Surfactant proteins A(SP-A)and D(SP-D),both members of the collectin family,play a well established role in apoptotic cell recognition and clearance.Recent in vitro data show that SP-A and SP-D interact with apoptotic neutrophils in a distinct manner.SP-A and SP-D bind in a Ca2+-dependent manner to viable and early apoptotic neutrophils whereas the much greater interaction with late apoptotic neutrophils is Ca2+-independent.Cell surface molecules on the apoptotic target cells responsible for these interactions had not been identified and this study was done to find candidate target molecules.Myeloperoxidase(MPO),a specific intracellular defense molecule of neutrophils that becomes exposed on the outside of the cell upon apoptosis,was identified by affinity purification,mass-spectrometry and weste blotting as a novel binding molecule for SP-A and SP-D.To confirm its role in recognition,it was shown that purified immobilised MPO binds SP-A and SP-D,and that MPO is surface-exposed on late apoptotic neutrophils.SP-A and SP-D inhibit binding of an anti-MPO monoclonal Ab to late apoptotic cells.Fluorescence microscopy confirmed that anti-MPO mAb and SP-A/SP-D colocalise on late apoptotic neutrophils.Desmoplakin was identified as a further potential ligand for SP-A,and neutrophil defensin as a target for both proteins.