The actin-binding protein, lymphocyte-specific protein 1, is expressed in human leukocytes and human myeloid and lymphoid cell lines.

Lymphocyte-specific protein 1 (LSP1) was originally reported as a lymphocyte-specific actin-binding protein using murine LSP1 probes. Subsequently, we identified LSP1 in polymorphonuclear neutrophils (PMN) and showed that it is the overexpressed 47-kDa protein in neutrophil actin dysfunction with 47- and 89-kDa abnormalities. This suggests that regulation of LSP1 expression in myeloid cells may be a functionally important event. LSP1 expression in human leukocytes, lymphoid cell lines, and myeloid cell lines (PLB985, HL60, and U937), uninduced (U) or induced to granulocytic (GI) or monocytic (MI) differentiation, was analyzed by Northern blot and immunoblot. By immunoblot, LSP1 is strongly expressed in PMN, less expressed in lymphocytes and monocytes (30-40% and 55-65% of the PMN level, respectively). By immunoblot and Northern blot, LSP1 is minimally expressed in U-PLB985 and U-HL60 (< 10% of the PMN level) and is weakly expressed in the B lymphoid cell line Daudi, but is not expressed in the pro-B, pre-B, T lymphoid cell lines tested, U-U937 or MI-U937. LSP1 mRNA and protein are up-regulated in GI-PLB985, GI-HL60, and MI-HL60. In HL60, LSP1 mRNA and protein increase in parallel to a maximum of eightfold the basal level on days 5 to 6 of granulocytic differentiation and four- to fivefold the basal level on day 3 of monocytic differentiation. The results show that LSP1 is expressed in all human leukocytes, and its expression is up-regulated during granulocytic and monocytic differentiation of myeloid cells in vitro. Since its overexpression is implicated in the functional pathogenesis of a novel human neutrophil motile dysfunction and microfilamentous cytoskeletal abnormality (NAD 47/89), finding LSP1 in all human leukocytes suggests that it plays a role in regulating microfilamentous cytoskeleton structure and motile function in all leukocytes. Since the protein is not lymphocyte specific and is an F-actin binding protein, and its isoforms are expressed in stromal and embryonic mesenchymal cells, we propose that the protein's name be changed to leufactin, as an abbreviated form of leukocyte F-actin binding protein.