T Cell Receptor Genetics and Bioinformatics

T Cell Receptors (TR): These serve as the specialized antigen receptors located on the T cells of the adaptive immune response.
Antigen Definition: Refers to any substance that the immune system identifies as being foreign to the body.
Receptors: These are specialized protein molecules. They can be located either on the surface of a cell or within its interior.
Evolutionary Origin: T cell receptors were acquired by jawed vertebrates (Gnathostomata, noted in the transcript as "grathostomata").
Membrane Anchoring: TR are anchored directly into the membrane of a T cell. They function as a integral part of the signaling apparatus known as the T cell Receptor (TCR) $\mathbb{CDR3}$ .
T Cells: These are specialized white blood cells that function to protect the "front" of the immune system.

TR Alpha Beta ($\alpha\beta$): These receptors recognize processed antigens. These antigens present themselves in the form of peptides.
Peptides: Defined as a short chain of $2$ to $50$ amino acids that are linked by chemical bonds. Peptides are responsible for regulating cellular function.
Polymorphic Major Histocompatibility Complex (MHC) Proteins: These are cell surface receptors that bind to foreign antigens and display them to T cells. This interaction triggers the adaptive immune response.
TR Gamma and Delta ( $\gamma\delta$ ): Unlike the Alpha Beta variety, Gamma and Delta receptors recognize non-peptidic antigens.

Chain Structure: TR are composed of two chains. These are categorized as either Alpha ( $\alpha$ ) and Beta ( $\beta$ ) or Gamma ( $\gamma$ ) and Delta ( $\delta$ ).
Genetic Loci: These chains are encoded by genes situated in four major loci on the chromosomes:
- TR Alpha (TRA): Located at $14911.2$ .
- TR Beta (TRB): Located at $7934$ .
- TR Gamma (TRG): Located at $714$ .
- TR Delta (TRD): Located at $14911.2$ .
Orphons: These are nonfunctional genes or DNA sequences that are located outside of the main chromosomal locus (orphons are genes outside of the major loci).

TR Gene Types: There are four specific DNA sequences that contribute to the synthesis of the T cell Receptor:
- Variable (V): Encodes the N-terminal portion of an antigen receptor in B and T cells.
- Diversity (D): Contributes to the diversity of the receptor.
- Joining (J): A short specialized DNA sequence crucial for developing a functioning vertebrate immune system.
- Constant (C): Represents the constant region of the receptor.
Inheritance: A child inherits the exact DNA sequences for the constant and variable regions of the TCR from their parents.
Early Development Rearrangement: During early development, T cells "snip and splice" the inherited gene segments. After this unique genetic process is complete, every T cell possesses a completely unique TCR capable of recognizing specific antigens.

Immunoglobulin Loci: The regions of DNA containing genetic instructions for creating components of B cells and antibodies include:
- IGH: Immunoglobulin Heavy Chain (a major protein subunit of antibodies).
- IGK: Immunoglobulin Kappa Locus.
- IGL: Immunoglobulin Lambda Locus (responsible for Lambda creation).
Subunits of the TCR Complex:
- Piece 1: Antigen Binding Subunit: Located on the TCR.
- Piece 2: Invariant Signaling Chains: These relay signals when an antigen is successfully recognized. These include the heterodimers CD3YE and CD38E.
- A2 Zeta ( $\zeta$ ) Chain Homodimer: This component is critical for launching the intracellular signaling cascade.

Lymphocyte Development: This is the process where multipotent hematopoietic stem cells located in the bone marrow differentiate into functional T cells and B cells.
V(D)J Recombination: This is a unique genetic process used by developing B cells and T cells to shuffle and assemble unique antigen receptor genes.
- It involves the recombination of many V and J segments.
- It assembles the Variable (V), Diversity (D), and Joining (J) gene segments.
- This process results in a massive variety of T cell receptors and antibodies.
Complementarity Determining Region 3 (CDR3):
- CDR3 is generated during the assembly of V, D, and J segments.
- It produces amino acid sequences that are between $8$ and $20$ amino acids in length.
- It generates the specific T cell receptor binding sites.
- This process is vital for making physical contact with the antigen.

Biological Diversity: The V(D)J process generates immense biological diversity in the CDR3 sequences of both B cell and T cell receptors.
Experimental Assays: These are used to identify which specific CDR3 sequences are diverse.
Curated Databases: These systems store and annotate CDR3s that are specific to certain antigens.
- VDJdb: A database used to match cancer patients to specific CDR3s. Researchers can compare the CDR3s found in cancer patients against those in the VDJdb.
Anti-Viral CDR3s: These reflect T cell receptor CDR3 sequences that specifically recognize viral peptides. Their function is defined by evidence from the hypervariable loop of the TCR that contacts the antigen.

RNA Sequencing (RNAseq): A powerful molecular biology technique utilizing high-throughput sequencing. It captures a "snapshot" of a cell's transcriptome.
Transcriptome: The set of all RNA molecules within a cell. RNAseq measures which genes are actively being transcribed and determines the precise quantity of these RNA molecules.
RNAseq Files: Digital datasets generated during sequencing that capture active genes and transcripts.
Exome Files: Utilized by clinical geneticists and researchers to uncover the genetic causes of conditions or rare diseases.
Common File Formats:
1. FASTQ
2. BAM
3. VCF
4. BED
Bioinformatics Pipeline: To find mutant amino acids, these files must be processed through a pipeline to align sequences and identify variants.