Overexpression of Human TLR8 Causes Fatal Anemia in SLE-Prone Mice By Altering the Bone Marrow Erythropoietic Niche

Anemia is a common hematologic abnormality in patients with active systemic lupus erythematosus (SLE), a disease characterized by innate immune system activation by nucleic acid containing immune complexes and cell debris. Work in mouse models has shown that overactivation of several cytoplasmic innate immune sensors by either self-DNA or self-RNA specifically leads to erythroid cell death in the fetal liver while sparing other hematopoietic cell lineages.

The endosomal receptors TLR7 and TLR8 both recognize ssRNA in humans. TLR7 overexpression in mice causes a lupus syndrome that includes the development of mild to moderate anemia (Hb >10) and thrombocytopenia and is associated with spleen histiocytosis, autoimmune hemolysis, erythrophagocytosis and compensatory stress erythropoiesis in the spleen. A recent study demonstrated that patients with TLR8 gain of function mutations present with immunodeficiency, inflammation and bone marrow failure. However the role of TLR8 in SLE has been difficult to study in mice because it has a 5 amino acid deletion that attenuates its RNA binding capacity.

To address the role of TLR8 in SLE, we overexpressed human TLR8 in a lupus mouse model (huTLR8tg.Sle1.Yaa) using a BAC transgene. 50% of homozygous huTLR8tg.Sle1.Yaa mice developed severe anemia (Hb<9) resulting in early mortality starting at 3-4.5 months of age. This phenotype required both the Sle1 and Yaa loci that promote the formation of high titer anti-chromatin and anti-RNA antibodies and onset of nephritis at >6 months of age. There was no difference in autoantibody titers between Sle1.Yaa wt and huTLR8tg mice and early death was not due to premature onset of renal disease. All mice had normal RBC indices prior to 10 weeks of age. Anemia was associated with an increase in bone marrow (BM) trabecular bone and a decrease in erythroblastic islands (EBI) in the BM with compensatory stress erythropoiesis leading to reticulocytosis and vast splenomegaly. RBC half-life was normal even after the development of reticulocytosis, but decreased in severely anemic mice as a pre-terminal event. Using CFSE labeling of RBCs we showed that hemophagocytosis occurred in vivo as a result of expansion of phagocytic red pulp macrophages.

Flow cytometry of BMs from transgenic mice showed normal erythroid progenitors but a block at the late CFU-E/early proerythroblast stage leading to overall decreased terminal erythroid differentiation. Single cell RNAseq of EBIs isolated from transgenic BMs confirmed the block in erythropoiesis. The erythroblast cluster proximal to the block had a signature of mitochondrial stress and decreased proliferation. Spleen EBIs from transgenic mice were characterized by a low frequency of early erythroblasts compared with BM EBIs. We found 6 related clusters of EBI central macrophages in the BMs of both wt and transgenic mice. In transgenic mice, most of these displayed an inflammatory and Type 1 IFN signature. One cluster (M2) expressed all the classical central macrophage phenotypic markers in wt mice but was characterized in transgenic mice by downregulation of multiple phagocytic receptors and a 5-fold decrease of VCAM1 expression. Loss of VCAM1 and downregulation of CD169 in central macrophages of transgenic BMs was confirmed by flow cytometry. By contrast spleen central macrophages from transgenic mice retained VCAM and CD169 expression.

Together, these results suggest that failure of BM erythropoiesis in huTLR8 transgenic SLE.Yaa lupus-prone mice, in which the acquisition of anti-nucleic acid autoantibodies drives excess innate stimulus through TLR7 and huTLR8, is due to a block in differentiation from CFU-E to the early proerythroblast stage; this is associated with an inflammatory phenotype specifically in BM erythroblastic island central macrophages and down regulation of adhesion and phagocytic receptors. Stress erythropoiesis in the spleens is associated with vast expansion of red pulp macrophages with phagocytic properties and fatal anemia is associated with a decrease in red blood cell half-life, suggesting that excessive RBC phagocytosis, coupled with insufficient erythroblast progenitors, eventually exceeds the capability of stress erythropoiesis to replace the RBC mass.