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Doctorials: Immunology

2021/2022

Róisín Darby, Doctorials 2021/22

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Learning Objectives

Róisín Darby, Doctorials 2021/22

Highlight the difference between lymph nodes and the spleen and how they are involved in tissue and blood infection, respectively Highlight key lymphatic drainage that is unique (e.g. testes and ovaries to para-aortic nodes) and why this is clinically significant. Briefly indicate the locations whereby lymph nodes are palpable on clinical examination Explain the role of opsonization and why asplenic patients are at risk from infection by encapsulated bacteria. List the clinically significant encapsulated bacteria and opsonins. Explain in simple terms the main differences between the innate and adaptive immune responses Review the process of inflammation Explain the role of cytokines in the formation of the systemic signs and symptoms of acute phase reactants and fever, emphasizing that cytokines must be constitutively produced in large enough amounts for this to occur Explain the difference between T-cell dependent and T-cell independent B-cell activation Emphasize the role of MHC II molecules on APCs and the role of MHC I molecules on all nucleated cells Explain, in detail, why polysaccharide antigens do not induce class-switching or memory, whereas protein antigens do. Explain what a conjugated vaccine is and why this is clinically important. Explain the process of positive and negative selection in the formation of self-tolerance. Briefly indicate the difference between central and peripheral tolerance and link this with the concept of autoimmunity.

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Geeky Medics, 2021

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INNATE IMMUNE SYSTEM

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INNATE

ADAPTIVE

Franks Lectures on Youtube

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An overview of the immune response

Róisín Darby, Doctorials 2021/22

Adapted from Janeway's Immunobiology, 9th edition pg 37

Innate Immune response: Immediate, non-specific first line defence involving inherited cell receptors and secreted proteins to detect and distinguish between pathogens and host tissues

Adaptive Immune response: A late, antigen-specific immune response following exposure to a microbial pathogen or foreign substance (e.g.antigen)

What are anatomic barriers?

What is an antigen?

Main cell types involved in the adaptive immune system?

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LO:Explain the role of cytokines in the formation of the systemic signs and symptoms of acute phase reactants and fever

Cytokines are cell signalling proteins that modulate immune responses, inflammation and hematopoietic cell proliferation and differentiation
Cytokines mainly secreted by hematopoietic cells
Cytokines can be divided into

Pro-inflammatory cytokines

Interleukins (IL-1, IL-6, IL-8, IL-12, IL-18)
Interferons (IFN)
Tumour Necrosis Factor (TNF)

Anti-inflammatory cytokines

Interleukins (IL-4, IL-10, IL-11, IL-13)
Transforming Growth Factor beta (TGFβ)

Róisín Darby Doctorials 2021/22

AMBOSS 2021

c

Adaptive immunity

Innate Immunity

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Cytokines continued

'What do you want for dinner....I'll have a Hot T-Bone stEAK!

Róisín Darby Doctorials 2021/22

First Aid for the USMLE Step 1 2021 pg 108

Interleukin-1	Causes fever Acute inflammation Activates endothelium to express adhesion molecules Induces chemokine secretion to recruit WBCs
Interleukin-2	Stimulates T cells
Interleukin-3	Stimulates bone marrow
Interleukin-4	Stimulates IgE production
Interleukin-5	Stimulates IgA production
Interleukin-6	Stimulates acute (aKute) phase protein production

IL-1: fever (Hot)

IL-2: Stimulates T cells

IL-3: Stimulates bone marrow

IL-4: Stimulates IgE production

IL-5: Stimulates IgA production

IL-6: Stimulates aKute- phase protein production

'Clean up on Aisle 8' (IL-8)

Neutrophils are recruited by IL-8 to help clear infections

What cytokines can dampen (attenuate) the immune response?

