All the Fabs kept their peptide-specific, MHC-restricted binding to the MOG-35-55 loaded empty RTL302-5D (Fig. 3B), excluding any binding dependence to non-native sequences of RTL1000. Additionally, we tested Fab binding to RTL1000 in different buffer conditions and found the Fabs to be conformationally sensitive, losing their ability to react with denatured RTL1000 (Supporting Information Fig. 1). Taken together, these data indicate selective Fab binding to the α1β1 DR2–MOG-35-55 native sequence of the folded RTL1000. We next tested the ability of the anti-RTL1000 Fabs to Olaparib bind the native full-length four-domain form of MHC-II complexes as expressed on APCs. L-cell DR*1501 transfectants
(L466.1 cells) were loaded with MOG-35-55 or control peptide. The loaded cells were incubated with the purified Fabs following anti-Fab-FITC incubation. As shown in Fig. 4A, no specific binding of Fabs was observed for MOG-35-55 loaded cells. MOG-35-55 and control-peptide loaded cells produced the same fluorescence
intensity as background. MHC expression on the APC surface was confirmed by anti-DR mAb (L243). A portion of the loaded cells that were used for the FACS analysis was incubated with the H2-1 T-cell hybridoma specific for the DR2–MOG-35-55 complex. Following 72 h incubation, cell supernatants were transferred to IL-2-dependent CTLL cells for detection of IL-2 levels secreted from the H2-1 hybridoma (Fig. 4B). H2-1 cells were activated U0126 in vitro only by the MOG-35-55 pulsed cells, secreting eightfold higher levels of IL-2 compared to non-pulsed or control peptide-pulsed APCs. Peptide-specific H2-1 activation confirmed a successful loading of MOG-35-55 peptide to the native MHC on the APCs used for the FACS analysis. Despite the presence of a biologically active determinant in the form of DR2–MOG-35-55
molecules presented by the APCs, no staining of such a complex was obtained by any of our anti RTL1000 Fabs. Considering the high affinity of the selected Fabs and the permissive conditions used for this experiment, we conclude that the Fabs do not bind the native DR2–MOG-35-55 complex presented by APCs. Further support for this finding came from blocking experiments which tested the Fabs ability to inhibit peptide-specific activation of the H2-1 hybridoma by DR2 Phosphoprotein phosphatase APCs pulsed with MOG-35-55 peptide (Fig. 4C). None of our selected Fabs were able to block this peptide-specific, MHC-restricted activation, as compared to a control TCRL Fab (D2) specific for RTL2010 (DR4–GAD-555-567) that also failed to block H2-1 activation. In contrast, complete blocking was achieved by the control anti-MHC-II mAb (TU39). The failure of the Fabs to interfere with MHC presentation to TCR implies an inability to bind native four domain DR2–MOG-35-55 complexes. This was indeed the case, as demonstrated by ELISA (Fig. 4D).