B-cell and humoral immunity

B-cell and humoral immunity

1. B-CELL AND HUMORAL IMMUNITY Santosh Yadav M.Sc. Clinical Microbiology Dept. of Microbiology Institute of Medicine Tribhuvan Univarsity Teaching Hospital, Nepal

2. B-cell development and maturation

3. Introduction Derives name from the organ it matures (bursa of fabricus in birds) In human it matures in bone marrow. The lymphocyte which arise and develop in bone marrow is called B-lymphocytes.

4. The discovery of B cell immunity 1954 - Bruce Glick Studies on the function of the bursa of Fabricius , a lymphoid organ in the cloacal region of the chicken Bursectomised chickens were later used in experiments to raise antibodies to Salmonella antigens None of the bursectomised chickens made anti-Salmonella antibodies Bursa was later found to be the organ in which antibody producing cells developed – antibody producing cells were thereafter called B cells Mammals do not have a bursa of Fabricius

5. Generation begins in the embryo and continues throughout life Before birth- Yolk sac Fetal liver Fetal bone marrow After birth Bone marrow

6. A mature and immunocompetent B-cell have three uniqueproperties :- It is specific only to one particular antigen. It produces antibodies specific to the antigen to which it was primed, and It generates memory for any similar antigenic attack in future.

7. B-cell Carry many surface marker: CD19 CD21 CD22 Membranous immunoglobulin Ig α and Ig β heterodimer

8. Stages of B cell Development Stem cell Progenitor B cell (Pro B cell) Precursor B cell (Pre B cell) Immature B cell Mature B cell Each stage characterized by distinct cell surface markers and a specific pattern of Ig gene expression.

9. Pro -B-cell Is the earliest recognisable B-cell stage. It has following surface marker:- CD -45 Ig α and Ig ᵦ heterodimer :-part of B-cell membrane receptor in later stage. CD-19 CD-24 CD-43 C-kit The heavy chain gene rearrangement begins in this stage and its completion signals the end of this stage.

10. Pre-B-cell Characterized by the beginning of the translation of heavy chain hene . Essentially require bone marrow stromal cell. Mu chain is usually first synthesized. No light chains are synthesized. Small peptide called Surrogate light chain are synthesized( not true light chain) They complex with mu chain and this complex is expressed on the cell surface. The mu chain and surrogate light chain complex associate with Ig α / Ig ᵦ hetrodimer to form pre-B-cell receptor .

11. Early pro-B Interleukin-7 receptor Late pro-B Pre-B Interleukin-7 Growth factor Stromal cell

12. Immature -B-cell Begins with light chain gene. Only one type of light chain is expressed(kappa or lambda) Light chain associates with mu chain and so a true IgM expression occurs on the B-cell Surface. This also results in B-cell commited to one particular antigenic specificity as determined by binding site of surface IgM . True BCR appears.

13. Mature B-Cell Characterized by simultaneous expression of IgM and Igd on B-cell surface. The mature B-cell then leaves the bone marrow and go to the peripheral lymphoid tissue where they get activated on encountering the antigen and produce two types of effector cells:- a)Plasma cell:- produce antibody, and b) Memory cell:- produce secondary immune response after re-encounter with the same antigen. If the mature B-cell cannot encounter antigen, they die within few days by apoptosis.

14. So the production of mature B-cell in bone marrow is an antigen independent phase while activation and effector cell production is an antigen dependent phase.

15. Negative selection of B-cells On an average, only 10 percent of total newly cells produced(5 x 10 ⁷ cells) i.e. 5 x 10⁶ cells are recruited into circulation everyday. Rest of 90 percent of newly produced B-cells die. This loss is due to the negative selection of those developing B-cells which express self reactive mIgM on their cell surface. Such cells are eliminated by programmed apoptosis. This process is called negative selection.

