Flashcards in Lecture 1 - B cells - Immune Deficiencies 1 Deck (70):
What are RSSs?
In which cells are they found in the DNA?
Recombination signal sequences
• These flank the V, D & J mini gene segments
• Nucleotide sequences recognised by the enzymes RAG-1 and RAG-2 → cleavage
RSS are found in all cells, as the immunoglobulin gene loci are present in all cells. The reason that the genes are not rearranged in all cells is that they do not express RAG1/RAG2
What are the various enzymes that perform gene recombination of immunoglobulins?
1. Site recognition and cleavage
• Recombination-activating genes (RAG-1, RAG-2
2. NHE joining
• PKcs (protein kinases)
• Ku70 and Ku80
• TdT (addition of nucleotides to form N regions)
• DNA Ligase IV
Describe the function of RAG1 & RAG2
1. Recognition of RSS sequences
• Induction of cleavage by making nicks in dsDNA
What are the three steps in VDJ recombination?
1. Site recognition and cleavage
2. NHE joining
Describe NHE joining
Enzymes bring the two ends together
• Ku70 and Ku80
These enzymes and the process are ubiquitous, i.e. this is how DNA is joined everywhere in the body
Describe the stage of ligation
1. TdT randomly adds extra nucleotides to the broken ends at VDJ junctions
2. DNA Ligase IV joins up the DNA
Which enzymes in the process of VDJ recombination are unique to B lymphocytes?
• TdT (NB Process unique to the heavy chain)
When do immature B cells leave the bone marrow?
Once they have a successful IgM and IgD on their surface
(i.e. once they are mature)
List the mutations that can disrupt B cell development
What two categories do they fall into?
-- Mutations in the enzymes that drive recombination and development--
-- Mutation in genes that transmit success signals --
Describe the structure of an immunoglobulin
Chains held together by disulphide bonds
• 1x between heavy and light chain
• 2x at hinge region between the two heavy chains
What are the features of large Pre-B cells?
Has Pre-B cell receptor:
• VDJ(h) + surrogate light chains
• Igα and Igβ signalling chains
NB Light chain in germ line configuration
What are the features of pre-pro B cells?
Rearranging heavy chain: D(h) → J(h)
What is a pro-B cell?
Successfully rearranged DJ(h)
Rearranging V(h) → DJ(h)
What are SLCs?
Where are they seen?
Which transcription factor is required for their expression?
Surrogate light chains
• They are the same in every B cell
• They are seen in pre-B cells
• E2A required for their expression
What are the different immunoglobulin loci in humans?
Which locus, κ or λ, is rearranged first?
What do the 'signalling' molecules do in VDJ rearrangement?
Describe the mechanism of the pre-BCR checkpoint
Signal that rearrangement of the heavy chain was successful
1. Heavy chain + SLC join with Igα and Igβ
2. Signal transduction, involving:
3. 'Success' signal
What is the structure of the pre-B cell receptor?
Heavy chain + SLC
The immunoglobulin is bound to the surface, but the light chains are yet to be made; SLCs take their place
What are Igα and Igβ?
Signal transduction molecules
Transmit signal from the surface by a sequential cascade into the nucleus
Describe the pathogenesis of agammaglobulinaemia
1. Mutation in part of 'success signal'
• Btk (→ leads to XLA)
• Igα & Igβ
• μ heavy chain
2. Even though Ig is successfully rearranged, the signal is not there
3. B cells development is halted
4. No B cells in circulation, no Ab
5. Recurrent infections
What is the pathogenesis XLA?
1. Mutation in Btk
2. No transduction of success signal at the pre-BCR checkpoint
3. B cell development does not occur
4. No B cells (and thus Ab in circulation)
NB No problem with rearrangement of immunoglobulin
What are the symptoms and features of XLA and other agammaglobulinaemias without B cells?
At what age do people with agammaglobulinaemia present?
• Recurrent infections: Strep. pneumonia, H. influenzae, otitis, sinusitis
• Serum lacking immunoglobulin
• No circulating B cells
Presentation at 3 years
What happens to T cells in XLA?
What is a combined immune deficiency?
Both humoral and cellular immunity is lost
Describe Intravenous immunoglobulin treatment
• IgG derived from sera of other people
• Given intravenously
• High dose once a month
• By end of month, the amount is still quite high despite decay
• Not great
• Only 20% of people remained infection free over a period of 5 years
• This is because the Ab is static and can not be improved over the course of an infection
What is Subcutaneous immunoglobulin treatment?
• Immunoglobulin self administered once a week
• Much more convenient
• Dosage can be better controlled
Which molecules in the pre-B cell receptor are vital for success signals?
