Reevaluation of quantitative flow cytometric analysis for TLR2 on monocytes using F(ab′) 2 fragments of monoclonal antibodies

In patients with refractory infections, reliable markers that monitor the severity and healing process are needed. The expression level of toll‐like receptor 2 (TLR2) on monocytes is such candidate. In the conventional assay system, the whole IgG (wIgG) form of anti‐TLR2 mAb has been used with control IgG, which blocks nonantigen‐specific bindings. However, the competitive reactions against Fcγ receptors (FcγRs) between labeled anti‐TLR2 mAbs and control IgG should be considered. Our goal was to precisely quantify TLR2 expression level on monocytes by flow cytometry (FCM). In this study, we prepared anti‐TLR2 mAbs, D45 (IgG2a), and D29 (IgG1), as well as their fragment antigen‐binding [F(ab′) 2 ] fragments to avoid nonantigen‐specific binding to FcγRs. And then, we determined TLR2 expression levels on monocytes by using these mAbs/fragments and our calibration system using recombinant TLR2 beads. The binding of PE‐labeled D45 wIgG to monocytes was completely blocked with unlabeled D45 wIgG, but not with unlabeled D45 F(ab′) 2 fragment. Although the nonantigen‐specific binding of D29 wIgG to nonstimulated monocytes was negligible, it was enhanced in interleukin‐10‐stimulated monocytes. It proved difficult to completely block nonantigen‐specific binding of D45 and D29 wIgGs by treatment with control IgG. It was demonstrated that the use of fluorescent‐labeled antigen‐binding region lacking the fragment crystallizable portion of anti‐TLR2 mAb [such as the PE‐labeled F(ab′) 2 fragment] is indispensible for quantification of TLR2 levels on monocytes in flow cytometry. © 2010 International Society for Advancement of Cytometry