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and also that the second derivative is negative to ensure a maximum. Taking the derivative
of Eq. 4.8, we nd
d
µ
µ
(4.12) =−
du
Gn2cu
0
21n−
=0
e glucose level G is just the glucose level for the wild-type case (Eq. 4.5) because a single
mutant cell can’t aect glucose levels much. Plugging in 𝐺st from Eq. 4.5, and u = 1, we nd
the condition for an evolutionary stable strategy (ESS):
c 1
=
µ
Gn21−
00
is equation connects the killing rate c, normalized by the natural turnover of beta cells,
to the steepness of the killing function n. Interestingly, the higher the T cell cooperativity (steepness) parameter n, the lower the killing rate c that is required for ESS. Since the
cooperativity of immune recognition is high (~ 3–5), killing rate should be small, about
µ
10%–20% of the natural turnover rate parameter
G . us, these secret-agent T cells can
00
work subtly and precisely.
Exercise 4.2 An exponential threat detector in the immune system
is exercise explores a circuit that can respond to an exponentially rising threat, such as a
virus or bacterium, and ignore signals that do not change with time, such as antigens from
self-tissues (Sontag 2017), see Figure 4.18.
a. Consider an antigen u(t) presented by antigen-presenting cells to T cells. is acti-
vates eector T cells T(t) that perform the response functions, and also regulatory T
cells denoted R(t) that inhibit the eector T cells. Interpret the following equations
and their parameters:
dR
=−aubR
dt
dT u
=−c dT
dt R
b. Solve the steady state of R and T for a given steady-state level of u. Explain why this
steady state does not depend on u.
c. Explain why a step-function change in u from level
u to level 2u leads to a pulse of T
1
activation that returns to its original level (Figure 4.18)
t
d. Shows that if antigen rises exponentially,
u tue
()
α
=
, the eector T-cell activity
0
does not return to the original steady-state level. Show that the activation level above
their original steady state is proportional to the antigen exponential growth rate
α
parameter
.
Autoimmune Diseases as a Fragility of Mutant Surveillance ◾ 107
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u
u = antigen
R =
regulatory T cell
T = effector T-cell
R
T
Step antigen, u
Pulse of T-cell activity
u
R
T
time
Exponential antigen, u
Prolonged T-cell activation
u
R
T
exponential
threat
activation
~
u~e
R~e
time
t
t
FIGURE 4.18 An exponential threat detector circuit in the immune system. Antigen activates
cytotoxic T-cells, T, and also regulatory T-cells, R, that inhibit T. A step increase in antigen causes a
pulse of T that adapts back to baseline. Only an exponentially increasing antigen level, as in pathogens or early cancer, causes a prolonged rise in T without adaptation to baseline.
e. What happens when the antigen rises linearly with time u tα= ?
f. Explain why an exponential threat detector might be useful for the immune system.
Exercise 4.3: Viral dynamics
Consider a model for the concentrations of virus, u(t), eector T cells, T(t), and T
du
α
=−
dt
dR
cT u
()
0
=−uR
dt
dT u
= −T
dt kR+
regs
, R(t):
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a. Explain the equations and the parameters ,kc, and
α
.
0
b. Calculate the steady-state solution.
c. Numerically solve the equations for various values of
and 01u
= . Explain the meaning of these initial conditions.
()
d. Assume that when the virus concentration goes below a minimal dose, 0.01
is killed by the innate immune system. What is the maximal viral growth rate
which the virus is killed by the immune system? What happens if
α
. Use 1,1ck==, 000RT
0
==,
() ()
u = , it
0
α
is larger than
0
α
for
0
this value?
Exercise 4.4: eories for autoimmunity
a. Read about the hypothesis of “molecular mimicry” for autoimmune diseases.
b. Read about the “hygiene hypothesis” for autoimmune diseases.
c. Discuss their pros and cons, and compare to the “surveillance of hyper-secreting
mutant” theory discussed in this chapter (200 words).
