Neuron-interacting satellite glial cells in human trigeminal ganglia have an APC phenotype

Satellite glial cells (SGC) in sensory ganglia tightly envelop the neuronal cell body to form discrete anatomical units. This type of glial cell is considered neuroectoderm-derived and provides physical support to neuron somata. There are scattered hints in the literature suggesting that SGC have an immune-related function within sensory ganglia. In this study, we addressed the hypothesis that SGC are tissue-resident APC. The immune phenotype and function of a large series (n = 40) of human trigeminal ganglia (TG) were assessed by detailed flow cytometry, in situ analyses, and functional in vitro assays. Human TG-resident SGC (TG-SGC) uniformly expressed the common leukocyte marker CD45, albeit at lower levels compared with infiltrating T cells, and the macrophage markers CD14, CD68, and CD11b. In addition, TG-SGC expressed the myeloid dendritic cell (DC) marker CD11c, the T cell costimulatory molecules CD40, CD54, CD80, and CD86 and MHC class II. However, the mature DC marker CD83 was absent on TG-SGC. Functionally, TG-SGC phagocytosed fluorescent bacteria, but were unable to induce an allogeneic MLR. Finally, TG-infiltrating T cells expressed the T cell inhibitory molecules CD94/NKG2A and PD-1, and the interacting TG-SGC expressed the cognate ligands HLA-E and PD-L1, respectively. In conclusion, the data demonstrate that human TG-SGC have a unique leukocyte phenotype, with features of both macrophages and immature myeloid DC, indicating that they have a role as TG-resident APC with potential T cell modulatory properties.