Immunohistological study of the cutaneous delayed type hypersensitivity reaction in sheep

The cutaneous delayed type hypersensitivity (DTH) reaction may be experimentally initiated both as an in-vivo technique for the study of the cell mediated arm of the immune system, and also as an accurate clinical test of the functional capacity of this part of the immune response. This study was performed to fully evaluate the immunohistological characteristics of the normal DTH reaction utilising an ovine model. Six clinically healthy sheep were inoculated with an intradermal Mycobacterium bovis vaccine. After 21 days, they were challenged with multiple intradermal injections of a purified protein derivative (PPD) of M. bovis in the hairless skin of the medial thigh. Simultaneous contralateral injections of sterile diluent were performed to provide control material. The resulting lesions were measured for increase in skin thickness and biopsied at 2, 7, 24, 48, 72, and 96 h post injection. The biopsies were divided, and stained both histochemically and with monoclonal antibodies directed against lymphocyte subsets, macrophages, and B cells. The DTH reaction was maximal at 72 hours post challenge, and was largely characterised by an initial influx of polymorphonuclear neutrophil (PMN) cells, after which there was an accumulation of αβ T cells. The number of macrophages within the lesion declined with the progression of the reaction. B cells and γδ T cells did not appear to play a major role in the response. Fibrin was a marked component of the reaction at later time points.