L12 – Humoral Immune Responses Flashcards
List the 5 Ab isotypes? Which are membrane bound? Which are secretory?
IgG, A, M, E, D
A = secretory and mucosal defense M,D = membrane bound
Compare secondary humoral response time, Ab titre and affinity, Ab class to primary.
Secondary:
- Shorter lag phase with exponential production of Ab from Ag-specific memory B cells
- Mainly IgG, little IgM
- Much higher Ab affinity to Ag
Describe the primary humoral response? (cell type, Ig type, time)
- long lag phase (~7-10 days), persistence depends on nature of Ag
- antigen-specific naïve B cells undergo clonal selection and differentiation into plasma cells and memory cells
- IgM first**, then IgG
Describe the B-cell receptor composition.
- Ig heavy and light chains
* Ig α/β chain for intracellular signal transduction
Summarize the intracellular signalling events to activate transcription after BCR activation by Ag? ***
- Antigens (Ag) crosslink B cell receptors (Ig) cluster on B cell surface
- Phosphorylate immunoreceptor tyrosine-based activation motifs (ITAM)**
- Recruit and phosphorylate tyrosine kinases** (especially LYN, SYK)»_space; initiate intracellular signaling
- Activate multiple pathways at the same time:
(i) Burton’s tyrosine kinase (BTK)
(ii) Phospholipase Cγ (PLCγ) , increase intracellular [Ca2+]
(iii) Phosphatidyl inositol-3-kinases (PI3K)/AKT - Activate transcription factors
List the gene transcriptions that occur after BCR signalling? *****
Activate genes for:
1) Cell proliferation (e.g. c-fos, cmyc)
2) Receptors for cytokines ** (e.g. IL4R, IL-6R, TGFBR, IFNGR)
3) Surface molecules e.g. MHC class II ** for Th cells
4) Specific enzymes: e.g. Activation induced deaminase (AID) – for somatic hypermutation in Ig
Describe how B cells present antigens after BCR activation/ crosslinking?
BCR crosslinking
> > Endocytosis BCR- Ag complex and break down
> > Use MHC - II from activated gene transcription to load Ag
> > Present MHC-II with Ag on surface to interact with Th
Classify Ag by their downstream reaction after binding to BCR?
Thymus dependent Ag»_space; Ag require contact help from T helper cell for optimal Ab response
Thymus- independent Ag»_space; no interaction with Th
What are the 2 ways in which T cells can induce B cell proliferation and class switching?
Cell-cell contact through CD40 and CD40L binding between Th and B cells
T cells secrete cytokines for Ig class switching and survival
Describe the cell-cell contact between Th and B cells and its purpose?
critical for Ig isotype class-switching to IgA, IgG, IgE
Signal 1: antigen binding to BCR increases the expression of MHC-II, CD40 (both constitutively expressed)»_space; IgM pentamer made immediately
Signal 2: MHC-II loaded with Ag bind to TCR on Th»_space;
upregulates CD40L on activated Th
Signal 3: CD40L on Th bind to CD40 on B cell»_space; induce Ig class switching
What disease is caused by CD40L deficiency in Th cells?
X-linked hyper IgM syndrome: no CD40/CD40L interaction = no Ig class switching
who also have a defect in macrophage function, immunodeficiency, more infection susceptibility
Describe the interaction between Thymus-independent Ag and B cells? (think TI-2 and TI-1 and their respective functions)
- antibody response without the need of Th cell contact
1. TI-1 Ag binds to BCR and other receptors (i.e. Toll- like receptors) »_space; Polyclonal B cell activation regardless of antigen specificity**
2. TI-2 Ag causes extensive Ig crosslinking on B cell»_space; Plasma cell with weaker Ab response (IgM mainly), No memory cells
Compare the source of TI-1 and TI-2 Ag?
TI-1 antigens (e.g. lipopolysaccharides (LPS)*** on Gramnegative bacterial cell wall components) = mitogens
TI-2 antigens (carbohydrates/polysaccharide*** with repeating subunits, e.g. pneumococcal polysaccharides, Salmonella polymerized flagellin)
Location of B cell activation and differentiation?
Germinal centers of lymphoid follicles of secondary lymphoid organs
List 2 processes that occurs in the dark zone of germinal centers in lymphoid follicles?
- B cell proliferation (clonal expansion)»_space; become centroblasts
- Somatic hypermutation in Ig gene»_space; change affinity to Ag
Describe the processes that occur in Class switch recombination (CSR)** in immunoglobulin gene to induce Ig class switching? (think about the enzymes involved)
Before switching, alternative splicing makes IgM, IgD
involves several enzymes:
1) Activation-induced deaminase (AID)*****: cytosine»_space; uracil
2) UNG: excises uracil from DNA
3) APE: nicks phosphodiester backbone in DNA
> > free ends of the DNA are rejoined by NHEJ
@ C SEGMENT GENES!!! NOT HYPERVARIABLE REGIONS IN SOMATIC HYPERMUTATION
Which Ig isotypes are made before and after class switching? Which cells make these isotypes?
- IgM or IgD are transcribed before switching by ALTERNATIVE SPLICING in naïve B cells
- IgM switched to other classes e.g. IgG, IgA or IgE by C GENE REARRANGEMENT in activated B cells and plasma cells
Which gene segment is involved in somatic hypermutation of Ig gene?
Hotspots of SHM = complementarity determining regions (CDRs)/ hypervariable region of V domains of H and L chains
= the antigen binding site
List the 4 processes that occur in the light zone of the germinal centers in secondary lymphoid organs?
1) Centroblasts»_space; centrocytes
2) Ig affinity maturation
3) Ig class switching
4) Differentiation of plasma cells and memory cells
Describe the maturation process that selects B cells with the correct Ig against the specific Ag?
Affinity maturation:
- Somatic hypermutation rearrange HV gene segment without changing overall specificity
- B cells making Ig with the highest Ag specificity are selected, the rest die
List the 2 cells responsible for B cell development and maturation
T cells (follicular T helper cells)
Follicular Dendritic cells
Describe how Follicular T helper cells help B cells maturation?
Cell-cell contact via CD40-CD40L»_space; Ig class switching
Make cytokines e.g. IL-4/IL-5/IL-13»_space; Critical of B cell survival and Ig class switching
Describe how Follicular dendritic cells help B cells maturation?
- Compete with B cells for antigen presentation
- Outcompete B cells with low affinity Ig
> > only B cells with high affinity Ig survive by expression of survival protein BCL-2»_space; Plasma and memory cells
Describe how antigen re-exposure causes increased humoral response and antigen specificity. (think about mutations in Fab)
- IgM made in primary response has little mutation in Fab, no effect on Ag binding
- IgG in late primary response has increased mutation in hypervariable region of Fab > increase binding affinity (low Kd)
- IgG in secondary/ tertiary response has increased somatic hypermutation in Fab»_space; much higher binding affinity
