[Immunohistochemistry of nodular vasculitis. A possible role of delayed cellular hypersensitivity].

The physiopathogenesis of nodular vasculitis is still unknown: there is probably an initial Arthus' phenomenon responsible for the lesions of the deep vessels, but the frequent recurrences and the long time duration of the hypodermic nodules are still not understood. Dendritic protein S100 positive cells had previously been observed within hypodermal granulomas. The purpose of this study was to confirm these observations in a large group of nodular vasculitis, and to compare the expression of S100 protein in other vascular diseases and in pure panniculitis. Immunohistochemical staining for S100 protein was performed on paraffine-embedded samples: 45 cases of nodular vasculitis, 21 of panniculitis, 10 of superficial leucocytoclastic vasculitis, 10 of periarteritis nodosa, and 10 of erythema nodosum were analyzed by means of optic microscopy. Numerous dendritic S100 protein positive cells were found within hypodermal granulomas in most of the cases of nodular vasculitis, but these cells were absent in the 3 cases of initial deep leucocytoclastic vasculitis lesions of nodular vasculitis. The mean percentage of these cells was 4.5 p. 100 in the hypodermal inflammatory areas; they were observed mainly around vascular lesions. Hypodermal dendritic cells were absent or less numerous in all other cases, and a high number of such cells seems to be observed only in nodular vasculitis. Because of their dendritic morphology, and the high expression of S100 protein, these cells could be interdigitated cells, which are known to present antigens to T-lymphocytes. They could play an important role in generating the chronicity of nodular vasculitis, by a mechanism of delayed cell-mediated hypersensitivity, after the initial lesions caused by immune complexes.