B-cell maturation begins with hematopoietic stem cells in the bone marrow, where they develop through pro-B cell, pre-B cell, and immature B cell stages. During this process, immunoglobulin genes undergo rearrangement and expression of B cell receptors occurs. Immature B cells then migrate to secondary lymphoid tissues to complete maturation. Mature B cells circulate and are activated by antigen to proliferate and differentiate into plasma cells or memory B cells through T cell dependent or independent pathways. T cell dependent activation induces affinity maturation, class switching, and generation of long-lived memory B 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

6. 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.

7. 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

8. STAGES OF B CELL DEVELOPMENT
1. PRO B CELL
2. PRE B CELL
3. IMMATURE B CELL
4. MATURE B CELL

9. 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

11. PRE B CELL
• Translation of heavy chain genes.
•
• μ chain is the 1st to be synthesized, no light chains are synthesized.
• V pre B and λ5 – 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.

13. 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

14. 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
a) Plasma cell
b) Memory cell

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

17. 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.

18. Primary and SecondaryAntibody Responses

19. B-Cell Development
Antigen-Independent (Maturation)
1) Pro-B stages
B-cell markers
2) Pre B-stages
H- an L- chain loci rearrangements surrogate light chain
3) Naïve B-cell
- functional B Cell Receptors.
 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.

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

21. HSC passes through progressively more delimited
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.

22. Overview of B – cell development

23. B cell receptor complex
 It is a multimeric protein complex with Ig and Igα – Igβ 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)

24. Activation of B cell

26. 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

27. B-CellActivation by
Thymus-Independent and DependentAntigens

28. 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.

29. T cell independent: 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).

30. Ti type-1 – e.g., LPS
-many are mitogens
-signal through TLRs
-CD14 is LPS receptor
 Sometimes coupled with BCR engagement
 Some can activate without BCR
engagement
Ti type-2 – e.g. capsule polysaccharides
bacterial flagellin
-crosslinkAbs
-AGs have repetitive, polymeric
structure
Two types

31. Helper T Cell-Dependent Activation of B Lymphocytes

32. B Cell Responses to Thymus-Dependent Antigens (T Cell-Dependent Antibody Responses)
1)Antigen crosslinking of antibodies
-antigen engagement
-Igα/Igβ signaling
-up-regulation of CD40 & MHC
2)TH cell engagement
-Cell/cell interactions
-MHC presentation
-TCR recognition
-CD40/CD40L coupling
3)Cytokine stimulation
-IL4, IL2, etc.
-class switching to IgG
-memory cell formation
Source: Source:Abbas et al,
2011

33. Phases of the Humoral Immune Response
AT-DependentAntigen Must
Contain Both B and T Cell Epitopes
Source:Abbas et al, 2011

34. Initial Contact T-B Conjugate
Note the broad area of membrane contact between
B and T Cells.

35. 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.
Somatic Hypermutation and Affinity Maturation of

36. 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.

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

38. • 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

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

41. Effector Functions ofAntibodies
Source:Abbas et al, 2011

42. THANK YOU

43.  Subhash Chandra Parija, Textbook of Microbiology and Immunology, 3rd edition
(2016).
 Kuby Owen, Immunology, 7th edition (2013).
 Abbas, Lichtman, and Pillai, Cellular and Molecular Immunology, 7th edition (2011).
 Tarlinton Nature Reviews Immunology 6, 785–790 (October 2006).
 Dr. S.K.Gupta, Essentials of Immunology, 1st edition (2014)
References