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RESEARCH REPORT |
1 Laboratory of Development of Dental Tissues, EA MENRT 1892, IFR 62, Faculty of Odontology, Lyon 1 University, G. Paradin Str., 69372 Lyon Cedex 08, France; and
2 Schering-Plough Corp., Laboratory for Immunological Research, 69571 Dardilly, France;
* corresponding author, farges{at}laennec.univ-lyon1.fr
| ABSTRACT |
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KEY WORDS: tooth pulp immune response HLA-DR TGF receptor chemokine
| INTRODUCTION |
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When dentin is being destroyed by caries or operative procedures, immature DC accumulate in the odontoblast layer and the subjacent pulp tissue close to the lesion in locations strategic to sample foreign antigens entering the dentinal tissue (reviewed in Jontell et al., 1998; Sakurai et al., 1999). Immature DC present in the interstitial (connective) tissues are characterized by a dendritic morphology, constitutive expression of class II major histocompatibility complex molecules that confer on them their antigen-presenting capacity, and expression of Factor XIIIa and CD68 markers (Banchereau and Steinman, 1998; Liu, 2001). After capture of foreign antigens at the dentin-pulp interface, DC migrate, while undergoing a process of maturation, via the afferent lymphatic to regional lymph nodes, to stimulate naïve T-lymphocytes, thus initiating a primary immune response (Banchereau and Steinman, 1998).
Factors favoring the accumulation of immature DC in the peripheral pulp in response to dentin injury are unknown, but TGF-ß1 diffusing intratubularly from damaged dentin could be implicated in this process, given its crucial role in the control of immune response and DC behavior (Strobl and Knapp, 1999). Here, we used a culture system of thick-sliced human teeth allowing for the transdentinal diffusion of TGF-ß1 (Melin et al., 2000; Lucchini et al., 2002) to analyze, by immunostaining, the distribution and activation status of DC in odontoblast, subodontoblast, and pulp core compartments.
| MATERIALS & METHODS |
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0.05). | RESULTS |
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| DISCUSSION |
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In our in vitro model, DC, or their precursors, were pre-existing in the pulp, given that there is no vascularization in the tooth slice culture system, and different mechanisms, not mutually exclusive, may have contributed to the apparent accumulation of DC in the odontoblast and subodontoblast layers: DC precursors, including monocytes residing in the tissue, may have differentiated into migratory immature DC under the influence of TGF-ß1 (Randolph et al., 2002). Furthermore, TGF-ß1 may have modified the spatial distribution of DC in the tooth. Indeed, for many years, TGF-ß1 has been known not only to inhibit DC proliferation, maturation, and apoptosis (Strobl and Knapp, 1999; Liu, 2001), but also to stimulate migration of human immature DC in vitro by increasing the expression of specific chemokine receptors constitutively present in the DC membrane (Sato et al., 2000). The migratory responses to Macrophage Inflammatory Protein-1
(MIP-1
), Macrophage Inflammatory Protein-3
(MIP-3
), RANTES, and Stromal Cell-Derived Factor-1
(SDF-1
) were all enhanced by TGF-ß1, while, reciprocally, co-stimulation of DC with TGF-ß1 and either of these chemokines increased their response to the cytokine (Sato et al., 2000). Chemokines are a family of nearly 50 small chemotactic cytokines produced by various cell types in response to pro-inflammatory cytokines and to bacterial and viral products (Baggiolini, 1998). They are mainly responsible for the recruitment of immune cells to sites of inflammation or infection. One of them, Interleukin-8, has been localized in cells of the odontoblast layer in vitro (Levin et al., 1999). Its expression increased after lipopolysaccharide stimulation, suggesting that odontoblasts would be capable of producing pro-inflammatory mediators, thereby actively participating in the recruitment of immune cells in response to caries-derived bacterial by-products. In this context, the identification of odontoblast-derived chemokines that could trigger DC accumulation at the dentin-pulp interface in association with TGF-ß1 in vivo would help improve our understanding of the mechanisms controlling the early immune response in the pathological pulp.
Like most cells present in human teeth (Sloan et al., 2001; Lucchini et al., 2002), pulp DC expressed TßRII, the type II receptor necessary for TGF-ß1 cell binding and signal transduction (Jayaraman and Massagué, 2000). Thus, pulp immature DC could also respond directly to TGF-ß1 stimulation.
A few mature DC were detected, in vivo and after culture, in both the deep pulp and the central part of the roots, but never in pulp horns. Those HLA-DR-positive cells were CD68-negative but expressed the lysosome-associated membrane protein DC-LAMP that is "turned on" upon activation of DC (de Saint-Vis et al., 1998). Mature DC-LAMP-positive DC are constitutively present in lymphoid organs but not in normal peripheral tissue. However, they have been observed at inflammatory lesions of both the skin and the joints. Interestingly, DC-LAMP-positive DC present in the tooth pulp in vivo still expressed Factor XIIIa, suggesting that they had been recently activated. The reason for the presence of those few mature DC in apparently sound pulp tissue is not known, and their potential role in either immunization or peripheral tolerance remains to be determined (Banchereau and Steinman, 1998; Huang et al., 2000).
In humans, TGF-ß1 is produced by secretory odontoblasts in sound teeth (Sloan et al., 2000; Lucchini et al., 2002), and this expression is increased under caries lesions (Sloan et al., 2000). Moreover, TGF-ß1 is stored in the dentinal matrix (Finkelman et al., 1990; Cassidy et al., 1997; Zhao et al., 2000), and, when dentin is being destroyed by caries or operative procedures, it is released from the matrix and could diffuse to the pulp through dentinal tubules (Finkelman et al., 1990; Magloire et al., 2001; Smith and Lesot, 2001). Thus, odontoblasts and/or damaged dentin, via a gradient of TGF-ß1 radiating from the pulp-dentin interface to the pulp core, could contribute, through differentiation from precursors and/or chemotactic recruitment, to the accumulation of immature DC in the peripheral pulp. In this way, TGF-ß1 would stimulate the hosts ability to respond rapidly to infection or injury and initiate repair, allowing for tertiary dentin formation by primary or replacement odontoblasts.
In conclusion, we present evidence that immature DC are attracted into the odontoblast layer by TGF-ß1 originating from dentin. This factor could thus direct DC trafficking in pathological conditions resulting from dentin injury. From a clinical point of view, the use of TGF-ß1 as a dentin/pulp-capping agent might represent a therapeutic strategy for modulating the early immune response and favoring healing in inflamed pulps, with the accumulation of immature DC at the pulp periphery further minimizing the risk of occurrence of a novel infection.
| ACKNOWLEDGMENTS |
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Received November 22, 2002; Last revision March 31, 2003; Accepted May 7, 2003
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