Endothelial proliferation in the delayed hypersensitivity reaction: an autoradiographic study.

The time-sequence of endothelial proliferation in the delayed hypersensitivity reaction was examined in the skin of guinea pigs. Animals were sensitized to tuberculin (PPD) and dinitrochlorobenzene (DNCB) and skin lesions were examined 12 hr to 7 days after challenge. Thirty minutes before sacrifice 3H-thymidine (3H-T) was injected i.p. and endothelial labeling was assessed in autoradiographs of 1.5-mu thick methacrylate sections. Endothelial labeling became manifest at 24 hr (PPD) and 12 hr (DNCB) after challenge, reaching a peak at 48 to 72 hr in PPD-sensitized animals and 36 to 48 hr in DNCB-sensitized animals. Peak labeling indices were 7.88 and 15.08%, respectively. Labeling indices for nonsensitized control groups were always less than 1%. Endothelial labeling was associated, in time of occurrence, distribution, and magnitude with mononuclear cell infiltration of the lesions. These findings thus document the occurrence of significant endothelial cell proliferation in delayed hypersensitivity responses and suggest that the events leading to the expression of hypersensitivity also result in endothelial replication.