Subversion of Keratinocyte Function by a Tick Salivary Protein

Summary: Tick feeding affects skin inflammation and allows the transmission of microbes that cause human diseases. Although host immunity functions to impair tick feeding, salivary glands secrete immunosuppressive molecules that compromise skin defenses. Among the characterized tick salivary gland components is sialostatin L2, a multifunctional, versatile, and immunomodulatory molecule involved in cathepsin inhibition and suppression of a non-canonical NLRC4 inflammasome scaffold. In this R21 application, we show a novel function of sialostatin L2 whereby this molecule binds to the mammalian high mobility group 1 (HMGB1) protein, an alarmin highly expressed in keratinocytes. Keratinocytes are the most predominant cells in the skin epidermis and express HMGB1. Notably, HMGB1 serves as a damage-associated molecular pattern (DAMP) triggering inflammation via cell-surface pattern recognition receptors. Thus, we postulate that a tick bite leads to the release of HMGB1 from damaged keratinocytes, and the interaction between HMGB1 and sialostatin L2 blocks downstream immune signaling events, preventing skin inflammation, and enabling tick feeding. Our central hypothesis is that the sialostatin L2-HMGB1 axis in keratinocytes countermeasures skin immunity. Our preliminary data show that sialostatin L2 mitigates the HMGB1-triggered proinflammatory response, suggesting a pivotal role of sialostatin L2 in suppressing mammalian inflammation during ectoparasite feeding. How HMGB1 signal transduction in keratinocytes is affected by sialostatin L2 remains elusive. Accordingly, Aim#1 of this R21 grant proposal will investigate the immunosuppressive function of sialostatin L2 through inhibition of HMGB1 signaling in keratinocytes. Aim#2 of this proposal will examine tick feeding in the context of HMGB1 released by keratinocytes. Collectively, we will: (1) uncover a previously unidentified mechanism of epidermal immunosuppression by a tick salivary molecule; and (2) shed light on the immunological role of alarmins during ectoparasite feeding.