Decellularized tissue is extracellular matrix removed cellular components from biological tissues obtained from animals. It is composed of extracellular components such as collagen, elastin, and glycosaminoglycan. Decellularized tissues are relatively new materials, which began to be energetically studied from 1990s. Especially, it was of strong interest in Europe and United States. The large number of venture companies have been established and already have been produced many kind of decellularized tissues on a commercial basis. Basically, decellularized tissues were developed as an alternative tissue to solve the shortfall of donor tissues. As the studies and clinical applications proceed, efficacy of decellularized tissue was gradually revealed unlike many of the other artificial materials. In our laboratory, we are pursuing the possibility of decellularized tissues in the fields of regenerative medicine and tissue engineering.
１） Decellularized tissue as tissue replacement materials and tissue remodeling materials
Many of decellularized tissue have been used in orthotopic tissue. In other words, decellularized blood vessel and decellularized heart valve are used as a blood vessel and a heart valve, respectively. Thus, decellularized tissue is used as tissue replacement materials as much as artificial materials. The characteristics that required for the tissue replacement materials are to exhibit its function immediately after transplantation, low inflammatory to a living body, and long-term stability. Many artificial materials remain over the long term without degradation in vivo. On the other hand, as biological materials are susceptible to the effect of tissue remodeling in vivo, it was stabilized by chemical crosslinking. However, decellularized tissue without chemical crosslinking can strongly recruit cells, which produce early integration to living tissue. In order to prepare a high performance decellularized tissue, decellularization technique is very important. Therefore, we have studied the new methodology without detergent, as it is called a high hydrostatic pressure (HHP) method. We have previously obtained favorable results with respect to blood vessel and cornea.
In recent years, decellularized tissues have been shown to be a promising materials for whole-organ tissue engineering. Namely, decellularized donor organs can be recellularized with mesenchymal stem cells, selected progenitor cell population, and/or cells induced from ES cells or iPS cells. In our laboratory, on the basis of the same concept, we have tried to regenerate the bone marrow, brain, cartilage using each decellularized tissues
２） A novel function of decellularized tissue complexed with artificial materials
Decellularized tissue is excellent material that possesses tissue compatibility and capacity of tissue reconstruction. To make active use of the characteristics of decellularized tissue, we have investigated the composite with an artificial materials. The candidate materials for complex are low-molecular compounds and drugs, naturally-derived materials such as lipids, proteins, polysaccharides, and nucleic acids, and artificial materials including polymers, ceramics, and metals. Although biological tissue has a densely packed structure, three dimensional structure after the decellularization is different depending on the composition of the tissue and organ. For example, in the blood vessel and dermis, as a volume fraction of cells occupying the tissue is extremely low, the tissue structure hardly changes before and after the decellularization. In contrast, in the organs with a high volume fraction of cells, for example, liver, heart, and kidney, extracellular matrix is too little to maintain the shape of them in air. The former decellularized tissue is expected to use widely as biomaterials, which have the same physical properties of the living tissue. However, it is necessary to contrive the ways of complex effectively with another components. We try to develop and characterize the composite materials composed of the decellularized tissue and polymer.
３） Contribution to understanding of disciplinary fields
When the decellularized tissues were transplanted into a living body, diverse biological reactions, unlike artificial materials will occur. One interesting phenomenon is infiltration of the host cells into the transplanted decellularized tissues. In the case of artificial materials, infiltrative cells are almost inflammatory cells or fibroblasts. In contrast, in the case of decellularized tissues, the same cells which presented originally in living tissues infiltrate into them and reconstruct the tissue. This phenomenon is called cell homing, and observed generally in a damaged tissue. However, the same phenomenon was observed in the transplanted decellularized tissue despite being subjected to various treatment. We have considered that this phenomenon maybe due to the structure of decellularized tissue. By investigating this phenomenon, we demonstrate the cause of recruitment and homing of cells in vivo, and contribute to the embryology and wound healing of tissue or organ as well as regenerative medicine.
Molecular aggregates formed by relatively weak interactions including hydrogen bond, hydrophobic interaction and electrostatic interaction have received a lot of attention in recent years. This is because the molecular aggregates exhibit unprecedented new functions. In medical engineering fields, molecular aggregates are considered for the application as drug and gene delivery carrier. The formation of molecular aggregates is affected by a concentration of polymer solution, pH, temperature, pressure as well as molecular composition. It is well known that hydrogen bond is stronger than electrostatic and hydrophobic interactions under the high pressure condition over 6000 atmosphere. Previously, we have shown to be form nanoparticles or hydrogels via hydrogen bonds enhanced by high-hydrostatic pressure processing, by using polyvinyl alcohol (PVA), which is synthetic hydrogen bonding polymers having simple hydrogen bonding structure.
We have conducted the development of drug and gene delivery system by hybridizing the hydrostatic pressure-induced molecular aggregates and bio-functional molecules such as drugs, proteins, nucleic acids, and inorganic compounds. Complex obtained by the high hydrostatic pressure treatment to a mixed solution of PVA and the plasmid DNA can efficiently delivered a gene into the cells. Pressurizing to the complex of liposome composed of a lipid bilayer membrane and plasmid DNA, conformational changes of the membrane structure is induced, which contributed to enhance the efficacy of gene delivery.
Extracellular matrix (ECM) is a complex fiber-composite materials in which collagen fibrils are major components. Many researchers have attempted to prepare a collagen-based gel materials to construct an ECM. However, the critical aspect in using collagen gel is that its mechanical strength is too small and easily deforms its triple-helix structure into a random coil structure due to external stimuli. Focusing on the architecture of biological tissue and its physiobiological property, we try to prepare an advanced ECM that possesses high mechanical strength and induction potency of wound healing using collagen. The feature of this study is to prepare an artificial tissue equivalent that replicated the structure of the native ECM from a quaternary structure of collagen molecules by fibrillogenesis. This process is important for the formation of microfibrils with regulated D-periodicity. We have successfully developed a monolithic and multilayered fibrillated collagen matrix that can be used for tissue implants. This fibrillated collagen matrix suppressed inflammation and remodeled the surrounding tissue, implying that regeneration of the tissue surrounding the collagen matrix occurred during the early phase of implantation and induced the healing response.
It is reported that cancer has a cancer antigens which are expressed specifically, and the immune reactions to them are induced in human bodies. Since then, immunotherapy that enhances the immune reactions against specific cancer antigens has been conducted to destroy the cancer by administrating the cancer antigens in a variety of forms of peptides, proteins, and DNA. However, in spite of the cancer antigen-specific immune reactions is induced in a number of immunotherapy, the sufficient clinical effects is not observed. The reason is considered due to the immune tolerance against cancer, because most of cancer antigens derived from self-antigens. In recent years, it revealed that regulatory T cells (Treg) that negatively regulate immune reactions play an important role in cancer immunotherapy and autoimmune disease. In immunotherapy, removing the Treg cells from the tumor-bearing body, anti-tumor immune reaction is enhanced, results in cancer rejection. On the other hand, in the therapy of autoimmune disease, injecting the Treg cells to inside the body, effector T cells that attack to self were suppressed. Therefore, a technology for collecting the Treg cells is very important to treat these diseases. In our laboratory, we have developed the novel methodology that can specifically and efficiently capture and release the Treg cells with intact condition.