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. 2020 Jul 10;18(10):587–600. doi: 10.1038/s41579-020-0400-5

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© Springer Nature Limited 2020

This article is made available via the PMC Open Access Subset for unrestricted research re-use and secondary analysis in any form or by any means with acknowledgement of the original source. These permissions are granted for the duration of the World Health Organization (WHO) declaration of COVID-19 as a global pandemic.

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Fig. 4 — a | Several environmental changes that occur at the onset of tick feeding are cues for spirochaetes in the gut to transition to a form that is infectious for vertebrates and to initiate migration to the salivary glands. Outer surface proteins important for this process include BBA52, which is upregulated during the early stages of feeding⁷², and BBE31, which interacts with the tick receptor TRE31 to enable the spirochaetes to exit the gut epithelial layer and migrate through the haemocoel to the salivary glands⁷¹. Ixofin3D and ISDLP are other proteins expressed by epithelial cells that bind spirochaetes and are thought to assist in exit from the gut^32,79. Spirochaetes outside the gut express OspA and OspC, which promotes binding to tick salivary glands and early dissemination in the vertebrate host^10,197. b | Transmission of Borrelia burgdorferi to a mammalian host is enhanced by the activity of several tick salivary proteins. As the tick feeds, several proteins are secreted into the host to modulate the host environment and to obtain a complete bloodmeal. These proteins also assist B. burgdorferi transmission. Complement is an important immune defence mechanism that restricts B. burgdorferi, as well as tick feeding. The tick salivary proteins ISAC, SALP20 and TSLPI inhibit activation of complement and increase B. burgdorferi transmission^164,179,180. Sialostatin L2 also modulates the immune response against the tick bite by impairing cytokine secretion by dendritic cells on exposure to B. burgdorferi¹⁶⁷. Tick histamine release factor (tHRF) is a salivary protein secreted during the late stage of tick feeding and triggers the release of histamine, presumably from mast cells or basophils¹⁸⁵. The best studied salivary protein is SALP15, which enhances B. burgdorferi transmission. B. burgdorferi expresses OspC on its surface during migration from the gut to the salivary glands. SALP15 binds OspC and can shield the spirochaete from antibody-mediated killing¹⁹⁰. Additionally, SALP15 suppresses CD4⁺ T cell function and IL-2 secretion¹⁶¹.