L7: lymphocyte activation Flashcards
What are lymphocytes?
Lymphocytes drive adaptive immune system: specifically recognise foreign molecules. Cells of innate have general ways but lymphocytes can specifically detect antigens, leading to expansion of antigen specific b and t cells (adaptations). This helps clear the infection. More effective response when pathogen invades again (memory) so more efficient.
clonal selection?
Key Elements of Clonal Selection
Unique Receptor Generation
Each B and T cell creates unique antigen receptor through cutting and pasting DNA. random combination of gene segments forms unique antigen binding domains (downside: can recognise self antigens)
No two cells have identical receptors
Selection Process
Cells with self-reactive receptors are deleted
Mature naive lymphocytes form primary repertoire
Activation Mechanism
Specific antigen binding triggers:
Lymphocyte activation
Proliferation
Only cells with specific antigen receptors respond
Clonal Expansion
From millions of lymphocytes
Only few recognize specific antigen
those cells multiply
Outcome
B cells → Plasma cells
T cells → CD4+ or CD8+ effector cells
Adaptive response phases?
- Antigen Recognition
B and T cells encounter specific antigen
Triggers initial immune response
Unique receptor recognizes antigen - Clonal Expansion
Lymphocytes rapidly divide
Exponential increase in antigen-specific cells
After a number of days they start differentiating. - antibodies seen at around 5 days.
- Peaks around 10 days
Two immunity types:
Humoral (antibody-mediated)
Cell-mediated - Antigen Elimination
Antibodies and effector T cells attack
Remove pathogen/infected cells
Neutralize threat - Contraction (Homeostasis)
14-21 days after initial response
Majority of activated lymphocytes undergo apoptosis
Prevents excessive immune response - Memory Formation
Small population of cells survive
Become memory B and T cells
Ensure faster response in future encounters
Time course of humoural immune response
4-5 days for antigen specific cells to be selected, proliferate and differentiate to produce antibodies. Most b cells die after response but memory remain for rapid reactivation next time (secondary immune response)
B-cell receptor (BCR) signalling mechanism/complex?
Immunoglobulin (Antibody) Limitation: Cannot signal by itself (Lacks signaling domains) Needs additional components for activation: CD79a and CD79b (Accessory proteins to the B cell receptor)
Contain ITAM (Immunoreceptor Tyrosine-based Activation Motif) in their cytoplasmic domains
ITAM contains tyrosine residues that can be phosphorylated for downstream signalling.
Cognate (specific) antigen binds to immunoglobulin causing signalling.
CD79a and b help:
Bring BCR to cell surface
Regulate BCR movement
Key Concept
The immunoglobulin needs CD79a/b to:
Signal
Move to cell surface
Initiate immune response
TCR complex?
T Cell Receptor (TCR) Complex Overview
Structure
Composition:
TCR consists of two main parts:
Alpha (α) and Beta (β) subunits
A small population with alternative subunit combinations
Signaling Limitations:
TCR cannot signal independently
Lacks ITAM (Immunoreceptor Tyrosine-based Activation Motif) motifs on its own
Requires additional complex partners for signal transduction
Relies on co-complexing with:
CD3 protein
CD247 protein (also known as CD3ζ)
Signal Initiation:
Antigen binding part alone does not trigger signaling
CD3 complex contains multiple ITAM motifs
Beta chains contribute to increased signaling complexity compared to B cell receptors (BCR)
Molecular Binding:
Complexes held together by electrostatic forces
Transmembrane domains typically involve charged molecules
Charge interactions depend on the lipid environment
Key Insights
The TCR is more complex in its signaling mechanism compared to the B cell receptor
Multiple proteins work together to enable effective immune cell signaling
Molecular charges and interactions are crucial for receptor function
Receptor signalling mechanism?
Inactive Receptor Characteristics:
No association with downstream:
Kinases
Signaling receptors
Molecular components are in a “resting” configuration
Activation Mechanism
Ligand Binding Trigger:
Ligand attachment causes conformational changes
Structural rearrangement of receptor molecules
Enables critical signaling processes
Molecular Signaling Cascade
Phosphorylation Process:
Kinases activate by phosphorylating ITAM motifs
(Immunoreceptor Tyrosine-based Activation Motif)
Phosphorylation creates binding sites for downstream molecules
Adaptor Protein Recruitment
Scaffold Protein Mechanism:
SH2 Domains (Src Homology 2 domains) act as molecular bridges
Bind and recruit secondary messengers:
Kinases
Enzymes
G protein exchange factors
Create complex signaling networks
BCR signalling initiation
in resting b cells: BCR weakly associates with three Src family kinases:
BLK (B lymphocyte kinase)
FYN
LYN
Src Kinases: Sarcoma (src) pro-oncogene family members
Antigen-Induced Activation:
BCR Crosslinking by antigen triggers:
Activation of BLK, FYN, and/or LYN
Kinase activation cascade begins
Src family kinases phosphorylate tyrosine residues of CD79A and CD79B ITAMs. phosphorylation causes:
Syk (Spleen Tyrosine Kinase) recruitment via:
Two SH2 (Src Homology 2) domains
SH2 domains bind to phosphorylated ITAMs
Trans-phosphorylation occurs due to:
Clustered BCR proximity
Neighboring cells in close contact
Syk kinases phosphorylate neighboring Syk molecules
BCR signalling proppogation?
