MICB 3301 Exam 4

Antibodies

Antibodies

-Glycoproteins produced by B cells
-Found in blood, mucosal surfaces, and tissues
-Bind antigens (neutralize or opsonize)
-5 classes or isotypes

Immunoglobulin

Another name for antibody

Antibody affinity

Strength with which antibody binds to its antigen at a given antigen-binding site

Epitope

The part of an antigen molecule to which an antibody attaches itself (there can be multiple of these on antigens)

IgM

First Ig produced after antigen exposure, cells that produce this immunoglobulin can switch and produce another Ig classes

IgG

-Major Ig in blood
-Can cross placenta
-Activate complement system

IgA

Major Ig in secretions (saliva, breast milk, tears)

IgE

Major Ig in allergic reactions

IgD

Ig found on B cell surfaces that participate in signaling

Half life of Ig

days

Half life of IgG

~20 days

Monomer

IgD, IgE, IgG

Dimer

IgA

Pentamer

IgM

Structure of antibody

2 heavy chains and 2 light chains

Heavy chain

2. the larger chain

Light chain

3. the shorter chain

Variable region (antigen binding site)

A

Constant region

D

Fc fragment

E

Disulfide bonds

Chemical bonds that link the heavy and light chains

Receptor binding site

Region that can bind Fc receptor on other cells

VDJ recombination

Mechanism of somatic recombination that occurs in developing lymphocytes during the early stages of T and B cell maturation

Clonal selection

1. Random rearrangement of antibody gene segments occurs as B cells develop in bone marrow early in embryonic life, before infection
2. Generates vast array of B cells, each preprogrammed to bind a specific antigen
3. Tolerance - removal of self-reactive B cells
4. Remaining B cells travel to lymphoid organs and tissues
5. Upon infection, antigen "selects" B cell with antibody that matches it
6. B cell proliferates, forming clone of identical cells, each with antibody for the antigen

Tolerance

-Removal of B cells recognizing self antigens
-Largely occurs in bone marrow
-Breakdown is one basis of autoimmunity

Helper T cells

Make cytokines that activate B cells, macrophages, and other T cells

Cytotoxic T cells

T cells that kill cells expressing foreign antigen using perforin and granzymes

Perforin

One of the proteins released by cytotoxic T cells on contact with their target cells. It forms pores in the target cell membrane that contribute to cell killing.

Granzymes

Cytotoxic enzymes that initiate apoptosis

Cluster of differentiation (CD)

Membrane proteins that function as co-receptors and can be used to determine a cell's identity

CD4

Helper T cells that interact with MHC class II molecules

CD8

Cytotoxic T cells that interact with MHC class I molecules

T Cell receptors (TCRs)

-Bind antigens (usually peptides) presented to them by other cells
-Only when presented by MHC molecule
-Expressed from gene segments rearranged in the thymus

Major histocompatibility complex (MHC)

Collection of genes encoding cell surface proteins for self/nonself recognition

Human leukocyte antigen (HLA) complex

MHC genes in humans

HLA typing

a process used to identify and compare HLA

Human chromosome 6

Where HLA is located

MHC Class I

-Presents peptides that originate in the cytoplasm from intracellular pathogens
-Expressed on surfaces of nucleated cells
-Presents peptides to CD8 cytotoxic T cells

MHC Class II

-Presents peptides from extracellular pathogens taken up by phagocytosis
-Expressed on surfaces of APCs
-Presents peptides to CD4 helper T cells

Antigen processing

1. Dendritic cell takes up pathogen for degradation
2. Pathogen is taken apart inside the dendritic cell
3. Pathogen proteins are unfolded and cut into small pieces
4. Peptides bind to MHC molecules and the complexes go to the cell surface
5. T-cell receptors bind to peptide: MHC complexes on dendritic cell surfaces

How big are antigen processed peptides?

8-25 amino acids

Pathogen

An organism that causes disease

Opportunistic pathogen

A microbe that normally doesn't cause disease in healthy people, but can become virulent in immunocompromised individuals

Carrier

An infected individual that may be a potential source of infection with no observable symptoms

Zoonoses

Diseases transmitted from animals to humans

Vectors

An organism (typically insects) that transmit disease to humans (e.g. mosquitoes, ticks, fleas)

Pathogenicity

Ability to cause disease

Virulence

Degree of pathogenicity

Virulence factors

Genetic, biochemical, or structural features that contribute to virulence

Latency

Pathogen stops reproducing, dormant, can become active again

Infectious dose 50

Number of microbes required to cause clinical disease in 50% of inoculated hosts

Model systems

Animal models or cell cultures to study disease

Human studies

Clinical trials, case studies

Epidemiology

The branch of medicine that deals with the incidence, distribution, and possible control of diseases and other factors relating to health.

