lymphocyte development

stages of lymphocyte development

1. stem cell

   • growth factor mediated commitment; proliferation

   • antigen independent

2. pro-lymphocyte

   • initiation of antigen receptor gene rearrangement

   • antigen independent

3. pre-lymphocyte

   • selection of cells that express pre-antigen receptors

   • antigen independent

4. immature lymphocyte

   • selection of repertoire

   • self-antigen dependent

5. lymphocyte subsets

   • maturation of functionally distinct T/B cell subset

6. mature lymphocyte

complementarity determining regions (CDR1, CDR2, CDR3)

hypervariable regions (HV) within the variable domain of the antigen receptor that contribute to antigen recognition

primary lineages of common lymphoid progenitors

follicular (FO) B cells

αβ T cells

antigen receptor expression

checkpoint in lymphocyte maturation following pre-B/T cell proliferation

• cells expresses complete antigen receptor

• failure to express antigen receptor leads to cell death

positive & negative selection

checkpoint in lymphocyte maturation following immature-B/T cell proliferation

• positive selection: cells with weak (self) antigen recognition are selected

• negative selection: cells with strong (self) antigen recognition are eliminated and eventually die

pre-B/T antigen receptor expression

checkpoint in lymphocyte maturation following pro-B/T cell proliferation

• cells expresses one chain of antigen receptor

• failure to express pre-antigen receptor leads to cell death

V(D)J recombination

antigen receptor genes are generated by recombination of variable (V), diversity (D), joining (J) gene segments

• germline Ig & TCR genes are composed of multiple DNA segments combined in developing lymphocytes

• D segment: only in BCR Ig heavy chain & TCR β chain

additional nucleotides are added in the processed (N/P)

intervening DNA sequences are removed = permanent

steps of V(D)J recombination

1. synapsis

2. cleavage

3. hairpin opening & end processing

4. joining

recombination-activating gene (RAG) 1 & 2

V(D)J recombination enzyme that is lymphocyte specific - creates double-strand breaks

ARTEMIS

V(D)J recombination endonuclease - opens (nicks) hairpins asymmetrically

• adds extra P nucleotides

terminal deoxynucleotidyl transferase (TdT)

V(D)J recombination enzyme that randomly adds nucleotide to broken ends

• adds extra N nucleotides

• does not require a DNA template to amplify DNA

combinatorial diversity

generated by different combinations of gene segments during V(D)J recombination

• creates 1 - 3 million different receptors

junctional diversity

generated by the addition or removal of nucleotides at the junctions of gene segments during V(D)J recombination

• asymmetric hairpin nicking by ARTEMIS → extra N nucleotides

• random addition by TdT → extra N nucleotides

• creates 10⁷ - 10⁹ different receptors

Omenn Syndrome

an autosomal recessive form of severe combined immunodeficiency (SCID) characterized by

• erythroderma (skin redness)

• desquamation (peeling skin)

• eosinophilia

• elevated serum IgE levels

• alopecia (hair loss)

• chronic diarrhea

• failure to thrive

• lymphadenopathy (enlarged lymph nodes)

• hepatosplenomegaly

autosomal mutations in genes encoding RAG1, RAG2, or ARTEMIS

patients are highly susceptible to infection & develop fungal, bacterial, & viral infections typical of SCID

pro-B cell

RAG1/RAG2 expressed → heavy chain starts recombination (no expression)

pre-B cell

μ heavy chain VDJ recombination completed (expressed) + surrogate light → pre-B cell receptor (pre-BCR)

• associates with Igα (CD79a) & Igβ (CD79b)

• pre-BCR expression = first checkpoint

immature B cell

IgM expression

• negative selection

• receptor editing (if needed) - substituting 1 light chain for another

mature B cell

IgM + IgD expression; several subsets; out to periphery

• follicular B = most common

pre-B cell receptor (pre-BCR)

a receptor complex composed of a heavy chain, Igα, and Igβ, expressed on the surface of pre-B cells

• inhibition of H chain recombination (allelic exclusion)

• proliferation of pre-B cells

• stimulation of κ light chain recombination

• shut off of surrogate light chain transcription

allelic exclusion

the process where only one allele of a gene is expressed in a cell, leading to the production of a single type of protein

• allows for each mature B lymphocyte to express only one type of immunoglobulin

tolerance

the state of unresponsiveness to self-antigens, preventing the immune system from attacking the body's own cells and tissues

BCR binding

the interaction between the B cell receptor and self-molecules, which can lead to deletion, anergy, or receptor editing to prevent autoimmunity

Burton’s tyrosine kinase (Btk)

a kinase crucial for signal transduction via the pre-BCR

• defect = B cell maturation arrests in the bone marrow at pre-B cell stage

X-linked agammaglobulinemia (XLA)

also called Burton’s agammaglobulinemia; caused by a defect in the BTK gene which codes for Burton’s tyrosine kinase (Btk)

• agammaglobulinemia = lack of serum antibodies

serum titers - IgM nondetectable; IgA & IgG low (from mother)

infants develop frequent infections of the ears, throat, lungs, & sinuses

thymus

major site of T cell maturation

• first cortex then medulla

double-negative thymocytes

most immature T cell; recent arrival from bone marrow

• do not express the T-cell receptor (TCR), CD3, CD4, CD8, & ζ chains

pre-T cells

expresses pre-T cell receptor (pre-TCR) = TCR β chain + invariant pre-Tα + CD3 + ζ chains

pre-T cell receptor (pre-TCR)

TCR β chain + invariant pre-Tα + CD3 (εγ + εδ) + ζ chains

• inhibition of β chain gene recombination

• proliferation of per-T cells

• stimulation of α chain recombination

• expression of CD4 & CD8

• shut off of pre-Tα transcription

double-positive thymocytes

CD4⁺ & CD8⁺ followed by chain expression (second wave of RAG expression)

• selection process = death by neglect, negative selection, positive selection

single-positive immature T cell

immature T cell that goes through additional negative selection in medulla

mature T cells

CD4⁺ or CD8⁺ or T_reg

double positive T cells selection

positive and negative selection based on the recognition of peptide-MHC complexes, ensuring the development of functional T cells and eliminating self-reactive T cells

death by neglect

cell death that occurs in T cells due to the absence of recognition signals (no recognition at all), absence of positive selection

positive selection

low avidity binding of TCR with self-peptide & self-MHC → stimulated to survive & differentiate

negative selection

process of cell death that occurs in single positive T cells due to high avidity binding of the TCR with self-peptide and self-MHC, preventing the development of self-reactive T cells

• high avidity → active death-promoting signals → apoptosis

regulatory T cells development

intermediate avidity

autoimmune regulator (AIRE) protein

a unique transcriptional regulator protein that induces the promiscuous expression of thousands of tissue-restricted antigens (TRAs) in medullary thymic epithelial cells (mTECs) → critical for the induction of immunological self-tolerance

autoimmune polyendocrine syndromes type 1 (APS-1)

also called autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED)

clinical conditions by impairment of multiple endocrine glands due to loss of immune tolerance

caused by mutations that inactivates the AIRE gene →failure to delete autoreactive thymocytes during negative selection