Tartrate‐resistant acid phosphatase as a differentiation marker for the human mononuclear phagocyte system

Human blood monocytes (BM) were stimulated with various immune modulators in short-term cultures. Tartrate-resistant acid phosphatase (TAcP) activity was demonstrated with an enzyme cytochemical method. Other members of the mononuclear phagocyte system (MPS), such as peritoneal (PM) and alveolar macrophages (AM), were also tested. Unstimulated BM and physiologic functional forms of macrophages, with the exception of AM, were invariably TAcP negative. On appropriate stimulation, particularly with media containing lymphokines, cultured BM became TAcP positive. The results suggest that TAcP is an inducible differentiation marker that indicates transformation of monocytes into cells belonging to a distinct subset of the MPS.