Viral attachment

Capsid and envelope spike proteins mediate attachment

How does influenza infect cells?

Hemagglutinin spikes bind to receptors containing sialic acid

How does HIV infect cells?

GP120 spike binds to CD4 and CCR5

How does SARS-CoV-2 infect cells?

Spike proteins bind to ACE2 receptors

How do viruses spread in the body?

Lymph, blood, neurons

Syncytia

Virus-induced cell-cell fusion, forming multinucleated cells

Respiratory syncytial virus (RSV)

A virus that causes an infection of the lungs and breathing passages that forms syncytia

Antigenic variation

Amino acid changes in virion spikes (common in RNA viruses)

Bacterial attachment

Pili and capsules

Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis

Bacterial capsules targetable by vaccines

Pseudomonas aeruginosa

-Opportunistic pathogen that forms biofilms in burn and cystic fibrosis patients
-Some strains called mucoid strains make capsules

Uropathogenic E. coli (UPEC)

Most common cause of UTIs

Bacterial virulence factors

Coagulase, streptokinase, IgA proteases, hemolysins, siderophores, DNase

Coagulase

Clots fibrinogen in plasma that protects pathogen

Streptokinase

Activates plasmin, digests fibrin clots- pathogen moves from clotted area

IgA proteases

Destroy IgA antibodies

Hemolysins

Lyse red blood cells to degrade hemoglobin and release iron

Siderophores

Bacterial iron-binding proteins

DNase

Degrades DNA, lowers viscosity of secretions, spread

Neutrophil extracellular traps (NETs)

Extracellular fibrillar networks produced by neutrophils in response to bacteria and fungi that are made up of DNA and antimicrobial proteins and enzymes

What bacterial virulence factor would help bacteria escape NETs?

DNase because DNA is part of NET backbone

How do bacteria evade innate immunity?

-Capsules block complement opsonization and membrane attack complex formation
-Proteases degrade complement C3b or C5a

How do bacteria evade adaptive immunity?

Capsules prevent antibody binding and some pathogens can degrade IgA antibodies

Chlamydia, Rickettsia, Mycobacterium, Salmonella, Listeria

Examples of intracellular bacterial pathogens (5)

Listeria monocytogenes

-Gram-positive, foodborne pathogen
-Found in produce, raw unpasteurized milk, cheese, and deli meat
-Psychrophile
-Can cross placenta
-Polymerizes host actin to move inside and between host cells (Actin rockets)

How does Listeria move?

Listeria propels itself by hijacking the actin cytoskeleton and can travel between different cells in a vacuole

Streptococcus pneumoniae

Bacteria that forms biofilms in the middle ear that causes otitis media (middle ear infections) and can also cause pneumonia

Staphylococcus and Enterococcus

Two bacteria that form biofilms on heart valves that cause endocarditis

Streptococcus mutans

Bacteria that causes dental plaque

Types of exotoxins

Membrane disrupting, superantigens, AB

Membrane-disrupting exotoxin

A type of exotoxin that lyses host cell by disrupting the integrity of the plasma membrane (e.g. hemolysin)

Superantigens

Cause T cells (>30%) to overexpress and release too many cytokines that can lead to multiple organ failure and Toxic Shock Syndrome

Toxic Shock Syndrome

Life-threatening condition caused by S. aureus superantigen

Where do superantigens bind?

TCR and MHC

AB exotoxins

-Composed of A and B subunits
-Many are ADP-ribosyl transferases
-Remove ADP-ribose group from NAD, attach it to host cell protein - protein inactivated or functions abnormally

A subunit

Causes a toxic effect

B subunit

Binds to target cell receptor

Diphtheria, Cholera, Botulinum

Bacteria with AB exotoxins (3)

Corynebacterium diphtheriae

Bacteria that produces diphtheria toxin

Diphtheria toxin

-Binds growth factor receptor
-Enters via endocytosis
-ADP-ribosyl transferase attaches ADP ribose from NAD onto EF-2
-Inhibits protein synthesis
-Inhibition of protein synthesis causes destruction of cardiac, kidney, and nervous tissues

Vibrio cholerae

Bacteria that produces cholera toxin

Cholera toxin

-Enterotoxin produced by Vibrio cholerae
-ADP-ribosylation of host G protein controlling cAMP production
-Increased cAMP causes water secretion from cells, leading to severe diarrhea

Clostridium botulinum

Bacteria that produces botulinum toxin

Botulinum toxin

-Neurotoxin produced by C. botulinum
-Blocks release of acetylcholine at neuromuscular junctions
-Muscles can't contract, causes flaccid paralysis

Antitoxin

Antibody that can neutralize exotoxins

Toxoid

Inactivated toxin that can still elicit an immune response