Biology of chronic wounds and new treatment strategies

Biology of chronic wounds

Wound healing is classically characterized by 3 phases of repair: inflammation, proliferation and remodelling. After an injury multiple biological pathways immediately become activated and the 3 phases of repair can overlap in time and space within a wound. Although cell-centric models are useful to simplify our understanding of wound healing, a model that provides a more comprehensive perspective is the ‘seed versus soil’ paradigm as described by Gurtner et al. ¹ : ‘seeds’ refer to the cells in the wound and the ‘soil’ represents the extracellular milieu (cytokines, matrix, oxygen tension e.g.). Another model is the concept of ‘dynamic reciprocity’ defined by Schultz et al. ² that describes the constant bidirectional communication between cells and their local microenvironment.

Wounds may be classified as those that repair themselves or can be repaired in an orderly and timely process (acute wounds) and those that do not (chronic wounds). The definitions used for both types of wounds vary in the medical literature. For example, in the Dutch relevant guideline, acute wounds are defined as wounds with an acute aetiology where loss of continuity of the skin is caused by a trauma or a surgical procedure, independent of the duration of the wound. ³ The terms ‘chronic’, ⁴ ‘hard-to-heal’ ⁵ and ‘complex’ ⁵ are used for wounds that fail to proceed through an orderly and timely repair process, or proceed through the repair process without establishing a sustained anatomic and functional result. The Dutch Society of Dermatology and Venereology prefers not to use the term ‘complex wound’, since a complex wound is not a disease entity per se or a diagnosis with a single aetiology. The aetiology of chronic wounds is diverse, but more than 80% are associated with venous or arterial insufficiency, decubitus, or diabetes mellitus.

Irrespective of the definitions used, key to effective treatment lies in recognizing the combination of factors that are involved in delaying or impairing the process of healing. In the European Wound Management Association (EWMA) position statement these factors were categorized into four groups: a) patient-related factors, b) wound-related factors, c) skill and knowledge of the healthcare professional and d) resources and treatment-related factors. ⁵ For a detailed discussion of these 4 groups we refer to the EWMA position statement. By understanding the interaction of these factors and their impact on healing we can develop effective treatment strategies to improve patient outcomes.

New treatment strategies

Generally, local treatment of a wound is secondary to the management of the underlying patient related pathology. For example, an ulcer in a patient with superficial venous insufficiency can be treated with endovenous ablative procedures and compression therapy. In addition to treating the underlying pathology and based on the increasing knowledge of wound biology and specific wound-related characteristics, tissue engineering is a promising therapeutic approach. Tissue engineering is defined as ‘an interdisciplinary field that applies the principles of engineering and the life sciences toward the development of biologic tissues that restore, maintain, or improve tissue function’. ⁶ Various tissue engineering approaches for cutaneous wounds have recently been reviewed.^7,8,9

Here we give a summary of these approaches, none of which is yet recommended for routine use by our national guidelines on venous ulcers, arterial ulcers, diabetic foot ulcers or acute wounds.

Biomaterials

Biomaterials are acellular substances used for the creation of the backbone of skin substitutes in clinical applications. They are either naturally derived (from autologous, cadaveric or xenogenic sources including skin, intestinal submucosa, bladder) or synthetically fabricated. Biomaterials can be used as temporary wound cover for all thickness wounds and stimulate cell proliferation and angiogenesis. To enable the repopulation and regeneration of a new natural matrix, cell-recognition signals can be added by incorporating adhesion peptides. Moreover, incorporation of growth factors like fibroblast growth factor, vascular endothelial growth factor, insulin-like growth factor and platelet-derived growth factor into matrix scaffolds has been studied. ⁸

Skin substitutes

Skin substitutes can be distinguished by their origin: xenogeneic (from other species often pigs), allogeneic (from a non-genetically identical individual of the same species) and autologous (from the patient itself). Xenografts, allografts, and epidermal or bilayer substitutes that do not provide autologous keratinocytes, can only provide temporary wound coverage. They are used in cases of extensive tissue loss or burns to limit immediate complications.

