Dose-dependent activation of Syk and SHIP1 by LynA and LynB at steady state creates a threshold for macrophage signaling in the absence of receptor engagement.

LynA-knockout and LynB-knockout mice, each expressing only one of two isoforms of the Src-family kinase (SFK) Lyn, have differential progression to autoimmunity. It is unclear, however, whether isoform-specificity or Lyn dose underlies differential signaling in the single-isoform knockouts. To address this question, we generated a series of Lyn-knockout mice with a varying LynA and LynB expression and tested macrophage signaling in response to pharmacological pan-SFK activation. We found that the magnitude of initiating signaling is a function of the combined basal expression of LynA and LynB, with the two isoforms equally capable of phosphorylating positive-regulatory Syk and negative-regulatory SHIP1. While expression of either isoform restored basal and SFK-initiated downstream signaling, WT-like levels of Erk and Akt signaling were enabled by expression of any amount of Lyn and insensitive to further upregulation of either isoform. Thus, either LynA or LynB expression at steady state leads to balanced activation of positive- and negative-regulatory signaling, setting a maximal response in the absence of a true microbial encounter.