Immuno 12-Regulatory T Cells & Tolerance

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Tolerance

Back 1

-lack of adaptive immune response to Ag despite exposure of lymphocytes to that Ag
-important in discrimination of non-self from self, harmless from dangerous
-ideal: focus response on dangerous non-self
-failure: autoimmunity, allergy, transplant rejection

Front 2

Autoimmunity: distinguishing native tissues from foreign pathogen

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-Innate Sys: inherent in the receptors directed at PAMPs

-Adaptive sys: not inherent in receptors, able to bind anything (protein, carb, lipid)
*need safeguard to ensure non-reactivity with native molecules--to maintain tolerance

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Mechanisms Regulating Adaptive response

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-Central (selection during development)
-B Cells: BM
-T cells: Thymus
Peripheral
-responder cell intrinsic"recessive"
Ignorance, AICD, anergy

-responder cell extrinsic "dominant"
Regulatory T cells

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Central Tolerance

Back 4

-positive and negative selection in Thymus

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Affinity

Back 5

-binding strength of one receptor to one ligand

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Avidity

Back 6

-aggregate binding strength of multiple receptors on one surface to ligands on another
-TCR avidity contributes to TCR triggering:
1. high avidity: triggered at low Ag conc
2. moderate avidity: requires higher Ag concentration
3. Low Avidity: largely "ignorant" (not activated)

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Thymocyte avidity "molding"

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-T cells mature from Thymic precursors
-specifities are "filtered" by positive and negative selection on self antigens
-->anti-self: TCR avidities rane from low to moderate
-->anti-foreign: TCR avidities are "unfiltered" (range from undetectable to high)

**think about slide with to overlappying graphs showing narrow range for self ag and wide range for foreign Ag

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Thymic Self-Representation

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-AIRE: Autoimmune Receptor
->Gene highly expressed in thymic epithelium
->encodes transcriptional activator
->induces expression of "ectopic" self proteins (Parathyroid, retina, ovary)

Front 9

APS

Back 9

Autoimmune Polyendocrine Syndrome
-clinical condition from AIRE mutation
-autoimmune attack on multiple endocrine structure (parathyroid, thyroid, adrenals, B-islets, gonads), vitiligo, alopecia

Front 10

Peripheral Tolerance

Back 10

-mechanisms intrinsic to the responder T cell
-Successful T cell activation requires 2 signals!
1)TCR crosslinking (Ag-specific)
2)Co-stim provided by APC, induced to mature by "danger" signals

*Signal 1 without signal 2 can lead to anergy in triggered T cell

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T cell Anergy

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-T cell unresponsiveness despite TCR engagement in the presence of co-stimulation

Proximal Events
-Reduced tyr phosphorylation
-Reduced Ca++ influx
Distal Events:
-Lack of NF-κB activation
-Lack of NFAT activation

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Ignorance

Back 12

-mechanisms of Peripheral Tolerance Intrinsic to the Responder T cell
-Low TCR avidity for Ag/MHC and/or low antigen abundance result in failure to produce “signal 1”
• Example: anatomically “privileged” Ags: CNS, eye (sympathetic ophthalmia)

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Clonal Deletion

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-Peripheral tolerance-mechanism intrinsic to T cell
-High antigen density
-persistent TCR triggering in the absence of costimulation

Front 14

Activation-Induced Cell Death (AICD)

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• Elimination of clonally expanded effector T cells
-Terminal phase of an antigen response--> homeostasis
-With persistent or repeated TCR triggering--> tolerance
• Cell death is apoptotic - largely fas mediated
-fas is upregulated following T cell activation
-fas- or fasL deficiency→lymphoproliferation/autoimmunity
-anti-fas or anti-fasL antibodies inhibit AICD (animal model)
• Inappropriate AICD has been proposed to account for
pathological T cell loss in HIV, EBV, Varicella

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CTLA-4

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-natural brake on T cell response
-expressed on activated T cells
-binds CD80/86 with higher avidity than CD28
-delivers an inhibitory signal to responder T cell
-critical to maintain overall T cell number

Front 16

Abatacept

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CTLA-4-Ig
-therapy in RA
-reduce T cell count

