| Review Article |
Open Access |
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| Are there Technical/Clinical Tools to Improve the Present Vascular Access
Outcome in Haemodialysis Patients? |
| Ezio Movilli |
| Division of Nephrology, Spedali Civili and University of Brescia, Italy |
| *Corresponding author: |
Ezio Movilli
Division of Nephrology
Spedali Civili and
University of Brescia, Italy
Tel: 237-7718 3510
E-mail: eziomov@libero.it |
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| Received October 31, 2011; Accepted November 18, 2011; Published November
20, 2011 |
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| Citation: Movilli E (2011) Are there Technical/Clinical Tools to Improve the Present
Vascular Access Outcome in Haemodialysis Patients? J Biotechnol Biomaterial
1:115. doi:10.4172/2155-952X.1000115 |
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| Copyright: © 2011 Movilli E. This is an open-access article distributed under the
terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited. |
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| When native vein and artery are not available due to previous harvest,
anatomical limitations, or disease progression, synthetic materials
such as Dacron or ePTFE have been used with varying degrees of success.
Synthetic graft materials are used with great success in larger diameter
applications such as aortic or iliac reconstruction, but they have
demonstrated unacceptably poor performance in most small diameter
applications (below 6 mm inside diameter). The poor efficacy of small
diameter synthetics is linked to short-term thrombosis, increased rate
of infection, chronic inflammatory responses to the foreign materials,
and compliance mismatch between the native tissue and the prosthetic
material. These problems are well illustrated in A-V access grafts, where
the intervention rates for synthetic grafts are three-fold higher than for
native vein fistulas [1]. Attempts to improve the durability of prosthetic
grafts began in the 1970s with the concept of seeding the luminal surface
of the graft, considered to be thrombogenic, with endothelial cells
[2]. The major technical feat overcome by extensive work in the 1980s
and 1990s centered on preventing the cells from being dislodged by luminal
blood flow on implantation of the graft. Strategies to overcome
this problem include precoating the graft with various adhesives, pressure
sodding, modification of the graft surface with RGD moieties, prolonged
culture of the graft, and flow conditioning. The field of Cardiovascular
Tissue Engineering has attempted to produce a clinically viable
synthetic conduit by using a variety of in vitro approaches that typically
combine living cells seeded into reconstituted scaffolds to create living
tissue engineered blood vessels (TEBVs) [3]. |
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| Scaffold Choice |
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| As noted, prosthetic material has served as the traditional scaffold
for vascular graft creation. Its availability and biocompatibility make
it attractive for use; however, in spite of seeding, it remains prone to
infection and anastomotic intimal hyperplasia owing to compliance
mismatch [4]. Bioresorbable scaffolds, such as poly glycolic acid, may
yield a more compliant construct. In theory, the extracellular matrix
proteins secreted by the seeded cells eventually replace the scaffold as it
dissolves. This has not proven to be the case as the microenvironment
of the decomposing scaffold has deleterious effects on the cells [5]. A
logical alternative is tissue allograft. Vascular transplants are prone to
rejection, however, and have not proven durable even with immunosuppression.
Methods to alter the immunogenicity of the transplanted
tissue include cryopreservation and removal of the cellular elements.
This latter strategy may mandate additional modifications of the graft
to restore endothelial and smooth muscle cell function. Decellularized
vein allograft as a scaffold for bypass graft creation has been studied
[6]. Type IV collagen, an important component of vascular basement
membrane to which cells adhere, appears to be preserved after cell removal.
When implanted into the arterial circulation for up to 2 months,
decellularized vein allografts remain sturdy; demonstrate reduced
hyperplasia and immunogenicity compared with nondecellularized
allograft controls [7]. However, the luminal surface of these scaffolds
was covered with a compact fibrin layer, suggesting a role for further
tissue engineering strategies, such as vascular cell seeding. Another approach
is to develop scaffolds which are partially resorbable [8]. Konig
and McAllister have recently developed an approach called sheet-based tissue engineering (SBTE) that uses dermal fibroblasts cultured in conditions
that promote the production of extracellular matrix (ECM) proteins
[9]. The fibroblasts, embedded in their own ECM, form a robust
sheet that can be rolled into tubes to make extremely strong conduits
that do not rely upon any exogenous scaffolds. The multi-ply roll is matured
to fuse into a cohesive tissue, which can then be seeded with endothelial
cells to make a completely autologous tissue engineered blood
vessel called the Lifeline™ vascular graft. |
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| Cell Choice |
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| The traditional cell used for luminal seeding is the differentiated
endothelial cell obtained from jugular or saphenous vein segments.