16. Development of Humoral immune response Humoral immune response refers to the production of secreted antibodies by plasma cells following antigenic stimulation of B-cells. Involves activation of B-cells by antigen followed by their proliferation and differentiation in to plasma cells and memory cell.

17. B-cell Activation Occurs in the peripheral lymphoid organ Antigen-driven activation and clonal selection of naive B cells Generation of plasma cells and memory B cells In absence of antigen-induced activation, naive B cells die within few weeks by apoptosis

18. 2 types of Antigens Thymus cell independent antigen(TI Ag) Do not require direct interaction of Ag specific helper T lymphocytes. Require cytokines secreted by TH cell. Multivalent nonprotein Ags Polysaccharides, lipids, nucleic acids Thymus cell dependent antigen (TD Ag) Require direct interaction with TH cells Proteins antigens

19. Multivalent Ag composed of repeated identical antigenic epitopes Maximal cross-linking of the BCR complex on specific B-cells Sufficient signalling for activation. Bcell co-receptor complex, further augment signaling T CELL INDEPENDENT RESPONSE

20. B cell co-receptor complex 3 protein CD19 CR-2/CD 21 TAPA-1 (Target of antiproliferative antibody-1)

21. T cell dependent antigen response

22. Memory Cells 26

23. Changes in germinal centres somatic hypermutation Selection Class switching

24. SOMATIC HYPERMUTATION B cells undergo rapid cell division Mutation of genes occurs randomly Ag binding affinity of mIg on B cells may increases Decrease No affinity

25. Selection Centrocytes compete with each other to bind Ag on follicular dendritic cells B cells bearing high-affinity mIg are most likely to compete Those that fail die by apoptosis Signal from TH cells for further survival Differentiate into Memory B cells or Plasma cells

26. Clonal selection of B cells Generates a clone of short-lived activated effector cells and a clone of long-lived memory cells Antigen molecules Antigen receptor B cells that differ in antigen specificity Antibody molecules Clone of memory cells Clone of plasma cells Antigen molecules bind to the antigen receptors of only one of the three B cells shown. The selected B cell proliferates, forming a clone of identical cells bearing receptors for the selecting antigen. Some proliferating cells develop into short-lived plasma cells that secrete antibodies specific for the antigen. Some proliferating cells develop into long-lived memory cells that can respond rapidly upon subsequent exposure to the same antigen. 30

27. Class switching or Isotype switching Process by which B cell changes class of IG produced while preserving antigenic specificity Allows any given VH domain to associate with the constant region of any isotype Enables antibody specificity to remain constant while the biological effector activities of the molecule vary Interaction between CD40 on the B cell and CD40L on the TH cell is essential Occurs only during active immune response

28. Clonal Selection Only one type of antibody—and one type of B cell—responds to the antigenic determinant That cell type then produces a large number of clones 32

29. Plasma cell Membrane form Ig changes to secreted form The rate of transcription of heavy and light-chain genes significantly greater

30. Memory cell Seen in T-dependent immune responses Survive for long periods High-affinity antigen receptors Capable of mounting rapid responses to subsequent introduction of Ag Remain in the lymphoid organ where generated or recirculates

31. Primary and secondary immune response Primary immune response The first contact of an exogenous antigen with an individual generates a primary humoral response, characterized by the production of antibody-secreting plasma cells and memory B cells. The kinetics of the primary response, as measured by serum antibody level, depend on the nature of the antigen, the route of antigen administration, the presence or absence of adjuvants , and the species or strain being immunized. Gradual rise in antibody production taking days to weeks Antibody level decline s

32. Secondary immune response Activation of memory cells by antigen results in a secondary antibody response that can be distinguished from the primary response in several ways. The secondary response has a shorter lag period, reaches a greater magnitude, and lasts longer. The secondary response is also characterized by secretion of antibody with a higher affinity for the antigen, And isotypes other than IgM predominate Second exposure to same antigen. Recognition of antigen is immediate. Results in immediate production of protective antibody, mainly IgG but may see some IgM

33. THANK U