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Objective. Bone marrow stromal cell antigen 1 (BST‐1) is a novel glycosyl phosphatidylinositol–anchored ectoenzyme, which is overexpressed on bone marrow stromal and synovial cell lines derived from patients with rheumatoid arthritis (RA). To investigate the pathophysiologic roles of BST‐1 in RA, we established an enzyme‐linked immunosorbent assay (ELISA) system to detect the soluble form of BST‐1 (sBST‐1) and examined levels of sBST‐1 in the sera of RA patients. Methods. Concentrations of sBST‐1 in sera from healthy donors and from patients with RA, osteoarthritis, Sjögren's syndrome, and systemic lupus erythematosus were measured with the ELISA. Results. In 7% of the RA patient samples (10 of 143), concentrations of serum sBST‐1 were higher (∼30–50‐fold) than in non‐RA samples. Serum sBST‐1 concentrations showed no correlation with age, C‐reactive protein level, or rheumatoid factor level. All RA patients with high concentrations of serum sBST‐1 had severe disease with involvement of several large joints. Conclusion. We believe the measurement of serum sBST‐1 may have prognostic value, but further analysis is necessary to clarify the clinical significance of elevated sBST‐1 in RA.

Bone marrow stromal cell Ag-1 (BST-1; CD157)-deficient mice were generated to examine the immunologic roles of the molecule in vivo. In BST-1−/− mice, the development of peritoneal B-1 cells was delayed, and CD38low/− B-lineage cells were increased in the bone marrow and spleen. Partial impairment of thymus-independent (TI-2) and thymus-dependent (TD) Ag-specific immune responses was noted in the systemic and mucosal compartments of BST-1−/− mice, respectively. Although serum Ig levels as well as TD and TI-1 Ag-specific systemic immune responses were normal, the TI-2 Ag-induced IgG3 response was selectively impaired. Oral immunization of BST-1−/− mice with cholera toxin, a potent TD Ag for the induction of IgA response, resulted in the poor production of Ag-specific Abs at the intestinal mucosa accompanied by the reduced number of Ag-specific IgA-producing cells in the lamina propria. These results indicate that BST-1 has roles in B cell development and Ab production in vivo.

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