TLR9 and systemic lupus erythematosus: Since different cell types are known to produce type I IFN in small quantities after microbial challenge [4], these observations raise the possibility that other circulating cells (for example, those involved in the innate immune system, such as monocytes or myeloid dendritic cells, and/or those expressing TLR-9 and/or TLR-7, such as B cells) are the source of IFN-α production in SLE.