Working group assignments immunology
Biomedical sciences year 2, Leiden University, Year 2021/2022
WG 1: Innate immunity 1
Objectives part I: Physical barriers to infection
- The students comprehend how physical barriers can function as a first line
of the innate immune defense.
- The students and can summarize examples of the mechanical, chemical
and biological element of these barriers.
- The students can explain why the mechanisms of this first line of defense
are considered as nonspecific and differ from mechanisms of the adaptive
immunity (adaptive immunity will be dealt with later).
Questions part I
1. Name the major features of innate immunity (in contrast to the adaptive
immunity)
a. Slow, non-specific, no memory, fixed, limited number of specificities,
constant during course of response
2. The bacterium Vibrio cholera is a major pathogen that causes life-
threatening acute diarrhoea and cholera. It is a Gram-negative bacterium
that infects mostly young children, the elderly or travellers after drinking
contaminated water in regions where sanitary provisions are poor. The
bacteria apparently are very capable to transfer to the small intestine to
exert its pathology. Discuss which physical barrier mechanisms are met by
V. cholera on their way from mouth to the intestinal tract and explain how
these bacteria combat these defensive mechanisms and manage to reach
the intestine to colonise the epithelial cell surface.
How do these bacteria subsequently breach through the intestine epithelial
barrier? (Hint: study figures 1.4, and 9.25)
a. Ingested trough drinking infected water. The physical barrier is
saliva in the mouth and the mucus at the gastro-intestinal tract. Also
the acidity in the stomach and the antimicrobial peptides and
enzymes. The peristaltic of the gastro-intestinal pathway also battle
the bacteria. Colonization = settling into body with their villi.
b. They defend themselves by activates adenylate cyclase, elevates
cAMP in cells, leading to changes in intestinal epithelial cells that
cause loss of water and electrolytes. The diarrhoea also flushes out
the commensal microbiota in the gut, making it easier for them to
colonize and multiply. The cholera toxin is compound secreted by
cholera to support water efflux. To battle this, you can ingest
charcoal to keep the water in the intestines. Can also use
neutralizing antibodies, so the toxin cannot bind to cell-surface
receptor. If the immune system is working properly, bacteria can
also be removed by TLR4 recognizing the bacteria, bacteria has a lot
of LPS, which TLR4 recognizes -> triggers inflammatory response ->
phagocytose.
, 3. Probiotic, such as Lactobacillus ssp., are often recommended by
nutritionists as dietary supplement after a course of antibiotics, or as part
of the treatment for gut-related candidiasis (fungal infections). What is the
rational for this recommendation
a. Antibiotic also kill the commensal bacteria in your gut, this can lead
to pathogenic bacteria being able to colonize in the gut because
they do not have the competition anymore. By supplementing with
probiotics, these will settle in the gut and prevent pathogenic
bacteria from settling.
4. Often during intestinal infection drugs are gives to supress diarrhea.
Discuss why this may not always be a good idea.
a. Because diarrhea is also useful the flush pathogens out of the gut.
Objectives part II: Humoral elements of the innate immune
response (Complement system)
- The student gains a clear understanding of the humoral elements of the
innate immune response and how these molecules or fragments thereof
act antimicrobial and /or modulate the immediate response of the immune
system against invasive micro-organisms and other foreign particles.
o Complement proteins (see further information in the other
objectives)
o Coagulation system
Cascade of enzymes in plasma that cooperates with platelets
to form blood clots. Pathogens get immobilizes in the clots
and cannot travel throughout the body trough the lymph and
blood. Clotting also serves to decrease blood and fluid loss.
Also during clot formation, agents are formed that contribute
to the recruitment of immune cells, antimicrobial defense and
tissue repair.
o Kinin system
Also enzymatic cascade that is induced by damage tissue. It
produces bradykinin that reduces hypertension, dilates blood
vessels, and relaxes smooth muscle in the damaged tissue.
These properties help elimination of the pathogens and the
repair of the damage tissue
o Protease inhibitors
Microbes secrete proteases that inactivate antimicrobial
peptides in the serum and makes them invisible to the
immune system. So there are protease inhibitors in the serum
that mimick the peptides and so the protease want the
inactive this. So the protease inhibitors trap microbial
proteases that bates proteases secreted by microbes and
cleaves it. This induces a conformational change in the
protease inhibitor which envelops the protease and
inactivates it. This complex can be bound by specific
receptors on macrophages, hepatocytes and fibroblasts.
o Defensins
Antimicrobial peptides that neutralize broad range of
structurally diverse microbial toxins
, There are defensins that are in the granules of the
neutrophils, also defensins that are secreted by Paneth cells
of the intestinal epithelium.
o Pentraxins
Function as bridging molecules -> they bind pathogens with
one binding site and use second binding site to human cell
surface receptors, for example CD89 (FcαR) on phagocytes.
