The supply of naive T cells by the thymus normally requires precursor T cell proliferation within the thymus and would be particularly important in the setting of HIV infection when both naive and memory T cells are progressively depleted. As a robust, quantitative index of intrathymic proliferation, the ratio of different T cell receptor excision circles (TRECs), molecular markers of distinct T cell receptor rearrangements occurring at different stages of thymocyte development, was measured in peripheral blood-mononuclear cells (PBMCs). This ratio has the virtue that it is a "signature" of thymic emigrants throughout their entire life and, thus, can be measured in peripheral cell populations that are easy to obtain. Using the new assay, we evaluated the effect of HIV infection on intrathymic precursor T cell proliferation by longitudinal analysis of PBMCs from recently infected individuals. Our findings reveal a substantial reduction in intrathymic proliferation. The analysis also indicates the existence of a compensatory mechanism acting to sustain the numbers of recent thymic emigrants (RTEs) in the periphery.

BackgroundIn the bone marrow, hematopietic and mesenchymal stem cells form a unique niche in which the oxygen tension is low. Hypoxia may have a role in maintaining stem cell fate, self renewal and multipotency. However, whereas most studies addressed the effect of transient in vitro exposure of MSC to hypoxia, permanent culture under hypoxia should reflect the better physiological conditions.ResultsMorphologic studies, differentiation and transcriptional profiling experiments were performed on MSC cultured in normoxia (21% O2) versus hypoxia (5% O2) for up to passage 2. Cells at passage 0 and at passage 2 were compared, and those at passage 0 in hypoxia generated fewer and smaller colonies than in normoxia. In parallel, MSC displayed (>4 fold) inhibition of genes involved in DNA metabolism, cell cycle progression and chromosome cohesion whereas transcripts involved in adhesion and metabolism (CD93, ESAM, VWF, PLVAP, ANGPT2, LEP, TCF1) were stimulated. Compared to normoxic cells, hypoxic cells were morphologically undifferentiated and contained less mitochondrias. After this lag phase, cells at passage 2 in hypoxia outgrew the cells cultured in normoxia and displayed an enhanced expression of genes (4-60 fold) involved in extracellular matrix assembly (SMOC2), neural and muscle development (NOG, GPR56, SNTG2, LAMA) and epithelial development (DMKN). This group described herein for the first time was assigned by the Gene Ontology program to "plasticity".ConclusionThe duration of hypoxemia is a critical parameter in the differentiation capacity of MSC. Even in growth promoting conditions, hypoxia enhanced a genetic program that maintained the cells undifferentiated and multipotent. This condition may better reflect the in vivo gene signature of MSC, with potential implications in regenerative medicine.

IntroductionCD5 is a lymphoid marker-one of the earliest acquired during T-cell ontogeny. Its expression increases coordinately with that of cell surface CD3. 1 Although expressed on lymphoid-committed progenitors, its expression is lost following natural killer (NK) cell differentiation. 2 In contrast to its being a pan-T-cell marker, CD5 is only expressed on some B cells. Based on its coexpression with CD11b, immunogloublin M (IgMϩ) B cells in mouse and human are classically separated into different subsets, termed B-1a and B-1b, each of which expresses CD11b and B-2. 3 The B-1a subset expresses CD5, and at a functional level these CD5 ϩ B cells frequently produce polyreactive antibodies, mainly IgM, that recognize a variety of self-antigens and foreign antigens. 4 The reason for the expression of CD5 on a particular B-cell subsetsomething Wortis and Berland 5 call "one of many intriguing and seemingly idiosyncratic features of B-1a cells"-has remained obscure. Two hypotheses are proposed to explain the origin of B-1 cells. The lineage hypothesis holds that only certain fetal progenitors are destined to become B-1 cells. 6,7 In contrast, the differentiation hypothesis holds that every B cell is able to acquire B-1 cell characteristics. In brief, the notion of B-cell lineage based on differential CD5 expression is controversial because of the lack of clear identification of a fetal progenitor destined to become a B-1 B cell and the demonstration that CD5 Ϫ cells can acquire CD5 expression in vivo 8 or in vitro 9 after B-cell receptor (BCR) stimulation This up-regulation supports the initial view that CD5 is an activation marker 10 ; however, a definitive function for this antigen remains to be established.It has been shown that CD5 up-regulation in B cells plays a role in tolerance to autoantigens. By setting the threshold level for activation signals, CD5 prevents B cells from activation-induced cell death and maintains tolerance in anergic B cells in vivo. 11 The reason for keeping potentially autoreactive cells alive is that these cells are also necessary for an effective immune response to some pathogens. 12 This supported the role of CD5 as a negative regulator of BCR signaling, which was later demonstrated by the generation of CD5-null mice. 13 In these animals, peritoneal B cells, which are poorly responsive to BCR stimulation, restored their capacity to fully proliferate to anti-IgM. 13 The molecular basis for BCR inhibition by CD5 has been extensively investigated. The physical association of CD5 to the BCR was demonstrated, 14 and our recent work further documented the structural basis of CD5 inhibition of BCR-mediated signals. Using a reconstitution approach, in a murine lymphoma B cell line we showed that Ca 2ϩ response, extracellular signal-related kinase-2 (ERK-2) activation, and the production of interleukin-2 (IL-2) induced by BCR activation were antagonized by CD5. 15 The role of the src autophosphorylationlike motif within the cytoplasmic domain was demonstrated. 16 Of interest, this domain is...