Lyn expression in macrophages promotes TLR activation and restricts proliferation in an isoform-independent manner.

Toll-like receptor (TLR) signaling is vital for antimicrobial macrophage function, and its dysregulation is associated with diseases such as lupus, multiple sclerosis, pulmonary fibrosis, and cancer. The Src-family kinase Lyn may have net activating or inhibitory effects on TLR signaling, yet distinct functions of the Lyn splice variants LynA and LynB in TLR signaling have not been investigated. We used isoform-specific Lyn knockout mice (LynAKO and LynBKO) to interrogate the contribution of each isoform to TLR signaling in bone-marrow-derived macrophages. Bulk RNA sequencing and cytokine analyses revealed that complete Lyn deficiency (LynKO) dampened TLR4- and TLR7-induced inflammatory gene expression and production of tumor necrosis factor but enhanced the expression of genes responsible for synthesizing the extracellular matrix and promoting proliferation. Despite reduced expression of total Lyn in single-isoform-knockout bone-marrow-derived macrophages, expression of either LynA or LynB alone was sufficient to preserve a wild-type-like transcriptome at steady state and after treatment with the TLR7 agonist R848. However, LynAKO and LynBKO macrophages did have impaired production of tumor necrosis factor in response to the TLR4 agonist lipopolysaccharide. Additionally, LynAKO and LynBKO macrophages were as hyperproliferative as LynKO cells. These data suggest that Lyn promotes macrophage activation in response to TLR signaling and restrains aberrant proliferation and matrix deposition in a dose-dependent rather than isoform-specific manner.