Effects of weightlessness on expression of TLR4/CD14 and chemotactic factors in gingival tissue of rhesus macaques

Background and objective: Healthy stomatognathic system is a guarantee for nutrient intake of astronauts during long-term spaceflight, but few researches have been carried out to investigate the influence of weightlessness on oral health. This study was aimed to explore the effects of simulated weightlessness on the histology and immune status of gingival tissues of rhesus macaques using a 6w ground-based weightlessness simulation model. Methods: An internationally recognized ground-based microgravity simulation model (-10° Head-down tilted bed rest, HDBR) on rhesus macaques was employed. Fifteen healthy male rhesus macaques weighing 4 to 8 kg were divided into three groups: control group (Group A), 6w -10° HDBR group (Group B) and 6w -10° HDBR followed by 4w recovery group (Group C), according to stratified random grouping design. After the macaques were sacrificed with humanistic care, oral gingival tissues were sampled immediately. The pathological changes of gingival tissues were investigated by hematoxylin and eosin (HE) staining. Histopathochemical method and real-time PCR were performed for the determination of protein and mRNA expression of TLR-4, CD14, MCP-1, MIP-1α and CCL20. Results: Inflammatory cells infiltration was found in the free marginal gingiva of all of the three groups, but it was more severe in group B than in the other two groups. In addition, the cornified envelope of gum in group B was also thinner than that in group A and group C. The expression of TLR-4, CD14, MCP-1, MIP-1α and CCL20 was all slightly reduced by 6w -10° HDBR at both protein and mRNA levels, but only the CD14 IHC scores showed statistically significant difference between Group A and Group B (P < 0.05). We also found that all of the reduced expression of TLR-4, CD14, MCP-1, MIP-1α and CCL20 were restored by 4w recovery, however, only MCP-1 IHC scores showed significant difference between Group B and Group C (P < 0.05). Conclusion: -10° HDBR for 6 weeks could promote the inflammatory cells infiltration by weakening the local immunity of oral gingiva. This study fills a gap in space medical researches and provides some experimental basis for oral healthcare of astronauts during their spaceflight.