killing of infected cells on a larger degree

Question 18

So all of this that we have been talking about is really intracellular, intracytoplasmic viruses.

What do we worry about when they are in vesicles within the cytoplasm…

Answer 18

So these guys get into phagocytes (like getting in your enemy’s tank) and are inaccessible to antibody (they are wearing a vesicle armor).

Question 19

What are the big intravesicular organisms we worry about?

Answer 19

• M tuberculosis
• M. leprae
• Listeria monocytogenes
• Leishmania

Question 20

Normally, macrophages endocytose a vesicle of micro material in order to fuse that sucker with a lysosome to degrade it.

However, these intravesicular pathogens reisst that second step, and just laugh at the macrophage for allowing it to come inside and persist.

What does this pissed off macrophage do to get rid of its unwanted guests?

Answer 20

Releases IL-12 to neighboring NK cells. NK cell binds to the macrophage, and together this pair turns on the NK cell, and causes it to produce Type II interferon, also called Interferon-y.

Interferon-y binds to its receptor on macrophages and activates them to increase phagocytosis and secretion of inflammatory cytokines (basically superactivating this macrophage to make it much better at digesting the stuff it has endocytosed)

Question 21

So in summary, what do NK cells do?

Answer 21

Kill intracytoplasmically infected cells.

Boost macrophages to kill the intravesicular pathogens they have ingested.

Question 22

NK cells are great, but I remember before when we were summarizing the responses against intracytoplasmic viruses that the innate response of NK cells wasn’t all that great, we needed the adaptive immunity to make a bunch of activated T cells to really do the damage.

What is the adaptive response for intravesicular pathogens?

Answer 22

CD4+ T cells that develop into TH1 effectors that produce WAYYY more IFN-y and take over as the macrophage booster.

Question 23

So CD4+ T cells make IFN-y only? Help me here I forget how these guys got into the picture…

Answer 23

Recall that Antigen presenting cells, like dendritic cells, present bad material to Naive CD4+ T cells in the lymph nodes that are draining.

If it’s an antigen that isn’t recognized, that T cell matures either into a TH1 cell (via IL-12, just like how we got NK cells to make IFN-y) or a TH2 cell.

TH1 cells, as we just learned, make WAYYY more IFN-y then NK cells. TH2 cells make IL-4 and IL-5

Question 24

Discuss positive feedback for IL-12 production

Answer 24

Recall that APCs make IL-12 for Naive T cells to turn to TH1 to make IFN-y. This produced IFN-y caused the APC to make more IL-12 to keep this response growing.

Also, CD4+ T cells are connected to APCs via their CD40L to the APC’s CD40 membrane protein. This connection further stimulates IL-12 production

Question 25

Discuss Tuberculoid vs. Lepromatous Leprosy

Answer 25

Leprosy lesions show what was mostly made in response to the leprosy virus.

The nice, not so horrible looking tuberculoid lesions are caused by Naive CD4+ T cells becoming mostly TH1, thus producing mostly IL-1, IFN-y and TNF-B, with only a little IL-4

In horrible, deadly, lepromatous leprosy, we got mostly TH2, not TH1, causing the production of destructive IL-4, IL-5, and IL-10.

Question 26

Alright, so when discussing extracellular pathogens, we need to talk about complement. Discuss very basically the three complement pathways and when they activate

Answer 26

Alternative Pathway - Pathogen surface creates local environment conducive to complement activation. This one is the first to act!

Lectin pathway - Mannose-binding lectin binds to pathogen surface. This is the second to act.

Classical Pathway - C-reactive protein or antibody binds to specific antigen on pathogen surface. This, ironically, is the last to kick in.

Question 27

“Complement activation,” through one of the three pathways discussed, leads to what exactly?

Answer 27

Cleavage of C3 protein to C3a and C3b.

C3b covalently binds to surface components of the pathogen, becoming an opsonin for the C3b receptor on macrophages.

Question 28

C3b binding to pathogen leads to what 3 effects?

Answer 28

• Recruitment of inflammatory cells (via C3a meeting up with C5a (from the cleavage of, you guessed it, C5) to recruit and activate leukocytes for microbe destruction)
• Opsonization of pathogens, facilitating uptake and killing of phagocytes (discussed earlier)
• Perforation of pathogen cell membranes in late phases of complement with C6 and C5b binding to C7 and C8 in the membrane of the pathogen, respectively, to make the membrane attack complex (MAC) which causes osmotic lysis of the microbe.

These all lead to the death of the pathogen!

Question 29

What cytokines are generated for defense against extracellular pathogens and what do they do?

Answer 29

Il-1/Il-6/TNF-a

Liver - Via C reactive protein and mannose-binding lectin, we get activation of complement and opsonization

Bone marrow endothelium - Neutrophil mobilization to lead to phagocytosis

Hypothalamus - Increase body temperature to decrease replication of the microbe

Fat/muscle - Protein and energy mobilization to increase body temperature to decrease replication of the microbe.

Question 30

What adaptive components do we see to fight extracellular pathogens?

Answer 30

B cells and antibodies! Wooh!

This is for the third complement pathway, the classical pathway (recall that the other two are really innate)

Antibody binding to the microbe causes C3a inflammation, C3b opsonization and phagocytosis, and C5a inflammation leading to C5b/C6 MAC lysis of the microbe!

Do you feel smart yet? Cause I do.

Question 31

Besides activating the classical pathway, what do antibodies do?

Answer 31

They can opsonize the microbe via IgG, which binds to the Fc receptors of phagocytes (specifically FcyRI)

Question 32

So what do we do with big ol worms that we can’t phagocytose, i.e., can not do the innate pathway on?

Answer 32

Need to use TH2 cells!

Question 33

Remind me again about TH2 cells…

Answer 33

Recall that APCs show bad stuff to Naive CD4+ T cells, causing them to become either TH1 or TH2 cells. TH2 cells produce multiple IL’s:

IL-4 - Tells B-Cells to mature to plasma cells that either make IgE to degranulate mast cells or produce neutralizing IgG antibodies.

Il-5 - Activate Eosinophils to attack helminths

IL-10/IL-4 - Suppress macrophage activation

Question 34

Discuss the eosinophil reaction against helminths

Answer 34

So all that IgE that was made thanks to IL-4 from TH2 cells telling B cells class switch to IgE plasma cells is now used to mark up the helminth. Eosinophils are like HEY!

Eosinophils bind to the IgE on the helminth via high affinity FceRI and kill it

Question 35

What are the helminth toxic components of eosinophil granules?

Answer 35

Eosinophil peroxidase - Poisons parasite and mammalian cells by catalyzing halogenation and triggering histamine release from mast cells

Eosinophil collagenase - Remodels connective tissue matrix

Major Basic protein - Poisons mammalian and parasite cells and also triggers histamine release from mast cells

Eosinophil cationic protein - Poisons parasites and is a neurotoxin

Eosinophil-derived neurotoxin - Degradation of RNA and antiviral effects