Disease in the Pld4(thss/thss) Model of Murine Lupus Requires TLR9

Phospholipase D4 (PLD4) is an endolysosomal exonuclease of ssRNA and ssDNA, rather than a phospholipase as its name suggests. Human polymorphisms in the PLD4 gene have been linked by genome-wide association studies to systemic sclerosis, rheumatoid arthritis, and systemic lupus erythematosus. However, B6.129 Pld4(−/−) mice develop features of a distinct disease, macrophage activation syndrome, which is reversed in mice mutated in TLR9. In this article, we compare a Pld4 null mutant identified on the BALB/c background, Pld4(thss/thss), which has distinct phenotypes: short stature, thin hair, and features of systemic lupus erythematosus. All phenotypes analyzed were largely normalized in Pld4(thss/thss)Tlr9(−/−) mice. Thus, Pld4(thss/thss) represents a rare model in which mouse lupus etiology is TLR9 dependent. Compared with PLD4-deficient B6 mice, Pld4(thss/thss) mice had elevated levels of serum IgG, IgG anti-dsDNA autoantibodies, BAFF, and IFN-γ and elevated B cell numbers. Overall, the data suggest that PLD4 deficiency can lead to a diverse array of rheumatological abnormalities depending upon background-modifying genes, and that these diseases of PLD4 deficiency are largely driven by TLR9 recognition of ssDNA.