Exercise 4.5: Bistability in a simple model for autoimmunity
Consider this simple model: e immune system attacks healthy tissue. is releases autoantigens, making the immune killing stronger, in a cooperative way, with Hill coecient
γ
n = 2. e variable is the amount of autoantigen a(t). e autoantigen is removed at rate
.
a. Explain the equation:
b. Draw a rate plot showing the xed points. Consider (graphically) dierent sce-
narios (dierent parameters) with dierent numbers of xed points. When is there
bistability?
c. Which scenario corresponds to an autoimmune disease? Which corresponds to no
autoimmune disease?
d. Suppose that individuals vary in their genetics in a way that aects the parameters of
the equation. Does an increase in the parameter c increase the risk for autoimmune
disease? Repeat for the parameters k and
da a
= c a
dt ka
n
nn
+
γ
.
−γ.
Autoimmune Diseases as a Fragility of Mutant Surveillance ◾ 109
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NOTE
1 It seems that the parathyroid has at least some autoimmune surveillance because autoim-
mune disease can be caused by certain drugs. In particular, drugs that enhance immune
response against cancer, by blocking immune checkpoints, have parathyroid autoimmunity
as a side eect in some patients. us, autoimmune surveillance in this organ may be tuned
to low levels that prohibit autoimmune disease under normal conditions.
REFERENCES
Culina, Slobodan, Ana Ines Lalanne, Georgia Afonso, Karen Cerosaletti, Sheena Pinto, Guido
Sebastiani, Klaudia Kuranda, et al. 2018. “Islet-Reactive CD8+ T Cell Frequencies in the
Pancreas, but Not in Blood, Distinguish Type 1 Diabetic Patients from Healthy Donors.” Science
Immunology 3 (20): 4013. https://doi.org/10.1126/SCIIMMUNOL.AAO4013/SUPPL_FILE/
AAO4013_SM.PDF.
Halle, Stephan, Kirsten Anja Keyser, Felix Rolf Stahl, Andreas Busche, Anja Marquardt, Xiang
Zheng, Melanie Galla, et al. 2016. “In Vivo Killing Capacity of Cytotoxic T Cells Is Limited and
Involves Dynamic Interactions and T Cell Cooperativity.” Immunity 44 (2): 233–45. https://doi.
org/10.1016/j.immuni.2016.01.010.
Hs, Wong, K. Park, A. Gola, Ap Baptista, Ch Miller, D. Deep, M. Lou, et al. 2021. “A Local Regulatory
T Cell Feedback Circuit Maintains Immune Homeostasis by Pruning Self-Activated T Cells.”
Cell 184 (15). https://doi.org/10.1016/j.cell.2021.05.028.
Korem Kohanim, Yael, Avichai Tendler, Avi Mayo, Nir Friedman, and Uri Alon. 2020. “Endocrine
Autoimmune Disease as a Fragility of Immune Surveillance against Hypersecreting Mutants.”
Immunity 52 (5): 872–84.e5. https://doi.org/10.1016/J.IMMUNI.2020.04.022.
Kracht, Maria J. L., Arnaud Zaldumbide, and Bart O. Roep. 2016. “Neoantigens and Microenvironment
in Type 1 Diabetes: Lessons from Antitumor Immunity.” Trends in Endocrinology and
Metabolism 27 (6): 353–62. https://doi.org/10.1016/J.TEM.2016.03.013.
Li, Jing, Maxim Zaslavsky, Yapeng Su, Jing Guo, Michael J. Sikora, Vincent van Unen, Asbjørn
Christophersen, et al. 2022. “KIR+CD8+ T Cells Suppress Pathogenic T Cells and Are Active in
Autoimmune Diseases and COVID-19.” Science 376 (6590): eabi9591. https://doi.org/10.1126/
science.abi9591.
Madi, Asaf, Eric Shifrut, Shlomit Reich-Zeliger, Hilah Gal, Katharine Best, Wilfred Ndifon, Benjamin
Chain, Irun R. Cohen, and Nir Friedman. 2014. “T-Cell Receptor Repertoires Share a Restricted
Set of Public and Abundant CDR3 Sequences at Are Associated with Self-Related Immunity.”
Genome Research 24 (10): 1603–12. https://doi.org/10.1101/GR.170753.113.
Martin-Blanco, N., R. Blanco, C. Alda-Catalinas, E. R. Bovolenta, C. L. Oeste, E. Palmer, W. W.
