The monoclonal antibody 1C11 recognizes human stabilin-1 interacting chitinase-like protein (SI-CLP). SI-CLP is a 39 kD member of the family of Glyco_18-domain-containing proteins. These chitinase-like proteins are involved primarily in Th2-associated disorders and tumor progression. Chitinase-like proteins are soluble mediators of cell differentiation, proliferation, activation, migration, and adherence.Chitinase like protein SI-CLP is a stabilin-1 interacting partner. SI-CLP gene expression was up-regulated by IL-4 and dexamethasone on the mRNA and protein levels in primary human macrophages in parallel with the expression of stabilin-1. The major site of SI-CLP intracellular localization is in Lamp1-positive and secretion commited CD63+ secretory lysosomes. The endocytic/sorting receptor stabilin-1 is a highly specific intracellular sorting receptor for SI-CLP. SI-CLP directly binds to, and colocalizes with stabilin-1. The lysosomal localization of SI-CLP in stabilin-1-negative macrophages, however, indicates that stabilin-1 may not be the only receptor mediating trafficking of newly synthesized SI-CLP.In vivo, the monoclonal antibody 1C11 recognizes SI-CLP in the cellular fraction of bronchoalveolar lavage specimens obtained from patients with chronic inflammatory disorders of the respiratory tract and in peripheral-blood leukocytes (PBLs) from these patients as well as healthy donors.