Entrevistas

Recently, cell transplantation has received significant research potential, not only due to the recognition of the substantial functional benefits possible, but also due to the development of human stem cell lines and technologies to control their differentiation and expansion.[19, 66, 80, 128] However, the immense potential of the technology is yet to be fully realised due to considerable variations in cell survival and innervation within the host tissue. This is likely due, in part, to the non-conducive regenerative environment within the (damaged) adult brain.

2.4.1 Insufficient trophic enrichment

A wide variety of neurotrophic factors delivered with appropriate timing and in an appropriate dose is required to promote cell survival, to reduce secondary tissue damage, and to promote tissue regeneration and remodelling post brain injuries.[129, 130] For example, endogenously secreted BDNF is widely understood to be responsible for neural survival, development, synapse formation, plasticity, and cognitive functions.[131] As a result, it has been extensively used for the treatment of injured brain.[132] However, the in vivo half-life of BDNF is short in the order of minutes,[133] similar to FGF (ca. 3 minutes),[134] and NGF (ca. 45min).[135] Therefore, the biological challenge is to attain the controlled and sustained delivery of trophic factors with an appropriate dose within the therapeutic window for the specific factor. Only then will it be possible to offer long-term protection with neurotrophic factors and to improve the regenerative outcomes.

This will most likely involve the incorporation of growth factors within the cell transplantation media. For example, the combination of NSCs and BDNF resulted in a more pronounced improvement in cognitive abilities of rats with AD compared to control animals only receiving NSC grafts.[97] To date, the most common approaches via direct addition of neurotrophic factors or by employing genetic modified cells to increase the secretion of relevant of growth factor, are used to present trophic factors within stem cell grafts.[21, 132] However, the genetic modified cells in gene therapy associated with issues including limited tropism and immunogenicity are still to be optimised by the field, which has limited their wide use in the clinic for brain repair.[136]

As a result, direct injection of neurotrophic factors would be a considerable focus, particularly, research targeting sustained and controlled delivery. This would ensure appropriate therapeutic effects of the delivered drug, while also minimising the invasiveness of multiple administrations to achieve the same outcome.[32]

2.4.2 Increased cell death post administration

Previous studies have demonstrated that cell survival has significant effects on the recovery of functions in patients.[23] Examples of grafted cells in PD patients

indicated the positive correlation between the number of surviving grafted cells and symptomatic relief of behavioral effects.[137] However, some of early studies reported that only 3-20% of grafted DA neurons survived in treatment of PD, [138, 139], indicating lower cell survival in cell transplantation challenging brain repair. This lower cell survival is attributed to a wide variety of factors. Both donors and recipients affect cell survival. When cells used in cell transplantation are sourced from the donors, there can be issues associated with tissue hypoxia and hypoglycaemia, and the mechanical and traumatic damage. These problems are compounded by issues associated within the recipients including free radical formation, growth factor deprivation, toxin accumulation, and presence of innate and/or adaptive immune response.[138] Aa a result, in many cases, the utility of anti-apoptotic and anti-inflammatory agents, and neurotrophic factors in cell transplantation has been studied to increase the ability of anti-cell death and cell survival.[119, 126, 138, 140]

In addition to factors from donors and recipients above, the poor transplantation procedure during cell transplantation results in considerable cell death. This process involves the primary cell dissection, cell culture in vitro, cell injection procedures, and development of grafted cells post, which cause the associated issues to hinder cell survival. For instance, direct administration of cells into brain is the most versatile and accurate methodology for cell grafting.[141] Such methods employ the use of a straight cannula and syringe for cell delivery, to ensure that the process is as minimally invasive as possible, thus avoiding an extensive iatrogenic injury. However, some shortcomings associated with this process still exist, such as a limitation on the scale-up volume and cell distribution post administration.[141] Recently, the novel radially branched deployment (RBD) device has been introduced to attempt to address some of these issues.[142] While some improvements such as the increased total delivery volume for relatively large target regions, the increase cell viability, and overcoming the reflux of infuscate along the penetration tract have been demonstrated post introduction of RBD, the same distribution of cells located in separate locations of big host brain(e.g. human) cannot be achieved compared to relatively small brain (e.g. mouse).[141] Furthermore, transplanted cells still experience a damaging shear regime and the different velocities from syringe to cannula/needle in the media,,[5]

