Bortezomib Inhibits Osteoclastogenesis and Bone Resorption Through Modulation of CHIT1 and YKL40 Expression

Background: VSIG4 acts as a co-inhibitory ligand to negatively regulate T cell proliferation and cytokine production by arresting cell cycle at G0/G1 phase and upregulating tolerance-inducing p27KIP-1. VSIG4 expression is restricted to macrophages. VSIG4 fusion protein also inhibits T cell production of Th1, Th2, and Th17 cytokines. Thus, we hypothesized that endogenous VSIG4 impairs helper T cell functions and then inhibits the subsequent antibody response. Materials and Methods: Mice were immunized subcutaneously with 100 g ovalumin (OVA) in a 1:1 emulsion with phosphate-buffered saline (PBS) and CFA in 100 l PBS. OVAspecific IgM or IgG subclasses were determined using culture supernatants or sera from immunized mice with OVA-coated plates and horseradish peroxidase-conjugated anti-mouse IgG1, -IgG2a, -IgG2b, and -IgG3. To study the VSIG4 role in antibody class switching via regulation of helper T cells, BALB/c mice were immunized intraperitoneally with 500 g of 2,4,6-Trinitrophenyl hapten (TNP)Keyhole Limpet Hemocyanin (KLH) for 14 days. Isolated B cells from TNP-KLH-immunized mice and purified CD4+ T cells from BALB/c wild-type or VSIG4 KO mice immunized s.c. with OVA/CFA were cocultured in the presence of TNPOVA (50 ng/ml) for 5 days. Culture supernatants were collected, and ELISA was performed on a TNP-CGG (100 g/ml)-coated plate to measure anti-TNP IgM and IgG titers. Results: Isotype switching of OVA-specific antibody subclasses to IgG1, IgG2a, IgG2b, and IgG3 was enhanced in OVA-immunized VSIG4 KO mice. 2,4,6-TNPe KLH-primed B cells cocultured with OVAprimed CD4+ T cells from OVA-immunized VSIG4 KO mice in the presence of TNP-OVA showed enhanced isotype switching to IgG subclasses compared to those cocultured with cells isolated from OVA-immunized wild-type (WT) mice. Furthermore, the levels of CD40L expression, the frequency of memory CD4+ T cells, and the production of isotype switching-inducing cytokines increased significantly in OVA-primed CD4+ T cells from VSIG4 knockout (KO) mice. T cells from OVA-specific T cell receptor transgenic mice produced more IFNwhen cocultured with macrophages from VSIG4 KO mice compared to WT mice. Conclusion: Our results demonstrate that macrophage-associated VSIG4 plays a negative role in helper T cell-dependent isotype switching by inhibiting helper T cell activation and differentiation, and suppressing the isotype switching-inducing cytokine production in antigen-primed CD4+ helper T cells.