During this process, randomly chosen V, (D), and J gene segments are brought into contiguity mediated by the coordinated activities of several enzymes, such as recombination-activating genes (RAG1/RAG2) and terminal deoxynucleotidyl transferase (TdT) (Gauss and Lieber 1996 Schatz and Swanson 2011), and the rearranged variable domain includes four framework regions (FR) and three complementarity-determining regions (CDR) (Fig. During the process of B cell development and maturation, the variable coding genes of BCR undergo a somatic recombination procedure called V(D)J recombination (rearrangement) (Fig. The number of functional genes of each gene family for human and mouse collected by IMGT ( ) is listed in Table 1. For light chains, only V and J gene families are involved in recombination. The immunoglobulin heavy chain variable region is encoded by three gene families, including variable (V), Diversity (D), and Joining (J) genes, which contain numerous gene copies with high sequence homology (Fig. The human IGH, IGL, and IGK genes locate on the chromosomes 14, 22, and 2, respectively. The variable region diversity of BCR determines the diversity and specificity of antigen recognition. Antibody is a Y-shaped protein composed of two identical large heavy chains (encoded by the gene IGH) and two identical small light chains (λ or κ) (encoded by the gene IGL or IGK) (Fig. Antibodies are encoded by a very unique protein-coding system, in which somatic rearrangements happen in the coding genes. Generation of antibody diversityĪntibody, also known as immunoglobulin (Ig), is the molecule produced by B cell that can specifically recognize and neutralize a variety of antigens. The precursor of T cells migrate through peripheral blood to the thymus, where they finish the differentiation and the maturation, while the maturation of B cells from the progenitors occurs mostly in the bone marrow. Both T and B cells originate from the pluripotent stem cells in the bone marrow. Bearing the significant functions in the immune system, T and B lymphocytes are derived, differentiated, and mature in different organs and tissues. Therefore, the diversity of TCR and antibody repertoire could reflect the adaptive immune status to some extent. In addition, pressures from both the internal and the outside environment, such as pathogen exposure, tumor formation, and autoimmune reaction, could affect the diversity of TCR and antibody by expanding the “pressure”-associated T/B cells. A powerful adaptive immune system relies on the generation of diversified TCRs and antibodies to recognize the enormous variety of antigens. T and B lymphocytes are two prolific and functionally important cells that play pivotal roles.
The adaptive immunity consists of two major components, cellular immunity and humoral immunity, which are highly specific to particular pathogens. However, challenges still remain both in the experimental pipeline and the analytical tools to handle the huge amount of data, and various potential solutions are being developed to fulfill the comprehensive application and translation of this technique.
The emergence of high-throughput sequencing (HTS) technique enables scientists to investigate the repertoire in an unprecedented level. Historically, scientific reports on the TCR and BCR repertoire are unlikely to grasp the whole picture due to the unavailability of high-throughput and sensitive tools. On the other hand, antibodies can bind to and neutralize the antigens directly. The diversified TCRs can recognize and bind to the various epitopes of the antigens presented by the major histocompatibility complex (MHC) molecule of the antigen presenting cells and form a TCR-antigen-MHC structure complex. The normal function of adaptive immunity relies on the generation of diverse T cell receptors (TCRs) that are expressed on the cell membrane of T lymphocytes, and B cell receptors (BCRs), also known as immunoglobulin (Ig) or antibody in the secreting form, which are expressed by B lymphocytes.
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