Schamel, et al. 2018. “A Window of Opportunity for Cooperativity in the T Cell Receptor.”
Nature Communications 9 (1): 2618. https://doi.org/10.1038/s41467-018-05050-6.
Pettmann, Johannes, Anna Huhn, Enas Abu Shah, Mikhail A. Kutuzov, Daniel B. Wilson, Michael L.
Dustin, Simon J. Davis, P. Anton van der Merwe, and Omer Dushek. 2021. “e Discriminatory
Power of the T Cell Receptor.” eLife 10: e67092.
Schwartz, Michal, and Irun R. Cohen. 2000. “Autoimmunity Can Benet Self-Maintenance.”
Immunology Today 21 (6): 265–68. https://doi.org/10.1016/S0167-5699(00)01633-9.
Schwartz, Michal, and Catarina Raposo. 2014. “Protective Autoimmunity: A Unifying Model for
the Immune Network Involved in CNS Repair.” Neuroscientist 20 (4): 343–58. https://doi.
org/10.1177/1073858413516799.
Semana, Gilbert, Rudolf Gausling, Richard A. Jackson, and David A. Haer. 1999. “T Cell
Autoreactivity to Proinsulin Epitopes in Diabetic Patients and Healthy Subjects.” Journal of
Autoimmunity 12 (4): 259–67. https://doi.org/10.1006/JAUT.1999.0282.
110 ◾ Sys te m s Me dicin e
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Sontag, E. D. 2017. “A Dynamic Model of Immune Responses to Antigen Presentation Predicts
Dierent Regions of Tumor or Pathogen Elimination.” Cell Systems 4(2): 231–241.
Yu, Wong, Ning Jiang, Peter J. R. Ebert, Brian A. Kidd, Sabina Müller, Peder J. Lund, Jeremy
Juang, et al. 2015. “Clonal Deletion Prunes but Does Not Eliminate Self-Specic Αβ CD8+ T
Lymphocytes.” Immunity 42 (5): 929–41. https://doi.org/10.1016/J.IMMUNI.2015.05.001.
DOI: 10.1201/9781003356929-10
Chapter 5
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Inflammation and Fibrosis
as a Bistable System
INTRODUCTION
Fibrosis, or excessive scarring, is a medical problem that cuts across medicine. In brosis,
scar tissue replaces healthy tissue and the organ loses function. Fibrosis occurs in the liver,
lung, kidney, heart, and other organs, and is a major contributor to age-related diseases.
It is always preceded by periods of intense inammation. ere is currently no cure for
progressive brotic diseases other than organ transplant.
In this chapter, we will understand the essence of inammation and brosis. Our basic
question is how a single biological process, tissue repair, can lead to two very dierent
results: healing or brosis. We will use this understanding to consider potential avenues
for therapy to prevent brosis and even to reverse it.
We will use two of our laws in this chapter, all cells come from cells, and biological processes saturate. e third law, cells mutate, will star again in the next chapters.
INJURY LEADS TO INFLAMMATION, WHICH
GOES TO EITHER HEALING OR FIBROSIS
As we know from our childhood injuries, the injured spot gets red, swollen, hot, and painful – this is inammation. e wound develops a scar over a few days. e scar usually
vanishes aer a couple of weeks, and the tissue is perfectly healed. But sometimes we get
permanent scars that last a lifetime. ese scars are examples of brosis.
ere is a universal sequence across tissues:
us, the process of tissue repair can lead to two dierent outcomes, depending on the
duration and intensity of the injury. In organs with poor ability to regenerate, like the
heart, brosis is triggered by almost any injury. In organs that can repair themselves to
injuryi→→nflammation fibrosisorhealing
111
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a certain extent, transient or small injuries lead to healing, but prolonged, repetitive, or
extensive injuries cause brosis.
Fibrosis has an essential physiological function: if there is a pathogen or a foreign object
that cannot be removed, the body tries to encapsulate it in brous scar tissue rich with
collagen. For example, the hepatitis C virus in the liver causes liver brosis (cirrhosis).
Likewise, a large wound that cannot be quickly healed needs to be lled in to maintain
tissue integrity. Fibrosis does the job.
