Introduction to the Immune system: Lectures 1 and 2

Hugh Bradyh.brady@imperial.ac.uk

Recommended textbook: Janeway’s Immunobiology (7th Ed) 2008, Garland Science

Why study immunology?

Several reasons, most important in terms of numbers:

Infectious diseases are a major burden worldwide

Major challenges are HIV/AIDS, TB and malaria• 42 million people living with HIV and AIDS worldwide

• Malaria causes more than 300 million acute illnesses and at least one million deaths annually

• An estimated two million deaths resulted from tuberculosis in 2002

• We need new strategies or vaccines to prevent or treat infectious disease

Do pathogens “want” to kill us? Does the

immune system (usually) “win” the fight

with pathogens, enabling survival of the

host?

Or does the pathogen evolve to adapt to

the host, including the host immune

response, enabling it to more effectively

persist?

Virulence and evolutionary fitness

Multiple theories:

• Virulence (damage to host) is detrimental to pathogen: a dead

host is less effective at transmission

• Highly virulent pathogens are recently emerged and will

evolve towards lower pathogenicity

• Virulence factors confer increased fitness to pathogen

• Virulence may increase as pathogens evolve. Are both true?

• Perhaps virulence increases to a limit, after which it is

counterproductive to the pathogen

• Must also consider that immune response is often responsible for

host damage

The role of the immune system

To protect us from infection:

• Viruses; 20-400nm (obligate intracellular)

• Bacteria; 1-5 m

• Fungi; 2-20 m

• Protozoan parasites; 1- 50m

• Metazoan parasites (worms) 3mm – 7m

Apart from infectious diseases, why else study immunology?

• Autoimmune disease – eg multiple sclerosis, rheumatoid arthritis

• Allergy – Allergy and asthma prevalence increasing

• 300 million people suffer from asthma and >180 000 die annually

• Asthma deaths are predicted to increase by almost 20% in the next 10 years

• Unwanted responses – transplantation.

The immune system

• Physical barriers

• Cells

• Soluble effector proteins

– complement, antibodies

• Cytokines – communication

Cells of the immune system

Lymphocytes are mostly small and inactive cells

An infection and the response to it can be divided into a series ofstages

Innate immunity

Many barriers prevent pathogens from crossing epithelia and colonizing tissues

Time frame for different stages of immune response

The innate immune response

• Mediated (initiated) by phagocytes, NK cells and soluble proteins

• Phagocytes• Cells specialized in the process of phagocytosis

» Macrophages Reside in tissues and recruit neutrophils

» Neutrophils Enter infected tissues in large numbers

• Recognize common molecules of bacterial cell surface using a few surface receptors

• Phagocytosis• Capture, engulfment and breakdown of bacterial pathogen

• The largest cells in the blood

• Monocytes are the precursors to macrophages in the tissue

• Can be tissue resident or recruited to sites of inflammation

• Engulf and kill viruses and bacteria

• Important for antigen presentation to T cells

Monocyte / macrophage

Macrophages are activated by pathogens and both engulf them and initiate inflammatory responses

Pattern recognition in innate immunity

Microrganisms have repeating patterns of molecular structure on their surface

in cell walls of Gram-positive and Gram-negative bacteria

Peptidoglycan molecules

Other microbial elements with repetitive structure

Bacterial DNA with unmethylated CpG dinucleotide repeats

The repetitive structures are known as pathogen-associated molecular patterns (PAMPs) and the receptors that recognise them as pattern recognition receptors (PRRs)

Phagocytes

• Most common leukocyte (white blood cell) in blood (up to 80%)

• Distinctive lobed nucleus and intracellular granules

• Highly motile, they respond rapidly to inflammatory stimuli by migrating out of the blood and into tissues in large numbers.

• Recognise, engulf and kill viruses and bacteria

• Short life span – about 24hrs

• Dead neutrophils are a major constituent of pus!

Neutrophils

The innate immune response

• Inflammatory response enhances phagocytosis through acute phase proteins• Mannose-binding lectin (MBL)

» Binds to bacterial surface with particular spatial arrangement of mannose or fucose

• C-reactive protein (CRP)

» Binds to phosphorylcholine on bacterial surface

• Complement

» Set of proteins which bind to bacterial surface

• Inflammatory response• Accumulation of fluid and cells at infection site (swelling, redness, heat and

pain)

Complement system

Complement system

Pre-formed protein cascade:

• Punches holes in pathogen membranes

• Alerts and recruits other components of immune system to danger

• Coats pathogens for uptake by phagocytes: ‘opsonisation’

Innate immunity

• First line of defence against infection

• Pre-existing or very rapid response (hrs)

• Recognition of pathogens is based upon pattern recognition (PAMPs) using germline encoded receptors (PRRs) or proteins.

