SummaryMesenchymal stem cells (MSC) are capable of differentiating into bone, fat, cartilage, tendon and other organ progenitor cells. Despite the abundance of MSC within the organism, little is known about their in vivo properties or about their corresponding in vivo niches. We therefore isolated MSC from spongy (cancellous) bone biopsies of healthy adults. When compared with the surrounding marrow, a fourfold higher number of colony-forming units was found within the tight meshwork of trabecular bone surface. At these sites, oxygen concentrations range from 1% to 7%. In MSC cultured at oxygen as low as 3%, rates for cell death and hypoxia-induced gene transcription remained unchanged, while in vitro proliferative lifespan was significantly increased, with about 10 additional population doublings before reaching terminal growth arrest. However, differentiation capacity into adipogenic progeny was diminished and no osteogenic differentiation was detectable at 3% oxygen. In turn, MSC that had previously been cultured at 3% oxygen could subsequently be stimulated to successfully differentiate at 20% oxygen. These data support our preliminary finding that primary MSC are enriched at the surface of spongy bone. Low oxygen levels in this location provide a milieu that extends cellular lifespan and furthermore is instructive for the stemness of MSC allowing proliferation upon stimulation while suppressing differentiation.

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D-amino acids are present in some peptides from amphibian skin. These residues are derived from the corresponding L-amino acids present in the respective precursors. From skin secretions of Bombinae, we have isolated an enzyme that catalyzes the isomerization of an L-Ile in position 2 of a model peptide to D-allo-Ile. In the course of this reaction, which proceeds without the addition of a cofactor, radioactivity from tritiated water is incorporated into the second position of the product. The amino acid sequence of this isomerase could be deduced from cloned cDNA and genomic DNA. After expression of this cDNA in oocytes of Xenopus laevis, isomerase activity could be detected. Polypeptides related to the frog skin enzyme are present in several vertebrate species, including humans.amphibia ͉ bombinin H ͉ isomerase ͉ chirality F rom amphibian skin, a wide variety of biologically active peptides have been isolated (1). One of these compounds, the heptapeptide dermorphin (present in the skin of the South American tree frog Phyllomedusa sauvagei), is a potent opiate that binds with high affinity and selectivity to -opiate receptors (2). Quite unexpectedly, dermorphin was found to contain D-Ala at position 2, and this residue is essential for binding to the receptor. Subsequently, several additional opiate peptides containing a D-amino acid were found in the skin of other Phyllomedusinae (3). The bombinins H, antibacterial and hemolytic peptides isolated from skin secretions of the frog Bombina variegata, contain either L-Ile or D-allo-Ile (D-aIle) as the second amino acid (4). Peptides with D-amino acids in their sequences were also found in different invertebrate species, such as snails (5-7), the venom of a spider (8), and crustaceans (9). The most recent addition to this group was a homologue of C-type natriuretic peptides isolated from the venom of the male platypus (10).In principle, the biosynthesis of these D-amino acids in animal peptides has been clarified. As shown for dermorphin (11) and subsequently for several cases of vertebrate and invertebrate peptides, the secreted products are derived from larger precursors encoded by mRNAs. At the positions where a D-amino acid is present in the end product, a codon for the corresponding L-isomer has been found (reviewed in refs. 12 and 13). This finding implies that, at some stage in the processing of the precursors, a conversion of certain L-amino acids to the D-form takes place. Indeed, in many cases, the L and D forms of the mature peptides coexist in the respective secretory glands and secretions, which can be interpreted as incomplete isomerization. An enzyme that catalyzes such an unusual reaction has been characterized from the venom of a funnel web spider (8). In this case, the chirality of a Ser close to the C terminus of a peptide termed -agatoxin IV is inverted. This ''isomerase'' is related to Ser proteases and functions without any added cofactors (14, 15).As mentioned above, skin secretions of Bombina species contain a group of hemolytic peptides tha...

We have recently described an IL-2/IL-4-producing CD8+CD25+ nonregulatory memory T cell population that occurs in a subgroup of healthy elderly persons who characteristically still have a good humoral response after vaccination. The present study addresses this specific T cell subset and investigates its origin, clonal composition, Ag specificity, and replicative history. We demonstrate that CD8+CD25+ memory T cells frequently exhibit a CD4+CD8+ double-positive phenotype. The expression of the CD8 αβ molecule and the occurrence of signal-joint TCR rearrangement excision circles suggest a thymic origin of these cells. They also have longer telomeres than their CD8+CD25− memory counterparts, thus indicating a shorter replicative history. CD8+CD25+ memory T cells display a polyclonal TCR repertoire and respond to IL-2 as well as to a panel of different Ags, whereas the CD8+CD25− memory T cell population has a more restricted TCR diversity, responds to fewer Ags, and does not proliferate in response to stimulation with IL-2. Molecular tracking of specific clones with clonotypic primers reveals that the same clones occur in CD8+CD25+ and CD8+CD25− memory T cell populations, demonstrating a lineage relationship between CD25+ and CD25− memory CD8+ T cells. Our results suggest that CD25-expressing memory T cells represent an early stage in the differentiation of CD8+ cells. Accumulation of these cells in elderly persons appears to be a prerequisite of intact immune responsiveness in the absence of naive T cells in old age.

Mesenchymal stem cells (MSCs) are able to differentiate into distinct lineages such as adipo-, osteo-, and chondrocytes. MSCs were isolated from three mouse strains, which are short- (SAMP6, 9.7 months), medium- (SAMR1, 16.3 months), or long-lived (C57BL/6, 28 months). We investigated primary colony-forming units with regard to bone marrow stroma and found differences that correlate with mean life expectancies of the particular genetic backgrounds. However, MSC derived from the various mouse strains behaved equivalently in vitro with respect to growth rate. By genomic means, we analyzed the cellular milieu in vivo and found considerable differences among the various mouse strains. This implies that, although individual MSCs show an equivalent differentiation potential in vitro, the primary stem cells are greatly influenced by their molecular environment.