Long-lived plasma cells are early and constantly generated in New Zealand Black/New Zealand White F1 mice and their therapeutic depletion requires a combined targeting of autoreactive plasma cells and their precursors.

INTRODUCTION: Autoantibodies contribute significantly to the pathogenesis of systemic lupus erythematosus (SLE). Unfortunately, the long-lived plasma cells (LLPCs) secreting such autoantibodies are refractory to conventional immunosuppressive treatments. Although generated long before the disease becomes clinically apparent, it remains rather unclear whether LLPC generation continues in the established disease. Here, we analyze the generation of LLPCs, including autoreactive LLPCs, in lupus-prone New Zealand Black/New Zealand White F1 (NZB/W F1) mice over their lifetime, and their regeneration after depletion.

METHODS: Bromodeoxyuridine-pulse chase experiments in mice of different ages were performed in order to analyze the generation of LLPCs during the development of lupus. LLPCs were enumerated by flow-cytometry and autoreactive anti-double strand (ds) DNA plasma cells by Enzyme-Linked ImmunoSpot (ELISPOT). For analyzing the re-generation of LLPCs after depletion, mice were treated with bortezomib alone or in combination with cyclophosphamide and plasma cells were enumerated 12 hours, 3, 7, 11 and 15 days after the end of the bortezomib cycle.

RESULTS: Autoreactive LLPCs are established in the spleen and bone marrow of lupus-prone mice very early in ontogeny, before week 4 and before the onset of symptoms. The generation of LLPCs then continues throughout life. LLPC counts in the spleen plateau by week 10, but continue to increase in the bone marrow and inflamed kidney. When LLPCs are depleted by the proteasome inhibitor bortezomib, their numbers regenerate within two weeks. Persistent depletion of LLPCs was achieved only by combining a cycle of bortezomib with maintenance therapy, e.g., cyclophosphamide, depleting the precursors of LLPCs or preventing their differentiation into LLPCs.

CONCLUSION: In lupus-prone NZB/W F1 mice, autoreactive LLPCs are generated throughout life. Their sustained therapeutic elimination requires both the depletion of LLPCs and the inhibition of their regeneration.