Generation and MHC class II loading of an endogenous influenza epitope revealed by a T cell receptor-like antibody

CD4⁺ T cells coordinate immune responses to infections and cancers and initiate many autoimmune diseases, yet the intracellular pathways that generate the peptide antigens they recognize remain incompletely understood. Progress has been limited in part by the scarcity of reagents that directly detect defined peptide:MHC class II (pMHC-II) complexes. Here, we established an mRNA vaccine-based pipeline to generate and characterize T cell receptor (TCR)-like antibodies. One result was 1D6, a monoclonal antibody specific for NA25:A^b, an influenza A/PR8 neuraminidase-derived epitope presented by MHC-II. 1D6 bound NA25:A^b with high affinity and specificity. 1D6 recognizes NA25:A^b through binding determinants partially distinct from those used by the cognate TCR. Using this reagent, we confirmed the proteasome dependence of NA25 presentation and showed that NA25:A^b accumulates in intracellular MHC-II loading compartments without a major requirement for canonical macroautophagy, pointing to an unconventional mode of antigen presentation. These findings establish TCR-like antibodies as powerful tools for dissecting noncanonical MHC-II antigen processing pathways.