Osteoclastic tartrate-resistant acid phosphatase (Acp 5): Its localizationto dendritic cells and diverse murine tissues

Tartrate-resistant acid phosphatase (TRAP) is a histochemical marker of the osteoclast. It is also characteristic of monohistiocytes, particularly alv eolar macrophages, and is associated with diverse pathological conditions, including hairy cell leukemia and AIDS encephalopathy. To study the biology of this enzyme, we investigated its expression and activity in mouse tissu es. Confocal fluorescence studies showed that TRAP is localized to the lyso somal compartment of macrophages. In adult mice, high activities of the enz yme were demonstrated in bone, spleen, liver, thymus, and colon, with lower amounts in lung, stomach, skin, brain, and kidney. Trace amounts were dete cted in testis, muscle, and heart. Expression of TRAP mRNA was investigated in tissue sections by in situ hybridization and protein expression was mon itored by histochemical staining or immunohistochemically. TRAP is widely e xpressed in many tissues, where it is associated with cells principally ori ginating from the bone marrow, including those of osteoclast/macrophage lin eage. The cellular distribution of TRAP mRNA and enzyme antigen in the tiss ues corresponds closely to that of cells staining with an antibody directed to the CD80 (B7) antigen. Therefore, to confirm its putative localization in dendritic cells, isolated bone marrow dendritic cells were matured in cu lture. These co-stained strongly for TRAP protein and the CD80 antigen. The se studies demonstrate that TRAP is a lysosomal enzyme that is found in div erse murine tissues, where it is expressed in dendritic cells as well as os teoclasts and macrophages, as expression previously shown.