Signaling through immune-system receptors Flashcards
Introduction
TCRs and BCRs have exquisite specificity for antigen but no intrinsic signaling capacity.
In the fully functional antigen receptor complex they are associated with invariant accessory proteins that initiate signaling when the receptors bind antigen.
Assembly with these accessory proteins is also essential for transport of the receptor to the cell surface.
TCRs and BCRs are able to respond to their specific antigen when it is present at very low levels.
This is especially important for T cells, as the antigen presenting cell will display many different peptides from both self and non-self proteins on its surface, and so the number of peptide-MHC complexes specific for a particular TCR is likely to be very low.
CD4 T cell is activated by 50 antigenic peptide:MHC complexes
B cells are activated when 20 BCR complexes are engaged.
TCRs
When cells were transfected with cDNAs encoding the TCRα and the TCRβ chains, the heterodimers where degraded and did not appear in the cell surface.
In the transmembrane region, there are residues that are positively charged, and through those charges, the receptor will interact with other molecules - the CD3.
CD3 complex
γ, δ, ξ, which are together by reciprocal charge interactions.
Through the ITAMs (immunoreceptor tyrosine-based activation motifs) fomains, signaling can occur.
CD3 proteins have an extracellular immunoglobulin-like domain
Assembly of CD3 with the α:β heterodimer
stabilizes the dimer during its production in the ER and allows the complex to be transported to the PM.
This is a T cell receptor complex, and it does not occur alone.
BCRs
The BCR associates to Igα and Igβ invariant protein chains are required for BCR transport to the PM and for the signaling function of the BCR.
They have an Ig-like domain.
Igα and Igβ that are bound by a disulfide bond, and they carry ITAMs, which will allow for signaling through the BCRs. The interactions here are hydrophobic.
The BCR is transported to the plasma membrane, and for that to happen it needs the Igα and Igβ, so there is always going to be a functional receptor, transported to the membrane through ITAMs.
More molecules for TCRs
The T cell has a T cell receptor complex, a CD4 and a CD8. On the antigen presenting cell, we have the MHC molecule with the peptide.
The CD4 in the T cell associates with Lck - a kinase.
Src family kinase
Constitutively associated with the cytoplasmic domains of CD4 and CD8
Phosphorylates ITAMs
It is brought near to the ITAMs when the co- receptor
binds TCR complex:peptide:MHC complex.
ZAP-70: ζ-chain-associated protein
Simultaneously engaged by two phosphorylated tyrosines
These molecules will produce a scaffold, that will recruit phospholipase C, and bring them where they need to be.
All of that will activate phospholipase gamma, which will break the PIP2 DAG and IP3. DAG diffuses to the plane of the membrane and recruits other signaling molecules to the membrane. DAG will bind PKC and RasGRP which will do signaling on their own. IP3 will have a receptor in the ER, which is a calcium channel, resulting in increased conc in the cytosol.
CRAC:
calcium release-activated calcium channels
ORAI1
Mutations in ORAI1 cause
Severe combined immunodeficiency (SCID).
All of these used to bring more calcium in the cytosol.
At this point the antigen signaling pathway splits into three different branches, each of which ends in the activation of a different transcription factor:
the calcium, PKC-theta and RasGRP.
All the different branches are important to produce transcription factors that will result in the activation of IL-2. IL-2 is essential for proliferation and differentiation of T cells into effector cells
Calcium transcription factor branch
Calcium - Ca2+ entry activates the transcription factor NFAT.
Calcineurin inhibitors
- Cyclosporin A
Binds to cyclophilin A, the resulting complex inhibits calcineurin.
- Tacrolimus
Binds FK-binding protein making a complex that inhibits calcineurin.
Both drugs prevent the formation of active NFAT
They are effective immunosuppressants widely used to prevent the rejection of organ transplants.
The scaffold of proteins that will phosphorylate and activate the transcription factor IL-2
RasGRP branch
Ras activation stimulates the mitogen-activated protein kinase (MAPK) relay and induces expression of the transcription factor.
