TLR9 and systemic lupus erythematosus: In addition to B cells, mtDNA–TLR9 may also facilitate SLE development through plasmacytoid predendritic cells (PDCs) as there has been a study showing that viral DNA or the DNA in the immune complexes can stimulate PDCs through TLR9 to express and secrete IFN-α, a cytokine that is involved in anti-dsDNA antibody production (38).