Antibacterial Properties of Oral Progenitor Cells: The role of Soluble Factors

Objectives: Oral mucosal lamina propria-progenitor cells (OMLP-PCs) are a multipotent PC population with known immunosuppressive properties. Many immunomodulatory soluble factors are also documented to be antimicrobial; leading to the hypothesis that OMLP-PCs, in addition to their immunoregulatory actions, may possess antibacterial properties. We have previously presented data demonstrating the broad spectral antibacterial properties of OMLP-PCs through a constitutive and contact-independent mechanism. The current aims of this study are to elucidate the soluble factors involved in mediating this effect.

Methods: OMLP-PCs (± pre-treatment with interferon (IFN)-g) were incubated with Gram positive or Gram negative bacteria for 7-14hrs (midlog of each bacterium). Genomic and protein levels of potential soluble antibacterial factors were examined by QPCR, ELISA and/or Western Blotting. Retained supernatants were filter-sterilised and cultured with live bacteria ± blocking antibodies to identified factors. The antibacterial effects of the identified factors were also confirmed by co-culture of the pure peptide form with each bacterium.

Results: Indoleamine 2,3 dioxygenase (IDO) expression and activity directly correlated with IFN- g exposure and did not contribute to OMLP-PC antibacterial activity. LL37 expression was not detected, irrespective of IFN- g and/or bacterial exposure. Haptoglobin was detected in the supernatant samples and found to be antibacterial against Gram negative bacteria at low levels (50pg/mL), with blocking restoring bacterial growth (P<0.01). OPG expression and secretion was constitutive and found to be antibacterial against Gram positive bacteria, with blocking significantly restoring bacterial growth (P<0.05).

Conclusions: OMLP-PCs secrete a range of soluble factors capable of mediating their antibacterial effects via a complex interplay of direct bacterial interactions and also potential signalling to innate immune cells. Unlike bone marrow mesenchymal stromal cells, LL37 and IDO do not play a role in mediating OMLP-PC antibacterial effects, confirming the role the in vivo microenviroment plays in modulating the phenotype of PCs.