B-cell development.pptx

B cell development is a process by which mature cells develop from the hematopoietic stem cells.

1. B-cell maturation Presenter - Dr. Vaishali.T Moderator - Dr. Vedavati B.I

2. History EDWARD JENNER - small pox vaccine – 1798. LOUIS PASTEUR and ROBERT KOCH – principles of infectious diseases and vaccination – 1800’s. EMIL VON BEHRING – antitoxin – 1890. ELVIN KOBAT – humoral immunity – 1930’s. BRUCE GLICK – B and T lymphocytes. GEORGE SNELL , JEAN DAUSSET – MHC – 1980 PETER C. DOHERTY – CMI response - 1996

3. Introduction The lymphoreticular system consists of an intricate networking of cells with diverse morphology, which spreads across various organs and tissues of human body. THE LYMPHOID SYSTEM – PRIMARY(central) AND SECONDARY (peripheral) lymphoid organs. TYPES OF IMMUNE RESPONSE: Humoral mediated immunity – mediated by antibodies from plasma cells, present in blood. Cell mediated immunity – mediated by sensitised lymphocytes. HEMATOPOIESIS: all immunological cells originate from hematopoietic stem cells (HSC)

4. Cells of lymphoreticular system

5. B cell development – Role of bone marrow The lymphocytes which arise and develop in bone marrow are called B lymphocytes or B cells. In foetal life – yolk sac, foetal liver and bone marrow. Bone marrow stromal cell environment – differentiation of Pro- B cell to Pre – B cell. Physical interaction of stromal cells with Pro – B cell. IL – 7 from stromal cells – completes the development.

6. VLA – 4 : Very late antigen -4 VCAM – vascular cell adhesion molecules – 1 SCF – Stem cell growth factor c-Kit– tyrosine kinase activity – pro b cell to pre b cell. IL -7 – down regulates VLA-4 – separate from stromal cells

7. STAGES OF B CELL DEVELOPMENT PRO B CELL PRE B CELL IMMATURE B CELL MATURE B CELL

8. PRO B CELL It is the earliest recognisable B cell stage. Surface markers CD 45 - signal transduction Ig α and Ig β heterodimer - part of cell membrane CD19 - part of co receptor. CD24 - heat stable antigen. CD43 - lymphocyte repulsion. c-Kit - receptor for SCF. Heavy chain rearrangement (D to Jh recombination) – VDJh joining. Requires transcription factor PAX5. Occurs in bone marrow

9. PRE B CELL Translation of heavy chain genes. μ chain is the 1 st to be synthesized, no light chains are synthesized. V pre B and λ 5 – surrogate light chains. Cells with this complex – proceed further development. μ and surrogate light chain complex with Ig α and Ig β heterodimer - pre B cell receptor. CD 43 and c Kit expression stops, CD25 starts. Pre B cell recognizes a ligand – signal to immature B cell formation. - pre B cell multiplies – clone of B cells. Each of these B cells will have different L chains.

10. Immature B cell Rearrangement of the L chain genes. One type κ or λ is expressed – due to allelic exclusion of Ig heavy chains. Along with μ chain – IgM expression occurs – true B cell receptor. CD 25 expression stops, pre B cell receptor disappears CD21 expression seen – C3d

11. MATURE B CELL IgD and IgM expressed on B cell surface. Differential splicing of H –chain mRNA – one for μ chain and one for δ chain. The mature B cell go to peripheral lymphoid tissue Immunoglobulin isotype switching – which reacts with possible epitopes Activated on encountering the antigen – clonal proliferation Produces effector cells Plasma cell b) Memory cell

12. Negative selection Only 10% of the B cells are recruited into circulation, rest 90% die due to Programmed apoptosis Clonal deletion or anergy

13. B1 CELLS Includes 5 % of mature B cells. Has CD5 marker on its surface. Predominant in foetal and neonatal life, migrate from foetal liver to peritoneal cavity. They multiply and secrete polyreactive antibodies including autoantibodies – IgM. They undergo little class switching and somatic hypermutation. May act as a contributing factor for initiation of autoimmune disorders.

14. Primary and Secondary Antibody Responses

15. B-Cell Development Antigen-Independent (Maturation) Pro-B stages B-cell markers Pre B-stages H- an L- chain loci rearrangements surrogate light chain Naïve B-cell - functional B C ell R eceptors . B cells are generated in the bone marrow. Takes 1-2 weeks to develop from hematopoietic stem (HSC) cells to mature B cells. Sequence of expression of cell surface receptor and adhesion molecules which allows for differentiation of B cells, proliferation at various stages, and movement within the bone marrow microenvironment.

