The presence of membrane bound CD99 ligands on leukocyte surface
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RESEARCH NOTE
The presence of membrane bound CD99 ligands on leukocyte surface Nuchjira Takheaw1,2, Supansa Pata1,2, Witida Laopajon1,2, Sittiruk Roytrakul3 and Watchara Kasinrerk1,2*
Abstract Objective: CD99, a leukocyte surface molecule, reportedly plays an important role in several cellular processes. However, the role of CD99 in T cell regulation remains unclear, as the CD99 ligand associated with T-cell regulation has not yet been identified. Our previous study showed that recombinant CD99 bound to CD99 ligands was expressed on monocytes, NK cells and dendritic cells. This interaction regulates the expression of IL-6 and TNF-α in CD3 + T cells following T cell activation. In the present study, we confirmed the presence of CD99 ligands in immune cells. Results: A recombinant CD99-human IgG fusion protein, CD99HIgG, was produced and used to search for CD99 ligand expression in various hematopoietic cell lines. Among several cell lines, THP-1 monocytic cell line showed strong positive reaction for CD99HIgG, and CD99 and CD99 ligand complexes were pulled-down using a DTSSP crosslinker. The study demonstrated the presence of the membrane bound CD99 ligand, and CD99 ligand candidates were identified via LC–MS/MS. These results may be useful to further identify the CD99 ligands, and to fully comprehend the role of CD99 in immunoregulation. Keywords: CD99 ligand, CD99, Recombinant CD99, LC–MS/MS, Leukocyte Introduction CD99 is a heavily O-linked type I transmembrane glycoprotein expressed on both hematopoietic and nonhematopoietic cells [1–3]. Up-regulation of CD99 expression has been demonstrated in activated and memory T cells [4, 5]. CD99 reportedly plays a key role in several cellular processes including cell adhesion, migration, differentiation and cell death [6, 7]. Furthermore, it has been proposed that CD99 may function as either an activating or inhibitory receptor in T cell regulation. Upon T cell activation, anti-CD99 monoclonal antibodies (mAbs) induced T cell signalling and functions was demonstrated [8–10]. In contrast, the inhibition of T cell responses using a distinct clone of anti-CD99 mAb has also been reported [11, 12]. In most studies of the role of CD99 in T *Correspondence: [email protected] 1 Division of Clinical Immunology, Department of Medical Technology, Faculty of Associated Medical Sciences, Chiang Mai University, Chiang Mai, Thailand Full list of author information is available at the end of the article
cell regulation, mAbs were used to mimic its ligands, and different mAbs resulted in diverse functional CD99 outcomes [2, 13]. Hence, the role of CD99 in immunoregulation remains controversial. This could be attributed to the fact that the CD99 ligand has not yet been identified. Recently, we demonstrated that putative CD99 ligands were expressed on the surfaces of monocytes, NK cells and dendritic cells, but not on those of B and T cells [14]. Recombinant CD99 protein enhanced the upregulation of IL-6 and TNF-α expression by T cells, mon
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