This strategy is disadvantaged by the need for ex vivo cell culture to
obtain the number of cells necessary to seed the graft lumen. Harvest
of micro vessel endothelial cells from liposuctioned adipose tissue appeared
promising in terms of immediate cell number [10]; however,
subsequent evaluation has suggested that contaminating cells in the
isolates leads to the development of hyperplasia within the seeded
grafts [7]. Endothelial progenitor cells isolated from peripheral blood
hold promise for vascular tissue engineering. These cells originate from
bone marrow and are a source of autologous cells for vascular repair.
Their presence in peripheral blood varies with patient characteristics
and may diminish with aging; therefore, use of this cell for vascular tissue
engineering would also likely require ex vivo expansion [11]. Many
researchers are currently investigating adipose tissue as a source of stem
cells for use in graft creation. Adipose derived stem cells (ASCs) can
be isolated in abundance from liposuctioned abdominal wall fat, making
them attractive for seeding. In a study of patients undergoing peripheral
vascular surgical procedures, an average of 210,000 ASCs/g of
adipose tissue was obtained [12]. The ASC is multi potent, having been
shown to differentiate into bone, cartilage, adipose, muscle, and neuron
cell lines. Studies from Di M Muzio, et al. [13] have begun to define its
ability to differentiate into cells with an endothelial phenotype. Endothelial
characteristics in these experiments were defined as realignment
in the direction of luminal flow, cord formation in response to extracellular
matrix (Matrigel), and the expression of endothelial cell message
and protein (endothelial nitric oxide synthase, von Willebrand’s factor,
CD31). ASCs have been seeded onto the luminal surface of decellularized
vein within a bioreactor that maintains the necessary culture
conditions for cell survival. Under gravitational force, cell attachment and spreading typically occur within 2 hours. Seeding with a minimum
of 2 x 105 cells/cm2, ASCs form a confluent monolayer on the luminal
surface. Preliminary study revealed a thin layer of fibrin on the graft
surface, suggesting that undifferentiated ASCs may not immediately
form a non-thrombogenic layer. These early results indicate that differentiation
of these cells prior to implantation may be necessary for
ultimate clinical success. |
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| Clinical applications |
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| McAllister, et al. [14] have recently reported the successful implantation
of a completely biologic tissue engineered graft for vascular access
in 10 patients with end stage renal disease receiving haemodialysis.
Patency rates at 1 and 6 months were 78 and 60% respectively. This
study is the first encouraging result of the use of a tissue engineered vascular
graft in a clinical setting. McAllister and colleagues used the cell
self assembly technique, as opposed to the cell-seeded gels or cell scaffold
technology, for the costruction of their tissue engineered vessel. As
previously described, these vessels are constructed by taking advantage
of the natural ability of cells to produce their own extracellular matrix
(ECM). Briefly, human fibroblasts, extracted from patient’s skin biopsies,
were cultured to form 15 sheets of living fibroblasts with associated
ECM. These sheets were then rolled over a stainless steel mandrel to
allow them to fuse. After 10 weeks of culture the vessels were dried and
the lumen seeded with autologous endothelial cells. Total time production
for the Graft ranged from 6 and 9 months. 7 days prior to implantation
the lumen of the vessel was seeded with autologous endothelial
cells and pre conditioned to flow and pressure. Grafts with an average
length of 23.2 cm (range 14-30 cm) were implanted into 9 patients (one
was excluded prior to surgery due to gastrointestinal haemorrhage) and
were assessed for both mechanical stability and effectiveness during a
safety phase (0-3 months long) and after haemodialysis was started.
While the patency rates were good, it was possible to use the graft for
haemodialysis for longer than 12 months in only 3 patients. |
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| The advantages of this approach are that the tissues are completely
autologous so that the grafts are non immunogenic and non thrombotic.
Moreover, since the graft develops in its own matrix and does not
need an external scaffold, there are not concerns about the use of xenogenic
scaffolds, especially cross infection. The major limitation of this
approach is the long time of culture required to develop the graft and
this will limit its clinical applicability especially in emergency. Other
concerns arise from the very high costs of production, the requirement
for patient specificity and the lack of off-the-shelf availability |
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| Conclusions |
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| Vascular tissue engineering is a rapidly developing discipline and
it likely will become a major modality for the treatment of advanced
cardiovascular disease. Encouraging in vitro and in vivo results show
that vascular engineering is now well established. Probably we are no
very far from the time when the use of engineered vascular tissues will
become an integral part of vascular surgical practice; we still need for
regulatory approval (CE marking, FDA approval) for adoption of these
very promising approaches. |
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| References |
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