Adaptive Immunology & Microbial Pathogenesis (I)

Biology 20

Case study:

• A patient presents with: – a recurring fever of 104◦F , – a debilitating headache, and – symptoms of gastroenteritis – Her symptoms have gotten worse over the past

several weeks.

• Suspecting a systemic infection might be a possibility, her physician orders a blood culture. The culture turns out to be positive for Gram negative bacilli.

Case study:

• A patient presents with: – a recurring fever of 104◦F , – a debilitating headache, and – symptoms of gastroenteritis – Her symptoms have gotten worse over the past

several weeks.

• Suspecting a systemic infection might be a possibility, her physician orders a blood culture. The culture turns out to be positive for Gram negative bacilli.

What test could be used to determine which kind of Gram negative bacilli is causing the infection?

Learning Objectives (I):

• Dual nature of the adaptive immune response – Humoral vs cell mediated immunity

• B cells and Antibodies – antigens

– VDJ recombination

– B cell activation

– Clonal expansion

• Antibody Activities – Agglutination, opsinization, neutralization,

interference with adherence, ADCC

Immunity

Dual nature of the adaptive immune response:

1) Humoral immunity 2) Cell-mediated immunity

Distinguishing ‘self’ from ‘non-self’

Microbe ?

Distinguishing ‘self’ from ‘non-self’

Pathogen-associated microbial patterns PAMPs

Innate immune response: • TLRs

Distinguishing ‘self’ from ‘non-self’

Pathogen-associated microbial patterns PAMPs

Antigens: unique proteins or polysaccharides

• Extracellular • Intracellular

Innate immune response: • TLRs

Antibodies made by activated

B cells

Humoral immunity

Adaptive immune cells: B lymphocytes

• B cells arise from hematopoietic stem cells in the bone marrow. – They undergo positive and negative

selection.

• ~ 10% of blood lymphocytes are B cells.

• B cells express B cell receptors.

• Plasma cells are specialized B cells that secrete antibodies.

• Antibodies are secreted B cell receptors. – Antibodies bind antigens.

B cells…

Many different B cells: • Expression of unique

B cell receptors • Recognition of unique

antigens

B cells & Adaptive Immunity

• The quality of the adaptive immune response (i.e., how well it defends against disease) is dependent on the quantity of antigen-specific cells (e.g., the frequency of unique B cell populations). – Analogy: Hide&Seek

1 hider and 1 seeker vs 1 hider and 100 seekers.

B cells & Adaptive Immunity…

Before infection During infection

Antigens Antigen: a substance (e.g., foreign protein) that stimulates an immune response and/or binds to antibodies.

E. coli O157:H7

• Gram-negative bacilli O157 refers to the particular structure of the O polysaccharide antigen

• H7 refers to type of flagellin antigen

Foodborne, gram neg, causes severe, acute hemorrhagic diarrhea

B cell receptor (BCR):

B cell Plasma membrane

Variable region (VDJ)

Constant region μ

Antibodies (Ab) aka immunoglobulins (Ig)

Antibody Gene Structure: V D J C

(hundreds of different

V genes) VDJ recombination

transcription

mRNA

B cell differentiation

Activation of B Cells

• First step required for B cell activation: antigen binding

• Second step: cytokine signaling

– T-dependent antigens.

– IL-4 is necessary for initial B cell proliferation.

Clonal Expansion

Memory Cells

Clonal Plasma Cells

Antibody Classes

Monomer

Dimer

Pentamer

IgD, IgE, IgG

IgA

IgM

Antibody Class Switching: different usage of Fc genes

Antibody Classes…

IgA IgM IgD IgE IgG

First Ab made in response to infection. Opsonization, phagocytosis, complement activation, agglutination of microbes.

80% of serum Abs (20d ½ life)

Opsonization, phagocytosis, complement activation, agglutination of microbes.

neutralizes toxins and viruses

Only Ab that crosses placenta

Binds to allergens and triggers histamine release from mast cells and basophils. Protects against parasitic worms. ADCC

Monomer or dimer

Found in mucosal areas such as the gut, and respiratory and urogenital tracts.

Found in saliva, tears, and breast milk.

Helps protect infants.

Major Functions of Antibodies

• Specific recognition of self and non-self molecules – Autoantibodies can cause autoimmune disorders (lupus

erythematosus, Grave's disease)

• Opsonization – Promotes phagocytosis

– Complement activation

– Interferes with adherence

• Antibody-dependent cellular cytotoxicity (ADCC)

• Neutralization of toxins and pathogens

• Agglutination

Opsonization

Antibody-dependent cellular cytotoxicity (ADCC)

• Killing of infected cells by antibody-triggered mechanism.

Classical Pathway of Complement

Activation

Toxin Neutralization

Agglutination

Case study:

• Considering the patient’s signs and symptoms and the presence of Gram-negative bacilli in the blood culture, the physician asks whether the patient has recently been to an area where typhoid fever is endemic. The patient’s answer is yes.

• The physician hypothesizes that the patient is ill with typhoid fever, and the Gram-negative bacilli are Salmonella typhi. How can the physician test this hypothesis? S. typhi produces a protein called invasin

that allows it to enter non-phagocytes.

Agglutination assay:

Agglutination assay:

Summary (I)

• Dual nature of the adaptive immune response – Humoral vs cell mediated immunity

• B cells and Antibodies – antigens

– VDJ recombination

– B cell activation

– Clonal expansion

• Antibody Activities – Agglutination, opsinization, neutralization,

interference with adherence, ADCC