IMMUNITY MEDIATED BY B LYMPHOCYTES AND ANTIBODIES

IMMUNITY MEDIATED BY B LYMPHOCYTES AND ANTIBODIES

IMMUNITY MEDIATED BY B LYMPHOCYTES AND ANTIBODIES

* B lymphocytes recognize extracellular pathogens and toxins transported to secondary lymphoid tissues

* Recognition stimulates proliferation and differentiation into* Plasma cells and memory B cells

* B lymphocytes generally require help from activated T lymphocytes for differentiation into plasma cells

* Plasma cells produce antibodies

ACTIVATION OF B LYMPHOCYTES

* Begins with antigen binding by receptors resulting in crosslinking of receptors

* Clustering and aggregation of receptors activates* Tyrosine kinases

* Tyrosine kinases phosphorylates Ig-alpha and Ig-beta proteins initiating intracellular signaling

* Additional signals are required and provided by* B cell co-receptor* CD4 TH2 lymphocytes

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Figure 7-2 part 2 of 2

SIGNAL ENHANCEMENT BY B CELL CO-RECEPTOR

* B cell co-receptor is complex of 3 proteins* CD21 [Complement receptor 2 (CR2)]

* Binds to complement on pathogen

* CD19

* CD81

* Signal enhancement results from juxtaposition of receptor and co-receptor* 1,000 to 10,000 X

* Additional signals required depending on nature of antigen and provided by CD4 TH2 cells

Figure 7-3

FINAL OUTCOME OF B CELL ACTIVATION

* Proliferation and differentiation of B cells into antibody producing plasma cells

* Morphology of B cells and plasma cells* B cells

* Large nucleus and small cytoplasm

* Plasma cells* Large nucleus and large cytoplasm packed with ER

THE NATURE OF ANTIGENS AND THE ANTIBODY RESPONSE

* Thymus independent antigens (TI antigens)* Activate naïve B cells without help from CD4 cells

* Classification into* TI-1 antigens (Lipopolysaccharides)

* TI-2 antigens (Polysaccharides)

* Minority of antibody production

* Thymus dependent antigens (TD antigens)* Activation of naïve B cells requires help from CD4 cells

* Majority of antibody production

THYMUS INDEPENDENT ANTIGENS AND ANTIBODY PRODUCTION

* TI-1 antigens* Lipopolysaccharide of gram-negative bacteria

* Stimulate production of IgM only* LPS specific activation

* LPS non-specific co-activation

* Repeating epitopes not required

* TI-2 antigens* Polysaccharides and proteins of bacteria

* Stimulate production predominately of IgM

* Repeating epitopes required

Figure 7-5

MECHANISM OF CD4 T CELL AND B CELL INTERACTION TO TD ANTIGENS

* TD antigens transported to secondary lymphoid tissues for meeting with CD4 T cells and B cells

* CD4 T cells are activated in T cell zone by APC

* B cells enter T cell zone and bind same antigen

* Antigen bound to B cell is internalized by* Receptor mediated endocytosis

MECHANISM OF CD4 T CELL AND B CELL INTERACTION TO TD ANTIGENS

* Antigen is processed and presented on B cell surface with MHC class II molecules

* T cell and B cell interact via CD40L and CD40

* T cell produce cytokine (interleukin-4) which activates B cell

* Cognate interaction* Interaction of B and T cells specific for same antigen

COGNATE INTERACTION OF CD4 TH2 CELLS AND B CELLS

* CI results in primary focus of B lymphoblasts in T cell area

* Some B lymphoblasts move to medullary cords and differentiate into plasma cells* IL-5 and IL-6 from TH2 cells

* Some B lymphoblasts move to primary follicles and differentiate into centroblasts

