Development of B and T lymphocytes Modified from “Serotonin” file Development of B and T lymphocytes Ali Al Khader, MD Faculty of Medicine Al-Balqa’ Applied University Email: ali.alkhader@bau.edu.jo

What will we discuss today? B-cell development T-cell development

B- cell development…overview Stem cell In periphery Pro-B cell Pre-B cell Memory B cell Immature B cell Mature B cell Plasma cell In bone marrow Modified from “Immunology for medical students, 2nd edition”

B- cell development Stem cells… - in fetal liver - in bone marrow soon after birth In bone marrow…radial maturation pathway B lineage cells develop into immature B cells with membrane-bound IgM molecules…these will leave the bone marrow These will acquire the ability to recirculate and populate all peripheral lymphoid organs Further maturation primarily in the spleen into follicular B cells expressing IgM & IgD on their surface

B- cell development, cont’d From lymphoid progenitor to mature B cells: 2-3 days The earliest committed B-lineage cell is pro-B cell -RAG proteins are first expressed at this stage -DJ & V to DJ recombination then occurs, and mu heavy chain is produced -TdT is most abundantly expressed here Characterized by appearance of B lineage surface markers, e.g., CD19

B- cell development, cont’d Remember that similar presence of retained segments is important when we will talk about receptor editing of the light chain

B- cell development, cont’d Rearrangements at IgH locus must be productive…1/3 of pro-B cells …now Ig mu heavy chain can be formed now the cell becomes: pre-B cell In early pre-B stage: two polypeptides associate non-covalently to form the surrogate light chain, and pair with the heavy chain -delta 5 -VpreB

B- cell development, cont’d Surrogate light chains are invariant Mu + surrogate light chains + Ig alpha + Ig beta = pre-B cell receptor Bruton’s tyrosine kinase (BTK gene) is activated downstream Its signaling is responsible for B lineage expansion (survival, proliferation and maturation) Its mutation will cause: …….

B- cell development, cont’d Pre-B cell receptor also inhibits rearrangement of heavy chain locus on the other chromosome If the rearrangement was non-productive, the other allele can compensate Pre-B cell receptor also inactivates surrogate light chain gene expression Rearrangement of kappa or lambda which associates with mu…here the cell becomes immature B cell

B- cell development, cont’d Kappa is rearranged after heavy chain and before lambda …rearrangement of kappa inhibits lambda rearrangement …2 conditions in which lambda will undergo recombination??? Allelic exclusion also applies for light chain Immature B cell will exit the bone marrow and complete maturation in spleen …otherwise they -undergo receptor editing -die -or become unresponsive By reactivation of Rag genes and new recombination of light chains (Negative selection) = clonal deletion = anergy

B- cell development, cont’d Mature naive B cells will die in a few months if they do not encounter antigens and recognize them with high affinity (respond to antigens) For mature B cells expressing IgM & IgD: binding with high affinity leads to proliferation & differentiation…leads to humoral immune responses B-1 and marginal zone B lymphocytes?????

T- cell development…overview Positive selection if recognize self MHC and negative selection if recognize self MHC with self-peptide antigens Double-negative (CD4- & CD8-)…in subcapsular zone Commitment to alpha beta or gamma delta lineage Double-positive cells…in cortex Selection of DP cells by MHC class I or class II molecules…in cortex Selection of single-positive cells depending on the MHC molecule….in medulla Release of naive mature T cells So those that are self-tolerant and self-restricted are retained = lineage commitment

T- cell development Congenital absence of thymus will cause immunodeficiency Thymus involutes with age…but memory T cells have long life span Precursors from fetal liver and adult bone marrow will seed the thymus … thymocytes

The TCR 2 lineages …95% alpha beta …the remainder: gamma delta Ch. 14 Ch. 7 2 lineages …95% alpha beta …the remainder: gamma delta Accessory molecules for TCR: -CD3 (4 transmembrane protein chains…gamma, delta, epsilon and zeta) -LFA-1 (= CD11a)…an integrin within Ch. 7 transmembrane *Beta & delta: VDJ *Alpha & gamma: VJ *A tyrosine kinase (Lck…of Src family) is brought into proximity of CD3 cytoplasmic part…by CD4/CD8

T- cell development, cont’d The most immature thymocytes: negative for TCR, CD4 or CD8 In cortex: expression of gamma delta and alpha beta TCRs…see next slide Alpha beta T cells begin to mature into CD4+ class II MHC-restricted or CD8+ class I MHC-restricted T cells…see slide 17 Then to the medulla then to the circulation Role of other cells in thymus (expressing MHC & secreting cytokines) In subcapsular sinus and outer cortex (+) for CD44 & CD25 (alpha chain of IL-2 receptor)

T- cell development, cont’d Cortical thymocytes… proliferation and apoptotic activity Rearrangement of beta, gamma, and delta -at DN stage -if functional gamma delta receptor produced…gamma delta TCR-expressing T cell results -if TCR beta chain…with invariant pre-T alpha chain and CD3: = pre-TCR complex…signal transduction: stimulation of proliferation, halting beta rearrangement and promoting alpha rearrangement 95% before reaching the medulla Sensitive to irradiation and steroids

T- cell development, cont’d Positive selection of DP thymocyte…with help of cortical epithelial cells Self-MHC restricted cells are produced …these need to be self-tolerant…otherwise?? With help of dendritic cells in medulla

Autoimmune regulator gene (AIRE) Regulate expression of antigens of endocrine cells in thymic epithelial and non-epithelial cells Mutation…autosomal recessive disease autoimmune polyendocrine syndrome I (APS-I)

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