B-cell Development and Activation Chapter 5 Self-Test Questions: Sections A, B & C: all (section D covered previously) A LPS a TI-type1 B-cell activator

What are the 2 phases of B-cell development? Antigen-Independent (Maturation) -- mostly in bone marrow vs Antigen-dependent (Activation) -- mainly in periphery Antigen-Independent (Maturation) 1) Pro-B stages -- B-cell markers 2) Pre B-stages -- H- an L- chain loci rearrangements 3) Immature B-cell -- functional BCR -- but… “Is it safe”?

How are self-tolerant B-cell selected? Screening on bone marrow stromal cells “2 nd chance” -- Receptor editing -- mainly LC -- alternative V segments Survival signals (<10%) or apoptosis

What happens during T-cell dependent B-cell activation? “Td antigens” 1) Antigen crosslinking of antibodies -- antigen engagement -- Igα/Igβ signalling -- up-regulation of CD40 & MHC 2) T H cell engagement -- cell/cell interactions -- MHC presentation -- TCR recognition -- CD40/CD40L coupling 3) Cytokine stimulation -- IL4, IL2, etc. -- class switching to IgG -- memory cell formation Why so complicated?

B-cell activation can occur without T-cell help “T-independent” B-cell activation Two types: Ti type-1 – e.g., LPS -- many are mitogens; -- bind to TLRs --can activate polyclonally without BCR engagement -- some activate through binding to BCR & TLR Yield only… Predominantly IgM, maybe some IgG No memory cells See Table 5-1

T-independent activation cont. Ti type-2 – AGs have repetitive, polymeric structure -- crosslink Abs -- e.g. capsule polysaccharides bacterial flagellin Yield only… IgM, maybe some IgG -- high avidity No memory cells- ?? See Table 5-1 Some are important bacterial AGs, but pose problems for vaccine development -- infants do not respond well

Revisiting the lymph node Development of 1 O to 2 O follicles With germinal centers What happens in GC? -- affinity maturation via “somatic hypermutation” -- memory cells produced -- class switching

How does affinity maturation occur? Key role of AID -- Activation-Induced Cytidine Deaminase -- induced DNA repair Several rounds of selection -- long-lived B-cells 1000 X Increased AG affinity Bind or Die!

How does class switching occur? Alternative RNA processing and... IgM and IgD Secreted IgM Gene splicing and switch to: IgG, IgE, and IgA Cytokines influence class switching

The humoral 1 O vs 2 O responses What are the key differences?... in lag... in magnitude... in isotypes How do properties of memory cells account for this? In peripheral tissues Other isotypes in BCR Higher affinity Ab Greater # of cells against AG More rapid T H cell stimulation More rapid class switch

Receptorantibody ligandCell distributionEffect following binding to antibody FcγRI (CD64) IgG1 and IgG3 Macrophages Neutrophils Eosinophils Dendritic cells Phagocytosis Cell activation Activation of respiratory burst Induction of microbe killing FcγRIIIA (CD16a) IgG NK cells Macrophages (certain tissues) Induction of antibody-dependent cell-mediated cytotoxicity (ADCC) Induction of cytokine release by macrophages FcεRIIgE Mast cells Eosinophils Basophils Langerhans cells Degranulation FcεRII (CD23) IgE B cells Eosinophils Langerhans cells Possible adhesion molecule FcαRI (CD89) IgA Monocytes Macrophages Neutrophils Eosinophils Phagocytosis Induction of microbe killing Fc Receptors and antibody effector functions What does “Fc γ RII” mean? Some example FcR mediated functions

B-cell malignancies can involve different stages of B-cells Types of lymphocyte malignancies Myeloma Leukemia lymphoma Chronic vs acute Clinical Diagnosis 1) Microscopy 2) gel electrophoresis of serum proteins