Antigen Receptors: Generation of Diversity and Lymphocyte Development

how can diversity of BCR and TCR be attained?

through recombination of gene segments

what is a recombination signal sequence (RSS)?

Antigen receptor gene segments that are "marked" for rearrangement

what are the gene segments that RSS flank?

the Variable (V), Diversity (D), and Joining (J) gene segments

What is the 12/23 rule?

The spacing and arrangement dictates that a 12 bp RSS must pair with a 23 bp RSS for recombination to occur

what are RAG-1 and RAG-2?

recombination activating genes

what does RAG 1 do?

binds to the RSS directly and cleaves DNA

what does RAG-2 do?

Associates with RAG-1 and increases DNA binding affinity to the RSS

what happens if RAG-1 or RAG-2 are missing?

both proteins are REQUIRED for VDJ recombination, so missing one or the other leads to no adaptive imunity

RAG-1 and RAG-2 are expressed only in....

precursors to T & B cells

what is the 5 step mechanism to DNA rearrangement generation of diversity (GOD) in the thymus and bone marrow?

1.) RSSs brought together and RAG-1 cleaves DNA where heptamer meets gene segment

2.) N-nucleotides added by TdT

3.) Single-stranded DNAs strands pair together

4.) DNA is processed

5.) Gaps are filled by double strand break repair enzymes

how do TCR genes rearrange in developing T cells?

the same RAG based mechanism

the V gene encodes for....

CDR1 and CDR2

CDR3 is encoded for by what piece of DNA?

V(D)J junctions

what are the 3 levels of diversity of antigen receptors?

Combinatorial diversity, Junctional diversity, Heterodimerization

where does the development of B cells occur?

in Bone marrow (pro, pre, and mature)

where does the development of T cells occur?

in the thymus (pro, pre, and mature)

for immune responses, a mature B cells moves to....

the spleen or lymph node

as we age, the thymus ______________

involutes (the cortex and medulla shrink and it becomes mostly fat)

what is positive selection for T cells?

The process that determines whether a T cell becomes a helper (CD4+) or cytotoxic (CD8+) T cell, while making sure they don't react to self cells, enabling them to function in the adaptive immune response.

what is the flow of T cell Selection events in the Thymus

T cells start as subscapular/double negative, then move to cortex where they become double positive, then move to the medulla where they become single positive, and then secondary lymphoid tissue

what % of thymocytes are subcapsular in the Thymus?

1% of thymocytes are subcapsular (double-negative/ don't have CD4 or CD8)

in the thymus cortex, what % of thymocytes can be found and what are they?

75% Double positive (aka have CD4, CD8, AND CD3 expressed)

in the thymus medulla, what % of thymocytes can be found and what are they?

13% single positive (only express CD4 OR CD8)

describe the migration pattern of cells into and through the thymus

starts in the medulla → cortex → subcapsular region → cortex → medulla → lymphoid tissues

thymocytes undergo selection based on....

the specificity of their TCR for self peptide-MHC ligands

recognition with low affinity for pMHC leads to..... and recognition with high affinity for pMHC leads to....

recognition with low affinity for pMHC leads to cell survival and lineage commitment, recognition with high affinity leads to death

why is positive selection needed?

it shapes the T cell repertoire, enriching for TCRs that recognize the MHCs of the individual, and ensures that T cells that T cells expressing class II also express CD4 and same for Class I and CD8

MHC is highly ___________

polymorphic, but we only express a few MHC types per individual

How are TCRs tested for their affinity/avidity?

with interactions with MHC-ubiquitous peptide complexes:

No/too weak interaction = Death by neglect

sufficient interaction = positive selection (survival)

Strong interaction = Negative selection (apoptosis)

deletion of autoreactive cells in the thymus leads to...

central tolerance

induction of clonal anergy by lack of co-stimulation leads to...

peripheral tolerance 1

generation of regulatory T cells leads to...

peripheral tolerance 2

where does negative selection take place?

at the cortico-medullary junction on specialized medullary epithelial cells (MTEC)

what does the human AIRE mutation lead to?

Endocrinopathy (Addison's disease, hypoparathyroidism, hypogonadism, diabetes)

Mucocutaneous cadidiasis

Alopaecia (autoimmune)