Splenectomized and asplenic patients have a high incidence of infections by encapsulated bacteria and do not respond to polysaccharide vaccines. To understand whether the absence of the spleen is associated with a defined B cell defect, we analyzed B cell subsets in the peripheral blood. We found that a population of B cells known as immunoglobulin (Ig)M memory is lacking in patients without spleen. The absence of IgM memory B cells correlates with an impaired immune response to encapsulated bacteria not only in splenectomized patients, but also in individuals with an intact spleen. We show that the physiological and transient predisposition to pneumococcal infections of young children (0–2 yr) is associated with the lack of circulating IgM memory B cells and of serum antipolysaccharide IgM. We also demonstrate that IgM memory B cells are undetectable in a fraction of patients with common variable immunodeficiency, who have recurrent and invasive infections by encapsulated bacteria. IgM memory B cells, therefore, require the spleen for their generation and/or survival and are responsible for the protection against encapsulated bacteria.

Angiogenesis is essential in bone fracture healing for restoring blood flow to the fracture site. Vascular endothelial growth factor (VEGF) and its receptor have been implicated in this process. Despite the importance of angiogenesis for the healing processes of damaged bones, the role of VEGF signaling in modulation of osteogenic differentiation in human mesenchymal stem cells has not been investigated in great detail. We examined the expression of VEGF-A and VEGFR-1 in human adult mesenchymal stem cells derived from trabecular bone (hTBCs). VEGF-A was found to be secreted in a differentiation dependent manner during osteogenesis. Transcripts for VEGF-A were also seen to be elevated during osteogenesis. In addition, transcripts for VEGF-A and the corresponding receptor VEGFR-1 were upregulated under hypoxic conditions in undifferentiated hTBCs. To investigate the signaling of VEGF-A on osteogenesis recombinant hTBCs were generated. High expression of VEGF-A stimulated mineralization, whereas high expression of sFLT-1, an antagonist to VEGF-A, reduced mineralization suggesting that VEGF-A acts as autocrine factor for osteoblast differentiation. In addition, VEGF-A secreted by hTBCs promotes sprouting of endothelial cells (HUVE) demonstrating a paracrine role in blood vessel formation. In summary, an in vitro analysis of transgene effects on cellular behavior can be used to predict an effective ex vivo gene therapy.

The complexity of the angiogenic cascade limits cellular approaches to studying angiogenic endothelial cells (ECs). In turn, in vivo assays do not allow the analysis of the distinct cellular behavior of ECs during angiogenesis. Here we show that ECs can be grafted as spheroids into a matrix to give rise to a complex three-dimensional network of human neovessels in mice. The grafted vasculature matures and is connected to the mouse circulation. The assay is highly versatile and facilitates numerous applications including studies of the effects of different cytokines on angiogenesis. Modifications make it possible to study human lymphangiogenic processes in vivo. EC spheroids can also be coimplanted with other cell types for tissue engineering purposes.

Given the need for robust and costefficient in vitro models to study angiogenesis and reproducibly analyze potential pro-and antiangiogenic compounds in preclinical studies, we developed a 3--dimensional in vitro angiogenesis assay that is based on collagen gel-embedded, size-defined spheroids generated from cultured human umbilical vein endothelial cells (HUVECs). Despite its wide distribution, limitations, sensitivity, robustness, and improvements, the capacity of this assay for functional screening purposes has not been elucidated thus far. By using time-lapse video microscopy, we show that tip cells lead the formation of capillary-like and partially lumenized sprouts originating from the spheroids. Angiogenic sprouting from spheroids generated from 5 different primary cultured human endothelial cell types was induced by physiologic concentrations of vascular endothelial cell growth factor 165. Based on this assay system, we determined the capacity of 880 approved drugs to interfere with or boost angiogenic sprouting, thereby assessing their putative angiogenesis-related side effects or novel applications. However, although this assay allowed for a rapid and reproducible determination of functional IC 50 values of individual compounds, the sprouting results were partially affected by the HUVEC passage number and donor variability. To overcome this limitation, immortalized HUVECs (iHUVECs) showing a more homogenous response in terms of proliferation and sprouting over multiple population doublings were used in the course of this study. Collectively, the spheroid-based angiogenesis assay provides a sensitive and versatile tool to study the impact of pro-and antiangiogenic determinants on multiple steps of the angiogenic cascade. It is compatible with different endothelial cell types and allows use of iHUVECs to improve its overall robustness.-Heiss, M