* Centroblasts proliferate and follicle changes morphology* Germinal center

GERMINAL CENTERS IN SECONDARY LYMPHOID TISSUES

* Mantle zone* Resting B cells present in follicle prior to arrival of

activated B cells and T cells

* Light zone* Non-dividing centrocytes associated with

* Follicular dentritic cells* Stromal cells of lymphoid follicles

* Dark zone* Proliferating centroblasts

GERMINAL CENTERS IN SECONDARY LYMPHOID TISSUES

* Site for somatic hypermutation and affinity maturation* Initiated by cytokines of T cells

* Begins with centroblasts in dark zone

* Results in centrocytes with mutated receptors in light zone

* B cells (centrocytes) which undergo somatic hypermutation* Produce receptor with range of affinities

* Highest affinity receptors are selected

* Must bind antigen or face apoptosis

* Antigen provided by follicular dentritic cells

PRESENTATION OF ANTIGEN BY FDC’S

* Follicular dentritic cells (FDC’s)* Bind antigen in form of immune complexes* Bound immune complexes are not internalized and become

clustered as* Iccosomes (Immune complex coated bodies)

* Iccosomes are shed from FDC’s and taken up by centrocytes

* Centrocytes must obtain, internalize and present antigen for differentiation into plasma cells

Figure 7-10

COMPARISON OF RESTING B CELLS AND PLASMA CELLS

* Differentiation based on intrinsic and inducible properties

* Intrinsic* Surface IG

* Surface MHC class II molecules

* High rate of antibody secretion

* Inducible* Growth

* Somatic hypermutation

* Isotype switching

ISOTYPE SWITCHING IN B CELLS

* Takes place primarily in germinal centers* Determined by

* Cognate interaction with CD4 T cells* Induction requires CD40L to CD40

* T cell cytokines* Induction or inhibition of isotypes

* Hyper-IgM Syndrome* Genetic immunodeficiency from no CD40L* B cells cannot switch isotypes* No response to TD antigens

GENERAL EFFECTOR FUNCTIONS OF ANTIBODIES

* IgM * Protection of blood

* IgG and IgA (monomeric)* Protection of blood and extracellular fluids

* IgA (dimeric)* Protection mucous membranes and secretions

* IgE* Protection of connective tissues

PROTECTION OF BLOOD BY IgM ANTIBODIES

* Primary function * Early protection against blood-borne pathogens

* Characteristics* First antibody produced

* Secreted form is pentamer with 10 binding sites

* Penetration of tissue fluids is limited

* Phagocytic cells have no IgM Fc receptors

* Fc region can bind complement

PROTECTION OF MUCOUS MEMBRANES BY DIMERIC IgA

* Dimeric IgA made by plasma cells in mucosal-associated lymphoid tissues* GALT and BALT

* Dimeric IgA transfer to epithelial surface* Transcytosis

* Receptor mediated transport of macromolecules across epithelial cells

* Mechanism of transcytosis* Binding to poly-Ig receptor on basolateral epithelium

* Endocytotic vesicle transport to apical epithelium

* Protease cleavage of receptor to secretory piece

IgG TRANSPORT FROM BLOOD TO EXTRACELLULAR FLUIDS

* Transport mediated by endothelial receptor* Brambell receptor (FcRB)

* Structure of receptor* Similar to MHC class I molecule

* Mechanism* Binding to FcRB on apical endothelium

* Endocytotic vesicle transport to basolateral endothelium

ANTIBODIES PROTECTING FETUS AND NEWBORN

* IgG* Protects fetus and newborn

* Maternal circulation to fetal circulation

* Transfer across placenta mediated by FcRB

* IgG levels similar in mothers and newborns

* IgA* Protects GI tract of newborn

* Transfer by breast milk

* Dimeric form (IgA2 subclass)

DISTRIBUTION OF ANTIBODIES IN HUMAN BODY

* Plasma* IgM, IgG and IgA (monomeric)

* Extracellular fluids* IgG and IgA (monomeric)

* Mucous membranes and secretions* IgA (dimeric)

* Connective tissues* IgE

ANTIBODIES PROTECT AGAINST BACTERIAL EXOTOXINS

* Number of bacteria cause disease by secreting exotoxins

* Many toxins have receptor-binding and toxic functions on separate polypeptide chains