When the receptors are cross-linked, it signals the phagocytes
to engulf and degrade the pathogen.
Similar role in the innate immune response as of that of Ab in
the adaptive response, also bind to the same surface
receptors on phagocytes.
Two types: Short (Serum amyloid P component, produced by
liver hepatocytes) and long (PTX3, produced by multiple cells
including MC, MP, DC, endothelial and epithelial cells)
- The student can make a general scheme of the interplay between the
serum proteins that constitute the complement system. In this scheme the
student can indicate the 3 different pathways of complement activation,
the central role of C3 and the common route of MAC formation.
o Pathways:
Alternative (spontaneous), Classical (Immune complexes),
Alternative (MBL)
o Central role C3
C3 has a central role because all three pathways, although
activated different, lead to C3 activation, deposition of C3b on
pathogens surface, and recruitment of similar effector
mechanisms for pathogens destruction
o Common route MAC formation
Fives components: C5,C6,C7,C8,C(
C5 is activated by C5 convertase (two C3b and one Bb,
C3b2Bb; is on pathogen surface because of earlier C3b
formation on surface) which cleaves it to C5b and C5a. C5b
initiates MAC formation -> C6 and C7 bind to C5b -> this
exposes hydrophobic region of C7, which inserts in lipid
bilayer of pathogen -> C8 binds C5b, which also exposes
hydrophobic region of C8, this insert into membrane of
pathogen -> these interactions lead to polymerization of C9
and formation of the pore in the membrane of the pathogen
- The student can give the function of complement receptors and can
describe at least four functional properties of the complement system with
respect to host defense.
o Function complement receptors:
Complement receptors have the function of helping
phagocytes to more efficiently taking up and phagocytosis of
pathogens that are coated with C3b or iC3b.
o Four functional properties:
Recruitment of inflammatory cells (C3a, C5a)
, Opsonisation of pathogens (C3b)
Perforation of pathogens cell membrane (MAC complex)
Immune complex clearance
- The student can name and recognize the cell types that produce
interferons and can explain the stimulatory action of these proteins on the
antimicrobial properties of phagocytes.
o Naming and recognizing cell types that produce interferons
and can explain the stimulatory action of these protein on
the antimicrobial properties of phagocytes
All nucleated cells can be infected with viruses and have
cytoplasmic proteins that can detect viral nucleic acids and
thus produce IFN.
Immediate effect of IFN is interfering with viral replication and
instruct nearby uninfected cells that they have to be ready to
fight infection
IFN also alert immune system cells that infection is present
and makes infected cells more vulnerable by attack of NK
cells by inducing upregulation of stress proteins (altered-self)
All cells have IFN-receptors.
Macrophages get activated by IFN-γ secreted by effector NK-
cells that have been activated by IFN of virus infected cells.
The IFN-γ lead to further activation of the macrophages which
can now better phagocytosis of virus particles and virus-
infected cells and increases the secretion of inflammatory
cytokines.
- The student understand how cytokines and chemokines may work.
(Cytokine function will also be dealt with in future WG 7)
o Cytokines activate or inhibit immune cells responses. They are
secreted by activated immune cells and can for example cause
fever, mobilize neutrophils from the bone marrow, acute the liver to
make more acute-phase proteins like CRP (can activate classical
pathway in absence of Ab) and MBL (activates lectin pathway)
Definition: wide variety of intercellular regulatory proteins
produced by many different cells in the body that ultimately
control every aspect of body defense.
o Chemokines are proteins in blood and tissue that promote
inflammation by enabling white blood cells to adhere to the inner
surface of blood vessels, migrate out of the blood vessels into the
tissue, and be chemotactically attracted to the injured or infected
site.
Questions part II
1. Fill in below: classical route, lectin route and/or alternative route:
a. Lectin route is activated by the interaction of microbial
carbohydrates with mannose- binding lectin (MBL) in the plasma and
tissue fluids
b. Classical route is activated by antigen-antibody complexes
c. Alternative route is activated by microbial cell walls
