Immunoglobulins

Structure of immunoglobulins (Ig) or antibodies: Immunoglobulins are glycoproteins made of heavy and light polypeptide chains. Specificity of antibody is determined by its antigen binding site while the effector function is determined by the heavy chain. Each monomer is made of 4 chains - 2 heavy chains and 2 light chains. Light chains in an antibody or Ig molecule are either kappa or lambda. Both heavy and light chains are further divided into constant or C and variable or V regions. Heavy chains are composed of one variable or Vh and either three (IgG and IgA) or four (IgM and IgE) constant Ch domains. Whereas, each light chain is composed of one variable and one constant domain. The variable domains of the heavy and light chain have hypervariable regions at the amino terminals that comprise the antigen-binding site. The Ch regions determine the class or isotype of the antibody and thus its effector functions. Five classes or isotypes of antibodies are seen - IgA, IgD, IgE, IgG and IgM. They are classed according to the heavy chain they contain – alpha, delta, epsilon, gamma or mu respectively. The carboxy terminal of heavy chains forms the Fc fragment. When an antibody is treated with the enzyme papain, it cleaves it into two Fab (antigen binding) segments and one Fc fragment.

Immunoglobulin genes and function in immune response: The antibody response to an antigen is diverse with the body producing many different antibody molecules with unique affinities and specificities to the antigen. Humans have an antibody repertoire > 100 billion.

The V region of light chains is coded by two DNA segments, V and J. These segments join by somatic recombination to form a complete V region exon. The constant region is coded by the C gene. The V and C exons join, followed by splicing to form the final mRNA transcript for light chains.

Heavy chain V regions are coded by three types of gene segments - V, D and J. They undergo a similar process as light chains (DNA rearrangement and recombination), to form the final mRNA transcript for heavy chains. The total number of V,D,J and C gene segments that code for immunoglobulins is more than 100, and they are located on chromosomes 2, 22 and 14.

The genes coding for the heavy chain C region are arranged in a series, each of which codes a different isotype. The first antibodies produced during an immune response are IgM, followed by “class switching” that allows different isotypes of antibodies to be produced like IgG, IgE and IgA. There are repetitive sequences of DNA called “switch regions” that lie in-between the gene clusters that code for immunoglobulins. During isotype switching, DNA undergoes recombination at the switch regions, and the intermediate DNA is excised out. The left out regions are then transcribed as any of the isotypes, depending on what type of antigen or pathogen initiated the immune response.

Apart from recombination and gene arrangements, immunoglobulin diversity is generated by somatic hypermutation and by different combinations of heavy and light chain V regions that pair to form the antigen binding site of the immunoglobulin. P and N nucleotides are added with the help of RAG and TdT enzymes to further increase diversity. Somatic hypermutation is the process of introducing point mutations in the rearranged V region genes of activated B cells. As a result multiple clones of B cells are produced in response to the same antigen. B cells that have greater affinity for the inciting antigen are preferentially selected and increase in number, a phenomenon called “affinity maturation”.

T cell receptor (TCR): It consists of disulfide-linked heterodimer of either variable alpha and beta chains or variable gamma and delta chains in alpha, beta T cells or gamma, delta T cells respectively. The heterodimers are associated with the variable chain or CD3 and homodimer of ζ chains to form the TCR. The intracellular tails of CD3 complex act as ITAMs (immunoreceptor tyrosine based activation motifs) and are involved in signal transduction. CD3 associates intracellularly with tyrosine kinases like ZAP 70, when activated by phosphorylation.

The structural diversity of TCR is attributable to gene rearrangements. Each chain has a variable or V region and a constant or C region. The exon for the V region of TCR, is formed by the joining of V (variable) and J (joining) gene segments to form alpha or gamma chains and by the joining of V, D (diversity) and J segments to form beta and delta chains. The variable gene segments join to each other by somatic recombination (gene rearrangement), during T cell development in the thymus. This is followed by transcription and splicing of the V exon to the exon for C region, to form the mRNA that is then translated to corresponding TCR chain. Structural diversity of TCRs is mainly caused by gene rearrangements between V, D and J gene segments. This is accomplished by more than 100 types of gene segments available for rearrangement. Hypervariable regions on the TCR differ between TCRs and contribute to recognition and binding to a wide variety of peptides presented by MHC. Nucleotides are added between the V and J segments by TdT or terminal deoxynucleotidyl transferase enzyme, which helps to increase diversity of TCRs.