Defined in situ enumeration of T6 and HLA-DR expressing epidermal Langerhans cells: morphologic and methodologic aspects.

An essential prerequisite for the in situ enumeration of epidermal Langerhans cells (LCs) is the unequivocal identification of the desired cell type. We have examined over 250 cryostat sections of normal human skin to analyze morphologic and methodologic problems underlying the quantification of epidermal LCs, defined by anti-T6 (OKT6) and anti-HLA-DR (OKIal) immunoperoxidase staining. Our findings show that OKT6 reactivity of dendritic processes in cross-sectioned epidermis yields microscopic images which are not easy to analyze objectively. The morphology that we find leads us to categorize dendritic cells into 3 arbitrary types of T6+ LC profiles. In addition we describe criteria for the assessment of OKT6 staining patterns relating to the dendritic state of epidermal LCs. Preliminary quantitative data on this issue are discussed in relation to: epidermal thickness; the thickness of skin tissue sections; and the discrepancy between the number of T6+ and HLA-DR+ LCs. We hope that the principles outlined in this report may serve to overcome potential methodologic problems with quantitation of T6+ epidermal LCs in skin sections.