Investigation into the role of La/SS-B in Interferon regulation and its relevance in health and human disease
The La/SS-B protein is well documented as an autoantigen for autoimmune conditions such as systemic lupus erythematosus (SLE). Recently, a link between La and viruses has been reported, an interesting observation given the fact that viral infection is a significant risk factor in SLE. However, the direct mechanism by which La functions in the innate immune response to viral challenge remains elusive. As such, we sought to investigate whether La directly regulates the induction of the anti-viral cytokines, type I Interferons, (IFNs) by direct modulation of innate immune toll-like receptor (TLR) or RIG-I-like receptor (RLR) signalling pathways.
Our findings have highlighted a dual and complex role for La in the regulation of IFN production. Initial studies demonstrated a role for La in the specific attenuation of IFN promoter transcriptional activity downstream of RLR activation. In confirmation of this, elevated IFN levels were observed following La knockdown in resting cells, compared with controls. Studies in a cohort of SLE patients supported these findings, with decreased La expression observed in SLE patients compared with healthy controls in basal cells, both at an mRNA and protein level. However upon activation of the anti-viral RIG-I receptor, La was shown to promote anti-viral responses by direct interaction with RIG-I receptor, enhancing complex formation between RIG-I and its ligand as well as downstream adaptor protein IPS-1, the overall consequence of which was increased IFN induction. Sendai virus infection experiments supported these findings with depletion of La leading to increased viral infection efficiency and decreased IFN and IFN stimulated gene (ISG) expression.
Thus a dual role exists for La in the context of RIG-I-mediated IFN production, with the protein capable of both inhibiting basal type I IFN in resting cells, most likely in an attempt to prevent inappropriate IFN induction and maintain innate immune balance. In the context of infection, however, La is a potent activator of IFN responses, promoting RIG-I binding to its ligand and enhanced type I and type III IFN production, in an effort to protect the host by limiting viral replication and promoting the clearance of the pathogen. Interestingly, we have observed that phosphorylation of La by casein kinase 2 (CK2) may be the regulatory switch that turns La from being active to inactive, and that in cells treated with a CK2-specific inhibitor, La has a higher affinity for RIG-I. With respect to a potential involvement of this protein in SLE we have observed that a higher ratio of expression of a potentially active form of La (neo-La) is observed in SLE patients, suggesting that this imbalance between full length and neo-La in SLE patients contributes to the dysregulated IFN expression observed in patients. Our work has important implications for our understanding of pathways regulating IFN production in the context of SLE.