About InVivoMAb anti-rat β-2-Microglobulin The 4C9 monoclonal antibody reacts with rat beta 2 microglobulin (β2M) a 12 kDa component of MHC class I molecules. β-2-microglobulin, a non-membrane-anchored glycoprotein, noncovalently associates with the polymorphic heavy chain of MHC class I molecules to form the HLA class I antigen complex. β-2-microglobulin is expressed on all leukocytes, platelets, endothelial cells and epithelial cells and plays an essential role in mediating proper folding and expression of MHC class I molecules. The importance of β-2-microglobulin is illustrated by β-2-microglobulin knockout mice which show a normal distribution of T lymphocytes, but have no mature CD4 or CD8 T cells and are therefore defective in CD4 and CD8 T cell-mediated cytotoxicity. The 4C9 antibody recognizes rat β-2-microglobulin alone or complexed with FcRn heavy chains. InVivoMAb anti-rat β-2-Microglobulin Specifications IsotypeMouse IgG1 Recommended Isotype Control(s)InVivoMAb mouse IgG1 isotype control, unknown specificity Recommended Dilution BufferInVivoPure pH 7.0 Dilution Buffer ImmunogenSoluble neonatal Fc receptor (FcRn) Reported Applicationsin vitro β2M blockade FormulationPBS, pH 7.0 Contains no stabilizers or preservatives Endotoxin<2EU/mg (<0.002EU/μg) Determined by LAL gel clotting assay Purity>95% Determined by SDS-PAGE Sterility0.2 μm filtered ProductionPurified from cell culture supernatant in an animal-free facility PurificationProtein G RRIDAB_10951294 Molecular Weight150 kDa StorageThe antibody solution should be stored at the stock concentration at 4°C. Do not freeze. Application ReferencesInVivoMAb anti-rat β-2-Microglobulin (CLONE: 4C9)Hilt, Z. T., et al (2021). "β2M Signals Monocytes Through Non-Canonical TGFβ Receptor Signal Transduction" Circ Res 128(5): 655-669. PubMedRATIONALE: Circulating monocytes can have proinflammatory or proreparative phenotypes. The endogenous signaling molecules and pathways that regulate monocyte polarization in vivo are poorly understood. We have shown that platelet-derived β2M (β-2 microglobulin) and TGF-β (transforming growth factor β) have opposing effects on monocytes by inducing inflammatory and reparative phenotypes, respectively, but each bind and signal through the same receptor. We now define the signaling pathways involved. OBJECTIVE: To determine the molecular mechanisms and signal transduction pathways by which β2M and TGF-β regulate monocyte responses both in vitro and in vivo. METHODS AND RESULTS: Wild-type- (WT) and platelet-specific β2M knockout mice were treated intravenously with either β2M or TGF-β to increase plasma concentrations to those in cardiovascular diseases. Elevated plasma β2M increased proinflammatory monocytes, while increased plasma TGFβ increased proreparative monocytes. TGF-βR (TGF-β receptor) inhibition blunted monocyte responses to both β2M and TGF-β in vivo. Using imaging flow cytometry, we found that β2M decreased monocyte SMAD2/3 nuclear localization, while TGF-β promoted SMAD nuclear translocation but decreased noncanonical/inflammatory (JNK [jun kinase] and NF-κB [nuclear factor-κB] nuclear localization). This was confirmed in vitro using both imaging flow cytometry and immunoblots. β2M, but not TGF-β, promoted ubiquitination of SMAD3 and SMAD4, that inhibited their nuclear trafficking. Inhibition of ubiquitin ligase activity blocked noncanonical SMAD-independent monocyte signaling and skewed monocytes towards a proreparative monocyte response. CONCLUSIONS: Our findings indicate that elevated plasma β2M and TGF-β dichotomously polarize monocytes. Furthermore, these immune molecules share a common receptor but induce SMAD-dependent canonical signaling (TGF-β) versus noncanonical SMAD-independent signaling (β2M) in a ubiquitin ligase dependent manner. This work has broad implications as β2M is increased in several inflammatory conditions, while TGF-β is increased in fibrotic diseases. Graphic Abstract: A graphic abstract is available for this article.
