Question

1)List the B-cell developmental stages from pre-pro to immature B cells describing B-cell receptor (BCR or Ig) rearrangements occurring and/or key events occurring at each stage.

2)What is the role of Artemis in V(D)J recombination and why is it important?

3)During B-cell activation some activated B-cells undergo differentiation into antibody secreting plasma cells outside of the germinal center and therefore never go through the germinal center responses. Other activated B-cells will return to the follicle forming a germinal center and undergo processes that alter the antibody that will be produced.  Describe the germinal center responses. Why are these responses important (hint: what is different about the antibody produced)? Why are T cells important in germinal center responses?

Answer 1

1.B cells are mainly associated with humoral immunity and it got its name from the site of maturation. In humans the maturation of B cells occurs at bone marrow. Development of B cells occur through various stages starting from bone marrow, which then progresses in peripheral lymphoid organs like lymph node and spleen.The development stages include

Lymphoid stem cell, Pro-B cell, Pre-B cell, Immature B cell, Mature B cell which then differentiate either into plasma cell and Memory cell.

At the pro B cell stage the lymphoid cell donot express immunoglobulin but it express a heterodimer Ig alpha or If beta which later forms a part of the b cell receptor or BCR. In the pre b cell stage they will undergo rearrangement of heavy chains. The synthesis of mew heavy chain occur first. Then the surrogate light chains can also be seen. These surrogate light chains+ mew chain+ heterodimer Ig alpha or Ig beta forms the pre B cell receptor. In the next stage pre B cell progresses into immature B cells which is characterized by the rearrangement of light chain genes which will result in the expression of light chain.By following the principle of allelic exclusion among the two light chains either Kappa or Lambda light chain will get expressed. By the joining of heavy chain mew and it's light chain forms complete IgM which them complexes with heterodimer Ig alpha or Ig beta on the b cell to form BCR.

2.VDJ stands for variable diversity joining genes . VDJ is also known as antigen receptor gene rearrangement. This rearrangement will result in the antigen receptors with unique specificities. Artemis is a protein complex that plays an important role in the VDJ recombination. In this process recombinase enzymes will detect the site of recombination, which is then followed by the flanking of genes to recombined. Then the DNA is cleaved by Rag 1 and Rag2 enzymes which result in a hairpin like structure.In short VdJ rearrangement is characterized by a break in the double strand of DNA which is resolved by a machinery. Artemis mainly functions in the repair of dna strand breaks during VdJ recombination. While rag enzyme cut the dna strands adjacent to v and j segments the intervening dna will between these two segments will get ligated to form circular dna molecule and the coding ends joins to forms hairpin like structure. Artemis will binds to these ends and aids in repair mechanisms.

3 .In germinal centres the events occur at two sites which include dark zone and light zone. In the dark zone of germinal centres the activated b cells will transform into centroblasts which then changes into centrocytes which are non dividing cells which express membrane Ig. The centroblasts by getting into somatic hypermutations randomly the centrocytes exhibits membrane Ig with low and high affinity. Those with low affinity undergo apoptosis which is then phagocytosed by tingible body macrophages. Those centrocytes possessing high affinity membrane Ig migrates to the light zone where they binds to follicular dendritic cells which differs from others dendritic cells by not acting as APC and do not express class II MHC. At this stage they undergo maturation and class switching and the selected centrocytes differentiate into plasma cells and memory cells. These memory cells are long lived cells which will respond to this the second antigenic exposure. T cells play an important role in the proliferation of b cells and somatic hypermutations.