16. Begins in the Bone Marrow and Is Completed in the Periphery

17. H S C passes through progr e s s iv e l y m ore de l imi t ed progenitor-cell stages until it reaches the pro-B cell stage. Pre-B cell is irreversibly committed to the B-cell lineage and the recombination of the immunoglobulin genes expressed on the cell surface Immature B cell (transitional B cell) leaves the bone marrow to complete its maturation in the spleen through further differentiation. Immune system must create a repertoire of receptors capable of recognizing a large array of antigens while at the same time eliminating self-reactive B cells.

18. Overview of B – cell development

19. B cell receptor complex It is a multimeric protein complex with Ig and Ig α – Ig β heterodimer. The heterodimer has cytoplasmic tails (61 and 48 respectively) – Immunoreceptor Tyrosine Based Activation Motif (IATM). Ig – extracellular – antigen binding unit. Heterodimer – intracellular – signal transduction unit. B cell co-receptor complex: Has CD19, CR2 (CD 21), TAPA 1(Target of antiproliferative Antibody 1)

20. Activation of B cell

21. B-Cell Activation and Differentiation Exposure to antigen or various polyclonal mitogens activates resting B cells and stimulates their proliferation. Two major types: T cell dependent (TD) T cell independent (TI) Source: Kuby

22. B - Ce ll Act i vation by Thymus-Independent and Dependent Antigens

23. T cell dependent : Involves recognition of epitopes by T cell receptor. CD28-B7 interaction is essential to produce IL2 CD40L (T)/CD40 (B) interaction for class switching from IgM to IgG and other immunoglobulins. Slow, more durable, high affinity IgG, IgA or IgE .

24. T cell independen t : Most TI antigens are polyvalent and induce maximal Crosslinking of membrane Ig on B cells, without a Need for T cell help. TI-1: e.g., LPS. Mitogenic at high concentrations to most B cells because of binding to pattern recognition receptors (PRRs) on B cell surface. At low concentrations, only activates those B cells that bind the antigen via the Ig receptor. TI-2: e.g., bacterial capsular polysaccharide. Highly repetitive determinants on multivalent antigen. Not mitogenic but can crosslink Ig receptors. Rapid, Low affinity IgM. Activate complement system (C3d).

25. T i t y p e - 1 – e.g., L P S many are mitogens signal th r ough TL R s CD14 is LPS receptor Sometimes coupled with BCR engagement Some can activate without BCR engagement Ti type-2 – e.g. capsule polysaccharides bacterial flagellin c r osslink A bs AGs have repetitive, polymeric structure Two types

26. Helper T Cell-Dependent Activation of B Lymphocytes

27. B Cell Responses to Thymus-Dependent Antigens (T Cell-Dependent Antibody Responses) Antigen crosslinking of antibodies antigen engagement Igα/Igβ signaling up-regulation of CD40 & MHC T H cell engagement Cell/cell interactions MHC presentation TCR recognition C D 40/C D 40L c o upling Cytokine stimulation IL4, IL2, etc. class switching to IgG memory cell formation Source: Internet Source: Abbas et al, 2011

28. Phases of the Humoral Immune Response A T - D ependent A ntigen M ust C ontain Both B and T Cell Epitopes Source: Abbas et al, 2011

29. Initial Contact T - B C o njugate Note the broad area of membrane contact between B and T Cells.

30. Antibodies  Affinity maturation is the process that leads to increased affinity of antibodies for a particular antigen as a result of somatic mutation in the Ig genes followed by selective survival of B cells producing the antibodies with the highest affinity . So m a t i c Hy p e r m u t a t i on an d A f f i n i t y Ma t ur a ti on of

31. B Cell activation can occur without T-cell help Rapidly mature into short-lived plasma cells without undergoing somatic hypermutation or class switching. Secrete IgM antibodies of low affinity. Do not contribute to memory B cell pools. B-1 cells may preferentially follow this non-follicular differentiation pathway as they appear to be much less dependent on T cell help for antibody production.

32. Isotype Switching Under the Influence of Helper T Cell-Derived Cytokines

33. Also called switch recombination. Ig heavy chain DNA in B cells is cut and recombined. So that VDJ exon that encodes V domain id placed adjacent to a C region. The intervening DNA is deleted. B cells change the isotypes by changing the constant region of the heavy chains. Specificity of the antibodies ( variable region) remains unaltered. Molecular Mechanism of Ig Isotype Switching

34. Summary of thymus-dependent vs thymus-independent antigens and B-cell differentiation Some antigen can produce memory cell.

35. Effector Functions of Antibodies Source: Abbas et al, 2011

36. THANK YOU

37. Subhash Chandra Parija , Textbook of Microbiology and Immunology, 3 rd edition (2016). Kub y Owen, Immunology, 7 th edition (2013). Abbas, Lichtman, and Pillai, Cellular and Molecular Immunology, 7 th edition (2011). Tarlinton Nature Reviews Immunology 6 , 785–790 (October 2006). Dr. S.K.Gupta , Essentials of Immunology, 1 st edition (2014) Ref e r enc e s