But brosis has a dark side in aging. Tissues progressively tend to show more brosis
than healing, as we will soon discuss in Part 3 of the book. Fibrosis can cause organs to fail.
For example, many types of kidney dysfunction are due to massive scarring of the kidney,
and cardiac failure is accompanied by scarring of the heart. Alcoholism leads to liver brosis, and liver brosis also occurs in non-alcoholic fatty liver disease (NAFLD), associated
with obesity, which aicts ~25% of the world population. A fraction of those with NAFLD
progress to chronic inammation and to brosis, with loss of liver function. Fibrosis is also
a risk factor for cancer in many organs. ere is currently no treatment for brosis except
for organ transplant. Treating brosis is a huge unmet need.
Because inammation always precedes brosis, physicians try to stop inammation
quickly to prevent brosis aer surgery, stroke, heart attack, and other medical situations.
ere is usually a time window of about 2 days in which stopping inammation can prevent brosis. If the time window is exceeded, brosis is inevitable, even if inammation is
stopped. Why this time window? In this chapter, we will try to nd out.
Another intriguing question is the slow timescale of healing and scar formation. Despite
the brief time window of days we just discussed, it takes months for the scar to mature –
that is, to reach its nal steady-state composition. Likewise, it can take 2 weeks for healing
to be completed. Where does this long timescale come from? is is another mystery we
will try to explain.
Inammation and brosis is a busy research eld in biology and medicine, which has
uncovered a large number of molecular facts. Many signaling molecules activate and
inhibit immune cells and broblasts, and these cells have many possible states.
We will take a big-picture view, putting the essential facts into a mathematical model
that captures the core features. is model has a basic property called multistability – the
ability to produce two or more dierent stable steady states. Multistability can shed light
on how inammation can lead either to healing, if the injury is brief, or to brosis, if the
injury is repetitive or prolonged. is understanding also points to potential strategies to
prevent and reduce brosis.
INFLAMMATION INCLUDES A MASSIVE INFLUX OF IMMUNE
CELLS AND ACTIVATION OF MYOFIBROBLASTS
Injury to a tissue causes cells to release factors that cause inammation. Some of the damaged cells die, and others become large metabolically active cells that stop dividing, called
senescent cells. e damaged cells and senescent cells secrete “alarm” proteins that ow in
the blood like IL6, IL1, and TNF that induce inammation.
Inammation and Fibrosis as a Bistable System ◾ 113
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e purpose of inammation is to ght pathogens and to start repair. Unlike the T-cells
of the previous chapter, which begin to matter only several days aer infection, here we are
talking about the innate immune system which is much faster and responds in minutes.
Inammation has four main features, which are easy to remember by a Latin rhyme –
rubor, calor, tumor, dolor: redness, heat, swelling, and pain. e h pillar of inammation is loss of tissue function.
Redness, swelling, and heat are caused by the dilation of nearby blood vessels, which
open up to let immune cells ow into the tissue, together with uids and proteins that
ght pathogens (Figure 5.1). e main immune cells are white blood cells that specialize in ghting bacteria called neutrophils. With them come blood monocytes that turn
into macrophages (“big eaters”), cells that can engulf pathogens and dead cells. ey also
help to remove the senescent cells, along with other innate immune cells called NK cells.
Macrophages play a big role in brosis and healing, which we will describe soon.
In parallel to letting in macrophages, injury sets o a process that lays down brous
material, mainly collagen, to seal up the injury. To do so, damaged cells as well as the
incoming macrophages secrete signals including TGF-β that activate a cell type found in
every tissue called broblasts (ber-forming cells). is signal causes the tissue-resident
broblasts to proliferate and change their shape to become super-ber-forming myobro-
blasts (myo = “muscle-bound”), Figure 5.1. eir muscle-like ability to generate force helps
to contract and close the wound.
e two main cell types in our story are thus the incoming macrophages, which we will
denote M, and the myobroblasts, F. ese two cell types activate each other’s proliferation. ey do so by secreting growth factors for each other – small proteins that diuse in
the tissue and are sensed by receptors on the cell surface. e binding of the growth factor
to the receptors makes the cells divide rather than die. Interestingly, F cells also secrete a
growth factor for themselves, in an example of an autocrine loop (Figure 5.2).
