## 4 Discussion
We found that NCs, although still negatively affected by low pH, maintained higher cell viability and synthesised appropriate matrix components such as sGAGs and collagens in a more appropriate ratio than NP cells. In comparison, NP cells exhibited diminished cell viability at pH 7.1, representative of a healthy disc, which also resulted in a reduction in the accumulation of sGAGs when compared to NCs. This correlates with results from [PERSON] et al., where the expression profile of bovine AC and NP cells cultured in acidic conditions was compared, demonstrating a significantly lower expression of matrix related genes with increasing acidity for NP cells [53]. Importantly, the sGAG:collagen ratio was considerably higher in NCs compared to NP cells for all culture conditions. A high sGAG:collagen ratio may be considered an appropriate parameter for the identification of NP-like tissue type, as it represents a tissue rich in sGAGs, replicating the composition typical of healthy NP [63]. In conclusion, the results of the present study demonstrate that among the plethora of characteristics that render the degenerated NP an extremely hostile niche, environmental pH was found to have the greatest impact on cell viability and matrix production. While NP cells appeared to be unaffected by the presence of inflammatory cytokines in the physiological range, low pH had a greater impact on viability and matrix synthesis, suggesting they may be a less effective cell type compared to NCs for cell-based NP regeneration. Although it is clear that environmental acidity is an important factor to consider in the context of disc regeneration, to the best of the authors knowledge, there are no clinically feasible approaches to neutralising or augmenting the intradiscal pH. A possible approach may involve injecting pH buffering biomaterials into the disc space. This may have some short-term benefit and create a permissible microenvironment to facilitate regeneration by transplanted cells, but the longevity of such an approach would need to be established. In parallel, techniques to non-invasively measure intradiscal pH in vivo would be required in order to tailor or adjust the treatment according to the severity of the degenerated state. Recent work has proposed a promising clinical MRI approach to detect pH changes associated with IVD degeneration in a swine model [64]. Overall, the presence of the key inflammatory cytokines IL-1\(\beta\) and TNF-\(\alpha\), that have previously been identified in human discs did not have any appreciable impact when supplemented in physiologically relevant concentrations. Therefore, developing imaging modalities to identify and characterise intradiscal pH in vivo in combination with pH buffering biomaterials may provide for an effective strategy to appropriately select and treat patients with cell-based therapies. Both the authors ([PERSON] and [PERSON]) have contributed equally to this manuscript and have read and agreed to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript. This work was supported by Science Foundation Ireland Career Development Award (15/CDA/3476).