Stat3 is a member of the signal transducer and activator of transcription family, which is important for cytokine signaling as well as for a number of cellular processes including cell proliferation, anti-apoptosis and immune responses. In recent years, evidence has emerged suggesting that Stat3 also participates in cell invasion and motility. However, how Stat3 regulates these processes remains poorly understood. Here, we find that loss of Stat3 expression in mouse embryonic fibroblasts leads to an elevation of Rac1 activity, which promotes a random mode of migration by reducing directional persistence and formation of actin stress fibers. Through rescue experiments, we demonstrate that Stat3 can regulate the activation of Rac1 to mediate persistent directional migration and that this function is not dependent on Stat3 transcriptional activity. We find that Stat3 binds to βPIX, a Rac1 activator, and that this interaction could represent a mechanism by which cytoplasmic Stat3 regulates Rac1 activity to modulate the organization of actin cytoskeleton and directional migration.

The rising prevalence of arthritogenic alphavirus infections, including chikungunya virus (CHIKV) and Ross River virus (RRV), and the lack of antiviral treatments highlight the potential threat of a global alphavirus pandemic. The immune responses underlying alphavirus virulence remain enigmatic. We found that pentraxin 3 (PTX3) was highly expressed in CHIKV and RRV patients during acute disease. Overt expression of PTX3 in CHIKV patients was associated with increased viral load and disease severity. PTX3-deficient (PTX3-/-) mice acutely infected with RRV exhibited delayed disease progression and rapid recovery through diminished inflammatory responses and viral replication. Furthermore, binding of the N-terminal domain of PTX3 to RRV facilitated viral entry and replication. Thus, our study demonstrates the pivotal role of PTX3 in shaping alphavirus-triggered immunity and disease and provides new insights into alphavirus pathogenesis.