These studies were undertaken to examine the status of the IFNAR signaling pathway in SLE. Western blot and immunoprecipitation analysis of SLE PBMC proteins showed constitutive phosphorylation of the IFNAR-associated signaling proteins Jak1 and STAT2, even in inactive patients without increased serum IFNα or IFN-I-like gene-inducing activity. We also found a decrease in SOCS1 expression which did not appear to be related with Jak1/STAT2 phosphorylation. In the absence of exogenous IFN-I, Jak1-STAT2 co-precipitated with IFNAR2, which increased by the addition of IFNβ.
Although SLE susceptibility is genetically heterogeneous 
, most SLE patients share increased expression of IFN-I-induced genes that is more pronounced during disease activity 
, indicating that the different genetic backgrounds leading to SLE converge on the IFN-I signaling pathway. However, serum IFN-I is not always increased in SLE 
and SLE induced by exogenous IFN-I only occurs occasionally. Thus, it seems reasonable to assume that increased IFN-I only leads to SLE in susceptible individuals. One possibility is that, rather that increased IFN-I production, SLE is associated with an increased responsiveness to IFN-I. The role of IFN-I signaling in SLE is further supported by the finding that absence of IFN-I receptor (IFNAR) in NZB mice prevents SLE-like disease but, interestingly, does not decrease over-expression of IFN-I-induced gene Ifi202 
. This does not appear to be universal, however, because in the MRL/lpr
model of murine SLE, the absence of IFNAR not only fails to prevent the disease but it leads to accelerated autoimmunity, whereas administration of IFN-I increases survival 
Although only a few of our active SLE patients had increased levels of plasma or serum IFNα with no correlation with SLEDAI scores, plasmas from active SLE patients induced expression of IFN-dependent genes in vitro
on control PBMC. Indeed, all active SLE plasmas tested (n
5) induced mRNA expression of the IFN-dependent genes 2′5OAS, MxA and EIF2α and there was a strong correlation with the SLEDAI scores, particularly for 2′5OAS. This has been previously shown, but it has been attributed to the presence of IFNα, as anti-IFNα can block such activity 
. We did not detect IFNα in these plasmas despite our ELISA kit is multivalent for IFNα. Thus, it is either a different IFN-I (β, ω), which we consider unlikely because others have failed to find these IFN-Is or, SLE plasma contains, as it has been suggested 
, IFN-I-inducing activity as it could be the case for some RNAs coupled to SLE autoantibodies. In such case, single or double stranded RNA could activate IFN-I transcription via TLR7 or TLR3, respectively TLR 
. Regardless of that, and more importantly, inactive SLE plasmas lacked such IFN-I-like activity and yet, they had constitutive phosphorylation of Jak1, pointing out again to increased sensitivity to IFN-I as part of the SLE diathesis.
The presence of the IFN signature in SLE has been known for several years, but little has been done to examine the IFNAR signaling pathway and its inhibitors in SLE patients. Dong et al 
found an increase in STAT1 and its phosphorylated form in kidneys and lymph nodes in MRL/lpr
mice. Moreover, SOCS1 and SOCS3, which are induced by IFN-I via STAT1 were also increased. However, another study found that inhibition of functional expression of Jak-STAT1 in the kidneys ameliorates the nephropathy in MRL/lpr
The most striking finding of the current studies was that PBMC from inactive SLE patients with normal serum IFNαα levels, have constitutive phosphorylation of IFNAR signaling molecules, which is not explained by intrinsic hypersensitivity to ligand binding as an exogenous IFNβ induced similar phosphorylation of Jak1 and STAT2 in SLE and control PBMC. As both IFNβ and IFNα bind to IFNAR and do not appear to differ in their biological activities, it seems reasonable to conclude that their activities are interchangeable. This suggests that constitutive activation of IFNAR in SLE is due to an impaired regulation in the absence of ligand.
SOCS1 is the main regulator of signaling through many cytokine receptors, including IFNAR 
. Thus, we examined whether SOCS1 expression was decreased in SLE as a possible explanation for increased phosphorylation of Jak1 and STAT2 in SLE. We found a decrease in SOCS1 expression, particularly at the mRNA level with decreased in vitro
response to IFNβ. However, Pearson correlation analysis failed to show association between decreased SOCS1 and pJak1 and/or pSTAT2. A study in Taiwanese SLE patients found an increased expression of CIS, but not SOCS1 or SOCS3 mRNA, whereas a second study in China, found that SLE patients as a whole had increased SOCS1 mRNA expression 
, looking at the latter study in depth it is clear that SOCS1 was increased only in active patients. The differences between these and our results could be explained in part by the genetic heterogeneity of SLE, given the different ethnic groups studied. It is of interest that two of our most active patients had a markedly increased expression of SOCS1 mRNA (but not of the IFN-I induced gene MxA). These two patients had the highest SOCS1 transcription in response to IFNβ, pointing again to the heterogeneity of SLE. However, given that we could not establish a statistical correlation between decreased SOCS1 and receptor phosphorylation, for the moment, the role of SOCS1 on increased IFNAR signaling in SLE should be considered only a possibility.
As previously known, in unstimulated PBMC, SOCS1 co-precipitated only with IFNAR1 
. However, upon addition of IFNβ, SOCS1 also associates with IFNAR2 to levels well above those of IFNAR1-associated SOCS1, which only increased slightly. This suggests that initial IFN-I signaling takes place mainly from the IFNAR2 chain until SOCS1 binds to it, when this protein is expected to block IFNAR2 signaling without significantly decreasing IFNAR1 signaling, which would be expected to be initially low, but to remain largely unaffected beyond the expected regulation achieved by constitutive SOCS1 binding.
If decreased SOCS1 turns to be a reproducible feature of SLE, it could explain many of the systemic inflammatory and clinical features of SLE, including weight loss and cachexia, which could be due to impaired regulation of cytokine receptors, including IL-2R 
, IFNγR 
, type III IFN’s 
, IL-12 family receptors 
, as well as toll like 
, and some non-immune receptors such as the leptin receptor 
. Regardless of its possible meaning, the basis of decreased SOCS1 expression in some SLE patients remains to be elucidated. It could be a SNP in the SOCS1 gene, which seems unlikely, because with such a large proportion of patients having the described phenotype, any GWAS study would have already found an association. Another possibility is that some combinations of gene polymorphisms associated to SLE lead to an impaired regulation of SOCS1 gene expression by a transcriptional factor. A third, more intriguing possibility, is that decreased SOCS1 mRNA is not due to decreased transcription but to increased regulation at the mRNA level, which could be the case if SOCS1 is regulated by microRNA’s. A recent study found that three mouse SLE strains with different genetic backgrounds and pathologic features share a common pattern of altered microRNA expression profiles 
, several of which, including miR-19a, miR-19b, and 155 target SOCS1 mRNA. Moreover, miR-155 is induced by IFN-I 
resulting in a positive feedback loop that increases IFN-I production.
In conclusion, the finding of constitutive phosphorylation of IFNAR/associated signaling proteins in SLE strongly suggest that overexpression of IFN-I-induced genes in this disease could be explained, at least in part, by an increased sensitivity of IFNAR. The role of decreased SOCS1 on IFNAR increased signaling in some patients remains a possibility that needs to be further examined.