Assignment 8 Flashcards
when progenitor T cells migrate to the thymic cortex for development/differentiation/maturation, what the this event marked by?
sequential expression of cell surface proteins (receptor/ligands)
-expression of TCR that is defined by its exquisite specificity in recognizing antigenic peptide displayed as a complex with MHC molecules
what influences the naive mature B cells to differentiate in the thymic microenvironment?
IL-7
during its migration through the thymus, the thymocyte expresses unique TCR capable of recognizing antigenic fragments ONLY when?
the peptide fragments are displayed in association with either a class I or II MHC molecule
what results in the acquisition of a unique TCR to an array of T cells each expressing a different TCR?
somatic recombination
what are TCR composed of (types of TCR..)?
2 transmembrane polypeptide chains - either alpha/beta (90% of T cells in human body) or gamma/delta
what is the TCR expressed in association with?
5 invariant polypeptides collectively termed CD3
what is the role of CD3
link the antigen binding receptor to the T cell with signaling pathways
what regions are found on each chain of the T cell receptor?
variable and constant regions
T cell clones express TCR with the _____ constant regions, each T cell clones express TCR with a ____ variable region
same
unique
*why - variable regions - VDJ
TCRalpha variable region = VJ
TCRbeta variable region = VDJ
what is the T cell repertoire a result of
- multiple copies of germ line VDJ gene segments
- random selection and combo of VDJ gene segments
- junctional diversity generated by the addition/deletion of bases
- random assortment of TCRalpha and TCRbeta chains
during T cell differentiation in the thymic cortex, what molecules are expressed on the cells surface
CD2 Pre-TCR-CD3 complex CD4 CD8 TCR-CD3 complex
what is the first molecule to be expressed and what it is considered
CD2
expressed throughout cell life
“pan marker”
why does CD3 continue to be expressed throughout cell life?
because TCRs are only expressed on cell surface complexed WITH CD4
although pre-TCR is only transiently expressed, what critical role does it play?
that the pre-TCR complex can only form on the developing thymocyte if the TCR beta chain variable region has been successfully recombined in the process of somatic recombination
expression of pre-TCR CD3 indentified developing thymocytes that have potential to express what?
viable TCR
what role does signaling via the CD3 molecules of the pre-TCR complex play in subsequent differentiation of the thymocyte
- proliferation of that developing thymocyte
- expression of CD4 and CD8 (Double positive thymocyte) –then become CD4+ or CD8+, but not both! (single positive)
- delivery of signal that direct somatic recombination of the TCR alpha chain
define somatic recombination in terns of TCR variability
process by which unique variable regions are created in the thymus
what initiates the recombination process of TCR?
recombinases, nucleoprotin products of the RAG-1 and RAG-2 genes
what happens during differentiation of T cell
the DNA in the locus that encodes variable regions is cut and recombined to make an intact gene for the variable region in the alpha chain and beta chain = rearranged gene can be transcribed to mRNA the translated into an alpha chain and bet polypeptide chain
how many identical TCRs are be on any given cells
25,000 - identical!
clones that express TCR receptors represent what
t cell repertoire
somatic recombination of the TCR ___ variable region occurs before TCR____
beta
alpha
how is TCR diversity made possible?
because of somatic recombination of the VDJ segments that encode the variable regions
- multiple germline genes
- combinational association
- junctional diversity - from Tdt
- random selection of alpha/beta chains
what is the role of recombinase activating genes (RAG-1 and RAG-2)
recombination process requires these to be acitvated
what is the role of terminal deoxynucleotidyl transferase
incorporation of nucleotides at junctions is mediated by template independent DNA polymerase called Tdt
explain allelic exclusion. What does it prevent?
-Inhibition of somatic recombination on the other allele
-Prevents two different α chains being expressed with a β chain which would result in two different
antigenic specificities—> bad
define T cell repertoire. What do the clines expressing TCR represent?
- There can be as many as 25,000 identical TCRs on any given cell
- Clones expressing these receptors represent the t-cell repertoire
Describe the steps of T-cell developmental pathway from stem cell (bone marrow) to mature CD4+, CD8+ T cell (thymus)
- Self renewing stem cell
- Pluripotent stem cell
- Lymphoid progenitor
- T-cell progenitor
- Thymus migration
- Growth only occurs in the thymus with presence of cytokine IL-7
define positive selection (cortex)
- When the t-cell receptor recognizes an MHC
- Proliferation occurs – appropriate interactive avidity
Define Negative selection for double positive (cortex/medulla)
- the t-cell receptor recognizes an MHC but the avidity is too high
- apoptosis – very high interactive avidity
explain death by neglect (cortex)
- When the t-cell receptor does not recognize an MHC
- Death – negligible interactive ability
what is the final phase a T cell has to go through?
