User:Sharmak141/sandbox

B-CELL AND ANTIBODY
Overview

B cell are lymphocytes of adaptive immunity that play key role in humoral response. B cells are distinguished from other immunity cells by receptors on the surface of the cell formally named B-cell receptor (BCR). BCR are the protein of certain gene segment that come together to form an antibody specific to B cells that help recognize and attach antigen to B cell for phagocytosis. As the name suggests B cells originate from bone marrow and complete their maturation where they become functional B cells, there is however, another stage of development outside of bone marrow in which primary B cell differenciates into plasma cell (antibody producing) and memory cell. There are many different isotypes and sub-isotypes of antibody that represent BCR on B cells. Organization of these antibody do no follow one gene one polypeptide rule, in fact they are collection of multiple gene segments that come from different location of the same chromosome cleaved and joint together by special enzymes and then translated into protein that represent different subclasses of antibodies on the surface of B cells. In order to protect a human body from millions on potential antigen, immune system needed to develop and diversify with changing environment. Diversity in Immunoglobulin (Ig) genes was observed during their development and even after maturity (class switching).

B-CELL Development

One of the secondary immune system cell is B cells, and its development is profound group of discussion today. The development of early B cells, which are generated from hematopoietic stem cells (HSCs) in a series of well-characterized stages in bone marrow (BM). Adult B lymphocytes develop in bone marrow (BM), where B lymphoid-specified cells are generated from hematopoietic stem cells (HSCs) and lose the potential to differentiate into other blood lineage cells (once committed they are unable to differentiate). B-Lymphoid progenitor cells receive signals from bone marrow stromal cells to initiate B cell development. Cytokines induce terminal deoxynucleotidyl transferase ( also known as specialized DNA) genes and recombinase genes (RAG-1 and RAG-2) synthesis in CD34+ lymphoid progenitors. This cell goes through D-J joining on the H chain chromosome to become early pro-B cells and the cell begins formation of CD45 (B220) and Class II MHC on the surface of the cell. Joining of a V segment also from the same chromosome to the D-J completes the late pro-B cell stage. It then become Pre B cell when surrogate light chain develops which resembles functional light chains however is non coding, it's function is to stabilize heavy chain. B cell develop receptor proteins Ig a and Ig b also know as signal molecule they help Ig cytoplasmic tails to bind to B cell surface by the help of molecule ITAMs (Immunoreceptor Tyrosine Activation Motifs). Late pre B cell initiates light chain and m chain formation on one chromosome once completed cell enters immature B cell. Immature B cell are very sensitive and need to be tested for self tolerance, they are introduced to self antigen and if binding occurs cell is eliminated by apoptosis.

Multi Gene Organization of Immunoglobulin protein

Two gene model was a mystery until 1965 when W. Dreyer and J. Bennett extrapolated two separate genes for variable and constant region and contradicted the existing one gene one polypeptide principle. S. Tonegawa and N.Hozumi in 1976 tested and confirmed this hypothesis by comparing Ig DNA from embryonic (germline) and adult myeloma (somatic) cells and won a nobel prize in 1987. κ and λ light chains and the heavy chains are encoded by separate multigene families situated on different chromosome. There are 31 functional Vλgene segments, 4 Jλ segments, and 7 Cλ segments in lambda light chain; and 40 Vκgene segments, 5 Jκ segments, and single Cκ segments in kappa light chain. Heavy chain consisted of 51 VHgene segments, 27 DH segments, 6 JH segments, followed by a series of CH gene segments, Each CH gene segment encodes the constant region of an immunoglobulin heavy-chain isotype.

 Variable Region Gene Rearrangements

In immunoglobulin, variable region genes generate antibody with different antigen specificity and reactivity. However specificity should change accordingly for different antigen and is achieved by gene rearrangement in the variable region of immunoglobulin. Diversity comes from the variation in combination of non-contagious gene segments of heavy and light chain and this diversity increases following any antigen exposure. Immunoglobulin gene rearrangement of variable (V), diversity (D) and joining (J) gene segments generates this large array of antigen receptors with different antibody specificities, all with specific antigen specificity. Approximately 10^9 different rearrangement sequence molecules can be generated by VDJ segment rearrangement. in domain of heavy chains, genes encoding lack all exon. Instead segments encoding the V region are split into arrays of gene segments. Genes of the light chain variable region also follow similar pattern on a different chromosome. "There are 51 functional VH genes and 41 Vk genes. D (diversity) and J (junctional) genes code for amino acids at the carboxyl end of V regions including CDR3" (Berg 2002).

Variable Region Gene Rearrangements Mechanism

Variability in DNA sequence can be due to number of reasons such as mutation or even deletion, however DNA sequencing has revealed that common form of rearrangement is due to looping out and re-ligation of the these segment sequences in the germ line. Recombination signal sequences (RSS) are key DNA sequences that take part in rearrangement and reassembling V, D, J segments that make up the heavy chain of the antibody. RSS functions as signal molecule for recombination of segments. These signal sequences are confined to one turn or two turn and may vary in sequence, however the length of RSS signal molecule is the same. When the looped out DNA sequence is lost nucleotide are sealed by a hair pin loop, when this loop is cleaved open few nucleotide basepairs are lost introducing P-N nucleotide addition by the TDT which also brings diversity to the coding sequence. P-N nucleotide addition can be as much 20 nucleotides. All these mechanisms can create diversity before the coding sequence (V,D,J segments) are formed, however occasionally (once per two cell division) somatic hypermutation can take place (replacing nucleotides) again creating diversity.

Immunoglobulin Diversity

It is suggested that approximately 10^9 different rearrangement are possible for any give type of Ig, and around 10^12 after reaching affinity maturation. Imprecise joining of coding segments can further increase diversity. on a given chromosome there are many coding sequence with may differ in size from one another, however these segment sequences makes their own contribution enormously to  antibody diversity. 7 means of generating antibody diversity are:
 * Multiple germ-line gene segments
 * Combinational V-D-J joining
 * Junctional Flexibility
 * P-region nucleotide addition (P-addition)
 * N-region nucleotide addition (N-addition)
 * Somatic Hypermutation
 * Combinational association of light and heavy chains

Class Switching

Before reaching affinity maturation heavy chain can undergo class switching, DNA rearrangement in which coding sequences can combine with any CH gene segment lying few thousand base pairs up stream in 5' direction. A typical B cell after maturation is equipped with IgM and IgD, often cytokines from helper T cells induces class switching to phagocytose foreign antigen adequately, in class switching the constant region of the immunoglobulin heavy chain rearranges but the variable regions remain the same keeping antigen specificity the same ( for ex. Inflammatory cytokines may with antibody class to IgE). It will always be cytokines and its environment that predicts what antibody class protein might switch to.

References