Weak tension accelerates hybridization and dehybridization of short oligonucleotides.

The hybridization and dehybridization of DNA subject to tension is relevant to fundamental genetic processes and to the design of DNA-based mechanobiology assays. While strong tension accelerates DNA melting and decelerates DNA annealing, the effects of tension weaker than 5 pN are less clear. In this study, we developed a DNA bow assay, which uses the bending rigidity of double-stranded DNA (dsDNA) to exert weak tension on a single-stranded DNA (ssDNA) target in the range of 2-6 pN. Combining this assay with single-molecule FRET, we measured the hybridization and dehybridization kinetics between a 15 nt ssDNA under tension and a 8-9 nt oligonucleotide, and found that both the hybridization and dehybridization rates monotonically increase with tension for various nucleotide sequences tested. These findings suggest that the nucleated duplex in its transition state is more extended than the pure dsDNA or ssDNA counterpart. Based on coarse-grained oxDNA simulations, we propose that this increased extension of the transition state is due to steric repulsion between the unpaired ssDNA segments in close proximity to one another. Using linear force-extension relations verified by simulations of short DNA segments, we derived analytical equations for force-to-rate conversion that are in good agreement with our measurements.

Human T lymphoblasts and activated dendritic cells in the allogeneic mixed leukocyte reaction are susceptible to NK cell-mediated anti-CD83-dependent cytotoxicity.

CD83 is a marker of dendritic cell (DC) differentiation/activation and its expression in the mouse thymus contributes to CD4(+) T lymphocyte development. Its extrathymic role remains unclear despite the functional effects observed with CD83 fusion proteins or CD83 antibody and recent reports of potential ligands. We investigated the previously observed and presumed functional blockade of the allogeneic mixed leukocyte reaction (MLR) with rabbit polyclonal anti-CD83 (RA83). RA83 inhibition of T lymphocyte proliferation stimulated with allogeneic immature monocyte-derived DC (iMoDC) was confirmed. However, we found it was due to antibody-dependent cellular cytotoxicity (ADCC) mediated by NK cells in the responder T cell preparation. The likely targets of the ADCC were MoDC that had up-regulated CD83 during the MLR. Using a (51)Cr-release assay, we confirmed that CD83(+) MoDC, but not CD83(-) MoDC, are lysed by NK cells in the presence of RA83. However, prior fixation of the stimulator MoDC in the allogeneic MLR did not abrogate RA83 inhibition, indicating that cells from the responder T lymphocyte preparation, involved in the MLR proliferative response, also expressed CD83. We found, after 3-4 days of culture with allogeneic MoDC, a subset of CD3(+) cells had up-regulated CD83 and CD25. These were blasting T cells and, when isolated from the MLR, were found to be lysed by autologous NK cells in the presence of RA83. Thus, CD83 is expressed by responding T cells as well as by stimulating cells in the MLR and both are susceptible to anti-CD83-mediated ADCC.