Introduction
Narcolepsy type 1 (NT1) is caused by 85-95% hypocretin (HCRT) neuron loss due to a combination of genetic and environmental risk factors and associated with DQA1*01:02/DQB1*06:02 (DQ0602, 98%) and T cell receptor (TCR)gene AJ24/AJ28/BV4-2. Pandemic influenza A H1N1 (pH1N1) and Pandemrix vaccination in 2009-2010 induced NT1onset. Antigen specific T cell immune response in NT1 supports adaptive (auto)immunity against HCRT and Pandemrix/pH1N1. Despite this progress, the mechanism behind HCRT neuron loss and how the flu triggers a response against HCRT are unclear.

Methods
Binding affinity of B/Victoria candidate peptides to DQ0602 was next tested using a competition binding assay. Antigen-DQ0602 restricted/reactive T cells were further isolated using tetramer DQ0602 and single-cell RNA sequenced with 10X. Lastly, TCR clones were transfected into Jurkat 76 for TCR activation by a certain peptide presented by DQ0602.

Results
With thorough analysis of their sequences, we found a B/Victoria PB1 epitope that is homologous to Pandemrix/pH1N1 PB1 segment with only one amino acid difference. Using tetramer DQ0602, we detected reactive CD4+ T cells to this epitope in two cohort studies using different subjects. PB1 cells from the first cohort were sorted out for single-cell RNA sequencing and over 6,000 cells were recovered. Over 90% were effector memory T cells, over 86% were from multiple NT1 patients, and more than 35 clones were enriched at least 20 times, including 2 with over 500 times. These clones were activated by both peptides from B/Victoria and Pandemrix/pH1N1 under NFAT and NF-κB pathways but not AP-1, suggesting molecular mimicry between influenza A and B through PB1 epitope. In addition, we found TCR clones from baseline PBMCs were activated by Pandemrix/pH1N1.

Conclusion
Our research showed influenza B plays a role in NT1 pathogenesis and cross-reactivity to PB1 between B/Victoria and Pandemrix/pH1N1.