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We study the effects of weak off-diagonal disorder on Peierls systems with a doubly degenerate ground state. We show that for these systems disorder in the electron hopping amplitudes induces a finite density of solitons in the minimal-energy lattice configuration of a single chain. These disorder-induced dimerization kinks are neutral and have spin 1/2. Using a continuum model for the Peierls chain and treating the lattice classically, we analytically calculate the average free energy and density of kinks. We compare these results to numerical calculations for a discrete model and discuss the implications of the kinks for the optical and magnetic properties of the conjugated polymer trans-polyacetylene., 28 pages, revtex, 5 Postscript figures, to appear in Phys. Rev. B

B cell motility within lymphoid tissue is precisely orchestrated by G-protein-coupled receptors (GPCRs) of the chemokine receptor family and other GPCRs, including the recently described 7α,25-dihydroxycholesterol receptor Ebi2. Here, we performed an exhaustive twophoton microscopy (2PM) analysis of the contribution of the chemokine receptors CXCR4, CXCR5 and CCR7, as well as Ebi2, during the dynamic migration of naïve B cells inside peripheral lymph nodes (PLNs) and the kinetics of B cell accumulation from the T cell area in B cell follicles. Using the intravital 2PM model of murine popliteal PLNs paired with genetic and pharmacological inhibition of GPCRs and their ligands, we observed a non-redundant role for CXCR5 for dynamic B cell motility in the T cell area and B cell follicles, while the functional absence of CXCR4 and CCR7 did not influence B cell migration parameters. Although CXCR5 was central for efficient B cell accumulation in B cell follicles, we did not find evidence for directed migration of T cell area-borne B cells towards B cell follicles, supporting a chemokinetic, and not chemotactic, role for the CXCR5 ligand CXCL13 during this process. Lack of Ebi2 confined B cells to the central part of B cell follicles, resulting in higher average migration velocities as compared to wild type B cells. In summary, our data provide novel insight into the contribution of GPCRs during the dynamic migration of naïve B cells in non-inflamed and inflamed PLNs.

Aspergillus fumigatus is a ubiquitous airborne fungus and opportunistic human pathogen. In immunocompromised hosts, the fungus can cause life-threatening diseases like invasive pulmonary aspergillosis. Since the incidence of fungal systemic infections drastically increased over the last years, it is a major goal to investigate the pathobiology of A. fumigatus and in particular the interactions of A. fumigatus conidia with immune cells. Many of these studies include the activity of immune effector cells, in particular of macrophages, when they are confronted with conidia of A. fumigus wild-type and mutant strains. Here, we report the development of an automated analysis of confocal laser scanning microscopy images from macrophages coincubated with different A. fumigatus strains. At present, microscopy images are often analysed manually, including cell counting and determination of interrelations between cells, which is very time consuming and error-prone. Automation of this process overcomes these disadvantages and standardises the analysis, which is a prerequisite for further systems biological studies including mathematical modeling of the infection process. For this purpose, the cells in our experimental setup were differentially stained and monitored by confocal laser scanning microscopy. To perform the image analysis in an automatic fashion, we developed a ruleset that is generally applicable to phagocytosis assays and in the present case was processed by the software Definiens Developer XD. As a result of a complete image analysis we obtained features such as size, shape, number of cells and cell-cell contacts. The analysis reported here, reveals that different mutants of A. fumigatus have a major influence on the ability of macrophages to adhere and to phagocytose the respective conidia. In particular, we observe that the phagocytosis ratio and the aggregation behaviour of pksP mutant compared to wild-type conidia are both significantly increased.

In this letter, we study the effects of weak off-diagonal disorder on conjugated polymers with a doubly degenerate ground-state. We find that disorder induces a finite density of neutral solitons in the lattice dimerization of a polymer chain. Interchain interactions result in a linear potential between the solitons and, if sufficiently strong, bind them into pairs resulting in an exponential suppression of the soliton density. As neutral solitons carry spin 1/2, they contribute to the polymer's magnetic properties. We calculate the magnetic susceptibility and suggest measurements of the magnetic susceptibility in {\it trans}-polyacetylene at low temperatures., 2 pages, 1 Postscript figure included, submitted to Synthetic Metals

We consider Peierls instability due to the interaction of electrons with both acoustic and optical phonons in metallic carbon nanotubes, resulting in a static twist in the nanotube lattice below the critical temperature T c . We study lattice excitations, the so-called solitwiston and polartwiston, over the ordered Peierls state for different types of boundary conditions. Furthermore, we calculate the electrical resistivity and find that our theory offers a possible explanation for the observed low-temperature rise in the electrical resistivity of carbon nanotubes.

SummaryGerminal centers (GCs) are specialized microenvironments where antigen-activated B cells undergo proliferation, immunoglobulin (Ig) class switch recombination, somatic hypermutation (SHM), and affinity maturation. Within GCs, follicular dendritic cells (FDCs) are key players in driving these events via direct interaction with GC B cells. Here, we provide in vivo evidence that FDCs express and upregulate Toll-like-receptor (TLR) 4 in situ during germinal center reactions, confirm that their maturation is driven by TLR4, and associate the role of FDC-expressed TLR4 with quantitative and qualitative affects of GC biology. In iterative cycles of predictions by in silico modeling subsequently verified by in vivo experiments, we demonstrated that TLR4 signaling modulates FDC activation, strongly impacting SHM and generation of Ig class-switched high-affinity plasma and memory B cells. Thus, our data place TLR4 in the heart of adaptive humoral immunity, providing further insight into mechanisms driving GCs arising in both health and disease.

Intravital two-photon microscopy is a powerful tool which allows to observe cell migration and interaction in living tissue. Recently this method was applied to lymphocytes in lymph nodes. The motility of lymphocytes has important implications on the functionality of immune responses. Different cell types entering and leaving lymphoid tissue have to home to their respective compartments and to find suitable interaction partners. Both processes crucially depend on chemotaxis which is mediated by corresponding chemokines and membrane bound receptors. While this knowledge was based on corresponding block- or knock-out-experiments, the aim of the recent two-photon experiments was to clarify the migration mode of lymphocytes in vivo under realistic conditions.Surprisingly two-photon experiments of lymphocyte motility in lymph nodes do not exhibit any sign of chemotactic activity. Three groups have found that the cells basically move in a random walk with directional persistence of 2 minutes (Schwickert etal Nature 2007 446:83; Allen etal Science 2007 315:528; Hauser etal Immunity 2007 26:655). These results induced a controversial discussion about functional implications that could not be resolved on the basis of the measured data alone. We show that a mathematical analysis of cell motility can clarify the interpretation of two-photon motility data for this specific case and in general. Data about tissue morphology and functionality are used together with a simulation of lymphocyte motility to understand the details of chemotaxis in lymphoid compartments. It is found that the chemotactic sensitivity of lymphocytes is tightly regulated and the simulations predict a frequent up and down regulation of chemokine receptors on the plasma membrane. It is shown that under these conditions both the motility data and lymphocyte functionality can be reconciled.