Comparative Danger Sensing1by Chris Forden Hypothesis: Individual immune cells correlate danger signals to antigens ● Proposed comparative danger sensing

Comparative Danger Sensing 1by Chris Forden

Hypothesis: Individual immune cells correlate danger signals to antigens

● Proposed comparative danger sensing● Existing circumstantial evidence● 3-party assay to test hypothesis● Tumors probably bifurcate by DS vs. TAA● Precedents in other signal processing systems● Further reading in appendices


Simple comparative danger sensing scheme

● Consider a primitive immune system of only Tc – no APCs

● Each Tc can receive exactly 3 signals all on cells

Danger (Fas (receptor), MICA, other ligands of NKG2D)

Cognate Ag MHC validity

● (presumably the presence of the endogenous, antagonist pMHC that positively selected the T cell in the thymus)

● Each Tc sequentially scans many tissue cells (for demo purposes, shown as few cells)


APC-less Tc scenario

Viral peptide

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Virus Healthy-self protein/peptide

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Danger Signal

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Scenario 1: Cognate peptide is viral

Viral peptide

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Danger Signal

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Scenario 2: Cognate peptide is healthy self

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Serendipitous evidence from published experiments

● Bystander cytotoxicity assays After Tc kill cognate targets, Tc kill bystanders with FasL.

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Cold, cognate targets

51Cr Labelled, non-cognate “bystander” or “reference” cells

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Theoretical objections

● Some organs such as the liver, constitutively display Fas.

● Why doesn't the presence of other cell types (example blood vessel lining) without Fas trigger toxicity against the liver?

Do Tc have receptors that can distinguish tissue types?

Are cells from interspersed organs (vascular, neurons, membranes, immune, etc.) to few to serve as effective reference cells?


Easy yes/no test of hypothesis

Similar to published, well-known 3-party assay


Similar to these previously performed tests

Standard, “2-party” cytotoxicity assay!

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51Cr Labelled, cognate targets


Similar to these previously performed tests

3-party “bystander” cytotoxicity experiment!

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Cold, cognate targets

51Cr Labelled, non-cognate “bystander” or “reference” cells

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3-party “reference-cell” experiment, which labels the cognate targets instead of high- and low-danger “bystanders.”

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Cold, non-cognate “bystander” or “reference” cells



Quantitatively greater amounts of Danger Signals on reference targets

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Cold, non-cognate “bystander” or “reference” cells

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Bifurcation of tumor cell populations

● Eventually two sub-populations emerge: Danger signal+, TAA- Danger signal-, TAA+

● Negative correlation between DS and TAA causes tolerance


Cytotoxicity synergy between danger signals and cognate Ag

Costimulation of CD8αβ T cells byNKG2D via engagement by MICinduced on virus-infected cells

by Veronika Groh, Rebecca Rhinehart*, Julie Randolph-Habecker*, Max S.Topp, Stanley R. Riddell and Thomas Spies

from

Targets

Blocking mAbs

march 2001 • volume 2 no 3 •

nature immunology


Bifurcation of tumor cell populations

● Cytotoxicity synergy between danger signals and cognate Ag

● Tumor cells have 3 ways to evade immune attack:

Reduce danger signals Reduce TAA display Both of the above

● Each of the 3 escape paths are followed by some mutant cells


Bifurcation of tumor cell populations:

danger sig++ vs. danger sig-

Broad tumor-associated expression and recognition by tumor-derived γδ T cells of MICA and MICB

from

Control Ig

MIC

by Veronika Groh, Rebecca Rhinehart*, HEATHER SECRIST‡,

STEFAN BAUER*§, KENNETH H. GRABSTEIN‡, and Thomas Spies

Proc. Natl. Acad. Sci. USAVol. 96, pp. 6879–6884, June 1999Immunology

Comparative Danger Sensing by Chris Forden

Bifurcation of tumor cell populations:

danger sig++ vs. danger sig-

Broad tumor-associated expression and recognition by tumor-derived γδ T cells of MICA and MICB

by Veronika Groh, Rebecca Rhinehart*, HEATHER SECRIST‡,

STEFAN BAUER*§, KENNETH H. GRABSTEIN‡, and Thomas Spies

from:

Proc. Natl. Acad. Sci. USAVol. 96, pp. 6879–6884, June 1999Immunology

Weakly bifurcated sub-populations?

