Cell-specific glycoforms of sialoadhesin and CD45 are counter-receptors for the cysteine-rich domain of the mannose receptor

We previously reported that CR-Fc, an Fc chimeric protein containing the cy steine-rich (CR) domain of the mannose receptor, binds to marginal zone met allophilic macrophages (MO) and B cell areas in the spleen and to subcapsul ar sinus Mr, in lymph nodes of naive mice (CR-Fc(+) cells). Several CR-Fc l igands were found in spleen and lymph node tissue lysates using ligand blot s. In this paper we report the identification of two of these ligands as si aloadhesin (Sn), an Mg-specific membrane molecule, and the leukocyte common antigen, CD45. CR-Fc bound selectively to Sn purified from spleen and lymp h nodes and to two low molecular weight isoforms of CD45 in a sugar-depende nt manner. CR-Fc binding and non-binding forms of Sn, probably derived from CR-Fc(+) and CR-Fc(-) cells respectively, were selected from spleen lysate s, Analysis of the glycan pool associated with the CR-Fc-binding form revea led the presence of charged structures resistant to sialidase, absent in th e non-binding form, that could correspond to sulfated structures. These res ults confirm the identification of the CR region of the mannose receptor as a lectin, We also demonstrate that the same glycoprotein expressed in diff erent cells of the same organ can display distinct sugar epitopes that dete rmine its binding properties.