Selection negative

Annunziato F, Romagnani P, Cosmi L, et al. Macrophage-derived chemokine and EBIl-ligand chemokine attract human thymocytes in different stages of development and are produced by distinct subsets of medullary epithelial cells possible implications for negative selection. J Immunol 2000 165(l) 238-246. 

Fisher, M., Nagarkatti, M., and Nagarkatti, P, 2,3,7,8-Tetrachlorodibenzo-p-dioxin enhances negative selection of T cells in the thymus but allows autoreactive T cells to escape deletion and migrate to the periphery, Mol. Pharmacol., 67, 327, 2004. 

Selection. Products designed for positive or negative selection of autologous or allogenic cells intended for therapy (e.g., purging of tumor from bone marrow, selection of CD34+ cells). 

Although central tolerance is the major mechanism to establish the T-cell repertoire by positive and negative selection mechanisms, thymic deletion of harmful T-cell populations is incomplete. Therefore, the immune system has developed mechanisms that deal with tolerance in the peripheral lymphoid organs providing the necessary... 

Figure 3 A general positive and negative selection strategy for evolving aminoacyl-tRNA synthetase variants specific for an unnatural amino acid.

This selection scheme was used to evolve the orthogonal E. coli tRNA u -TyrRS pair in yeast. A synthetase library (10 in size) was similarly constructed by randomizing five active-site residues in E. coli TyrRS corresponding to the five residues randomized in the Af/TyrRS. Mutant synthetases were identified after several rounds of positive and negative selection that incorporate a number of unnatural amino acids into proteins, albeit with rather low protein yields (about 0.05 mgl A similar approach has been used to evolve orthogonal E. coli leucyl tRNAcuA LeuRS pairs that selectively incorporate photochromic and fluorescent amino acids into proteins in yeast. ... 

Primary CLL cells purified from peripheral blood of volunteer patients (see Note 3) CLL cells are purified by Ficoll gradient centrifugation, by negative selection, or by a combination of both (see Note 4). CLL cells can be used immediately or frozen in 90% heat/inactivated FBS supplemented with 10% DMSO and stored at -80°C or in liquid nitrogen for short-and long-term storage, respectively. CLL cells are cultured in complete RPMI medium and maintained in a humidified atmosphere of 5% CO at 37°C (see Note 5). 

Finally, in this brief overview of lymphocyte defects, mention should be made of mutations affecting major histocompatibility-complex (MHC) Class II molecules. These mutations affect a multiprotein transcription factor complex that regulates the expression of MHC Class II molecules (121). Affected patients have undetectable levels of MHC Class II antigens HLA-DP, DQ, and DR on the surface of monocytes and B cells. Lack of these antigen-presenting molecules leads to impaired immune response. Affected individuals have moderate lymphopenia with a severely reduced number of CD4+ T cells and normal or increased numbers of CD8+ T cells. Since MHC molecules in the thymic epithelium play a key role in positive and negative selection of primitive T cells, selection of competent T cells is also affected in the absence of MHC Class II antigens. 

Fig. 3. Representation of (A) the two basic types of targeting vectors, (B) positive and negative selection markers, (C) the hit-and-run approach to introduce subtle mutations, and (D) the CRE/loxP recombinase system.

Homologous recombination occurs approximately 1000-fold less frequently than non-homologous recombination, therefore methods have been developed to enrich for homologous recombination events. A widely applied method includes the use of a positive and a negative selection marker and does not require the expression of the target gene in ES cells (Mansour et al., 1988). The targeting construct is based on a replacement-type vector... 

Both positive and negative selection of cells can be achieved... 

Lymphoid dendritic cells promote negative selection in the thymus. This may be attributed to their ability to induce fas-mediated apoptosis. Based on their ability to cause apoptosis and their ability to eliminate self-reactive T cells, lymphoid dendritic cells exhibit a regulatory function instead of a stimulatory immune effector function. Myeloid dendritic cells also have differential effects. For example, T cells can be primed to selectively activate THi responses by CD14-derived myeloid dendritic cells. Naive B cells can be activated in the presence of CD40L and IL-2 to secrete IgM by CD34+, CD14-derived myeloid dendritic cells. This effect on naive B cells is not observed with CD la-derived dendritic cells. 

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