The Development and Survival of Lymphocytes Flashcards
B/T cell generation
The B/T cells come from the pluripotent hematopoietic stem cells in the bone marrow, then that differentiates into a common lymphoid progenitor that can generate the B/T cells. The naive cells move to the lymph nodes, and they can engage the antigen, so they can be activated for clonal expansion and produce effector cells upon antigen binding.
Lymphopoiesis
The process to generate lymphocytes
Bone marrow cells and Stromal cells
Adipocytes - the level of fat deposition depends on age, and they can take up the space of the medulla.
The stromal cells have a lot of projections, which increases the surface area that they cover, and there are stem cells associated to them. They are non-lymphoid cells that are specialised for the connective tissue. They:
- Form specific adhesive contacts with the developing lymphocytes
- Provide cytokines and chemokines that control lymphocyte differentiation and proliferation
The stromal cells will be fundamental in the development of the B cells.
The B cells will move throughout the surface of the stromal cells until they are ready to move from the bone marrow, and get to the lymph node.
The B cells leave the bone marrow through the central sinus.
Main phases of a B lymphocyte’s life
The stromal cel provide adhesive sites for the developing B cell, and there is going to happen the rearrangement of the Igs.
The progress of gene rearrangement is monitored, success leads to production of a protein signal to progress to the next stage. This process is antigen independent, but the next processes will be antigen dependent.
As the B cell will move across the stromal cell, selection will happen, meaning that the random receptor that was generated will be tested for self antigen binding (negative selection).
The selected cells become mature B cells, which are naive. They will migrate from the bone marrow to the peripheral lymphoid organs (intestine or spleen). This is when they will face the foreign antigen, which will activate the naive cells, and will become the activated B cells, which differentiate further into cells able to release the receptor - antibody, called plasma cells.But at the same time a population of memory cells is generated - cells that will not produce antibody, but if there is a second round of infection, they will be activated.
The antibody secretion happens at the level of bone marrow, meaning that the activated cells will move from the peripheral organs back to the bone marrow.
Stromal cell - B cell connection
The HSC will produce a multipotent progenitor, which will be attracted to the stromal cell by the cytokines, and it will bind the FLT ligand. As they have initial attachments, they will produce mono-attachments.
The stromal cell keeps on producing factors like IL-7. At this point, the common lyphoid progenitor could be a T or B cell.
But at some point the stromal cell and the early pro B cell will interact through KIT and SCF because of the E2A transcription factor. Meaning that if the cell will produce the E2A, it will stay for further production of the B cell. If it fails to produce it, it will detach from the stromal cell and move into the thymus as a T cell.
The B cell continues interacting with the stromal cell to produce a late pro-B cell and be part of the definitive cell fate. Then the pre-B cell is generated and it looks similar to a normal B cell. Until we finally have a fully functional B cell receptor that will generate the immature B cell.
Rearrangement and expression of immunoglobulin genes
Only one gene locus is rearranged at a time - the heavy chains first and then the light chains.
B-cell development begins by the rearrangement of the heavy-chain locus.
In the large pre-B cell we can only have a pre-B receptor;
In the Immature B cell we can only have IgM;
In the Mature B cell we can have IgM and IgD.
Allelic exclusion
Alleles are the different versions of the genes in the same locus., so there can be one or another, but not both of them at the same time.
Mechanisms through signaling by the pre-B-cell receptor that prevents successful arrangements at both heavy-chain alleles
1) V(D)J recombinase activity reduction by directly reducing the expression of the RAG-1 and RAG-2 genes.
2) Further reduces levels of RAG-2 by indirectly targeting RAG-2 for degradation
3) Reduces access of the heavy-chain locus to the recombinase machinery
Immunoglobulin heavy and light chain BCR loci
The first chain to be rearranged is the heavy one. The rearrangement occurs first in one chromosome. But if there is no functional protein expressed, then we can move to another allele. The early pre-B will continue to a late pre-B cell that will express a pre-B receptor, which will result in a surrogate light chain which will always be the same.
When the signalling is proper, then the cell will proliferate in the large pro-B, because we are testing that the heavy chain arrangement is successful. If it is, we can make multiple copies of that cell, and then each cell will carry its own process of light chain rearrangement.
If the heavy chain was successful, it can be used as a template for a lot of others to make a different receptor, just by the rearrangement of the light chain.
The light chains will do the cross-linking for the signalling to occur.
That results in the RAGs expression inhibition to occur and the proliferation of the large B cells.
Once the proliferation stops, we have a RAG expression again, and then we can undergo the light chain rearrangement.
Light-chain rearrangement also exhibits allelic exclusion - one allele at a time;
Many cells that reach the pre B-cell stage succeed in generating progeny (immature B cells).
B cells are allowed to progress to the next stage when a productive rearrangement has been achieved
B cell selection
Once we generate an immature B cell it has the chance to test the cell agains self-antigen. If it does cross-link to the the self antigen, there will be a negative selection, so the cell cannot leave the bone marrow. A process called central tolerance.
Immature B cells are tested for autoreactivity before they leave the bone marrow.
If the self-antigen is a multivalent self-molecule and has a repetitive epitope, then the cell will have clonal deletion (and elimination by apoptosis) or receptor editing.
If there is a soluble self-molecule, which could be a hormone, the cell still migrates to the periphery as an anergic B cell, which is a cell that has variably reduced IgM expression, shortened life span, and localize to the T-B interface; variably impaired response to LPS and reduced response to BCR ligation. At some point we will get rid of these cells, but there is still a chance that they can activate an immune response.
