Antigen Recognition by T Cell Receptors

What must happen for a T cell to recognize an antigen?

The foreign cell must be processed by APC and its peptides must be presented on the host cell surface in association with MHC

T cell receptor are very structurally similar to…

Arm of the antibody

2 classes of t cell receptors

alpha:beta T cells & gamma:delta T cells

Mechanism of diversity generation of TCRs is identical to…

diversity generation in BCRs

What is the mechanism by which diversity is generated in TCRs?

Somatic recombination & junctional diversity

The alpha chain on the TCR is very similar to the…

light chain of antibodies (V & J recombination)

The beta chain on the TCR is very similar to the…

heavy chain of antibodies (VDJ recombination)

The gamma chain on the TCR is very similar to the…

light chain of antibodies (V & J recombination)

The delta chain on the TCR is very similar to the…

heavy chain of antibodies (VDJ recombination)

Which class of TCR makes up the majority of TCRs in the body for most domestic species?

alpha:beta TCRs

Which species have a more even distribution between the two classes of TCRs?

Young ruminants & chickens

Are BCRs membrane bound?

Yes

Are TCRs membrane bound?

Yes

Once the TCR recognizes the antigen, does it undergo any further changes?

No

Once the BCR recognizes the antigen, does it undergo any further changes?

Yes, can exhibit isotype switching & somatic hypermutations

Do BCRs undergo secretion following antigen recognition?

Yes

Do TCRs undergo secretion following antigen recognition?

No

What is the primary different between the mechanism of BCRs and TCRs?

TCRs act only as a sensor for antigens, then send signals to the T cell to cause target cell death, while BCRs undergo changes following antigen recognition to directly cause the target cell death

T cell receptor complex

-TCR -CD3 εδ -CD3 γε -CD3 ζζ

The T cell receptor is always…

presented in complex with CD3 receptors

T helper (TH)

Helps B cells, macrophages, & some other cells

Cytotoxic T cells (CTL)

Kill virus- and other intracellular pathogen-infected cells or tumor cells

Major co-receptors expressed on T cell membranes

CD4+ & CD8+

Which co-receptor functions as a T helper cell?

CD4

Which co-receptor functions as a cytotoxic T cell?

CD8

Double positive

A T cell that expresses both CD4 & CD8

Double negative

A T cell that has neither CD4 or CD8

Which species has a significant proportion of double positive T cells?

Swine

___________ can bind to CD4 and infect the CD4 T cells

Virus particles

A depletion of CD4 T cells can cause…

immunodeficiency & eventually death

What are key viruses that can cause depletion of CD4 T cells?

HIV & FIV

2 types of MHC molecules

MHC class I & MHC class II

What is the key structural difference between MHC classes?

MHC I has one polypeptide bound to the cell membrane & a microglobulin and MHC II has 2 polypeptides bound to the cell membrane

CD4 T cells recognize antigens bound to…

MHC II

CD8 T cells recognize antigens bound to…

MHC I

CD8 binds the ____ domain of MHC I

alpha 3

CD4 binds the ____ domain of MHC II

beta 2

How long are the MHC I peptide complexes?

8-10 AAs long

How long are the MHC II peptide complexes?

13-25 AAs long

Promiscuous (degenerate) binding

Principle that one MHC molecule can bind a variety of peptides

Motif

A common sequence in a peptide antigen that binds to an MHC molecule

Anchor residues

Amino acids common to many peptides that tether the peptide to structural features of the MHC molecule

MHC I are expressed by…

all nucleated cells

MHC II are expressed by…

-B cells -Macrophages -Dendritic cells -Epithelial cells of the thymus