Background and Objectives Long-lived plasma cells (LLPC) contribute significantly to the production of pathogenic autoantibodies in Systemic Lupus Erythematosus (SLE), the prototype of systemic autoimmune diseases. These cells are refractory to conventional immunosuppressive therapies and thus represent an unmet therapeutic challenge. In the current view, LLPC are generated early in ontogeny and no longer formed later in disease pathogenesis, when constant generation of short-lived plasma cells is supposed to be a hallmark of pathology. However, the homeostasis of autoreactive LLPC-compartment has not been characterised so far. Therefore, in this study we rigorously analysed the dynamics of generation, maintenance and replacement of LLPC in NZB/W mice, a murine model of SLE.
Materials and Methods NZB/W mice of different ages (4 to 29 week) were fed with bromodeoxyuridine (BrdU) in their drinking water for two weeks in order to discriminate between cells generated during and before the feeding period and to track the cells and their replacement over time. Next, LLPC were depleted in mice with a stable splenic LLPC-compartment by two injection of bortezomib (0.75 mg/kg BW) in combination or not with a dose of 35 mg/kg BW cyclophosphamide every fourth day. The mice were sacrificed at different time points after BrdU feeding or LLPC-depletion and bone marrow and spleen LLPC were enumerated by FACS and ELISPOT.
Results Generation of autoreactive LLPC in spleen and bone marrow starts very early in ontogeny before the onset of symptoms. LLPC-generation continues and is maintained throughout life, reaching a plateau in the spleen but showing persistently increasing numbers and replacement in bone marrow. When LLPCs are efficiently depleted by bortezomib, their numbers fully recover within 2 weeks after its withdrawal. After termination of bortezomib, a persistent depletion of LLPC was only maintained if the precursors of the LLPC were continuously targeted e.g. using cyclophosphamide.
Conclusion Our results provide relevant insights into the disturbed homeostasis of B cell and LLPC showing that, in autoimmunity, the LLPC-compartment manifest an unforeseen dynamism that is related to B cell hyperactivity. Our data disprove the idea of a “therapeutic window of opportunity” as the generation of LLPC starts very early in ontogeny. Moreover, the continuous generation and replacement of autoreactive-LLPC has an impact on the treatment of systemic autoimmune diseases suggesting that the depletion of autoreactive-LLPC has to be connected with the prevention of regeneration of these cells through the targeting of B cell activation and differentiation.