Leu-M2 (Mac-120) is a platelet antigen and is not constitutively expressed by macrophages

Leu-M2 (Mac-120) is a mouse anti-human monoclonal antibody that reacts with an antigen on the surface of most peripheral blood monocytes. The presence of Leu-M2 positivity has been associated with the capacity of monocytes to present antigen to T cells. We found that anti-Leu-M2 does not react with an intrinsic monocyte antigen but instead binds to platelets present on the surface of monocytes. Leu-M2 staining is lost from monocytes after they are treated with calcium chelating agents which remove platelets bound to the monocyte surface. Staining for Leu-M2 can be reintroduced by incubating monocytes with autologous platelets. Human alveolar, peritoneal, and tissue macrophages and the U937 cell failed to stain for Leu-M2 antigen. Leu-M2 does not appear to represent either platelet glycoprotein IIb/IIIa or thrombospondin. Our study underscores the importance of carefully screening antimonocyte antibodies for their reactivity with platelets.