Front 17

peripheral tolerance: extrinsic to responder T cells

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Regulatory T cells
-concept of "dominant tolerance" has existed for 40 yrs
-new research in last decade has revealed many Tregs
-all Tregs defined FUNCTIONALLY: when mixed with responder T cells, Tregs inhibit the proliferative response to ag

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Regulatory T cells

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• CD4+/CD25+ Treg’s
• “Natural” - thymus-derived
• “Adaptive” - conversion of naive CD4’s in periphery
• Other CD4+
• TH3
• Tr1
• CD8+ - Avidity model of peripheral Treg’s
*CD25 = IL-2Rα chain, required for the high-affinity IL-2R

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CD4+/CD25+ History

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1969 - Early thymectomy (day 3-4) --> autoimmune organ attack
-Prevented if mice received splenic CD4+ T cells from a normal
adult
1995 - Identification by Sakaguchi
• Transfer T cells into athymic mice -->normal immune function
• Transfer CD25-depleted T cells into athymics-->autoimmunity
-thyroiditis, insulitis, gastritis, adrenalitis, arthritis, etc.
-short time window to rescue by transfer of CD25+ fraction

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CD4+/CD25- Tregs

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- 10% of circulating mouse CD4+ cells bear CD25 (<1% of
CD8’s)
-Upon triggering with CD3/CD28 crosslinking in vitro:
• no proliferation
• no secretion of IL-2, IL-4, or IFN-γ
• contact-dependent inhibition of local “responder” T cells
-Capable of self-renewal in vivo
-Constitutively express CTLA-4
-Dependent on IL-2 for maintenance of CD25 expression
and regulatory phenotype
-IL-2-deficiency and CD25-deficiency are both associated
with autoimmunity

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"Natural" Tregs

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-arise in Thymus
-proposed "third role" of thymus (in addition to pos & neg selection)
-Alternative to cell death for thymocytes with significant avidity for self
-Thymocyte TCR triggering is required for Treg differentiation
-CD28 co-stim required for normal Treg levels
-3rd, Treg-specific signal postulated
-Treg induction presumably driven by self recognition (Treg repertoire unk)

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FoxP3

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-Transcription factor implicated in autoimmunity
-discovered via ID of common gene in 2 naturally occuring diseases (Human-IPEX, Mouse-Scurfy)
-TF that binds DNA using a protein domain termed "forkhead box"
-Highly conserved b/w human, mouse and rat
-FoxP3-deficient subjects have no CD4+/CD25+ T cells

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CD4+/CD25+ "lineage marker"?

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-FoxP3
-possible master controller for Treg program
-FoxP3 represses TCR-triggered IL-2 transcription, induces CTLA-4 and CD25 transcription
-Foxp3+ Treg function is stable (transferable in mice)
-Mouse: FoxP3+T cells are >90% CD4+/CD25+ and functionally suppressive

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"Adaptive" T regs

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-arise in the periphery
-Naive CD4+ T cells-->CD4+CD25+FoxP3+ under following circumstances:
1. TCR x-linking in presence of TGF-b (in vitro)
2. slow infusion low dose soluble antigen (in vivo)

-Function
->Mouse: potent suppressors of responder T cells (indistinguishable from natural T regs)
->humans: despite FoxP3 expression, suppression is LESS consistently demonstrated (role unclear)

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Role of TGF-B and Tregs

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-TGF-b converts naive CD4+/CD25- T cells to CD4+/CD25+ T regs

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Proposed Mechanisms of CD4+/CD25+ Treg suppression

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A) CTLA-4 contact: "reverse-signaling" through CD80/86
B) Secreted or surface IL-10 and TGF-b
C)"Soaking Up" local IL-2
D)Killing via perforin/granzyme

-not clear if TCR triggering is required to activate suppressive function

-suppression can be over-riden by strong stimulus

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Other CD4+ Regulatory T cells

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1. Th3: TGF-b secreting CD4+ T regs
2. Tr1: stimulation of CD4+ T cells with Ag in vitro in presence of IL-10 yields a regulatory T cell that secretes predominantly IL-10, and some TGF-b
**simply be aware that other CD4+ regulatory T cells have been
described, and appear to function via secretion of TGF-β and/or IL-10