Autoantibody-mediated diseases like myasthenia gravis, autoimmune hemolytic anemia and systemic lupus erythematosus represent a therapeutic challenge. In particular, long-lived plasma cells producing autoantibodies resist current therapeutic and experimental approaches. Recently, we showed that the sensitivity of myeloma cells toward proteasome inhibitors directly correlates with their immunoglobulin synthesis rates. Therefore, we hypothesized that normal plasma cells are also hypersensitive to proteasome inhibition owing to their extremely high amount of protein biosynthesis. Here we show that the proteasome inhibitor bortezomib, which is approved for the treatment of multiple myeloma, eliminates both short- and long-lived plasma cells by activation of the terminal unfolded protein response. Treatment with bortezomib depleted plasma cells producing antibodies to double-stranded DNA, eliminated autoantibody production, ameliorated glomerulonephritis and prolonged survival of two mouse strains with lupus-like disease, NZB/W F1 and MRL/lpr mice. Hence, the elimination of autoreactive plasma cells by proteasome inhibitors might represent a new treatment strategy for antibody-mediated diseases.

The current view holds that chronic autoimmune diseases are driven by the continuous activation of autoreactive B and T lymphocytes. However, despite the use of potent immunosuppressive drugs designed to interfere with this activation the production of autoantibodies often persists and contributes to progression of the immunopathology. In the present study, we analyzed the life span of (auto)antibody-secreting cells in the spleens of NZB × NZW F1 (NZB/W) mice, a murine model of systemic lupus erythematosus. The number of splenic ASCs increased in mice aged 1–5 mo and became stable thereafter. Less than 60% of the splenic (auto)antibody-secreting cells were short-lived plasmablasts, whereas 40% were nondividing, long-lived plasma cells with a half-life of >6 mo. In NZB/W mice and D42 Ig heavy chain knock-in mice, a fraction of DNA-specific plasma cells were also long-lived. Although antiproliferative immunosuppressive therapy depleted short-lived plasmablasts, long-lived plasma cells survived and continued to produce (auto)antibodies. Thus, long-lived, autoreactive plasma cells are a relevant target for researchers aiming to develop curative therapies for autoimmune diseases.

Long-lived plasma cells in the bone marrow produce memory antibodies that provide immune protection persisting for decades after infection or vaccination but can also contribute to autoimmune and allergic diseases. However, the composition of the microenvironmental niches that are important for the generation and maintenance of these cells is only poorly understood. Here, we demonstrate that, within the bone marrow, plasma cells interact with the platelet precursors (megakaryocytes), which produce the prominent plasma cell survival factors APRIL (a proliferation-inducing ligand) and IL-6 (interleukin-6). Accordingly, reduced numbers of immature and mature plasma cells are found in the bone marrow of mice deficient for the thrombopoietin receptor ( IntroductionAntibody-secreting plasma cells are found in many tissues. However, the plasma cells that provide antigen-specific systemic antibodies for up to decades after immunization or infection predominantly reside in the bone marrow. [1][2][3] There are multiple lines of evidence that individual plasma cells can survive in humans and mice for many months at the least. [4][5][6][7] These long-lived plasma cells are important for maintaining protective antibody memory. However, autoantibody-secreting long-lived plasma cells are refractory to conventional immunosuppressive therapy and therefore represent a therapeutic challenge in autoimmune diseases. [8][9][10] Plasma cell survival is not cell-autonomous but depends on signals provided by their environment. The most potent plasma cell survival factors identified so far are a proliferation-inducing ligand (APRIL), interleukin-6 (IL-6), tumor necrosis factor-␣ (TNF-␣), stromal-derived factor-1␣, and signals transduced via CD44. [11][12][13][14] The bone marrow contains multiple microenvironmental niches that stimulate cellular proliferation, differentiation, and survival. [15][16][17][18][19][20] Each niche seems to support specifically one or a few particular hematopoietic stem or precursor cells. In this way, the sizes of these populations are limited by the number of available niches. 16,21 Similarly, competition for a limited number of survival niches may also control the turnover rate within the bone marrow plasma cell compartment. 12,[22][23][24] One or multiple niches may exist that have the capability to support the terminal differentiation and survival of bone marrow plasma cells. 25 As indicated by strong colocalization between a particular subtype of stromal-derived factor-1␣ ϩ reticular stromal cells and immunoglobulin G ϩ (IgG ϩ ) bone marrow plasma cells, the former seems to be an important element of plasma cell niches in that tissue. 26 However, in culture, bone marrow stromal cells support plasma cell survival only for a limited time, 13 suggesting that additional cell types contribute to the formation of plasma cell niches.In addition, it has been shown that macrophage-derived APRIL is required to support differentiation/survival of bone marrow plasma cells during early life, suggesting that factors ...

Proper liver function is crucial for metabolism control and to clear toxic substances from the bloodstream. Many small-molecule therapeutics accumulate in the liver, negatively impacting liver function and often resulting in hepatotoxicity and cell death. Several analytical methods are currently utilized to evaluate hepatotoxicity and monitor liver function. To date, none of these methods have specifically targeted protein phosphorylation-mediated signal transduction pathways which should be altered in response to toxic effects of small molecule therapeutics. To develop novel assays to probe specific signaling pathways in the liver, identification and quantification of specific protein phosphorylation sites in this complex organ is necessary. Here, we have utilized an optimized immobilized metal affinity chromatography (IMAC) protocol to enrich phosphorylated peptides from a tryptic digest of proteins isolated from whole liver lysate. LC-MS/MS analysis of IMAC-enriched peptides resulted in the identification of more than 300 phosphorylation sites from over 200 proteins in rat liver, a significant advance over previously published analyses of the liver phosphoproteome. Previously characterized phosphorylation sites and potentially novel sites were identified in the current study, including sites on proteins implicated in metabolism regulation, transcription, translation, and canonical signaling pathways. Moreover, protein phosphorylation analysis was performed without prior fractionation of the sample, enabling analysis of small sample amounts while minimizing analysis time, potentially allowing for high-throughput assays to be performed with this methodology. From these data, it appears that this methodology can be used to identify new phosphorylation sites and, in combination with a stable isotope-labeling step, to investigate the effects of liver diseases, cancer and evaluate potential toxicology of new drug substances.

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