1,720,956 research outputs found
HUMAN HEMATOPOIETIC-CELLS AND THYMIC EPITHELIAL-CELLS INDUCE TOLERANCE VIA DIFFERENT MECHANISMS IN THE SCID-HU MOUSE THYMUS
No full textTo study the role of thymic education on the development of the human T cell repertoire, SCID-hu mice were constructed with fetal liver and fetal thymus obtained from the same or two different donors. These animals were studied between 7 and 12 mo after transplantation, at which times all thymocytes and peripheral T cells were derived from stem cells of the fetal liver graft. Immunohistology of the thymus grafts demonstrated that thymic epithelial cells were of fetal thymus donor (FTD) origin. Dendritic cells and macrophages of fetal liver donor (FLD) origin were abundantly present in the medullary and cortico-medullary areas. Thymocytes of SCID-hu mice transplanted with liver and thymus of two different donors (FLD(A)/FTD(B) animals) were nonresponsive to Epstein-Barr virus-transformed B cell lines (B-LCL) established from both the FLD(A) and FTD(B), but proliferated vigorously when stimulated with third-party allogeneic B-LCL. Mixing experiments showed that the nonresponsiveness to FTD(B) was not due to suppression. Limiting dilution analysis revealed that T cells reacting with the human histocompatibility leukocyte antigens (HLA) of the FLD were undetectable in the CD8+ T cell population and barely measurable in the CD4+ subset. On the other hand, CD4+ and CD8+ T cells reactive to the HLA antigens of the FTD were readily detectable. These results indicate that FLD-reactive cells were clonally deleted, whereas FTD-reactive cells were not. However, the frequencies of FTD-reactive T cells were consistently twofold lower than those of T cells specific for any third-party B-LCL. In addition, the cytotoxic activity and interleukin 2 production by FTD-specific T cells were lower compared with that of third-party-reactive T cell clones, suggesting that FTD-specific cells are anergic. These data demonstrate that T cells become tolerant to autologous and allogeneic HLA antigens expressed in the thymus via two different mechanisms: hematopoietic cells present in the thymus induce tolerance to "self"-antigens by clonal deletion, whereas thymic epithelial cells induce tolerance by clonal anergy and possibly deletion of high affinity clones
THYMIC SELECTION OF THE HUMAN T-CELL RECEPTOR V-BETA REPERTOIRE IN SCID-HU MICE
No full textImplantation of pieces of human fetal liver and thymus into SCID mice results in the development of a human thymus-like organ, in which sustained lymphopoiesis is reproducibly observed. In this model, T cell development can be experimentally manipulated. To study the influence of thymic selection on the development of the human T cell repertoire, the T cell receptor (TCR) Vbeta gene repertoire of double-positive (CD4+CD8+) and single-positive (CD4+CD8- and CD4-CD8+) T cells was analyzed in the SCID-hu thymus using a multiprobe ribonuclease protection assay. TCR diversity in double-positive SCID-hu thymocytes was found to be comparable with that present in the thymus of the fetal liver donor, did not change with time, and was independent of the origin of the thymus donor. Thymic selection in SCID-hu thymus induces changes in Vbeta usage by the single-positive CD4+ or CD8+ T cells comparable with those previously reported for single-positive cells present in a normal human thymus. Finally, significant differences were observed in the Vbeta usage by CD4 or CD8 single-positive T cells that matured from genetically identical stem cells in different thymic environments. Collectively, these data suggest: first, that the generation of TCR diversity at the double-positive stage is determined by the genotype of the stem cells; and second, that polymorphic determinants expressed by thymic epithelium measurably influence the Vbeta repertoire of mature single-positive T cells
BACTERIAL SUPERANTIGENS MEDIATE T-CELL DELETIONS IN THE MOUSE SEVERE COMBINED IMMUNODEFICIENCY HUMAN LIVER THYMUS MODEL
No full textThe ability to analyze T cell receptor (TCR) thymic repertoire shaping in humans by self and foreign ligands is hampered by the lack of suitable models. We recently documented that the mouse severe combined immunodeficiency (SCID)-human fetal liver/thymus model recapitulates the TCR Vbeta gene repertoire of human thymocytes. Here, we show that an exogenous superantigen, staphylococcal enterotoxin B, administered to such mice induces clonal deletions in both CD4+8- and CD8+4- cells involving the same human Vbeta clones that are selected in vitro by this toxin. This model, therefore, may allow comprehensive studies into the effects of microbial and other agents on human T cell thymic selection processes in a biologically relevant setting
CHIMERISM AND TOLERANCE TO HOST AND DONOR IN SEVERE COMBINED IMMUNODEFICIENCIES TRANSPLANTED WITH FETAL LIVER STEM-CELLS
No full textWe have studied the peripheral T cell repertoire of two patients with severe combined immunodeficiency who were successfully treated with human histocompatibility leukocyte antigen (HLA)-mismatched fetal liver stem cell transplantation. The patients presented a split chimerism. T cells were of donor origin, whereas the B cells/monocytes were of the host phenotype. Interestingly, the natural killer (NK) cells in one patient were donor derived and in the other patient of host origin. The NK cells were functional but did not have antihost or donor reactivity. Despite the HLA mismatch between donor and host cells, complete tolerance was achieved in vivo, and a specific unresponsiveness of peripheral blood mononuclear cells from both patients toward the host cells was demonstrated in vitro. Nevertheless, we could isolate T cell receptor (TCR)alphabeta, CD4+ or CD8+, T cell clones specifically reacting with HLA class I and II molecules of the host. The CD4+ host-reactive T cell clones from both patients produced interleukins 2 and 5, interferon-gamma, granulocyte / macrophage colony-stimulating factor but are specifically defective in interleukin 4 production. The frequencies of CD8+ host-reactive T cells were high, and were in the same range as those observed for CD8+ alloreactive T cells. In contrast, no donor-reactive CD8+ T cells or host or donor-reactive TCRgammadelta+ T cells were detected. These data indicate that, after fetal stem cell transplantation, donor-reactive, but not host-reactive cells, are deleted from the T cell repertoire. Therefore, a peripheral mechanism of suppression or clonal anergy, rather than clonal deletion, is involved in maintaining in vivo tolerance toward the host
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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