The apical papilla is a poorly vascularized tissue present in the apical portion of immature teeth (Retana-Lobo & Reyes-Carmona, 2023). Embryologically, it is derived from the dental papilla, a condensation of ectomesenchyme, which, induced by the overlying oral epithelium, evolves into the dental pulp that secretes dentin (D’Souza, 2002) (Tucker et al., 2004). As the root continues to develop, the dental papilla becomes apical to the pulp tissue, giving rise to the apical papilla (Sonoyama et al., 2008).

In 2008, Sonoyama et al. isolated and characterized the resident stem cells in the human apical papilla (SCAPs). These cells express mesenchymal stem cell markers, are highly proliferative, and possess a high osteo/dentinogenic potential (Sonoyama et al., 2008). Moreover, they can differentiate into odontoblast-like cells and produce pulp-like and dentin-like tissues in a non-infected environment in vivo (Huang et al., 2010).

Over the last decades, SCAPs have been of particular interest because they are believed to transfer into root canals in regenerative endodontic procedures of immature necrotic teeth (Lovelace et al., 2011) and to be responsible for the continued root development observed in most treated teeth (Chrepa et al., 2020). Necrotic teeth are generally associated with chronic periapical infection and evidence indicates that bacterial antigens remain present in dentin even after adequate debridement (Diogenes & Hargreaves, 2017). Tissue inflammation causes elevated levels of cytokines, including IL-6 and several others (Cooper et al., 2011), which, if sustained, affect the tissue's ability to regenerate (Cooper et al., 2014). Therefore, anti-inflammatory agents, including IL-10, are produced to limit tissue damage and promote a return to homeostasis (Cooper et al., 2014). Although it has been demonstrated that SCAPs can survive, retain stemness, and present increased osteogenic and angiogenic potential after periapical infection (Chrepa et al., 2017), alterations due to infection and presence of toxins in the environment may explain why after regenerative endodontic procedures the new mineralized tissue responsible for root maturation is reparative tissue, distinct from dentin (Meschi et al., 2021).

There is a great variation in the literature with respect to the effect of bacteria and bacterial by-products on the biological characteristics of dental stem cells. Some in vitro studies have shown that the presence of bacterial antigens, such as lipopolysaccharides (LPS), either enhances mineralization (Chrepa et al., 2017) or does not affect cell viability, mineralization capacity, or the expression of odontogenic and mineralizing genes (DSPP and OPN) of SCAPs (Lertchirakarn & Aguilar, 2017). However, others indicate that LPS have a negative effect on stem cell fate by upregulating the release of proinflammatory mediators (Zhang et al., 2013; Widbiller et al., 2018; Bucchi et al., 2023) and by interfering with odontoblast-like cell differentiation and mineralization of dental stem cells (Widbiller et al., 2018; Lei et al., 2020). Besides these variations in the literature, the effect of an inflammatory microenvironment on other biological properties of SCAPs, such as the release of anti-inflammatory interleukin-10 and the expression of some mineralizing genes (i.e. collagen type I alpha 1 chain (Col1A1)), have not been studied so far.

From a clinical perspective, in the field of regenerative endodontics, previous studies have demonstrated that differentiation of stem cells into odontoblast-like cells and the consequent secretion of the dentin to complete root development is relevant, since it has a positive impact in the biomechanical performance of teeth (Bucchi et al., 2019), and therefore, in their prognosis. Similarly, to reestablish the pulp tissue after pulp necrosis allows the tooth to recover its sensitive and defensive capacity to respond to new caries lesions (Couve et al., 2013). Thus, due to the role and use of SCAPs in regenerative therapies, a better understanding of the effect of bacterial toxins on their biological characteristics is required.

This study aimed to assess whether the biological characteristics of stem cells from the apical papilla (SCAPs), such as viability, adhesion to dentin, expression of mineralizing and odontogenic genes, mineralization, and release of immunomodulatory cytokines, are affected by the inflammatory environment (LPS).