* Exotoxin disease prevented by antibodies that block toxin binding* Neutralizing anitbodies

* Antibodies raised by vaccination against toxins using* Toxoids (modified toxins)

ANTIBODIES PROTECT AGAINST VIRAL AND BACTERIAL INFECTION

* Initial step in microbial pathogenesis is attachment to host tissues mediated by adhesins

* Microbial adhesins * Molecules

* Gp120 of HIV* Hemagglutinin (HA) of Influenza viruses

* Organelles* Fimbriae of Escherichia coli

* Neutralizing antibodies against adhesins prevent attachment

DESTRUCTION OF ANTIBODY COATED PATHOGENS

* Phagocytes are agents of destruction* Macrophages and neutrophils

* Mechanism* Antibodies bind to pathogens* Phagocytes have Fc-gamma receptors on surface * Fc-gamma receptor of phagocytes binds to Fc region of antibody

* Low affinity binding

* Antibody coating of pathogens enhances phagocytosis* Encapsulated pathogens

DESTRUCTION OF ANTIBODY COATED PARASITES

* Adult parasites are the largest of microorganisms* Diphyllobothrium latum (5 - 10 meters)

* Ascaris lumbricoides (20 – 35 cm)

* Mechanism of destruction* IgE coating of parasites

* Binding of mast cells, basophils and activated eosinophils * Release of granules contents onto surface

* Mechanism of elimination* Inflammatory mediators promote physical removal

* Constriction of smooth muscle

* Increased blood vessel permeability

CASE STUDY

* 45 year old white female

* Presents to family physician with complaint* Mild fatigue for one month

* Passed “Ribbon-like worm” with bowel movement

* Patient history * No exotic travel

* No pets

* Eats out 3 to 4 times a week (patron of sushi / sashimi)

CASE STUDY

* Laboratory testing* CBC with differential normal* Basic metabolic panel normal

* Physical examination* Extraction from rectum of ribbon-like worm* Approximately 80 cm

* Specimen (worm) sent to laboratory

CASE STUDY – QUESTIONS

* What is the diagnosis

* What is the etiological agent

* How is the etiological agent transmitted

* What is the recommended treatment* United States* Japan

CASE STUDY – ANSWERS TO QUESTIONS

* Diagnosis* Diphyllobothriasis

* Etiological agent* Diphyllobothrium latum

* Transmission* Ingestion of raw fish containing plerocercoid larvae

CASE STUDY – ANSWERS TO QUESTIONS

* Treatment in the United States* Praziquantel* Niclosamide

* Treatment in Japan* Amidotrizoic (diatrizoic) acid (Gastrografin)* Administered either orally or by injection into

duodenum by endoscopy

CASE STUDY

* 87 year old female presents to family physician with

* Erythematous lesion on left thigh

* Three weeks duration

* Lesion biopsy submitted for pathology exam

* Pathology report

* Subcutaneous Dirofilaria infestation with granulomatous and eosinophilic dermatitis

MAST CELLS, EOSINOPHILS, BASOPHILS AND IgE ANTIBODY

* Mast cells, basophils and activated eosinophils have IgE receptor* Fc-epsilon-RI

* High affinity for Fc region of IgE

* Mast cells * Fc-epsilon-RI receptors and cytoplasmic granules are constitutive

* Cytoplasmic granules contain “inflammatory mediators”* Histamine

* Antigen binding initiates degranulation of mast cells

Fc RECEPTORS AND NATURAL KILLER CELLS

* NK cells * Express Fc-gamma-RIII receptor for IgG1 and IgG3

* Fc-gamma-RIII important in ADCC

* ADCC* Mediated primarily by NK cells* Illustrates that antibody can direct specific attack by effector cells that

have no specificity for antigen

* Mechanism* Virus infected cells express viral proteins on surface* Antibodies bind viral proteins* NK cells bind to antibodies, release granules, kill cells