Maturation and Selection of B and T cells

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Questions and Answers

What is the fate of Pre-B cells that cannot express a pre-BCR or receive pre-BCR signals?

Mature into B cells immediately

Die by apoptosis (correct)

Undergo positive selection

Transform into T cells

When does the final maturation step of B cell development occur?

During the pro-BCR stage

Upon entering the bone marrow

When IgM is co-expressed with IgD (correct)

After positive selection

What characterizes the mature B cell capable of responding to antigens in peripheral lymphoid tissues?

Expression of T cell receptors

Expression of IgM only

Co-expression of IgM and IgD (correct)

Absence of any surface markers

When does the co-expression of IgD with IgM typically take place in B cell maturation?

After leaving the bone marrow Signup and view all the answers

What is the consequence if an immature B cell remains self-reactive even after receptor editing?

Apoptosis (deletion) Signup and view all the answers

What stimulates the proliferation of pro-T cells in the thymus?

IL-7 Signup and view all the answers

What is the role of CD4 and CD8 in T-cell maturation?

Displaying specificity for MHC class II and I respectively Signup and view all the answers

What leads to the death of T cells by apoptosis?

Failure to express antigen receptors Signup and view all the answers

What helps promote the survival and proliferation of pre-T cells?

Expression of pre-TCR complex Signup and view all the answers

What is the result of successful V(D)J recombination in T-cell maturation?

Formation of the pre-TCR complex Signup and view all the answers

What is the primary function of positive selection in T cells?

Recognizing MHC-self peptide complex Signup and view all the answers

What is the role of the pre-BCR complex in B cell maturation?

Promotes survival and proliferation of B lineage cells with productive Ig heavy-chain rearrangement Signup and view all the answers

During B cell maturation, what cells give rise to a large number of precursors known as pro-B cells?

Stem cells Signup and view all the answers

What is the function of surrogate light chains during B cell maturation?

Associate with Ig μ heavy chain on the cell surface Signup and view all the answers

What is the result of a T cell recognizing a weak binding to self MHC I-self peptide complex during positive selection?

Formation of CD8+ T cell and loss of CD4+ T cell Signup and view all the answers

What happens if a T cell strongly binds to self MHC (class I or II)-self peptide complex during negative selection?

Apoptosis of the T cell Signup and view all the answers

Why do T cells that do not recognize an MHC molecule in the thymus die by apoptosis?

They fail to receive survival signals Signup and view all the answers

Study Notes

B Cell Maturation

e-B cells express a functional μ heavy chain, an essential component of the pre-BCR and BCR.
Pre-B cells that cannot express a pre-BCR or receive pre-BCR signals die by programmed cell death (apoptosis).
The IgM-expressing B lymphocyte is the immature B cell.
The final maturation step involves coexpression of IgD with IgM through alternatively spliced H chain transcripts.
Coexpression of IgM and IgD enables B cells to respond to antigens in peripheral lymphoid tissues.
This coexpression occurs either in the bone marrow or after B cells leave the bone marrow and go to the spleen.

Selection of Mature B Cells

Positive selection is based on the expression of complete antigen receptors, not on the recognition specificity of these cells.
Negative selection: immature B cells that bind self-antigens in the bone marrow may reexpress the VDJ recombinase enzyme and undergo additional light-chain V-J recombination to change their receptor specificity (receptor editing).
If B cells are still self-reactive after receptor editing, they may die by apoptosis (deletion).

Maturation and Selection of T Lymphocytes

T cell progenitors migrate from the bone marrow to the thymus for maturation.
Pro-T cells (double-negative thymocytes) do not express CD4 or CD8; IL-7 produced in the thymus stimulate their proliferation.
CD4 and CD8 are co-receptors that display strong specificity for MHC class II and I, respectively.

Maturation of T Lymphocytes

Successful V(D)J recombination leads to the expression of TCR β-chain protein on the cell surface with pre-Tα protein, forming the pre-TCR complex of pre-T cells.
The pre-TCR complex delivers intracellular signals that promote survival, proliferation, and TCR α gene recombination.
Surviving cells express the complete αβ TCR and both CD4 and CD8 coreceptors (double-positive T cells or double-positive thymocytes).

Positive Selection of T Cells

MHC I and MHC II molecules are recognized by CD8+ and CD4+ T cells, respectively, in the thymus.
Positive selection of T cells involves recognition of MHCII-self peptide complexes by CD4+ T cells and MHCI-self peptide complexes by CD8+ T cells.

Negative Selection of T Cells

T cells that strongly interact with class I or II MHC-peptide complexes in the thymus undergo apoptosis.
Negative selection removes autoreactive T cells that recognize self-MHC-self peptide complexes with high affinity.

Summary

Lymphocytes undergo multiple selection steps during maturation to preserve useful specificities based on the expression of intact antigen receptor components and what they recognize (self or nonself).
Both positive and negative selection are mediated by recognition of the same set of self MHC-self peptide complexes in the thymus.

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