Lecture 21 - Immune Regulation Flashcards
What can ‘immune regulation’ refer to?
For the most part, refers to immune suppression
Can also refer to:
• Th1 vs. Th2 balance
• Cellular vs. humoral response
• Immunity vs. tolerance
Which cell types can Tregs suppress?
Helper T cells CTLs B cells DCs Macrophages et al
Why do we need immune suppression?
• To protect against autoimmunity (i.e. protection of self tissues against the immune system)
- Treg defects can cause autoimmunity
- Treg therapy can mitigate autoimmunity
- Regulation of responses against commensal bacteria
- Achieve a balance in immune responses to pathogens: enough to clear the pathogen, but not causing tissue damage
Describe the role of Tregs in tolerance
Treg Suppression is an important component of peripheral tolerance
Central tolerance is not completely effective at eliminating autoreactive cells, so peripheral tolerance is also required
Which important markers do Tregs express?
CD4+
FoxP3+
CD25+ CTLA4 GITR CD62L(hi) Folate receptor 4 CD127(lo)
What are some other suppressor T cells?
- Tr1 (secrete TGFβ in response to IL-10)
- Th3 (TGFβ and IL-10)
- NKT cells
These cells play a far lesser role than CD4+FoxP3+ Tregs
What is XLAAD?
Why was it significant?
An autoimmune disease in which FoxP3 was mutated, which is vital for development of Tregs
Showed the clinical manifestation of absence of Tregs, as well as the importance of them
XLAAD: X-linked autoimmune disease • One of the few autoimmune diseases that is inherited in a Mendelian fashion • Mutation in FoxP3, Tregs were absent • Clinical characteristics, multiple organ autoimmunity: - Failure to thrive - Eczema - Food allergy - Autoimmune cytopaenia etc.
Scurfy mouse:
• Naturally occurring mouse FoxP3 mutant
• Lack Tregs (epiphenomenon, or causing the disease?)
• Same clinical phenotype as XLAAD (i.e. multi organ autoimmune phenomena)
• Same phenotype as FoxP3 KO mouse
• Transfer of Tregs resolved the disease in these mice
This confirms that it was the absence of Tregs that caused the disease
What is the function of CD25?
α-chain of the IL-2 receptor
When it is present in the IL-2R, the receptor has high affinity for IL-2
What is the function of FoxP3?
Transcription factor, essential for expression of the genes in Tregs
• FoxP3 can complex with NFAT to bind to DNA (thus influencing transcription)
FoxP3 is part of a large transcriptional complex; other genes include:
• In general, genes involved with gene transcription:
• Transcription regulation
• RNA processing
• Chromatin modification
etc.
Over 700 targets of FoxP3 have been identified
Effect of FoxP3 can be:
• Up-regulation of genes
• Down-regulation of genes
How can FoxP3 expression in Tregs be visualised?
FoxP3 is a transcription factor, thus only present in the nucleus, not on the cell surface
→ difficultly with flow cytometry
- GFP-FoxP3 chimaeric protein
• GFP gene inserted at N terminus of FoxP3 gene
• Chimaeric protein expressed in Tregs
• However, function of FoxP3 was interrupted, leading to increase in autoimmune phenomena
• Allowed further analysis with flow cytometry to show that the vast majority of FoxP3+ cells were CD4+ (not CD8+ or DP or DN)
- Bicistronic mRNA
• GFP gene inserted downstream from FoxP3, but has IRES which results in transcription of GFP whenever FoxP3 is transcribed.
• The protein was not, however, chimaeric
• Allowed further study of Tregs, without interrupting the function of FoxP3
List some genes that are regulated by FoxP3
Upregulated:
• CD25
• CTLA4
• PRDM1, Crem etc.
Down-regulated:
• Zap70, Jak2, IL-2, Myc
• Genes that are involved in T cell signalling and proliferation
• Tregs are continuously seeing self-Ag, and thus should not be activated against these
• Down regulation of these genes adapts the Tregs to chronic stimulation (preventing apoptosis)
NB chronically stimulated cells are prone to dying
Outline the various ways that Tregs suppress
Summary
Suppression of effector T cells': • Proliferation • Function • Migration • Killing of effector cells
Influence DCs
• Killing; thus reducing T cell activation
• Prevent maturation
Promote expansion of Treg cell repertoire:
• “Infectious tolerance”
• Mediated by TGF-β
Which molecules do Tregs express?
Cytokines: • IL-10 • TGF-β • 'IL-2 sink' • IFN-γ
Cell surface markers:
• CTLA4
• ICOS
Cytotoxins:
• Perforin
• Granzyme B
• HO-1
Other:
• cAMP
• IDO
Describe how Tregs become functional
Tregs must be activated by antigen to mediate suppression
Experiment:
• Two populations of Tregs:
- Recognise HA
- Don’t recognise HA
• T cells are stimulated by HA in presence of either of the two groups of Tregs
• T cells + Tregs that recognise HA have suppressed proliferation
• T cells + Tregs that don’t recognise HA proliferate
Describe Treg influence on DCs
– Suppressive cytokines –
IL-10:
• Inhibits DC and macrophage Ag presentation
– CTLA4 –
- CTLA4 on surface of Treg interacts with CD80/86 on APCs
(more avidly than CD28 on T cells) - Results in a negative signal transmitted into the APC
- APCs then down-regulate co-stimulatory molecules (CD80/86)
- DCs are less able to stimulate T cells