Strongly bifurcated sub-populations


What if tumor cell populations split into

danger sig+, TAA- vs. danger sig-, TAA+ ?

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TSA peptide

TSA molecular chain

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Total tolerance eventually results


Bifurcated tumor populations may depend on growth factor inter-transmission

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Growth factor Growth factor receptorTSA molecular chain


The low-danger sub-population can express any protein without attack from the immune system.

Bifurcated tumor populations may depend on growth factor inter-transmission

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Growth factor Growth factor receptorTSA molecular chain

The TAA- sub-population is constrained against expressing any detectable TAA to avoid attack from the immune system, so it may require external stimulation to thrive, while it protects the TAA+ cells.


Precedents from other disciplines

● Neuroscience A single dendritic branch of an amacrine cell in the

eye, can distinguish between stripes moving radially inward from stripes moving radially outward.

Such computation is probably as complex as the proposed correlation function for Tc

AEuler T, Detwiler PB, Denk W. 2002. Directionally selective calcium signals in dendrites of starburst amacrine cells.Nature. Aug 22;418(6900):845-52.



● Neuroscience Comparisons are common signal processing

techniques in many levels of neural processing


Precedents from neuroscience



● Electronic signal processing

Electrical differential amplifier

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RR


● Further reading in appendices

● Comparative Tc model predicts DC● Historical context: Matzinger's original danger

model● Evidence supporting hypothesis● CMV vs. comparative danger sensing● Mathematics of correlation


Predicting dendritic cells

● Vulnerability of a Tc-only immune system to well-adapted viruses



● Vulnerability of a Tc-only immune system to well-adapted viruses

● The immune system must correlate danger signals to Ags, even when separated in time.



DC

!Danger SignalViral peptideCMV MHCHealthy-self protein/peptide !Danger SignalCMV Healthy-self protein/peptide

Early viral peptide

Chaperone



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Now that danger signals have appeared, viral peptides no longer appear on MHC. However, the DC has already sequestered viral fragments and now matures in response to the danger signals...






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● A Tc-only immune system is vulnerable to well-adapted viruses

● The immune system must correlate danger signals to Ags, even when they are separated in time.

● DC can overcome temporal separation between DS and Ags, by monitoring cells over a long period of time.

● DC's signal is predicted to be very strong—enough to override DS-Ag separation seen by Tc.


Matzinger's 1994 danger model

● Dangerous viruses burst cells necrotically● DC capture pathogenic Ags and fragments of

dead cells Exposed hydrophobic moieties and other PAMPs

(e.g. LPS) indicate danger, trigger (e.g. via TLRs) costimulation

● Tolerance from long-term/high quant. contact● The immune system as a whole correlates DS

to Ags


Matzinger's danger model

● Did not consider budding viruses dangerous “There is also no need to make a response to a virus

that enters a cell, makes few copies of itself and then leaves without doing any damage. (We might even want to welcome such viruses for the genes that they could bring us.)”

● Matzinger P. Essay 1: The Danger Model in its historical context. Scand J Immunol 2001; 54: 4-9.


But... non-lyzing viruses are really bad dudes, too!

● More selection pressure to Escape senescence Increase cell division Ignore stop receptors Metastasize

● Much more likely to integrate into host DNA (R. T-W-Fiennes. 1982. Infectious Cancers of Animals and Man)

● Uniquely trigger Th1 response (Kagi D, Seiler P, Pavlovic J, Ledermann B,

Burki K, Zinkernagel RM, Hengartner H. The roles of perforin- and Fas-dependent cytotoxicity in protection against cytopathic and noncytopathic viruses. Eur J Immunol. 1995 Dec;25(12):3256-62.)



● Bystander cytotoxicity assays As Tc kill cognate targets, Tc also kill bystanders, using

FasL. “Bystanders” are probably reference cells;

● Only bystanders syngeneic with Tc are killed (1).● Conventional explanation: bystanders might be infected

but silenced. But why syngeneic requirement?● Syngeny would be required of reference cells, to exclude

viral manipulation of reference.