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Cells of the immune system

Róisín Darby Doctorials 2021/22

Ross and Paulina Histology 6th ed, pg 277-284

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Cells of the immune system�

Geeky Medics, 2021

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Róisín Darby, Doctorials 2021/22

Geeky Medics, 2021

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EMQ Past Question:

1.Responsible for fast secondary immune response

2.Secretes antibodies

3.Releases mediator that causes vasodilation

4.Matures in the thymus

5.Agranular phagocyte

Neutrophil
Macrophage
B cell
T cell
Plasma cell
NK cell
Basophil
Memory cell

Róisín Darby, Doctorials 2021/22

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LO: Review the process of inflammation

A non-specific defensive response to tissue damage (e.g. pathogens, abrasions, chemical & temperature damage) 4 characteristic signs and symptoms: Redness Pain Heat Swelling (May also be loss of function, e.g. sensation)

AMBOSS/ Dr Catriona Dowling lecture, 2020

Róisín Darby Doctorials 2021/22

Charecteristic	Cause	Mediators
Redness (Rubor)	Release of vasoactive mediators by immune cells and endothelium- arteriolar smooth muscle relaxation- causing vasodilation and increased blood flow	Histamine Bradykinin Prostaglandins (PGE2, PGD2 and PGF2) NO
Heat (Calor)	Same process and mediators as redness (more blood flow- bringing heat)	"
Pain (Dolor)	Stimulation of free nerve endings by certain mediators and H⁺	Bradykinin PGE2 Histamine
Swelling (Tumor)	Release of mediators from immune cells/ damage to the endothelium -> separation of endothelial junctions -> Increased vascular permeability -> leakage of protein rich fluid from postcapillary venules -> Increased oncotic pressure in the interstitium -> Accumulation of fluid in the interstitium	Histamines Leukotrines Serotonin
Loss of Function (Functio Leasa)	Caused by the combined effect of other cardinal signs

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LO:Highlight the difference between lymph nodes and the spleen and how they are involved in tissue and blood infection, respectively

Róisín Darby Doctorials 2021/22

Janeways Immunobiology 9th ed pg 20/ AMBOSS 2021

Lymph nodes are throughout the body, near organs and large vessels
They carry out non-specific drainage of lymph via macrophages
Store and circulate T and B cells
Antigen presentation in the lymph nodes induces the proliferation and differentiation of B lymphocytes and activation of T lymphocytes

_______________________________________________________

The spleen is an intraperitoneal organ in the left upper quadrant of the abdomen
Important roles in the filtration of abnormal RBCs and platelets in the red pulp
White pulp- lymphatic sheath and lymphoid follicles which produce antibodies
Marginal zone- macrophages are involved in opsonization of encapsulated bacteria

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Róisín Darby Doctorials 2021/22

AMBOSS 2021

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LO:Explain the role of opsonization and why asplenic patients are at risk from infection by encapsulated bacteria. List the clinically significant encapsulated bacteria and opsonins.

Róisín Darby Doctorials 2021/22

Dr Catriona Dowling lecture, 2020/ First Aid for USMLE step 1 2021 pg 104

Name three clinically significant encapsulated bacteria?

What prophylaxis treatment should asplenic patients be given?

Opsonization increases the susceptibility of bacteria to phagocytosis:

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Encapsulated bacteria: High yield for GEMS!

Róisín Darby, Doctorials 2021/22

Rosh Review, 2021

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ADAPTIVE IMMUNE SYSTEM

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Innate vs Adaptive Immune System

Róisín Darby, Doctorials 2021/22

First Aid for the USMLE Step 1 2021, pg. 99

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Geeky Medics, 2021

Cells of the Adaptive Immune system

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Cells of the Adaptive Immune system

Geeky Medics, 2021

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Cells of the Adaptive Immune system

Geeky Medics, 2021

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T cell differentiation

Róisín Darby Doctorials 2021/22

First Aid for the USMLE 2021 pg 102

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Role of MHC II molecules on APCs and the role of MHC I molecules on all nucleated cells

First Aid for the USMLE 2021 pg 100

Róisín Darby Doctorials 2021/22

Remember: CD4+ T cells interact with MHC II CD8 + T cells interact with MHC I Rule of 8: MHC I x CD 8 = 8. MHC II x CD 4 = 8

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�The difference between T-cell dependent and T-cell independent B-cell activation�B cell activation and class switching require an initial signal and a co-stimulatory signal