• Non-adaptive

• No memory

• Evolutionarily early

Acquired / adaptive immunity

• Specific to a particular antigen – detected by specific receptors on T and B cells

• Response improves with time

• Results in MEMORY =Protection against re-infection with the same pathogen

• Evolutionarily late – only present in vertebrates

• Involves T and B lymphocytes

B cells

• Produce antibodies

• Sometimes called “humoral immunity”

Proliferation Differentiation

B cell receptors

Long lived

T cells

T cell receptor (TCR)

only recognises foreign antigen as a peptide on an MHC molecule presented by an antigen presenting cell (APC)

Pathogen Recognition: T and B cell receptors

• T and B cells have receptors specific for only one antigen

• Genes for the antigen receptors are re-arranged in each cell – every cell is different

• Gene rearrangement results in receptors that can recognise a huge variety of antigens (108 different specificities in a human at one time)

T and B cell responses are initiated in secondary lymphoid organs - lymph nodes and spleen

HaematopoiesisB cellsNeutrophils etc

T cell development

Primary and secondary immune responses

Smallpox - an immunology success story

Evolution of smallpox lesions on proximal upper

extremity on days 3, 5, and 7 of lesions

1796 Exposure to cowpox protects against smallpox

Jenner

SmallpoxVaccinationwell known in16th centuryChina

Smallpox is the only major human disease to have been eradicated.

The Adaptive Immune response

• Creates millions of different B and T cells for specific antibody-mediated and cell-mediated immunity

• Antibody-Mediated Immunity (AMI)• Involves B lymphocytes, plasma cells and antibodies

• Humoral immunity

» Name derives from antibodies found in body fluids (humors - old medical term)

• Cell-Mediated Immunity (CMI)• Involves T lymphocytes, antigen-presenting cells and MHC (major

histocompatibility complex) molecules

• Cellular immunity

Antibody-mediated (humoral) immunity

Directed against extracellular microorganisms

and toxins

B-lymphocytes (B cells)• Differentiate into plasma cells which produce antibodies

• Function as antigen-presenting cells (APC’s)

Classification of Antibodies (Immunoglobulins)• Immunoglobulin M (IgM)

• Immunoglobulin G (IgG)

• Immunoglobulin A (IgA)

• Immunoglobulin D (IgD)

• Immunoglobulin E (IgE)

Cell-mediated immunity

• Directed against intracellular microorganisms • Phagocytic cells and nonphagocytic cells

• T-lymphocytes (T cells)• Differentiate into effector cells following antigen presentation by

antigen presenting cells (APC’s)

• Activate B lymphocytes

• Functional types of T cells• Helper (CD4 T cells)

» TH1 and TH2 cells

• Cytotoxic (CD8 T cells)

• Regulatory (Suppressor)

» CD4 Tregs

» CD8 Tregs

The nature of antigens

• Historically named as antibody generators• Molecule which stimulates production of and binds specifically to an

antibody

• Contemporary view distinguishes between• Antigen

» Molecule which can bind to specific antibody but cannot elicit adaptive immune response

• Immunogen

» Molecule which can stimulate adaptive immune response

• Best immunogens are proteins with MW > 10,000

The nature of antigens

• Carbohydrates, nucleic acids and lipids are also potential antigens / immunogens

• Hapten• Small (low MW) molecule unable to elicit immune response

• Combines with larger carrier molecule which together function as immunogen

• Antibody may react independently with hapten following hapten/carrier adaptive immune response

• Example

» Penicillin G (MW of 372)

» Albumin (MW of 66,000)

Antigens are the molecules

recognized by the immune

response

Epitopes are sites within

antigens to which antigen

receptors bind

An antibody binds an antigen directly whereas a T-cell receptor binds

a complex of antigen fragment and self molecule

The nature of antibodies

• Antibodies are glycoproteins• Exist as monomers, dimers or pentamers of

basic structure• Basic antibody structure has 4 polypeptide

chains• 2 identical light chains

• 2 identical heavy chains

• Regions of heavy and light chains• Variable

• Constant

The nature of antibodies

• Also referred to as• Immune globulins / Immunoglobulins (IG)

