Significant progress has been made during the past decade towards the clinical adoption of cell-based therapeutics. However, existing cell-delivery approaches have shown limited success, with numerous studies showing fewer than 5% of injected cells persisting at the site of injection within days of transplantation. Although consideration is being increasingly given to clinical trial design, little emphasis has been given to tools and protocols used to administer cells. The different behaviours of various cell types, dosing accuracy, precise delivery, and cell retention and viability post-injection are some of the obstacles facing clinical translation. For efficient injectable cell transplantation, accurate characterisation of cellular health post-injection and the development of standardised administration protocols are required. This review provides an overview of the challenges facing effective delivery of cell therapies, examines key studies that have been carried out to investigate injectable cell delivery, and outlines opportunities for translating these findings into more effective cell-therapy interventions.

Bronchial thermoplasty is a treatment for asthma. It is currently unclear whether its histopathological impact is sufficiently explained by the proportion of airway wall that is exposed to temperatures necessary to affect cell survival.Airway smooth muscle and bronchial epithelial cells were exposed to media (37–70°C) for 10 s to mimic thermoplasty. In silico we developed a mathematical model of airway heat distribution post-thermoplasty. In vivo we determined airway smooth muscle mass and epithelial integrity pre- and post-thermoplasty in 14 patients with severe asthma.In vitro airway smooth muscle and epithelial cell number decreased significantly following the addition of media heated to ≥65°C. In silico simulations showed a heterogeneous heat distribution that was amplified in larger airways, with <10% of the airway wall heated to >60°C in airways with an inner radius of ∼4 mm. In vivo at 6 weeks post-thermoplasty, there was an improvement in asthma control (measured via Asthma Control Questionnaire-6; mean difference 0.7, 95% CI 0.1–1.3; p=0.03), airway smooth muscle mass decreased (absolute median reduction 5%, interquartile range (IQR) 0–10; p=0.03) and epithelial integrity increased (14%, IQR 6–29; p=0.007). Neither of the latter two outcomes was related to improved asthma control.Integrated in vitro and in silico modelling suggest that the reduction in airway smooth muscle post-thermoplasty cannot be fully explained by acute heating, and nor did this reduction confer a greater improvement in asthma control.

Endoscopic treatment of Barrett's oesophagus often leads to further damage of healthy tissue causing fibrotic tissue formation termed as strictures. This study shows that synthetic, self-assembling peptide hydrogels (PeptiGelDesign) support the activity and function of primary oesophageal cells, leading to epithelialization and stratification during in vitro 3D co-culture. Following buffering in culture media, rat oesophageal stromal fibroblasts (rOSFs) are incorporated into a library of peptide hydrogels, whereas mouse oesophageal epithelial cells (mOECs) are seeded on the surface. Optimal hydrogels (PGD-AlphaProC and PGD-CGD2) support mOEC viability (>95%), typical cell morphology (cobblestone-like), and slower migration over a shorter distance compared to a collagen control, at 48 h. Positive expression of typical epithelial markers (ZO-1 and cytokeratins) is detected using immunocytochemistry at day 3 in culture. Furthermore, optimal hydrogels are identified which support rOSF viability (>95%) with homogeneous distribution when incorporated into the hydrogels and also promote the secretion of collagen type I detected using an enzyme linked immunosorbent assay (ELISA), at day 7. A 3D co-culture model using optimal hydrogels for both cell types supports a stratified epithelial layer (expressing involucrin and AE1/AE3 markers). Findings from this study could lead to the use of peptide hydrogels as a minimally invasive endoscopic therapy to manage oesophageal strictures.

Interactions of anaerobic gut bacteria, such as Clostridium difficile , with the intestinal mucosa have been poorly studied due to challenges in culturing anaerobes with the oxygen-requiring gut epithelium. Although gut colonization by C. difficile is a key determinant of disease outcome, precise mechanisms of mucosal attachment and spread remain unclear. Here, using human gut epithelial monolayers co-cultured within dual environment chambers, we demonstrate that C. difficile adhesion to gut epithelial cells is accompanied by a gradual increase in bacterial numbers. Prolonged infection causes redistribution of actin and loss of epithelial integrity, accompanied by production of C. difficile spores, toxins, and bacterial filaments. This system was used to examine C. difficile interactions with the commensal Bacteroides dorei , and interestingly, C. difficile growth is significantly reduced in the presence of B. dorei . Subsequently, we have developed novel models containing a myofibroblast layer, in addition to the epithelium, grown on polycarbonate or three-dimensional (3D) electrospun scaffolds. In these more complex models, C. difficile adheres more efficiently to epithelial cells, as compared to the single epithelial monolayers, leading to a quicker destruction of the epithelium. Our study describes new controlled environment human gut models that enable host–anaerobe and pathogen–commensal interaction studies in vitro .

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