with cell suspension being strongly influenced by the strain rate associated with the velocity flow field.[20] This leads to mechanical membrane disruption and deformation, which accounts for the large percentage of cell death immediately following transplantation.[141] Apart from those factors, the continued cell death post transplantation also considerably depends on the lack of trophic factor discussed in the above section (2.4.1) and the presence of inflammation/immune response discussed in the subsequent section (2.4.3). Therefore, a delicate procedure should be employed to optimise the efficacy cell transplantation in the brain. Specifically, extensive optimisation of the delivery media that cells are suspended within, can aid in mechanical protection during administration. Importantly, post administration, the media should provide an environment that is capable of physically and biochemically supporting transplanted cells to enhance their survival and integrations.

2.4.3 Immune response

Grafted cells being recognized as foreign materials after cell transplantation are expected to reside within the targeted regions of brain. As such, there is often an immune and/or inflammatory response, associated with not only the iatrogenic injury but also other introduction of foreign material/cells. This response is initiated by the major histocompatibility complex (MHC) present in translated cells in the immune system as antigen from graft against host, which can last for several weeks. This often compromises the function of surviving endogenous cells and even diminishes the grafted cells with subsequently immune-mediated rejection,[9] despite their advanced survival during administration. Therefore, this immune response poses a significant challenge for cell transplantation.

For clinic therapy, immunosuppressive agents, called immunosuppressants, have been illustrated to reduce both immune system and immune response for anti-rejection effects of transplanted cells.[143] This regime designed to reduce the activation of T cells, however, is not effective in vivo and even results in severe side effects such as an increased risk of infection caused by the compromise of the immune system.[144, 145] Such issues have resulted in major studies focusing on the employing autologous cells, and stem and progenitor cells. These cells selected are primarily due to the superior compatibility and less issues

associated with the presentation of MHC antigens.[8, 146] Autologous cell transplantation isolated from individuals’ own cells such as MSCs, has been investigated, which has demonstrated the remarkable phenotypic plasticity into neuronal cell from adult bone marrow.[147-149] MSC transplantation in TBI has also significantly improved the functional recovery, because of the anti-inflammatory and immunomodulatory benefits of MSCs. These benefits are derived from an immuno-privileged environment formed via the de-activation of dendritic cells (DCs) and T cells, and the secretion of trophic factors such as anti-inflammatory cytokines, e.g. interleukin(IL)-10 and transforming growth factor-beta 1(TGF-b1).[150-152] Furthermore, transplanted allogeneic MSCs have also survived for the extended duration in vivo, implying the lack of immune recognition and clearance,[153] whereby they can hamper the presentation of pro-inflammatory cytokines and improve cell survival.[154] In line with MSC acting on immunomodulatory cells, NSPCs and NPCs function in the similar way to work for cell therapy.[155] For example, NPCs transplanted has provided the neuroprotection in chronic CNS inflammation by inducing the apoptosis of blood-borne CNS-infiltration encephalitogenic T cells, which indicates their therapeutic effects in brain injuries associated with chronic inflammation such as stroke.[156]

In spite of these advantages of autologous grafts, their limited availability and the difficulty of the rapid expansion of autologous cells in vitro, meaning that obtaining appropriate cell numbers within a short duration which has provided the stimulus to consider the other novel protocol approaches, have challenged their wide use. [157] Therefore, it is essential to find the novel approaches to avoid the related immune complications. The approaches either modulating the immune response via attenuation of immune activation or providing a supportive environment for cells to physically escape the immune response are prefered. The employment of biomaterials as the delivery vehicle is one of the most potential approaches to achieve by researchers. There are many candidate biomaterials that have been considered, and the interested readers are directed to the following insightful reviews.[3, 15, 158-162] While many tissue engineering scaffolds have been trialled to progress cell transplantation towards clinical applications, here we will focus on peptide-based hydrogels mimicing the ECM due to their inherent

shear-thinning property to encapsulate and protect grafted cells during administration, and their ability to be rationally designed to offer the long term support for cell survival..