Syk phosphorylates multiple sites on the scaffold adaptor protein BLNK.
BLNK phosphorylation recruits and activates BTK, Phospholipase C-γ and G protein exchange factors (GEFs).
Phospholipase c-y (gamma) :
PLC-γ: key signalling molecule recruited following BCR/TCR activation.
Breaks down PIP2 in the cell membrane to secondary messengers: IP3 and DAG.
IP3 causes the release of Ca2+ from the ER, raising intracellular [Ca2+]
Activates Nuclear Factor of activated T cells (NFAT)
Intracellular [Ca2+] and DAG activate the NF-κB pathway via Protein Kinase C (PKC)
NF-kB activation?
DAG: Activates PKC (phospholipase C)
Activated PKC phosphorylates CARMA1
CARMA1 recruits BCL10 and MALT1 (CBM complex).
CBM complex recruits and activates IκB Kinase (a.k.a. IKK/NEMO).
NEMO phosphorylates IκB, which causes ubiquitination of IκB by ubiquitin ligases.
Ubiquitinated IκB is degraded by the proteasome, releasing NF-κB for translocation to the nucleus.
NFAT activation?
Activated by ca release from the er. Increased ca binds to calmodulin which binds and activates calcineurin (a phosphatase). When nfat is in cytoplasm it is phosphorylated and this phosphorylation is what keeps it in the cytoplasm. So once calcineurin is activated it removes phosphate groups of nfat (dephosphorylation) allowing nfat to move to nucleus where it can induce transcription.
propogation of bcr signalling (2)
3rd pathway: activates mapk cascade to induce a 3rd trasncription factor :AP-1 where it along with nf-kb and nfat will induce tf to activate cells, make survival better and make them proliferate.
Syk phosphorylates multiple sites on the scaffold adaptor protein BLNK.
BLNK phosphorylation recruits and activates BTK, G protein exchange factors (GEFs) and PLC-γ.
PLC-γ cleaves PIP2 into DAG and IP3.
IP3 induces release of Ca2+ to activate NFAT.
DAG and Ca2+ activate the NF-κB pathway.
GEFs activate MAPK cascade to induce AP
Together NF-κB, NFAT and AP-1 induce gene transcription that leads to activation, survival and proliferation.
Initiation of tcr signalling?
TCR signaling requires recognition of the MHC by the co-receptors CD4 (MHCII)
or CD8 (MHCI). They stabilize the TCR-MHC interaction and recruit Lck (a kinase) to the TCR.
Lck phosphorylates ITAMs on zeta chain (CD247) and CD3.
Phosphorylated ITAMs recruit ZAP-70 (Zeta-associated protein-70) tyrosine kinase, which is activated by Lck.
Propogation of tcr signalling?
This all leads to the recruitment of a scaffold protein called: LIT recruits SLP- 76 activates PLC-gamma will cleave phosphoinositol biphosphate to DAG+ip3
Get ca release and activation of nfat and also mapk cascade activation. Together they reg activation, survival and proliferation of t cells.
of activated T cells) and SLP-76.
- SLP-76 activates PLC-γ.
-PLC-γ cleaves PIP2 to DAG and IP3.
-IP3 raises Ca2+ which activates NFAT.
-DAG activates NF-κB via PKC.
-DAG activates GEFs and MAPK cascades.
-NF-κB, NFAT and AP-1 activate gene transcription, leading to activation, survival and proliferation.
b and t cells before activation?
When activated more of blast cells? Enter cell cycle, bigger, condensation of chromatin to facilitate more gene transcription, expand rough and smooth er to produce more proteins. Divide up to 4 times a day which drives clonal expansion. B cells become antibody secreting cells. Effector t cell has condensed chromatin, produce cytokines: cd4 or cd8.memory cells look like small resting lymphocytes.