(1) MARK J. SMYTH,* ERIKA KRASOVSKIS, AND RICKY W. JOHNSTONE. 1998. Fas Ligand-Mediated Lysis of Self Bystander Targets by Human Papillomavirus-Specific CD8+ Cytotoxic T Lymphocytes. JOURNAL OF VIROLOGY, p. 5948–5954 Vol. 72, No. 7


When can NKG2D ligands costimulate Tc? Constrasting cytotoxicity assays Without reference cells, no costim

●

Lauren I Richie Ehrlich, Kouetsu Ogasawara,* Jessica A. Hamerman,* Rayna Takaki, Alessandra Zingoni, James P. Allison,† and Lewis L. Lanier

● Engagement of NKG2D by Cognate Ligand or Antibody Alone Is Insufficient to Mediate Costimulation of Human and Mouse CD8 T Cells

● The Journal of Immunology, 2005, 174: 1922–1931

In viral infections in a monolayer of fibroblasts, MICA did costimulate Tc

●

Veronika Groh, Rebecca Rhinehart*, Julie Randolph-Habecker*, Max S.Topp, Stanley R. Riddell and Thomas Spies

● Costimulation of CD8αβ T cells by NKG2D via engagement by MIC induced on virus-infected cells

● march 2001 • volume 2 no 3 • nature immunology

Confounding differences: Ag, cytokine concentrations?



Fas+ cells killed when transplanted into Fas- animals●

JEFFREY C RATHMELL and CHRISTOPHER C GOODNOW, then at Stanford● The in vivo balance between B cell clonal expansion and elimination is regulated by CD95 both on B cells and in their micro-environment

● Immunology and Cell Biology (1998) 76, 387-394

But: Fas- animals have massive upreg of FasL

Anyway: upreg of FasL as necessary, is still a kind of comparision-based cytotoxicity? Furthermore partially Fas- animals still killed Fas+ grafts.

But: Killing occurred when targets were APC conjugated by CD4+ Tc, not typical target scenario



● Natural Killer cells might also compare danger signal presentation

NK cells conditioned 1-3 days with only targets displaying NKG2D ligands, lose cytotoxicity but not IFN-γ, and regain NKG2D expression.

● Jerome D. Coudert, Jacques Zimmer, Elena Tomasello, Marek Cebecauer, Marco Colonna, Eric Vivier, and Werner Held

● Altered NKG2D function in NK cells induced by chronic exposure to NKG2D ligand–expressing tumor cells

● BLOOD, 1 SEPTEMBER 2005 VOLUME 106, NUMBER 5



Vulnerability of a Tc-only immune system to well-adapted viruses

● CMV separates danger signals from antigen in time


march 2001 • volume 2 no 3 • nature immunology

24h

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Costimulation of CD8αβ T cells by NKG2D via engagement by MIC induced on virus-infected cells

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Anti-CMV Tc killed 2 of 4 infected cells. Partial success like that might benefit chronic pathogens like CMV. However, some well-adapted pathogens are more lethal than CMV, typically those that spread when the host is dead.



● CMV also spatially separates danger signals from antigen


march 2001 • volume 2 no 3 • nature immunology

Costimulation of CD8αβ T cells by NKG2D via engagement by MIC induced on virus-infected cells

from:

CMV IE-1 peptide

MIC (danger sig.)



● CMV also spatially separates danger signals from antigen.

● Does this interfere with correlation calculation taking place in the 2D calculation surface of an immune synapse?

● Is that why CMV benefits from being as big as a cell—to use its cytoskeleton control to thwart cross-products (danger x Ag) from being formed in the immune synapse, which otherwise might function as a 2D calculation manifold as well as transmission area?


Mathematics of Correlation

● Math definition of “sample correlation”

● For cell i: X

i is quantity of antigen displayed

Yi is quantity of danger signals displayed


Mathematics of Correlation

● Summations will probably be implemented as integrators with decay.

● Non-linearities (squares and roots) do not have to be exact.

● Subtraction is the math essence of comparison.● Cross products (xy signals) were proven to be

available in Tc by V.Groh et. al.'s proof of synergy between DS and Ag.


Precedents from neuroscience

● We discover danger signals at close to conscious level by doing pattern recognition.

● But first, our visual pre-processing neural nets compare object to background, adjust for overall bright/darkness of scene, adjusts shapes for shadows and leaves, etc., at a level below conscious analysis. Many of these steps are kinds of comparison operations.

● Perhaps that is one reason why we thought of pattern-recognition before comparison, as the fundamental signal-processing technique the immune system might use.

Documents

Comparative Danger Sensing1by Chris Forden Hypothesis: Individual immune cells correlate danger signals to antigens ● Proposed comparative danger sensing