Róisín Darby Doctorials 2021/22

AMBOSS, First Aid 2021, pg 103

T- cell dependent B cell activation 1. Prior activation of CD4+ T helper cell is needed 2. B- cell receptor mediated endocytosis after antigen recognition 3. Exogenous antigen is presented on MHCII , recognized by TCR on Th cell 4.CD40 on B cell binds CD40L (ligand) on Th cell 5. Th cells secrete cytokines that determine Ig class switching of B cells B cell activated, produce immunoglobulins and undergo class switching and affinity maturation Thymus- dependent antigen are antigen containing a protein component. Class switching and immunologic response can occur as a result of direct contact of B cells with Th cells	T- cell INDEPENDENT B cell activation Occurs in immediate response to non-protein antigens (e.g gram negative LPS, polysaccharides) NO prior T Cell activation as MHC not able to present non protein antigen to T cells In the absence of T cells, most activated B cells rapidly mature into short lived plasma cells without undergoing somatic hypermutation or class switching These B cells produce IgM antibodies of low affinity Thymus independent antigens are considered only weakly immunogenic

T- cell dependent B cell activation

Requires activation of CD4+ T-helper cells, which require prior antigen presentation via MHC II by B cell
Response to protein or peptide antigens
B lymphocytes recognize antigens via their BCR(a membrane bound IgG or IgM)
B cell receptor mediated endocytosis occurs of the BCR/antigen complex
Antigen broken down into small fragments by lysosomal proteases
Presentation of antigen fragment (via MHC II) on Th cells
Co-stimulation of the B cell CD40 by CD40L on Th cell
Immunoglobulin production occurs
T cell-activated B cells migrate under the influence of chemokines to the mantle zone of germinal centers where they acquire CD5 expression transiently, and then enter the germinal center
https://next.amboss.com/us/article/x50Emg?q=t%20cell-dependent%20activation#Z96f15daceb6669363fbf7f762ed57703

T- cell INDEPENDENT B cell activation

Initiated as an immediate response to nonprotein antigens e.g. gram positive LPS
Leads to production of IgM antibodies
No prior T cell activation required as MHC are unable to present nonprotein antigens to T cells

More information on the use of adjuvants in vaccines: https://www.nature.com/articles/s41573-021-00163-y
https://www.cdc.gov/vaccinesafety/concerns/adjuvants.html

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Antibodies

Róisín Darby Doctorials 2021/22

Geeky Medics, 2021

With so many antibody possibilities….How is a limited number of genes coding for them?

What type of antibodies can cross the placenta?

A limited number of genes are encoding for millions of antibody types because antibodies can generate diversity and specificity by several mechanisms : The antigen INDEPENDENT ways of generating antibody diversity are:

VDJ recombination
Random addition of nucleotides to DNA during recombination by deoxynucleotidal transferase (TdT)
Random combination of heavy and light chains

Generation of antibody specificity which is antigen DEPENDENT occurs by

4. Somatic hypermutation and affinity maturation in the variable region

5. Isotype switching in the constant region

NOTE: In the next few slides I will focus on the three main strategies of generating antibody diversity which are important to know for GEMS, but for USMLE Step 1 if you plan on taking it you should know all 5 (see pg 104 of First Aid for the USMLE 2019 for more details)

IgG is the only isotype that crosses the placenta- It provides infants with a passive immunity that starts to wane at about 6 months.

NOTE: Other immunoglobulins like IgA and IgM can pass from mother to child via the breast milk and these play an important role in this early infancy passive immunity too

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Antibodies continued

3 main strategies for producing antibody variability
Gene rearrangement (VDJ recombination)
Somatic hypermutation
Class- Switch recombination (Isotype switching)

Róisín Darby Doctorials 2021/22

Dr Caitriona Dowling lecture, 2020

Gene rearrangement (VDJ recombination) Genes for Heavy and Light Chain Recombine to generate surface receptor for antigen	Somatic Hypermutation Somatic hypermutation in Complementarity Determining Region (CDR) (hypervariable region) refines affinity of antibodies	Class switch recombination (Isotype switching) A biological mechanism that changes a B cell’s production of antibody from one class to another (e.g. IgM to IgG) Inactivated B cells express IgM/ IgD but once activated they may express IgA, IgE, IgG or retain IgM expression This is done by excision of unwanted isotypes