What is Btk?
A kinase involved in signal-transduction at Pre-BCR checkpoint
What is BLNK?
• An adaptor protein
• Involved in signal transduction of the success signal in Ig gene rearrangement
What is a PID?
How many are there?
What are the classes?
Give some examples of each class
Primary immune deficiency
• Immune deficiency, not secondary to something else (e.g. infection, treatment, environmental exposure etc.)
There are over 200
1. Combined immune deficiencies
• Both humoral and cellular immunity affected
2. Antibody deficiencies
• Humoral immunity affected
Are there such things as solely T cell deficiencies?
No, there aren't.
B cell function is dependent on T cell function (especially CD4 T cells)
If there is a T cell deficiency, the B cell compartment will also be affected → combined immune deficiency
What is gammaglobulin?
Where do RAGs cut?
Between the gene segment and the RSS
i.e. the RSSs are cleaved out, leaving only the gene segments
What are the different classes of heavy chain?
Gamma1, Gamma2, Gamma3, Gamma 4
What are the different classes of light chain?
Is Ig gene rearrangement conserved?
What does this mean?
Why would this be so?
This means that similar Ig gene loci are seen in many different mammals:
This is because it is a very effective system
How many different VDJ(h)/VJ(k) combinations are there?
Which things contribute to Ig diversity?
1. Ig gene rearrangement
2. Combinatorial diversity: (heavy and light chain combination)
3. Junctional diversity
What other molecule is important for Ig gene recognition and cleavage?
How is it different from RAG?
It is not unique to Ig gene rearrangement
What is the function of Artemis?
Recognition and synapsis of NHE
Describe the components of the BCR
• Heavy chain
• Light chain
• Igα and Igβ
What is the difference between XLA and HIGM?
XLA: no antibody or mature B cells
HIGM: only IgM, no CSR or SHM
What is the mean age of diagnosis of XLA?
What normally leads to the diagnosis?
Mean age of 3
• S. pneumoniae
• H. influenzae
What determines which cells are affected in combined immune deficiencies?
Which gene is mutated
i.e. if the gene product is vital for NKs, B and T cells, then there will be a deficiency of all three
Which mutation leads to T-B-NK-?
Mutations in which genes lead to T-B+NK-?
Mutations in which genes lead to T-B+NK+
Mutations in which genes lead to T-B-NK+?
What is the phenotype of mutation in ADA?
What is the phenotype of mutation in JAK3?
What is the phenotype of mutation in IL7Ra?
What is the phenotype of mutation in Artemis?
What is the phenotype of mutation in γc chain?
What is the phenotype of mutation in RAG?
What are the various treatments for XLA and HIGM?
What is contained in Intragam?
IgG (all subtypes), with trace amounts of IgM and IgA
Describe the features of SCID
What mutations give rise to SCID?
Which mutation is most common?
(Severe combined immunodeficiency disease)
A combined PID, thus affecting both humoral and cellular immunity
Susceptible to recurrent infections
• Common gamma chain (most common)
• ADA (2° most common)
Describe diversity of odorant receptors
Odorant receptors are germ line encoded (unlike BCRs / Ig, which are rearranged)
Human have 400 functional odorant receptor genes (600 pseudogenes)
This is 1/30th of our genes (if we accept we have 30 000 genes)
V regions differ between ...
C regions differ between ...
... all Ig molecules (unless they are from the same B cell)
... different classes of Ig
How are the heavy and light chains of Ab joined?
• 1x between Heavy and Light chain
• 2x at hinge region between heavy chains
Describe the process of differentiation into mature B cells
At which stage does heavy chain rearrangement commence?
What about light chain rearrangement?
2. Pre-pro-B cell
3. Pro-B cell
4. Pre-B cell large
5. Pre-B cell small
6. Immature B cell
7. Mature B cell
Heavy chain rearrangement: pre-pro-B cell
Light chain rearrangement: Pre-B cell small
At which stage is Ig rearrangement complete?
Immature B cell
What is the difference between immature and mature B cells?
Immature: completed Ig rearrangement
Mature: expressed IgD and IgM on the surface
Why do predominantly boys get XLA?
Because the gene for Btk is on the X-chromosome
What is the most common mutation causing agammaglobulinaemia without B cells?
XLA: i.e. Btk mutation
How rare is XLA?
Rare: 1/250 000
What is XLA due to?
Mutation in Btk
List genes in which mutation causes combined immune deficiencies
• Common γ chain
Compare SCIg and IVIg
• SCIg might be better because it is administered weekly
• SCIg much more convenient, because it can be done at home