Abstract Introduction MHC class II molecules present exogenous antigen-derived peptide to CD4+ T cells leading to T-cell activation and signaling into the antigen presenting cell. Class II molecules are alpha beta heterodimers which form two distinct conformers via differential pairing of transmembrane domain GxxxG motifs (i.e. M1 and M2-paired class II). Methods Previous studies in mouse I-Ak and human HLA-DR class II used conformer specific mAbs to define unique immunobiological properties of each conformer. This report utilizes site directed mutagenesis, immunoprecipitation, and genetic sequence alignments for mAb characterization. As well as flow cytometry, and in vitro and in vivo T cell stimulation assays. Results Here we report that AMS-32.1, a widely-used anti-I-Ad mAb, is specific for the 20% of M1-paired I-Ad class II expressed by the cell, and 34-5-3S anti-I-Ad mAb is non-conformer specific. While AMS-32.1 engagement of M1-paired I-Ad elicits B-cell calcium signaling, 34-5-3S co-engagement of all class II blocks this response, indicating M2-paired class II inhibits M1-paired class II signaling. In vitro, AMS-32.1 blocks majority of T-cell activation by ova-peptide—I-Ad complexes, even under conditions where peptide is loaded onto both conformers. In studies using a murine model of hepatitis B virus replication, where MHC class II-restricted T-cell-dependent humoral responses are key to antigen clearance, AMS-32.1 administration blocks HBsAg seroconversion, indicating that M1-paired class II is central to the in vivo development of CD4+ T-cell-dependent antibody responses. Conclusion Overall, this study reveals AMS-32.1 is specific for M1-paired I-Ad class II and M1-paired class II molecules are key to CD4+ T-cell activation. Funding Source Research reported was supported by Albany Medical College Institutional Funds and the National Institute of Allergy and Infectious Diseases of the National Institutes of Health under award numbers R03AI165365 (JRD) and R01AI148354 (MDR). The content is so Topic Categories Classical and Non-Classical Antigen Presenting Cells (APC)
Cangialosi et al. (2026) studied this question.