Epitope spreading of Lyme autoantigen apoB-100 and CD4+ T cell responses to Borrelia burgdorferi Mcp4 are regulated by IL-10 in murine Lyme disease

Borrelia burgdorferi , the causative agent of Lyme disease (LD), has evolved immune evasion mechanisms to establish a persistent infection in their vertebrate hosts, resulting in chronic inflammation and autoimmune T and B cell reactivity in many B. burgdorferi -infected individuals. In this study, we used an unbiased immunopeptidomics approach to identify foreign and self MHC class II peptides isolated from inguinal and popliteal lymph nodes from B. burgdorferi - infected C57BL/6 (B6) mice, which develop mild, self-limiting LD; and from infected B6 Il10-/- mice, which develop severe, persistent LD. Nearly 10,000 MHC-II peptides were identified by LC-tandem MS analysis which included many peptides derived from proteins abundant in arthritic joints that are associated with inflammation, tissue repair, and extracellular matrix remodeling. Notably, the number and variety of unique peptides derived from apolipoprotein B- 100 (apoB-100); a validated autoantigen in human Lyme arthritis (LA), atherosclerosis, and liver disease; was greatly expanded in lymph nodes of infected mice, particularly in Il10-/- mice at 4 weeks (6-fold increase) and 16 weeks (15-fold increase) post-infection, compared with uninfected mice, indicating epitope spreading. One of the apoB-100 peptides identified in infected, but not uninfected, B6 and Il10-/- mice was APOB3500-3515, an immunogenic cryptic epitope in murine autoimmune atherosclerosis. No apoB-100 peptides had sequence homology to any B. burgdorferi antigens. Surprisingly, only six peptides derived from B. burgdorferi proteins were validated in this study. One of these B. burgdorferi epitopes, derived from methyl- accepting chemotaxis protein Mcp4 (BB0680), was an immunogenic target of CD4+ T cell responses in B. burgdorferi -infected Il10-/- mice, but not in B6 mice. In conclusion, this study has shed light on the importance of IL-10 in suppressing epitope spreading and limiting B. burgdorferi -specific CD4+ T cell responses. Furthermore, this study supports epitope spreading and exposure of cryptic antigens as likely mechanisms of infection-induced apoB-100 autoimmunity in LD. AUTHOR SUMMARY Lyme disease is caused by infection with the spirochetal pathogen Borrelia burgdorferi, and affects ∼500,000 individuals in the U.S. annually. T cell responses to both host and pathogen are dysregulated during infection, resulting in chronic infection and frequent development of autoimmunity. To assess the immune-relevant CD4+ T cell epitopes presented during development of Lyme disease, we used an unbiased, immunopeptidomics approach to characterized the MHC class II immunopeptidome in mice infected with Borrelia burgdorferi . We identified nearly 10,000 unique peptides. Peptides derived from apoB-100, a known human Lyme autoantigen, were highly enriched in infected mice, compared with uninfected controls, and showed evidence of epitope spreading. Furthermore, we identified several peptides derived from Borrelia burgdorferi , including one immunogenic peptide from a methyl-accepting chemotaxis protein, Mcp4. Interestingly, both apoB-100 epitope spreading and immune responses to Mcp4 were observed in mice lacking the anti-inflammatory cytokine IL-10, indicating an important role of IL-10 in suppressing T cell responses to Mcp4 and epitope spreading of Lyme autoantigen apoB-100. ### Competing Interest Statement The authors have declared no competing interest.