If there is low-affinity-non-cross-linking self molecule, the cells will still migrate to the periphery, but they will also be different from the normal mature B cells and they will be clonally ignorant, meaning that they assume a normal follicular B cell phenotype, so they will respond less to the antigen. They don’t circulate as long as the B cells.
Development of T cell lymphocytes
The committed progenitor for the T cell travels through blood to the thymus.
The thymus has many lobules, which contain a cortex and a medulla. There will be different cell types in the cortex and medulla, therefore different environments will be provided for the development of the thymus.
The T cell precursor rearranges its T-cell receptor genes in the thymus. This is a random process which does not require an antigen. Then the immature T cells will be selected. For the T cell the antigen will be complex connected to the MHC molecules, and therefore they will need to recognise the MHC molecules - positive selection. Then they will undergo a process of negative selection, where they will not recognise the cell peptide.
Both selection types will happen in the thymus.
Once that happens the mature T cells can leave the thymus to the peripheral lymphoid organs. They are activated to produce effector cells. The activated T cells will migrate to the site of infection and will proliferate and eliminate infection.
Notch1 produced by the stromal cell instructs the precursor to commit to T-cell development
The epithelial cells in the thymus will provide the molecules with nurturing signals, and the antigen for selection of the receptors (MHC).
The dendritic cells will also carry the MHC molecules for the selection.
A lot of the antigen presentation happens at the level of the medulla, while the arrangement of the receptors will happen at the level of the cortex.
Deficiencies
The importance of the thymus in immunity was first discovered through experiments on mice.
Thymectomy at birth results immunodeficiency.
In DiGeorge syndrome (humans) thymus does not form and the affected individuals produces B lymphocytes but few T lymphocytes.
If we remove the epithelial cells from the thymus, there will be no thymus.
Due to deletions in chromosome 22q11.
Cardiac and endocrine defects are also associated.
Nude mouse
Thymus does not form
Defect in Foxn1
Transcription factor required for terminal epithelial cell differentiation
The role of thymic stroma in inducing differentiation of the T-cell progenitors was demonstrated by tissue grafts.
A thymectomy will affect a younger human rather than an older one, because the thymus is still going to be in development. Also, with age the thymus will be surrounded and covered by adipose tissue, so there will be a decreased ability in T cell production, and therefore if we encounter a new disease later in life, we will have to rely on the T cells that are already produced.
Stages of differentiation in the T cells
The T cell receptors will have a TCR complex, containing CD3,4 and 8.
The thymocytes will start as a double negative, they will have no receptors/co-receptors/CD4/CD8. 60% of the cells in the thymus will be like that.
They can split to gamma-delta receptors, which will be exported to the periphery, or alpha-beta receptors, which will continue to the production of CD3 and further to CD4 and CD8, so they will become double positives and can be moved to the periphery.
The rearrangement of the segments for each separate type (gamma, alpha, beta, delta) happens at the same time.
Their route will be dependent on signalling. If one of them is present in the first place, like beta with surrogate alpha, or delta/gamma, then the rearrangements of the other ones will stop.
In the early development of a fetus only the gamma-delta formation succeeds.
Later in the development alpha-beta segments will be rearranged though the VDJ recombination.
The development pathway of the T cells starts with the stem cells which develop into double negatives, there they go though several stages of CD 44 and CD25, and then they go to double positives finally ending up with a single positive.
Positive/negative selection
A positive selection means that the receptor alpha-beta is able to interact with the MHC molecules.
Positive selection also determines the cell-surface phenotype and functional potential of the mature T cell.
Thymic cortical epithelial cells mediate positive selection.
The double positives need to be transformed into single positives, and this happens by the interaction to the MHC molecules. In the stromal cells which expresses MHCI molecules there is only going to be CD8 T cells - positive selection. If there is no MHCI molecules and only MHCII expressed by the stromal cells, there is not going to be any positive selection.
The second step is the negative selection, because the T cells have to be able to interact with the MHC molecules, but not with the peptide, since there is going to be cell recognition. IF it happens then it results in negative selection, and the cell will undergo apoptosis.
Clonal deletion can occur at double- or single- positive stage
In order to ensure that there is no autoimmunity for tissue specific cells, we have the autoimmune regulator (AIRE).
AIRE is a molecule that will interact with the transcription factors, and other molecules that will allow the chromatin to open, and the transcription to be initiated, meaning that many different peptides from molecules that normally wouldn’t express in the thymus, will do it now. And the T cells will interact with different peptides from different tissues to reduce autoimmunity. Tissue specific molecules now can be expressed in the thymus because of the AIRE expression.
The positive and negative selection have different avidity, to produce different amounts of mature T cells.
Peripheral lymphoid organs
In the lymph nodes, different cells from the bone marrow will acquire different positions. Dendritic cells in the lymph node express a receptor or cytokine, so they migrate into the developing lymph node via the lymphatics. The dendritic cells secrete other factors that will attract the T cells.
The B cells will induce follicular dendritic cells, that will secrete chemokines to attract even more B cells.Then they will wait for the antigen in there.
If there is no positive selection of the B cells, then they will have a half life of 3 days.
But if there is positive selection of the B cells, then they will be long lived cells that will have a half life of about 3-8 weeks.
If they are stimulated by the antigen, then they will be even longer-lived mature recirculated memory B cells.and they will express the high affinity IgA, IgG and IgE.
This is specific for the conventional B-2 cells. But there are also other types of B cells such as B1 and marginal B2 cells.
