Salp15

Salp15 is a 15-kDa salivary protein that has been identified in Ixodes scapularis and named Salp15 after its calculated molecular mass. This protein has previously been shown to bind specifically to the dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN/CD209) to upregulate the expression of CD73 in Treg cells (Wen S et al. 2020). These data suggest that Salp15 has properties that suppress the host immune system.

Tick saliva contains a mixture of bioactive molecules with a wide range of properties. One of the most extensively studied components of tick saliva is a 15-kDa salivary gland protein (Salp15) from Ixodes scapularis. This multifunctional protein suppresses the host immune response through a pleiotropic effect on several important defense mechanisms. Salp15 and its homologue from I. ricinus"Iric1" have been shown to bind to the outer surface protein C(OspC) of Borrelia burgdorferi sensu stricto, allowing the spirochetes to evade antibody-mediated killing in the human host.

OspC is the most variable protein on the outer surface of Borrelia burgdorferi, which can bind other ligands such as plasminogen, fibrinogen, fibronectin or complement factor 4 in addition to Salp15. So far it has been shown that several OspC variants produced by B. burgdorferi s. l. spirochetes are able to bind Salp15 or its homolog. In B. burgdorferi, Salp15 has been shown to provide protection against borrelizid antibodies. The binding constant in the nanomolar range indicates very strong interactions.

Salp15 also binds to CD4 molecules on the surface of T lymphocytes (Juncadella IJ et al. 2009), thereby interfering with TCR-mediated signal transduction and inhibiting the activation and proliferation of CD4+ T cells. Furthermore, the secretion of interleukin-2 (IL-2) is prevented.

Salp15 Iric-1 is a Salp15 homolog that has been identified in I. ricinus.

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