Tagged: Tissue (biology)

Current bioprosthetic heart valves substitutes: 100% biocompatible?

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Over 400.000 heart valves are replaced worldwide every year. Distrophic calcification and inflammation lead these prostheses to failure in the middle-long term in over 50% of patients, especially in the youngers that require recurrent re-operations [1]. When an aortic valve must be replaced, 80% of the times a biological heart valve substitute (BHVs) is used. These valves are engineered using porcine aortic valves or pericardium from different animal species like pig, sheep and horse that is properly assembled to build a prosthesis. Biological valves are of course different from mechanical valves that are made of pyrolytic carbon or other different materials.

BHVs are subjected to a treatment, called fixation, before being commercialised and implanted into humans. This process, performed with a chemical agent named glutaraldehyde (GLU), is fundamental for many reasons, for example in order to increase tissue mechanical strength, to sterilize the prostheses and preserve it prior to use but mainly to act as a shield for molecules that, if not suitably inactivated, cause a rejection of implanted tissue. Despite its importance, GLU is not able to mask completely these molecules and in the mid-long term it faces a chemical binding degeneration due to the mechanical stress to which bioprostheses are subject when implanted and put into operation. Multiple factors lead to the replacement of GLU-treated BHVs approx. after 10 years following the original implant due to tissue calcification and consequent stenosis of the valve, i.e. a narrowing of the internal diameter of the valve preventing an effective blood flow. BHVs degeneration was recently confirmed to be correlated to GLU masking efficiency of xenogeneic tissues which contain proteins, sugars and molecules that are proper of derivation species, whose complete elimination or inactivation is necessary to meet the requirements for clinical use. Particularly, the residual presence of a specific molecule called alpha-Gal (epitope), significantly increases the level of antibodies against galactose, starting from day 10 following BHV implantation, reaching a maximum peak at around 3 months after it [3].

This sugar moiety is expressed in most mammalian tissues, except humans and higher primates. In humans, continuous antigenic stimulation by the gastrointestinal flora (expressing the epitope) ends with the production of anti-alpha-Gal antibodies accounting for 1-3% of the total amount in the blood stream. A new developed test demonstrated that around 30% of alpha-Gal epitopes in BHVs are still reactive even after fixation with GLU, prior to be implanted. It’s likely the time to try to produce alpha-Gal free BHVs that are likely to longer-lasting, resulting in a better quality of life for patients. The removal of the alpha-Gal molecules detected in a tissue might also provide new insight of deleterious effects possibly related to the presence of secondary substances whose role is currently overshadowed by the preponderant reactions of the alpha-Gal [4].

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References:

1. Zilla P et al. – Prosthetic heart valves: catering for the few. Biomaterials 2008;29(4):385-406.

2. Bloch O et al. – Immune response in patients receiving a bioprosthetic heart valve: lack of response with decellularized valves. Tissue Eng Part A 2011;17(19-20):2399-2405.

3. Naso F et al. – First quantitative assay of alpha-Gal in soft tissues: presence and distribution of the epitope before and after cell removal from xenogeneic heart valves. Acta Biomater. 2011;7(4):1728-1734.

4. Naso F et al. – First quantification of alpha-Gal epitope in current glutaraldehyde-fixed heart valve bioprostheses. Xenotransplantation. 2013;20(4):252-261.

Is rejection of human transplanted animal tissues avoidable?

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The demand for organ and tissue transplantation (heart, kidney, cardiac valves, cartilage) already far surpasses the number of available donors. The lack of resources has led biomedical researchers to explore the use of animal donors as an attractive and unlimited source for biological devices (xenotransplantation). Xenogeneic tissues, the tissue that are harvested from common animals such as pigs, sheep, cows, horses, are currently employed in clinical practice to create biological prostheses for the substitution of specific structures like heart valves, ligaments, pericardium etc.. and in the repair of various damaged body costituents (gastric-mucosa, nerves, cartilage). It is well know by many scientific studies that xenogeneic tissues express superficial molecules, called epitopes, alpha-Gal in primis, but also the major histocompatibility complex (MHC), capable of triggering hyperacute and acute vascular rejection phenomena. An epitope is definied as an antigenic determinant, something that it is not recognized as a “personal effect” by the immune system cells (B cells, T cells) and antibodies of the recipient organism. Once a tissue is recognised as non-self, a series of events are triggered ending in vascular thrombosis.

Currently, commercially available xenogeneic bioprosthes are processed with treatments which have not been proven able to completely mask or inactivate such epitopes. Most common methods involves the use of chemicals to stabilize matrices (the main component of tissues); these agents penetrate deep into the substrate and cover it quite permanently like a molasses. Glutaraldehyde fixative is one of them and even if it allows the clinical use of tissues, it is toxic for cells sorrounding the site of implantation and for the graft itself, therefore prostheses biological integration is not possible.

The ability to ascertain alpha-Gal and MHC epitopes removal from a xenogeneic tissue is closely related to the possibility of its quantitative determination. Recently, a new patented test has been processed by prof. Gino Gerosa’s research team at the Cardiac Surgery Reserch Lab of the University of Padova. The test developed assesses the presence of still unmasked alpha-Gal epitopes that rather than triggering a hyperacute rejection, due to reduction in number, lead to a chronic phlogosis that have dramatic effects in the mid-long term after the bioprostheses has been implanted, especially when this xenogeneic tissue is used to construct heart valves substitutes.

Analysis of currently commercial heart valve bioprosthes assessed, for the first time, the presence of the alpha-Gal
epitope used for about 40 yrs for surgical reasons. Such quantification might provide indications of biocompatibility relevant for the selection of bioprosthetic devices and an increase in the confidence of the patient. It might
become a major quality control tool in the production and redirection of future investigation in the quest for alphaGal-free long-lasting substitutes.

These are the first scientific results that shed light on one of the most critical cause for mid-long term failure of heart valve bioprosthese.

Related article:

First quantification of alpha-Gal epitope in current glutaraldehyde-fixed heart valve bioprostheses by Naso F e al. Xenotransplantation. 2013 doi: 10.1111/xen.12044