• Immune serum globulins (ISG)

• Gamma globulins

• Contemporary immunology• Antibody

»Secreted form of IG made by plasma cells

• Immunoglobulin»Antigen binding molecules of B cells

(B cell antigen receptors)

Classification of Antibodies (Immunoglobulins)

• Five (5) classes (isotypes)IgA IgG IgM IgD IgE

• Based on structural differences in constant regions of heavy chains

• Classes have specialized effector functions

High affinity IgG and IgA antibodies neutralize bacterial

toxins and can inhibit infectivity of viruses

IgE has specialized role activating Mast Cells

B cells and antibody-mediated immunity

• B lymphocytes originate from stem cells in bone marrow

• Maturation takes place in bone marrow followed by migration to secondary lymphoid tissue

• Antigen exposure in secondary lymphoid tissue

• Following exposure to antigen, B lymphocytes differentiate into plasma cells and memory cells

• Plasma cells produce antibodies of all IG classes

1. Neutralisation

Antibodies block the interaction of the virus with its receptor

2. Opsonisation

Antibodies label the virus and it is now recognised by phagocytes

NB Abs can also label the whole virally infected cell for phagocytosis

3. Complement activation

Membrane Attack Complex damage to the viral envelope, MAC contains activated complement proteins, forms pore

Activation of antibody producing cells by clonal selection

• B lymphocytes recognize intact pathogenic microorganisms and toxins

• B lymphocytes possess specific surface receptors for recognition of specific antigen

• IgM and IgD

• Binding of specific antigen results in proliferation of a clonal population of cells

• Antigen determines clonal proliferation

T and B cell responses are clonal

Receptor diversity;• T and B cells have

receptors specific for only one antigen

• Each cell is unique

Specific antigen recognition

Precursors

Clonal Proliferation

Memory cells ‘Effector cells’

Activation of antibody producing cells by clonal selection

• Proliferation of activated cells is followed by differentiation into

• Plasma cells

»Life span of

4 to 5 days

1 to 2 months

»Produce 2,000 antibody molecules / second

• Memory cells

»Life span of years to decades

»Differentiate into plasma cells following stimulation by same antigen

Primary and secondary immune responses

Primary and secondary antibody response

• Primary Response

• Following exposure to an antigen, there is a slow rise in IgM followed by a slow rise in IgG

• Secondary Response

• Following exposure to previously encountered antigen, there is a rapid rise in IgG and slow or no rise in IgM

»Memory response

T cells and cell-mediated immunity

• T lymphocytes originate from stem cells in bone marrow followed by migration to thymus gland

• Maturation takes place in thymus gland followed by migration to secondary lymphoid tissue

• T lymphocytes respond to antigens on the surface of antigen presenting cells (APC’s)

• Antigen presenting cells (APC’s)

• Macrophages

• Dendritic cells

• B lymphocytes

T cells and cell-mediated immunity

• Antigen presenting cells (APCs)• Ingest and process antigens then display fragments (short

peptides) on their surface in association with molecules of major histocompatibility complex (MHC)

• Major histocompatibility (MHC) molecules• MHC class I molecules

» Present antigens to CD8 T cells

• MHC class II molecules

» Present antigens to CD4 T cells

• T cells which encounter antigen differentiate into effector T cells

Pathogen Recognition: T and B cell receptors

• T and B cells have receptors specific for only one antigen

• Genes for the antigen receptors are re-arranged in each cell – every cell is different

• Gene rearrangement results in receptors that can recognise a huge variety of antigens (108 different specificities in a human at one time)

How T-cells are made

Mechanism of host defence against intracellular infection by viruses

Mechanism of host defence against intracellular infection by Bacterium

MHC class I molecules present antigen derived from proteins in the cytosol

Cytotoxic T cells

• CD8+ cells

• Kill by releasing granzymes and perforin or

• by engagement of Fas on target cells by Fas Ligand

• Granule contents cause apoptosis in the target cell

MHC class II molecules present antigen originating in intracellular

vesicles

Summary

The immune system protects us against a huge variety of different pathogens

Innate immunity is our first line of defence

• Physical barriers

• Complement

• Phagocytes

Acquired / Adaptive immunity

Involves T and B lymphocytes

Responses are antigen specific and clonal

Memory responses

Secondary responses are faster and better (more and higher affinity antibody) than primary responses – the basis of vaccination