The Lyme disease agent, Borrelia burgdorferi, is maintained in a tick-mouse cycle. Here we show that B. burgdorferi usurps a tick salivary protein, Salp15 (ref. 3), to facilitate the infection of mice. The level of salp15 expression was selectively enhanced by the presence of B. burgdorferi in Ixodes scapularis, first indicating that spirochaetes might use Salp15 during transmission. Salp15 was then shown to adhere to the spirochaete, both in vitro and in vivo, and specifically interacted with B. burgdorferi outer surface protein C. The binding of Salp15 protected B. burgdorferi from antibody-mediated killing in vitro and provided spirochaetes with a marked advantage when they were inoculated into naive mice or animals previously infected with B. burgdorferi. Moreover, RNA interference-mediated repression of salp15 in I. scapularis drastically reduced the capacity of tick-borne spirochaetes to infect mice. These results show the capacity of a pathogen to use a secreted arthropod protein to help it colonize the mammalian host.

The Lyme disease agent Borrelia burgdorferi naturally persists in a cycle that primarily involves ticks and mammals. We have now identified a tick receptor (TROSPA) that is required for spirochetal colonization of Ixodes scapularis. B. burgdorferi outer surface protein A, which is abundantly expressed on spirochetes within the arthropod and essential for pathogen adherence to the vector, specifically bound to TROSPA. TROSPA mRNA levels in ticks increased following spirochete infestation and decreased in response to engorgement, events that are temporally linked to B. burgdorferi entry into and egress from the vector. The blockade of TROSPA by TROSPA antisera or by the repression of TROSPA expression via RNA interference reduced B. burgdorferi adherence to the I. scapularis gut in vivo, thereby preventing efficient colonization of the vector and subsequently reducing pathogen transmission to the mammalian host. Identification of an I. scapularis receptor for B. burgdorferi is the first step toward elucidating arthropod ligands that are required for survival of spirochetes in nature.

Summary There is a current imperative to unravel the hierarchy of molecular pathways that drive the transition of early to established disease in rheumatoid arthritis (RA). Herein, we report a comprehensive RNA sequencing analysis of the molecular pathways that drive early RA progression in the disease tissue (synovium), comparing matched peripheral blood RNA-seq in a large cohort of early treatment-naive patients, namely, the Pathobiology of Early Arthritis Cohort (PEAC). We developed a data exploration website ( https://peac.hpc.qmul.ac.uk/ ) to dissect gene signatures across synovial and blood compartments, integrated with deep phenotypic profiling. We identified transcriptional subgroups in synovium linked to three distinct pathotypes: fibroblastic pauci-immune pathotype, macrophage-rich diffuse-myeloid pathotype, and a lympho-myeloid pathotype characterized by infiltration of lymphocytes and myeloid cells. This is suggestive of divergent pathogenic pathways or activation disease states. Pro-myeloid inflammatory synovial gene signatures correlated with clinical response to initial drug therapy, whereas plasma cell genes identified a poor prognosis subgroup with progressive structural damage.

Tick saliva has pleiotropic properties that facilitate persistence of the arthropod upon the host. We now describe a feeding-inducible protein in Ixodes scapularis saliva, Salp15, that inhibits CD4(+) T cell activation. The mechanism involves the repression of calcium fluxes triggered by TCR ligation and results in lower production of interleukin-2. Salp15 also inhibits the development of CD4(+) T cell-mediated immune responses in vivo, demonstrating the functional importance of this protein. Salp15 provides a molecular basis for understanding the immunosuppressive activity of I. scapularis saliva and vector-host interactions.

Inactivation of the TNFAIP3 gene, encoding the A20 protein, is associated with critical inflammatory diseases including multiple sclerosis, rheumatoid arthritis and Crohn's disease. However, the role of A20 in attenuating inflammatory signalling is unclear owing to paradoxical in vitro and in vivo findings. Here we utilize genetically engineered mice bearing mutations in the A20 ovarian tumour (OTU)-type deubiquitinase domain or in the zinc finger-4 (ZnF4) ubiquitin-binding motif to investigate these discrepancies. We find that phosphorylation of A20 promotes cleavage of Lys63-linked polyubiquitin chains by the OTU domain and enhances ZnF4-mediated substrate ubiquitination. Additionally, levels of linear ubiquitination dictate whether A20-deficient cells die in response to tumour necrosis factor. Mechanistically, linear ubiquitin chains preserve the architecture of the TNFR1 signalling complex by blocking A20-mediated disassembly of Lys63-linked polyubiquitin scaffolds. Collectively, our studies reveal molecular mechanisms whereby A20 deubiquitinase activity and ubiquitin binding, linear ubiquitination, and cellular kinases cooperate to regulate inflammation and cell death.