Basal phosphorylation of SHIP1 by Lyn suppresses proinflammatory signaling in the absence of a phagocytic synapse.

Microscale engagement of the hemi-immunoreceptor tyrosine-based activation motif-containing receptor Dectin-1 by fungal particles activates Src-family kinases (SFKs) and Syk, drives second-messenger generation, and induces downstream Erk and Akt signaling and proinflammatory responses in macrophages. To avoid inappropriate activation in the absence of a pathogenic threat, macrophages restrict signaling in response to low-valency ligands. To examine how SFKs regulate this sensitivity threshold, we compared signaling induced by pharmacological SFK activation with signaling triggered by depleted zymosan, a high-valency β-glucan particle that engages Dectin-1 to form a phagocytic synapse. We found that particulate engagement of Dectin-1 protected the inhibitory ITIM-associated phosphatase SHIP1 from phosphorylation by SFKs, allowing robust activation of Erk and Akt and proinflammatory induction. In contrast, receptor-independent SFK activation induced phosphorylation of SHIP1 and failed to amplify signaling downstream of PLCγ2 and PI3K. Although multiple SFKs could phosphorylate SHIP1, Lyn uniquely maintained the basal set-point of SHIP1 phosphorylation, thereby keeping PIP3 levels low and suppressing basal Erk and Akt signaling. This Lyn-dependent regulation was essential for suppressing Akt activation and balancing signaling through the Erk and Akt pathways in the absence of a phagocytic synapse. In contrast, antimicrobial responses to particulate stimuli, including second-messenger signaling, Erk/Akt, and proinflammatory outputs, did not strictly require Lyn expression. These findings highlight the unique role of Lyn in limiting spurious proinflammatory signaling and shed light on a mechanism by which macrophages selectively respond to high-valency particulate ligands that override this basal inhibitory program.