Long-lived plasma cells in immunity and immunopathology.

Following tetanus vaccination, a wave of antibody-secreting cells appear in the peripheral blood composed of vaccine-specific, migratory plasmablasts and plasma cells secreting antibodies specific for other antigens. The latter probably were tissue resident plasma cells formed in earlier immune responses that are mobilized due to competition with the newly formed tetanus-specific plasmablasts. Newly formed plasma cells secreting antibodies specific for a particular antigen/vaccine are accommodated in the bone marrow likely at the global expense of the pre-existing long-lived plasma cell population providing humoral memory for other antigens. Plasmablasts but not mature plasma cells are attracted by the ligands for the chemokine receptors CXCR4 and CXCR3. While CXCR4 and its cognate ligand is important for plasma cell homing to the bone marrow, CXCR3 and its ligand IP10 are likely to be involved in attracting them to inflamed tissue. In NZB/W mice, a model for systemic lupus, long-lived autoreactive plasma cells are present not only in bone marrow, but also in inflamed tissues and spleen. Autoreactive plasma cells in the spleen are present long before the onset of the disease, suggesting that these cells contribute to induction of immunopathology.

The role of long-lived plasma cells in autoimmunity.

Recent results on the biology of plasma cells have shown that these cells can survive as long as memory B cells. Possibly, such long-lived plasma cells are also involved in the production of autoantibodies. Here, we discuss the potential involvement of long-lived plasma cells in the pathogenesis of autoimmune disease and the consequences it has for the development of effective therapeutic strategies.

BMP7 promotes adipogenic but not osteo-/chondrogenic differentiation of adult human bone marrow-derived stem cells in high-density micro-mass culture.

The objective of our study was to elucidate the potential of bone morphogenetic protein-7 (BMP7) to initiate distinct mesenchymal lineage development of human adult mesenchymal stem cells (MSC) in three-dimensional micro-mass culture. Expanded MSC were cultured in high-density micro-masses under serum-free conditions that favor chondrogenic differentiation and were stimulated with 50-200 ng/ml BMP7 or 10 ng/ml transforming growth factor-beta3 (TGFbeta3) as control. Histological staining of proteoglycan with alcian blue, mineralized matrix according to von Kossa, and lipids with Oil Red O, immunostaining of type II collagen as well as real-time gene expression analysis of typical chondrogenic, adipogenic, and osteogenic marker genes showed that BMP7 promoted adipogenic differentiation of MSC. Micro-masses stimulated with BMP7 developed adipocytic cells filled with lipid droplets and showed an enhanced expression of the adipocyte marker genes fatty acid binding protein 4 (FABP4) and the adipose most abundant transcript 1 (apM1). Development along the chondrogenic lineage or stimulation of osteogenic differentiation were not evident upon stimulation with BMP7 in different concentrations. In contrast, TGFbeta3 directed MSC to form a cartilaginous matrix that is rich in proteoglycan and type II collagen. Gene expression analysis of typical chondrocyte marker genes like cartilage oligomeric matrix protein (COMP), link protein, aggrecan, and types IIalpha1 and IXalpha3 collagen confirmed chondrogenic differentiation of MSC treated with TGFbeta3. These results suggest that BMP7 promotes the adipogenic and not the osteogenic or chondrogenic lineage development of human stem cells when assembled three-dimensionally in micro-masses.

Generation of migratory antigen-specific plasma blasts and mobilization of resident plasma cells in a secondary immune response.

Maintenance of protective humoral immunity depends on the generation and survival of antibody-secreting cells. The bone marrow provides niches for long-term survival of plasma cells generated in the course of systemic immune responses in secondary lymphoid organs. Here, we have analyzed migratory human plasma blasts and plasma cells after secondary vaccination with tetanus toxin. On days 6 and 7 after immunization, CD19(+)/CD27(high)/intracellular immunoglobulin G(high) (IgG(high))/HLA-DR(high)/CD38(high)/CD20(-)/CD95(+) tetanus toxin-specific antibody-secreting plasma blasts were released in large numbers from the secondary lymphoid organs into the blood. These cells show chemotactic responsiveness toward ligands for CXCR3 and CXCR4, probably guiding them to the bone marrow or inflamed tissue. At the same time, a population of CD19(+)/CD27(high)/intracellular IgG(high)/HLA-DR(low)/CD38(+)/CD20(-)/CD95(+) cells appeared in the blood in large numbers. These cells, with the phenotype of long-lived plasma cells, secreted antibodies of unknown specificity, not tetanus toxoid. The appearance of these plasma cells in the blood indicates successful competition for survival niches in the bone marrow between newly generated plasma blasts and resident plasma cells as a fundamental mechanism for the establishment of humoral memory and its plasticity.

BMP2 initiates chondrogenic lineage development of adult human mesenchymal stem cells in high-density culture.

Human bone marrow-derived mesenchymal stem cells (MSCs) have been shown to differentiate into distinct mesenchymal tissues including bone and cartilage. The capacity of MSCs to replicate undifferentiated and to mature into cartilaginous tissues suggests these cells as an attractive cell source for cartilage tissue engineering. Here we show that the stimulation of human bone marrow-derived MSCs with recombinant bone morphogenetic protein-2 (BMP2) results in chondrogenic lineage development under serum-free conditions. Histological staining of proteoglycan with Alcian blue and immunohistochemical staining of cartilage-specific type II collagen revealed the deposition of typical cartilage extracellular matrix components. Semi-quantitative real-time gene expression analysis of characteristic chondrocytic matrix genes, such as cartilage link protein, cartilage oligomeric matrix protein, aggrecan, and types I, II, and IX collagen, confirmed the induction of the chondrocytic phenotype in high-density culture upon stimulation with BMP2 and transforming growth factor-beta3 (TGFbeta3). Histologic staining of mineralized extracellular matrix with von Kossa, immunostaining of type X collagen (typical for hypertrophic chondrocytes), and gene expression analysis of osteocalcin and adipocyte-specific fatty acid binding protein (aP2) further documented that BMP2 induced chondrogenic lineage development and not osteogenesis and/or adipogenesis in human MSCs. These results suggest BMP2 as a promising candidate for tissue engineering approaches regenerating articular cartilage on the basis of mesenchymal progenitors from bone marrow.