Depending on the layer of skin that needs to be replaced, bioengineered skin substitutes are available as epidermal-, dermal- and bilayer substitutes. Epidermal substitutes are usually created by expansion of patient-derived keratinocytes in the laboratory. These may be attached to a carrier material before being transferred back to the patient. Available methods to deliver keratinocytes to a wound include application as cultured epithelial autografts, delivery on carrier dressings ranging from bovine collagen to a chemically defined polymer and conversion into a suspension that can be sprayed onto wound sites.

Dermal substitutes are based upon a 3-dimensional matrix material of synthetic or biological origin, which behaves like extracellular matrix. Acellular products are used to minimize immunogenic responses. They provide a scaffold that will be repopulated, re-vascularised, and re-modelled with the patient’s own fibroblasts and endothelial cells. They may also incorporate cells (e.g. neonatal foreskin cells) or bioactive molecules into this structure. Provision of these key factors described in the dynamic reciprocity model of wound healing to a wound bed may provide the necessary stimulus to rebalance the wound microenvironment in favour of healing.

Bi-layer substitutes are the most complex and advanced class of substitutes. They are difficult to manufacture, have a very limited shelf life and are the most expensive substitutes available. They behave as physical wound cover but also induce the production of a number of cytokines and growth factors. At our department of Dermatology at the Vumc (Amsterdam, the Netherlands) we are currently performing a randomized controlled trial for comparison of a full thickness bi-layer autologous skin substitute with acellular donor skin in patients with venous ulcers not healing with standard therapy. This skin substitute has proven effective and safe in over 60 patients with non healing ulcers of various aetiology. ¹⁰

There is still a requirement for more large randomized trials to validate the efficacy of the skin substitutes observed in preliminary studies in chronic wounds such as diabetic and venous ulcers. For the evidence relating to the use of skin substitutes in chronic wounds we refer to the reviews by Dieckman et al. ⁸ and Greaves et al. ⁹

Future perspectives for stem cell therapy

In contrast to the situation early in gestation, as human adults, we have lost the potential to regenerate injured tissues without fibrosis. The study of tissue regeneration in simpler developmental systems such as planarians (head or tail), fish (fin), and amphibians (limbs) teaches us that undifferentiated mesenchymal elements play a key role in regeneration after transection of these specific sites. ¹¹ Stem cells from various sources are studied for wound healing and tissue repair and have demonstrated varying success in preclinical settings. ¹² Presently, epithelial keratinocytes are applied with success for burn resurfacing. ⁸ However, recent research focuses on progenitor cells that maintain epithelial homeostasis instead of on the more differentiated keratinocytes. In the skin two distinct subpopulations of epidermal stem cells are present: a basal keratinocyte population in the interfollicular epithelium and stem cells in the bulge region of the hair follicle. Before clinical application of these stem cell populations is possible many questions remain to be answered, such as how reliably and specifically to identify and discriminate the various stem cell populations, and what culture conditions are necessary to prevent stem cell loss.

Mesenchymal stem cells (MSCs) either from bone marrow or adipose tissue present further suitable candidates for stem cell therapy. BM-MSCs can only be obtained in a limited amount and their differentiation abilities decrease with age. Moreover, they can have immunosuppressive properties and their isolation is extremely painful for patients. Adipose tissue-derived stem cells may provide a superior source: they are isolated from lipoaspirates, obtained by suction-assisted lipectomy and are easily obtainable in adequate quantities with little patient discomfort. Last but not least induced progenitor cells can be generated from adult human dermal fibroblasts and from keratinocytes by transduction with a combination of various transcription factors, involved in reprogramming. These cells could potentially be used in the construction of tissue engineered skin, after considerations of potential tumorgenicity have been resolved.

Conclusions

The concepts of ‘seed versus soil’ and ‘dynamic reciprocity’ provide more comprehensive perspectives on wound healing than the simpler cell-centric models. Tissue engineering using biomaterials, skin substitutes and stem cells offers the potential to improve significantly the clinical outcome in wound healing approaches. However, many questions remain to be answered regarding experimental and clinical application which need to be addressed in future research.