Neuronal-immune Changes During Pulpitis

Abstract

Introduction: Dental pulp tissue is densely innervated and it has unique immunological characteristics. Upon injury such as deep caries, exposing the pulp, a strong neuronal and immune response occurs which modulates pain and the tissue damage. Our knowledge on the dynamics of initial innate immune response, whether this response is modulated by sensory afferents of the pulp, and how this innate immune response alters tissue damage and/or pain outcome is limited. Materials and Methods: Dental pulp exposure in mice was used as the pulpitis model. To investigate the innate immune response, pulp tissue was collected from the permanent molars at different time points and flow cytometry was performed. To investigate tissue damage, H&E staining was performed. Immunohistochemistry and in-situ hybridization were performed to capture spatial changes of innate immune cells, sensory afferents, and bacteria invasion, respectively. Finally, mechanical pain sensitivity was captured by facial Von Frey stimulation and spontaneous pain was captured using the Mouse Grimace Scale. Data were analyzed using two-way ANOVA, t-tests, or repeated measure of ANOVA with appropriate multiple comparison tests. Results: We found that neutrophils constituted 70-90% of immune cell populations up to day 7. We also found that the neuropeptide CGRP, released from sensory afferents, contributed to the recruitment of myeloid cells (p=0.017) while also increasing spontaneous pain at day 1 (p=0.02). Moreover, when we depleted neutrophils and monocytes, we found that there was more tissue necrosis at day 6 (p=0.002) and bacteria had penetrated deeper parts of the tissue, whereas mechanical pain was lower compared to control group (p=0.03). Conclusions: Neutrophils and monocytes are crucial to protect dental pulp tissue from further bacterial penetration and tissue damage while also contributing to mechanical pain sensitivity. We also found that CGRP modulates innate immune responses either by causing vasodilation or by directly mediating neutrophil recruitment while contributing to spontaneous pain behavior during pulpitis.