TLR2 has a prominent role in host defense against a wide variety of pathogens. Stimulation of TLR2 triggers MyD88-dependent signaling to induce NF-κB translocation, and activates a Rac1-PI 3-kinase dependent pathway that leads to transactivation of NF-κB through phosphorylation of the P65 NF-κB subunit. This transactivation pathway involves tyrosine phosphorylations. The role of the tyrosine kinases in TLR signaling is controversial, with discrepancies between studies using only chemical inhibitors and knockout mice. Here, we show the involvement of the tyrosine-kinase Lyn in TLR2-dependent activation of NF-κB in human cellular models, by using complementary inhibition strategies. Stimulation of TLR2 induces the formation of an activation cluster involving TLR2, CD14, PI 3-kinase and Lyn, and leads to the activation of AKT. Lyn-dependent phosphorylation of the p110 catalytic subunit of PI 3-kinase is essential to the control of PI 3-kinase biological activity upstream of AKT and thereby to the transactivation of NF-κB. Thus, Lyn kinase activity is crucial in TLR2-mediated activation of the innate immune response in human mononuclear cells.

The liver is an organ that, when dysfunctional in a septic patient, is strongly associated with morbidity and mortality. Understanding the pathophysiology of liver failure during sepsis may lead to improved diagnostics and potential therapeutic targets. Historically, programmed cell death receptor (PD) ligand 1 (PD-L1) has been considered the primary ligand for its checkpoint molecule counterpart, PD-1, with PD-L2 rarely in the immunopathological spotlight. PD-1 and PD-L1 contribute to liver dysfunction in a murine cecal ligation and puncture (CLP) model of sepsis, but virtually nothing is known about PD-L2’s role in sepsis. Therefore, our central hypothesis was that sepsis-induced changes in hepatic PD-L2 expression contributed to worsened liver function and, subsequently, more pronounced morbidity and mortality. We found that although PD-L1 gene deficiency attenuated the hepatic dysfunction seen in wild-type mice after CLP, the loss of PD-L2 appeared to actually worsen indices of liver function along with a trend toward higher liver tissue vascular permeability. Conversely, some protective effects of PD-L2 gene deletion were noted, such as reduced liver/peritoneal bacterial load and reduced IL-6, IL-10, and macrophage inflammatory protein 2 levels following CLP. These diverse actions, as well as the unique expression pattern of PD-L2, may explain why no overt survival advantage could be witnessed in the septic PD-L2−/− mice. Taken together, these data suggest that although PD-L2 has some selective effects on the hepatic response seen in the septic mouse, these factors are not sufficient to alter septic mortality in this adult murine model. NEW & NOTEWORTHY Our study shows not only that ligands of the checkpoint protein PD-1 respond inversely to a stressor such as septic challenge (PD-L2 declines, whereas PD-L1 rises) but also that aspects of liver dysfunction increase in septic mice lacking the PD-L2 gene. Furthermore, these differences in PD-L2 gene-deficient animals culminated in the abrogation of the survival advantage seen in the septic PD-L1-knockout mice, suggesting that PD-L2 may have roles beyond a simple immune tolerogen.