Rodent epidermal Langerhans cells demonstrate greater histochemical specificity for ADP than for ATP and AMP

Langerhans cells (LCs) in mammalian epidermis possess the ectoenzyme Ca++/Mg++-dependent adenosine triphosphatase (ATPase), which has served as a useful histochemical marker for these dendritic cells in a variety of tissue preparations. Since ATPase represents only one of several potential cell surface polyphosphatases, we investigated the capacities of 3 related adenine nucleotide substrates to identify rodent epidermal LCs. Cell surface ATPase activity was not inhibited in the presence of ouabain and was observed to be strictly divalent cation-dependent, with complete interchangeability between Ca++ and Mg++. Optimal staining in the presence of either cation occurred at a 20 mM concentration. Substrate concentration dependence was also observed, with optimal staining at 0.33 mM adenosine 5'-triphosphate (ATP). On an equimolar basis, however, adenosine 5'-diphosphate (ADP) was superior to ATP for the identification of LCs both in whole mounts of epidermis and in suspensions of disaggregated epidermal cells. The substrate adenosine 5'-monophosphate (AMP) stained follicular epithelial cells in both rodent species but failed to identify epidermal LCs in the mouse and only weakly stained these dendritic cells in rat epidermis. We conclude from these studies that ADP demonstrates greater specificity for LC surface polyphosphatase activity than ATP and that the inadvertent inclusion of AMP during identification procedures for epidermal cell suspensions will falsely identify cells other than LCs.