RasGRP is a guanine-nucleotide exchange factor that specifically activates Ras
ERK: extracellular signal-related kinase
KSR: kinase suppressor of Ras
The scaffold of proteins which will phosphorylate will result in the activation of the AP1 transcription factor.
The transcription factor AP-1 is formed as a result of the Ras/MAPK signaling pathway . This can activate cyclin, which is important for cell cycling.
PKC branch
PKC activates the transcription factors NFκB and AP-1.
PKC-θ is restricted to T cells and muscle
CARMA1 is a large membrane-localized scaffold protein.
NEMO deficiency leads to X-linked hypohidrotic ectodermal dysplasia and immunodeficiency, characterized by developmental defects in ectodermal structures such as skin and teeth, as well as immunodeficiency.
B cell signalling
For the B cells the identity of the enzymes and other molecules in the signalling cascade is different, but the overall process is similar to that of the T cells.
The way it works is that the antigen will crosslink the B cell receptors, and the signal becomes activated, because that will allow an intracellular kinase to phosphorylate the ITAM domains. Once the these domains are phosphorylated, there will be a Syk molecule that will bind to the domains.
B-cell antigen receptor signaling is modulated by a co-receptor complex of at least three cell-surface molecules:
CD19, CD21, CD8, C3dg.
The way it works is that C3dg and the antigen will bind the CD21. Once the CD receptors are activated, there will be signalling, which has the PLC-gamma pathway, this will result in DAG, Ca2+ activation, G proteins, etc, which will result in NFkB, NFAT and AP-1 transcription factor that will produce IL-2, and there will be clonal expansion of the B cell.
In the signalling pathway there is a Btk
Bruton’s tyrosine kinase
- Deficiency prevents the development and functioning of B cells
- X-linked agammaglobulinemia
Virus are able to induce B-cell proliferation by the acquisition of ITAMS-containing receptors from their hosts.
- Epstein-Barr virus (EVB)
LMP2A
- Kaposi sarcoma herpes
They are able to cause malignant transformation and proliferation of the cells they infect
Antigen activation of both B and T cells
Upon antigen activation of the B cell and T cell, there will be a clonal expansion, so it is important that the clonal expansion doesn’t go unregulated, which can result in malignant transformation or proliferation. Overexpression of the immune cells can be detrimental to our bodies. So the proliferation needs to be regulated by co-receptors. In the T cells, they express the CD28 that engage with B7.1 on the antigen presenting cell, which is a necessary signal for the activation of the T cell, however B7 can also interact with CTLA-4 that is also present in the T cells, but engages in a different way, and there will be cross-linking of the receptors, and provides high avidity clustering, when it happens - the signalling terminates.
Another way to regulate the number of clones produced is through apoptosis, which could be activated in different ways.
One of the ways is through receptor and ligand. The receptor (Fas) carries a death domain. So the cell that contains the Fas will carry apoptosis, because it can activate other molecules like caspases to induce apoptosis.
The ligand for the Fas is the FasL(igand), which is expressed in another cell.
For the T cells: they carry both the Fas ligand and the Fas receptor, if there are too many cells present for clonal expansion, then it is very likely that the cells will engage the ligand and the receptor and they will die by apoptosis.
Summary
The B and T cells express their receptors, which are associate with other molecules that will form the T/B cell complexes. They are important because they will have the signalling capacity, because the T/B cells don’t have any on their own.
Upon antigen binding, it has to be cross-linking of the B cell receptors, and the co-receptors also need to be engaged, which are different for B and T cells.
Signalling will be activated, and they involve different proteins and phosphorylation of different molecules that will end up with activation of PLC that will branch the signalling in three ways and result in the activation of the transcription factors, that will be important for the production of IL-2.
Once IL-2 is produced there will be a clonal expansion, so we amplify the cells (B/T cells) that are able to engage with the antigen.
The clonal expansion is a process that is self-regulated. If there are too many cells they can die by apoptosis or termination of the signalling through other molecules that bind to the co-receptors on the antigen presenting cell.
