Development of Immune Cells; B and T Lymphocytes and Immune Receptor Diversity Flashcards
• Hematopoietic stem cells (HSCs) can be identified as CD[] cells that lack any mature lymphocyte phenotypic markers (e.g. CD3 and CD4/CD8 for T cells, CD19 or CD20 for B cells, etc.)
• Hematopoietic stem cells (HSCs) can be identified as CD34+ cells that lack any mature lymphocyte phenotypic markers (e.g. CD3 and CD4/CD8 for T cells, CD19 or CD20 for B cells, etc.)
Cytokines that induce hematopoietic activity in the Myeloid lineage?
IL-3, IL-5 and GM-CSF: Induce hematopoietic activity in the Myeloid lineage..
Cytokines used to increase neutrophils, platelets and monocytes in patients receiving drugs that kill progenitor cells?
G-CSF, GM-CSF (and IL-3)
Cytokines that induce hematopoietic activity in the Lymphoid lineage?
IL-3 and IL-7: Mediate hematopoietic activity in the Lymphoid lineage. IL-3 for B cell and IL-7 for B, T and NK cells development/survival.
B cell development takes place in the [], while T cell development takes place in the [].
B cell development takes place in the bone marrow, while T cell development takes place in the thymus.
For the heavy chain of the BCR/Ig and the TCR b chain, 3 segments must be successfully recombined: [].
For the heavy chain of the BCR/Ig and the TCR b chain, 3 segments must be successfully recombined: variable (V), diversity (D) and joining (J) segments.
For the light chain of the BCR/Ig and the TCR a chain, 2 segments must be successfully recombined: [].
For the light chain of the BCR/Ig and the TCR a chain, 2 segments must be successfully recombined:variable (V) and joining (J) segments (no D gene segments).
Name of the enzyme that combine different V-D-J segemnts?
V-D-J recombinase
V-D-J Recombinase Components
- RAG-1 and RAG-2 proteins (Recombinase-Activating Gene) are lymphoid-specific, and work like scissors to cut the DNA open, and initiate the recombination of V-D-J gene segment.
- DNA repair enzymes come in next, to join the ends and repair the double stranded DNA breaks.
* The convention is to refer to this process as V-D-J rearrangement, even though the D gene segments are not present in Ig light chain or TCRa chain rearrangement.
V-J recombination is for…
…Ig light chain or TCRa chain.
V-J-D recombination is for…
…heavy chain of the BCR/Ig and the TCR b chain.
junctional diversity
There is a second, very important strategy for creating receptor diversity also occurring during this somatic recombination process, termed junctional diversity - removal and/or additiona of nucleotides.
exonucleases
Exonucleases may non-specifically remove nucleotides from the ends of the VD-J segments during the recombination process.
terminal deoxyribonucleotidyl transferase (TdT)
The terminal deoxyribonucleotidyl transferase (TdT) enzyme can randomly insert nucleotides into the sites where the VD-J segments are rejoined (called “Ninsertions” for nontemplate encoded insertions).
Although all three CDRs that make up the regions that recognize antigen (CDR1, CDR2, and CDR3) are encoded within the V-D-J recombined segment, [] are entirely contributed by the V segment.
Although all three CDRs that make up the regions that recognize antigen (CDR1, CDR2, and CDR3) are encoded within the V-D-J recombined segment, CDR1 and CDR2 are entirely contributed by the V segment.
•After the common lymphocyte progenitor commits to the B cell lineage, the cell undergoes proliferation in response to the cytokines IL-3 and IL-7 to generate B cell precursors called pro-B cells. These cells have not yet begun to recombine and generate their antigen receptors.
- Pro-B cells then undergo somatic gene recombination in the Heavy chain. All B cells first rearrange their heavy chain locus to express µ. If this is successful, these cells are allowed to continue their development as pre-B cells. Some of the heavy µ chain is expressed on the cell surface with surrogate light chains as a pre-BCR.
- The expression of the pre-BCR is a survival checkpoint for the pre-B cell. If the pre-BCR can successfully make it to the surface and form a complex with the Iga and Igb signaling components, the cell is allowed to undergo another round of proliferation. This also signals the cell to shut down all concurrent rearrangement at the second heavy chain locus on the other chromosome – a process called allelic exclusion – to make sure that each cell only expresses a single antigen receptor
•Pre-B cells next rearrange their light chains, which are then paired with µ and delivered to the cell surface for a second survival checkpoint - if the cell has generated a functional IgM on the surface, the cell is allowed to live as an immature B cell.
•The final maturation step for B cells is the co-expression of IgD along with IgM on the B cell surface. Note that these receptors still express the same Variable domains, thus are still specific for the same antigens. Once the cell is IgD+ IgM+ it is considered a mature naïve B cell, and is now able to respond to antigen in the secondary lymphoid organs.
The most immature progenitors arriving from the bone marrow are called []. They do not express signature molecules that mark a T cell: the CD4 or CD8 T cell coreceptors or the CD3/TCR complex – thus they are often called []. Like their pro-B cell counterparts, pro-T cells are highly dependent on the cytokine [] for their proliferation and maintenance.
The most immature progenitors arriving from the bone marrow are called pro-T cells. They do not express signature molecules that mark a T cell: the CD4 or CD8 T cell coreceptors or the CD3/TCR complex – thus they are often called double (or triple) negative (DN) thymocytes. Like their pro-B cell counterparts, pro-T cells are highly dependent on the cytokine IL-7 for their proliferation and maintenance.
After initial proliferation, DN thymocytes first rearrange their TCRb chain using the V-D-J recombinase machinery described above. Similar to B cells, the DN T cell will then attempt to express the rearranged TCRb chain on the cell surface with a surrogate a chain in a complex known as the [].
After initial proliferation, DN thymocytes first rearrange their TCRb chain using the V-D-J recombinase machinery described above. Similar to B cells, the DN T cell will then attempt to express the rearranged TCRb chain on the cell surface with a surrogate a chain in a complex known as the pre-TCR.
Where is the first developmental checkpint for T cell maturation?
•Expression of the pre-TCR is a developmental checkpoint, ensuring that the TCRb chain V-D-J recombination resulted in a functional protein. If the pre-TCR can signal (indicating a “good” TCRb chain), then the cell undergoes several rounds of proliferation. Just like in B cells, this checkpoint closes the b chain locus on the other chromosome (allelic exclusion). If the pre-TCR is no good, the cell will try again with the other locus. Continued failure to pass this checkpoint leads to death by apoptosis.
After passing this checkpoint, each cell expresses both CD4 and CD8, becoming []. These cells now independently rearrange their TCRa chain – potentially pairing the good TCRb chain with lots of different TCRa chains, in order to diversify the population.
After passing this checkpoint, each cell expresses both CD4 and CD8, becoming double positive (DP) thymocytes. These cells now independently rearrange their TCRa chain – potentially pairing the good TCRb chain with lots of different TCRa chains, in order to diversify the population.
- This ensures that the selected TCR will be able to work with the set of MHC molecules available in the organism.
- Both the TCR and correct CD4 or CD8 coreceptor must recognize a selfpeptide/MHC complex to pass this second developmental checkpoint
At the DP stage, cells also undergo [] selection, where T cells that are highly responsive to self-peptides are eliminated, to prevent autoimmunity.