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Isotope switching (Class switching) in more detail

Polysaccharide antigens do not induce class switching as most are T-lymphocyte independent antigens

Polysaccharides therefore induce a T cell-independent B-cell response and therefore achieve sufficient immunization only in adults and older children

In conjugate vaccines inactivated polysaccharide are linked to a carrier protein activates T cell dependent memory response which stimulates a more rapid and long- lasting immune response, especially in children under 2

Róisín Darby Doctorials 2021/22

AMBOSS/ Tuomenan 2021

Remember: MHC not able to present non protein antigen to T cells

Immunoglobulin class switching (From AMBOSS)

NOTE: YOU DO NOT NEED THIS LEVEL OF DETAIL BUT MIGHT HELP SOME UNDERSTAND THE CONCEPT BETTER!

Genes that encode for the heavy chains of immunoglobulins feature three different types of regions: C regions, VDJ regions, and S regions (not shown). C and VDJ regions are exons (i.e., they encode amino acid sequences); S regions (switch regions) have a regulatory function.

S regions are located upstream of their corresponding C regions (e.g., the S_ε region is upstream of the C_ε region) and, in turn, each S region possesses an upstream promoter.�Class switching begins with the initiation of transcription of a particular S region. A complex and not yet fully understood molecular mechanism ensues that involves enzymatic breaking and subsequent repairing of DNA strands. Depending on the desired immunoglobulin class, two different S regions interact with each other to rearrange or join the two C regions and bring them in proximity to the VDJ region.

The pre-mRNA produced from the rearranged gene undergoes alternative splicing and mature mRNA is formed. The mRNA is then translated into the corresponding immunoglobulin protein of the desired class.

Article on pneumococcal conjugate vaccines: https://www-uptodate-com.proxy.lib.ul.ie/contents/pneumococcal-vaccination-in-children?search=conjugate%20vaccine%20&source=search_result&selectedTitle=2~150&usage_type=default&display_rank=2

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Self-tolerance

Immune tolerance is the state of unresponsiveness of the immune system to substances or tissues that have the potential to induce an immune response. Self tolerance to an individual's own antigens is achieved through both central tolerance and peripheral tolerance mechanisms.
Central tolerance- Takes place in primary lymphoid organs where development of lymphocytes takes place (thymus, bone marrow)
Peripheral tolerance- Takes place in secondary or peripheral lymphoid organs (spleen, lymph nodes)

Can you give an example of a ‘self antigen’?

What happens when self tolerance goes array…

Róisín Darby Doctorials 2021/22

Nature, 2021

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Autoimmunity

Autoimmunity is an immune reaction against the body’s own cells that occurs as a result of the loss of immune tolerance/ self tolerance
Women have higher incident of autoimmune disease than men
If our selection mechanisms fail, autoimmune disease can result

Róisín Darby Doctorials 2021/22

NIH, 2021

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Thank you for listening! Questions?

Róisín Darby Doctorials 2021/22

My email: 20276974@studentmail.ul.ie

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References

AMBOSS

First Aid for the USMLE, 2021 edition
Janeways Immunobiology, 9th edition
Zero to Finals: Understanding the immune system in one video https://www.youtube.com/watch?v=_jBpv9fYSU4
Dr Catriona Dowling ‘Overview of Immunology’2020 lecturehttps://sulis.ul.ie/access/content/group/ad2f3402-6b55-4500-a91a-4d6d0f52a963/Year%201%20Lectures/Week%2017%20-%20Henry%20Fitzgibbon/Overview%20of%20Immunology.pdf

https://geekymedics.com/immune-response/

https://www.nature.com/subjects/immune-tolerance

Doctorials: Immunology 2020 by Alisha Jaffer
https://www-uptodate-com.proxy.lib.ul.ie/contents/pneumococcal-vaccination-in-children?search=conjugate%20vaccine%20&source=search_result&selectedTitle=2~150&usage_type=default&display_rank=2
https://twitter.com/RoshReview
https://www.youtube.com/watch?v=rHx30H3dUKQ&t=338s
https://www.niehs.nih.gov/health/topics/conditions/autoimmune/index.cfm

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