What are B Lymphocytes?

Bruce Glick’s experiments on chicks specify that, lymphocytes derived from bursa of fabricus are responsible for humoral immunity. The cells derived from bursa are referred as “B Lymphocytes” to specify the region of their development.

Since bursa is absent in mammals and other vertebrates bursa equivalent regions involved in B lymphocytes production are identified in bone marrow. Rabbits are an exception; their B cells develop in the appendix-sacculus rotundus.

Generation of B cells starts from the embryo stage and continues through out life. In early stages of embryo development B cells production takes place in yolk sack and liver.

In later stages bone marrow starts the production of B cells and continues through out the life.

Development of B cells:

Development of B cells occur in several stages, each stage represents some rearrangement in the genome content at the antibody loci, to produce unique immunoglobulin’s from each individual B cell and the change is known as ‘class switch’ on the whole, differentiation of B cells can be studied in two segments, one is antigen independent and the other is antigen dependent.

(1) Antigen independent development of B cells:

The haematopoietic stem cells of bone marrow first differentiate into “Progenitor B cells” or “Pro B cells” and the progenitor B cells with transmembrane tyrosine phosphate (CD 45R marker) move to bursa or bursa equivalent areas of bone marrow and multiply to increase their number.

The stromal cells of bone marrow provide suitable environment for the differentiation of pro B cells into “Precursor B cells (pre-B cells)”.

The interaction of stromal cells and their secretions with C kit receptors of pro-B cells play a crucial role in initiation of differentiation process.

The differentiated pro B cells start expressing IL-7 receptors (interleukin-7) on their surface. The interleukin-7 (IL-7) secreted by surrounding stromal cells enhances maturation process and detachment of matured B cells from the surrounding cells.

By the time of their release, each B cell carry membrane bound immunoglobulins with unique binding sites or paratopes. Rearrangement and random recombination in immunoglobulin or antibody gene, during maturation of B cells is responsible for the production of immunoglobulins with unique binding site or paratopes from each B cell of a group (Refer genetical basis of immunoglobulin diversity for further details).

The initial rearrangements that occur in the genome of the antibody loci are designed to produce a functional Ig heavy chain. Once a progenitor B cell has made a productive rearrangement for n heavy chain (IgM is the first immunoglobulin to be produced by B cells) they synthesize the heavy chain and retain it in the cytoplasm.

The pre B cells with diffuse heavy chains of IgM immunoglobulin class are large in size. The cytoplasmic heavy chain later joins with “surrogate light chain” to become expressed at the surface as the “pre-B receptor.

Surrogate light chains resemble actual light chains but are alike on every pre-B cell. When Pre-B cells express membrane (lx heavy chains with surrogate light chains they divide rapidly and are referred as “Lymphoblast cells”.

Lymphoblast cells then cease dividing and become small resting pre-B cells, at which point they cease expression of the surrogate light chains and express the heavy chain alone in the cytoplasm. When the cells become small again, they start to rearrange the light-chain (L-chain) genes.

If a productive light chain rearrangement is made, it is expressed with p. heavy chains on the surface of the cell. The cell with surface bound immunoglobulin loose BCR, CD25 markers of pre B cells and are called as “immature B cell”. Immature B cells produce a 8 heavy chain in addition to p heavy chain, by a mechanism of alternative mRNA splicing, and express on their surface along with IgM.

The B cells with immunoglobulins that can bind with self antigens die before reaching the secondary lymphoid organs through negative selection. It is estimated that about 80-90 per cent of the B cells produced by the bone marrow die before they enter in to the circulation.

B cells that acquired membrane bound IgM and IgD leave the bone marrow and pass to secondary lymphoid organs via blood. Rearrangement in genetic material of immunoglobulins during maturation process helps in the expression of immunoglobulins that can suit the limitless array of antigens.

Alternative splicing (or differential splicing) is a process by which the exons of the RNA produced by transcription of a gene (a primary gene transcript or pre- mRNA) are reconnected in multiple ways during RNA splicing.

The resulting different mRNAs may be translated into different protein isoforms; thus, a single gene may code for multiple proteins.

The surface markers of B cells also changes along with the genetical changes of the developing B cells. The pro B cells express C kit receptor, CD45R, CD19, CD 43, and CD24 markers on the surface to support their differentiation activity.

During metamorphosis from pro B to pre B cells, C kit & CD43 receptors disappear with the gradual development of new CD25, 1L-2 and pre BCR receptors.

The pre B cells become immature B cells by loosing pre BCR, CD25 markers and express surface immunoglobulin IgM. The immature B cells become mature B cells by the addition of IgD immunoglobulin on their surface.

(2) Antigen dependent B cell development:

Mature and immuno competent B cells in secondary lymphoid organs prior to antigenic interaction are referred as “naive B lymphocytes” and they are in G()of their cell cycle.

Since the membranebound immunoglobulin in each B cell is unique in its binding site (paratope) interactions of naive B lymphocytes with appropriate antigens result in differentiation and proliferation of B cells to produce clones of antibody synthesizing plasma cells and memory cells. Plasma cells secrete different immunoglobulin isotypes, through class switching in Ab gene loci (See genetical basis of immunoglobulin diversity for clarification).

In the absence of antigen interaction naive B cells die within a few days (about 3-8 days) through apoptosis.

The antigens giving stimulation to B cells may be thymus dependent or thymus independent. The response to thymus independent antigen is generally weak and results in the production of IgM type of antibodies without forming any memory cells. Thymus dependent antigens are responsible for humoral immunity.

C-kit receptor or cytokine kit receptor or CD117 is also called KIT, is a cytokine receptor expressed on the surface of hematopoietic stem cells as well as other cell types.

This receptor binds to stem cell factor (a substance that causes certain types of cells to grow). Altered forms of this receptor may be associated with certain cancer cells.

CD45 is also called “common leukocyte antigen”. It is a protein tyrosine phosphatase (PTP) located in hematopoietic cells except erythrocytes and platelets.

Protein tyrosine kinases constitute a family of receptors and enzymes that catalyze the dephosphorylation of phosphor tyrosine residues. It has several isoforms and the hematopoietic cells express one or more of the isoforms.

The specified expression of the CD45 isoforms can be seen in various stages of differentiation of normal haematopoietic cells; some isoforms of CD45 are CD45RO, CD45RA, and CD45RB.