negative selection for single positive thymocytes (medulla)
what occurs during negative selection for single positive thymocytes?
high interactive avidity with bone marrow derived APC
CD4 pr CD8 interaction with MHC occurs
explain lineage selection
-refers to the transition of a double positive thymocyte (CD4+,CD8+) to a single positive thymocyte expressing either CD4 or CD8 but not both
-silencing of either the CD4 or CD8 gene occurs in each individual T cell
-t-cell lineages are classified by the expression of CD4 or CD8 – reflective of subsequent
biological activity (cytokine secretion or killing)
define double positive thymocytes. when does it occur?
cell surface expression of both CD4 and CD8 on the developing thymocyte – occurs early in phase II
define single positive thymocytes
-transition of a double positive thymocyte (CD4+,CD8+) to a single positive thymocyte
expressing either CD4 or CD8 but not both
explain the term interactive avidity and list four factors on which interactive avidity depends
- Interactive avidity determines fate of double positive T-cell based on
- # ofTCRonT-cell
- # of MHC molecules on APC
- amount of intrinsic affinity of TCR for MHC peptide
- # adhesion molecules
- # antagonistic molecules
- There are approximately 25,000 t-cell receptors on a cell
how do naive T cells get into lymph?
HEV
glycocalyx of endothelial cells trap T cells
describe the t cell movement through the lipid bilayer by extravasation
• l-selectin - grabs t-cell
• integrins (LFA-1) – makes t-cell sticky
• matrix metalloproteinases – degrade collagen to make channels in basement
membrane for t-cells
• transendothelial migration of lymph into tissue
What happens in the absence of antigen recognition ?
lymphocytes exit via efferent lymphatics and enter main lymphatic circulation
when T cells express TCR that are autoreactive, how are they killed?
die passively in the thymus
tolerance induction and screening/selection can only occur after what?
thymocyte expresses the TCR-CD3 complex
screening occurs on _____ cells
double positive - CD8+ and CD4+
what does interactive avidity of double positive thymocytes and their TCR with self-antigen/MHC complex depend on?
- intrinsic affinity of the TCR for self antigen/MHC complex
- density of TCR
- density of self antigen/MHC complexes on thymic epithelium
- density of antagonist peptide complexes
what occurs during negative selection
if recognition exceeds a pre-determined threshold
thymocytes are clonally eliminated or inactivated
consequence of high avidity interactions of TCR for self-antigen/self MHC
occurring in the absence of CD4/CD8 interaction with appropriate MHC
where does negative selection take place?
thymic cortex
where does a more finely tuned process of negative selection occur
thymic medulla
note that the process is acting on already positively selected single positive thymocytes
does occur with interaction of CD8 and CD4 with class I and II MHC
define death by neglect
insignificant avidity with thymic epithelial cells results in death by neglect
define positive selection
occurs when the avidity of recognition is intermediate (between the 2 extremes of death by neglect and negative selection)
death by neglect — positive sel — neg sel
where does the silencing of either the CD4 or the CD8 gene occur
in each individual T cell => classifies the T cell as CD4 or CD8!
the function of the T cell is determined at this stage!
in negative selection in the cortex, where are the APC dervied?
thymic epithelium
in negative selection in the medulla, where are the APCs derived?
bone marrow
the number of self proteins to which the developing T cell is exposed in the thymic medulla is much ____ as a result in the expression of what?
greater
AIRE gene
what is the role of the AIRE gene?
plays critical role in the promiscuous/ectopic expression of tissue-restricted proteins in the thymic epithelial cells (TECs), particualrly medullary TECs
describe APECED syndrome (autoimmune polygrandular syndrome type 1)
AIRE gene is mutated
mucocutaneous candidiasis
hypoparathyroidism
autoimmune adrenal insufficiency
primary effects endocrine system
where do naive T cells undergo antigen-induced differentiation?
in secondary lymphoid tissue following intiial encounter with antigenic peptide-MHC — then their roles are defined
what does lymphocyte secretion of matrix metalloproteinases do?
degrade collagen to generate channels in the subendothelial basement membrane
what are nTregs?
naturally produced by the normal thymus as a functionally distinct subpopulation of T cells that leave the thymus where they play an active role in est. and maintaining self tolerance - peripheral tolerance
they are regulatory cells, not effector cells
what make up the nTregs? What is their role?
CD4+(phenotypical)
CD25+ (phenotypical)
FOXP3+ (transcription factor necessary for development and function of nTregs)
players in controlling self reactive T cells
what do humans with mutations in FOXP3 develop?
multi-organ autoimmune inflammatory disease (IPEX) - immune dysregulation, polyendocrinopahty, enteropathy, X linked syndrome)
= water diarrhea, eczema, type 1 DM
TX with immunosuppresents
depletion of nTregs leads to what
tumor rejection
