Summary regenerative medicine
Lecture 1: Introduction
Loss of organ function can be treated in several ways: transplantation, implantation of a prosthetic device,
autografting, stem cells, or in vivo induction of regeneration.
Regenerative medicine is not the same as stem cell therapy (where stem cells are injected into a lesion). A
challenge of stem cell therapy is to keep them in the right spot. A solution could be to use collagen
(extracellular matrix, ECM). In regenerative medicine, often the BBB principle is used: from bed to bench
and back.
- A patient has either acute (bone fraction, skin cut, MI) or chronic (non-healing wounds, hypertrophic
scars, fibrosis, diabetes, smoking, obesity) damage. This damage causes a repair trigger, and the goal of the
body is to fully regenerate the tissue. The process of regeneration occurs through wound healing,
rebuilding and training/maturation.
Tissue
For tissues, the most important thing is communication.
There is homeostasis in which cells communicate with
each other via signaling. If there is disturbance of the
homeostasis, regenerative medicine can help by
augmenting the existing processes of wound healing,
dampening pathological adverse repair processes like
fibrosis, or re-educate the microenvironment. In some
cases, the tissue damage is so big, that regenerative
medicine is not sufficient anymore, in this case
replacement tissue is made using tissue engineering.
Regenerative medicine is in general used to repair
damage from the inside, whereas the outside is more
repaired using tissue engineering.
Restoring of the damaged body has been an ancient dream, and the myth of Prometheus, where an eagle
picks out the liver as a punishment, but the liver regenerates, is based on reality, because the liver has in
fact the ability to regenerate. Also in lower organisms, regeneration occurs when a blastema is formed
using dedifferentiation.
Important molecules
Molecular signaling helps pattern formation. There are regulatory molecules that provide instructive
signals for proliferation, differentiation and function, and there are adherence molecules that provide
support, instructive signaling, and sequestration of regulatory molecules.
Definitions of regeneration
Regeneration is the ability of an organism, organ, or tissue to regain its original structure and function after
damage. In the 90s, Robert Langer and Joseph Vacanti already described tissue engineering as an
interdisciplinary field that applies the principles of engineering and life sciences toward the development
of biological substitutes that restore, maintain, or improve tissue function.
So, regenerative medicine (RM) is seen as an interdisciplinary field of research and applications focused on
the repair, replacement, or regeneration, using converging technological approaches that stimulate and
support the body’s own self-healing capacity. The aim and targets of RM are to devise new therapies for
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,patients with severe injuries or chronic diseases, in which the body’s own responses do not suffice to
restore functional tissue. Ageing magnifies the need for RM.
Tissue engineering and regenerative medicine
In severe burn wounds, or diabetic ulcers, the body needs help in the form of tissue engineering. What is
necessary to build an organ is a scaffold (the matrix), cells, and molecules (adhesion molecules and growth
factors). This is called the triad of RM. We already established that regenerative medicine was used to
improve communication, re-educate the local microenvironment, and reverse adverse processes. If this all
fails, replacements are built using tissue engineering. From destruction, we go to instruction, construction,
and reconstruction. It is a multidisciplinary field of tissue engineering, stem cell biology, materials science,
genetics and molecular biology, transplantation, and developmental biology. However, this course is
focused on cell and matrix and material regenerative medicine.
Choice of cells
• Autologous cells: cells coming from the patient, either stem (or progenitor) or somatic cells.
o Advantages: no rejection
o Disadvantages: underlying disease affects the cells, only small numbers, excessive culturing
needed (expensive), unavailable off-shelf (taken and used directly)
• Allogenic cells: stem or progenitor cells coming from donors, can be obtained from various tissues.
o Advantages: obtained in large numbers, bulk culture and uniform batches, banking is
possible, available off-the-shelf
o Disadvantages: rejection, needing immune suppression
Immune compatibility
The ideal scenario is a bioengineered construct without the need of an immune response. Other options
are the use of immune privileged cells (ESCs express low MHCI and no MHCII), cell banking, which allows
histocompatibility matching, and cells with immunoregulatory function (mesenchymal stem cells induce
tolerance in grafts).
Stem cells
Stem cells can be taught to become specialized cells, using the right signal transduction pathways.
Characteristics of stem cells are that they are unspecialized, can self-renew, and can differentiate into
various functional phenotypes. There are different types of stem cells:
• Embryonic stem cells (ESCs): are derived from the inner cell mass of early embryos. They have
unlimited self-renewal and are pluripotent. However,
the challenge is to control this multipotency.
o The germ layers that arise from ESCs are
ectoderm (neuronal and pigment cells),
mesoderm (muscle cells, blood cells, kidney
cells), and endoderm (beta cells, lung cells,
pancreatic cells). Also germ cells are
generated.
• Adult stem cells: are derived from various organs,
because everyone has their own stem cells. An
example is mesenchymal stem cells (MSCs), but also organ-specific stem cells are found in the gut,
skin, and bone marrow. These are multipotent (cannot develop into everything, but still to cells in
the same category), more differentiated, which makes further differentiation easier, can be patient
derived (autologous) and therefore are well tolerated. However, in the case of disease, the stem
cell is affected as well.
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, • Induced pluripotent stem cells (iPSCs): are generated from normal cells from the patients (e.g. from
fibroblasts or cells in the urine). They are well tolerated and can again be compromised by an
underlying disease. An adult tissue cell is de-differentiated to an embryonic stem cell stage.
Differentiation
Differentiation of cells is driven by paracrine factors and calcium,
but also surface matters (e.g. fibronectin). If stem cells are on soft
surfaces, fat cells are formed, whereas if they are on stiff surfaces,
bone cells are produced. Also topography (the outside of a cell)
matters, and influences adhesion and differentiation.
Stem cells are instructed by chemical (growth factors, chemokines,
cytokines, ECM, chemicals), physical (scaffolds, scaffold roughness
and wettability) and mechanical cues (pressure tension, shear
stress).
Scaffolds
Scaffolds can be natural or chemical polymers that mimic the ECM, and are there for support, architecture,
and biomechanical features (promote adhesion, survival, cell function). Scaffolds are often biodegradable
and replaced by physiological matrix. Biological factors and patient derived cells go into a scaffold to form
a tissue-engineered graft and are put in the patient.
The right biomaterials must be used for scaffolds. However, all
implanted (bio)materials lead to a foreign body reaction (FBR):
inflammation. This is the case because an incision activates
coagulation and coats the implant with serum proteins. The innate
immune system is activated, and macrophages are attracted. The
implant is encapsulated by fibroblasts, which (if possible) is degraded
by macrophages. Macrophages have a lot of functions in wound
healing (phagocytosis, angiogenesis, cell recruitment etc).
RM in the clinic and on the market
Finding things in mice does not mean that it works for humans as well.
On top of that, clinical trials are time-consuming and expensive. Some
RM products on the market right now are Epicel (Genzyme), which is a
bio-artificial skin graft of keratinocytes and a polymer scaffold.
Another one is Carticel (Genzyme), which is a cartilage graft made of
chondrocytes.
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