is circuit of two cell types was characterized in detail by growing the cells together
in a plate by Ruslan Medzhitov and colleagues (Zhou et al. 2018). Growing cells in a plate,
FIGURE 5.1 Inammation at an injury site involves inux of uids and macrophages together
with dierentiation of broblasts into myobroblasts which lay down scar tissue.
wound
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2
1
FIGURE 5.2 Macrophages M and myobroblasts F secrete growth factors c
and myobroblasts also secrete growth factors for themselves in an autocrine loop.
and c2 for each other,
1
called an “in vitro” approach, allows careful measurement of the parameters and dynamics of the circuit, such as cell growth rates and growth-factor secretion and removal rates.
us, we have estimates for the rate parameters in this circuit.
e situation in the body, in vivo, is certainly more complex than in vitro. Still, in vitro
studies can provide principles to help us understand the in vivo process. Generally, in systems medicine there are four approaches: in vivo, in vitro, in silico (computer simulation),
and in envelopo (back of the envelope calculations, like we do here). Don’t expect your
friends to know this last term, I invented it for this book.
If the injury is transient, inammation is resolved within a couple of weeks. M and F cell
populations shrink (die by programmed cell death called apoptosis) and vanish. e scar is
removed. e tissue cells, such as epithelial cells, divide, and the injury is healed.
If the injury is repetitive or prolonged, however, or if the tissue cannot regenerate (such
as heart and brain tissues), M and F populations rise and a permanent scar is formed, made
of bers and cells. is is brosis.
Our purpose is to understand the dynamics of the inammation process and how it can
“decide” to show healing or brosis.
MATHEMATICAL MODEL FOR MYOFIBROBLASTS SHOWS BISTABILITY
Let’s begin by considering only the myobroblasts, F. is will help us explain the equations and will be useful soon when we add in the macrophages. e main point is that one
equation for myobroblasts can show two dierent behaviors, a property called bistability.
But rst, to clear the mind, let’s take a nice deep sigh of relief.
e myobroblasts produce and secrete a growth factor for themselves, which we will
(Figure 5.3). c1 is degraded at rate
denote
c
1
minus removal
γ
. us the rate of change of c1 is its production
1
(5.1)
e parameters in these equations, based on in vitro data, are as follows: secretion rates
like a are about 100 molecules/cell/min, and degradation half-lives ln(2)/
Since the production and removal processes take minutes to hours, and cell division and
death take a day or longer, we can use separation of timescales as in the previous chapters.
dc
dt
1
=−aF
γ
c
11
γ
are hours.
1
Inammation and Fibrosis as a Bistable System ◾ 115
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d
1
1
FIGURE 5.3 Circuit describing how myobroblasts secrete their own growth factor and are
1
1
removed.
1
1
FIGURE 5.4 e concentration of the growth factor is proportional to the concentration of the F
cells that produce it at quasi steady state.
On the rapid timescale of hours in which c1 levels reach their steady state, cell density F
hardly changes. us, growth factors like c
pute using dc
/0dt = . We nd that c1 is proportional to the cells that make it, F (Figure 5.4):
1
(5.2) c
are in quasi-steady-state, which we can com-
1
a
= F
1
γ
1
We now turn to the equation for the rate of change of F cells, given by cell proliferation (all
cells come from cells), which increases with c
(5.3) =−pF
dF
dt
Plugging in the quasi-steady-state Eq. 5.2 for c
dF
dt
, minus cell removal at rate d1:
1
cd1F
11
results in a proliferation that rises like F
1
pa
1
2
Fd
=−
γ
F
1
2
Let’s nd the xed points where dF/0dt = . For this purpose, we use the rate plot, a useful
method for equations with one variable, F in this case. We employed the rate plot for beta
cells in Chapter 2. On the x-axis we plot cell density F and on the y-axis we plot the total
pa
1
2
F
cell proliferation
that rises quadratically with F and is thus a parabola (black line
γ
in Figure 5.5). We next plot the total cell removal dF
, a line that rises with F (blue line in
1
Figure 5.5).
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