Shu Takahashi

Cell surface‐anchored SR‐PSOX/CXC chemokine ligand 16 mediates firm adhesion of CXC chemokine receptor 6‐expressing cells

Direct contacts between dendritic cells (DCs) and T cells or natural killer T (NKT) cells play important roles in primary and secondary immune responses. SR‐PSOX/CXC chemokine ligand 16 (CXCL16), which is selectively expressed on DCs and macrophages, is a scavenger receptor for oxidized low‐density lipoprotein and also the chemokine ligand for a G protein‐coupled receptor CXC chemokine receptor 6 (CXCR6), expressed on activated T cells and NKT cells. SR‐PSOX/CXCL16 is the second transmembrane‐type chemokine with a chemokine domain fused to a mucin‐like stalk, a structure very similar to that of fractalkine (FNK). Here, we demonstrate that SR‐PSOX/CXCL16 functions as a cell adhesion molecule for cells expressing CXCR6 in the same manner that FNK functions as a cell adhesion molecule for cells expressing CX3C chemokine receptor 1 (CX3CR1) without requiring CX3CR1‐mediated signal transduction or integrin activation. The chemokine domain of SR‐PSOX/CXCL16 mediated the adhesion of CXCR6‐expressing cells, which was not impaired by treatment with pertussis toxin, a Gαi protein blocker, which inhibited chemotaxis of CXCR6‐expressing cells induced by SR‐PSOX/CXCL16. Furthermore, the adhesion activity was up‐regulated by treatment of SR‐PSOX/CXCL16‐expressing cells with a metalloprotease inhibitor, which increased surface expression levels of SR‐PSOX/CXCL16. Thus, SR‐PSOX/CXCL16 is a unique molecule that not only attracts T cells and NKT cells toward DCs but also supports their firm adhesion to DCs.

Cutting Edge: SR-PSOX/CXC Chemokine Ligand 16 Mediates Bacterial Phagocytosis by APCs Through its Chemokine Domain

SR-PSOX and CXC chemokine ligand (CXCL)16, which were originally identified as a scavenger receptor and a transmembrane-type chemokine, respectively, are indicated to be identical. In this study, we demonstrate that membrane-bound SR-PSOX/CXCL16 mediates adhesion and phagocytosis of both Gram-negative and Gram-positive bacteria. Importantly, our prepared anti-SR-PSOX mAb, which suppressed chemotactic activity of SR-PSOX, significantly inhibited bacterial phagocytosis by human APCs including dendritic cells. Various scavenger receptor ligands inhibited the bacterial phagocytosis of SR-PSOX. In addition, the recognition specificity for bacteria was determined by only the chemokine domain of SR-PSOX/CXCL16. Thus, SR-PSOX/CXCL16 may play an important role in facilitating uptake of various pathogens and chemotaxis of T and NKT cells by APCs through its chemokine domain.

Purification, Molecular Cloning, and Characterization of TRP32, a Novel Thioredoxin-related Mammalian Protein of 32 kDa

We purified a protein of 32 kDa from human thymoma HPB-ALL cells that was co-purified with a catalytic fragment of MST (mammalian STE-20-like), a kinase of the STE20 family, which is proteolytically activated by caspase in apoptosis (Lee, K.-K., Murakawa, M., Nishida, E., Tsubuki, S., Kawashima, S., Sakamaki, K., and Yonehara, S. (1998)Oncogene 16, in press). Molecular cloning of the gene encoding this 32-kDa protein (TRP32) reveals that it is a novel protein of 289 amino acid residues and contains an NH2-terminal thioredoxin domain with a conserved thioredoxin active site. The human and mouse TRP32 proteins have 99% homology, and the thioredoxin domains are completely identical. The thioredoxin domain of TRP32 has thioredoxin-like reducing activity, which can reduce the interchain disulfide bridges of insulin in vitro. However, the thioredoxin domain of TRP32 is more sensitive to oxidation than human thioredoxin. Northern blot analysis showed that TRP32 is expressed in all human tissues. Expression of TRP32 was also confirmed in all mammalian cell lines tested by Western blot analysis using anti-TRP32 monoclonal antibody. Subcellular fractionation and immunostaining analysis showed TRP32 is cytoplasmic protein. These findings suggest that TRP32 is a novel cytoplasmic regulator of the redox state in higher eukaryotes.

Upregulation of SR-PSOX/CXCL16 and Recruitment of CD8+ T Cells in Cardiac Valves During Inflammatory Valvular Heart Disease

Objective—SR-PSOX/CXCL16 is a transmembrane chemokine and is implicated in activated CD8+ T cell trafficking. In the present study, we examined the expression pattern of SR-PSOX/CXCL16 in the heart and investigated a potential role of SR-PSOX/CXCL16 in inflammatory valvular heart disease. Methods and Results—Initial expression of SR-PSOX/CXCL16 in murine embryos was detected in endothelial cells lining endocardial cushions in the forming heart at E11.5. From mid-gestation to adult, expression of this gene in the heart was exclusively observed in valvular endothelial cells. Examination of SR-PSOX/CXCL16 expression in human cardiac valves demonstrated that SR-PSOX/CXCL16 was strongly expressed in valvular and neocapillary endothelial cells in patients with infective endocarditis. SR-PSOX/CXCL16 expression in neocapillary endothelial cells was also observed in patients with rheumatic and atherosclerotic valvular disease. Moreover, CD8+ T cells were distributed closely to endothelial cells expressing SR-PSOX/CXCL16. In vitro adhesion assays showed that SR-PSOX/CXCL16 induced adhesion of activated CD8+ T cells to vascular cell adhesion molecule-1 (VCAM-1) through very late antigen-4 (VLA-4) activation. Furthermore, SR-PSOX/CXCL16 stimulated interferon-&ggr; (IFN-&ggr;) production by CD8+ T cells. Conclusions—SR-PSOX/CXCL16 may be involved in CD8+ T cell recruitment through VLA-4 activation and stimulation of IFN-&ggr; production by CD8+ T cells during inflammatory valvular heart disease.