Complete analysis of the B-cell response to a protein antigen, from in vivo germinal centre formation to 3-D modelling of affinity maturation

Somatic hypermutation of immunoglobulin variable region genes occurs within germinal centres (GCs) and is the process responsible for affinity maturation of antibodies during an immune response. Previous studies have focused almost exclusively on the immune response to haptens, which may be unrepresentative of epitopes on protein antigens. In this study, we have exploited a model system that uses transgenic B and CD4<sup>+</sup> T cells specific for hen egg lysozyme (HEL) and a chicken ovalbumin peptide, respectively, to investigate a tightly synchronized immune response to protein antigens of widely differing affinities, thus allowing us to track many facets of the development of an antibody response at the antigen-specific B cell level in an integrated system <i>in</i> <i>vivo</i>. Somatic hypermutation of immunoglobulin variable genes was analysed in clones of transgenic B cells proliferating in individual GCs in response to HEL or the cross-reactive low-affinity antigen, duck egg lysozyme (DEL). Molecular modelling of the antibody–antigen interface demonstrates that recurring mutations in the antigen-binding site, selected in GCs, enhance interactions of the antibody with DEL. The effects of these mutations on affinity maturation are demonstrated by a shift of transgenic serum antibodies towards higher affinity for DEL in DEL-cOVA immunized mice. The results show that B cells with high affinity antigen receptors can revise their specificity by somatic hypermutation and antigen selection in response to a low-affinity, cross-reactive antigen. These observations shed further light on the nature of the immune response to pathogens and autoimmunity and demonstrate the utility of this novel model for studies of the mechanisms of somatic hypermutation

Optimality of mutation and selection in germinal centers

The population dynamics theory of B cells in a typical germinal center could play an important role in revealing how affinity maturation is achieved. However, the existing models encountered some conflicts with experiments. To resolve these conflicts, we present a coarse-grained model to calculate the B cell population development in affinity maturation, which allows a comprehensive analysis of its parameter space to look for optimal values of mutation rate, selection strength, and initial antibody-antigen binding level that maximize the affinity improvement. With these optimized parameters, the model is compatible with the experimental observations such as the ~100-fold affinity improvements, the number of mutations, the hypermutation rate, and the "all or none" phenomenon. Moreover, we study the reasons behind the optimal parameters. The optimal mutation rate, in agreement with the hypermutation rate in vivo, results from a tradeoff between accumulating enough beneficial mutations and avoiding too many deleterious or lethal mutations. The optimal selection strength evolves as a balance between the need for affinity improvement and the requirement to pass the population bottleneck. These findings point to the conclusion that germinal centers have been optimized by evolution to generate strong affinity antibodies effectively and rapidly. In addition, we study the enhancement of affinity improvement due to B cell migration between germinal centers. These results could enhance our understandings to the functions of germinal centers.Comment: 5 figures in main text, and 4 figures in Supplementary Informatio

Natural killer cell-mediated lysis of T cell lines chronically infected with HIV-1.

The susceptibility of HIV-1-infected CD4+ T cell lines to natural killer (NK) cell-mediated lysis was examined. Non-adherent peripheral blood mononuclear cells (PBMC) of healthy adults lysed HUT cells chronically infected with the IIIB or WMJ1 strains of HIV-1 to a significantly greater extent than uninfected HUT cells. In contrast, Sup-T1 cells chronically infected with these two strains of HIV-1 were not lysed to a greater extent than uninfected Sup-T1 cells. Clone A1.25-infected Sup-T1 (A1.25/Sup-T1), derived from IIIB-infected Sup-T1 cells (IIIB/Sup-T1), were susceptible to non-adherent PBMC-mediated lysis, as were A1.25-infected HUT cells (A1.25/HUT). When non-adherent PBMC were depleted of CD16 (Leu-11b)+ NK cells by treatment with anti-Leu-11b plus C, lysis of HIV-1-infected HUT or Sup-T1 cells was reduced to low levels, indicating that the lysis was mediated by NK cells. Expression of HIV antigens on these target cells did not correlate with their susceptibility to NK cell-mediated lysis. Depletion of interferon-alpha (IFN-alpha) producing HLA-DR+ cells from non-adherent PBMC had no effect on the magnitude of NK cell-mediated lysis of IIIB or WMJ1-infected HUT cells. In contrast, lysis of A1.25/Sup-T1 or A1.25/HUT cells required the presence of HLA-DR+ cells. IFN-alpha production appeared to be required for NK cell-mediated lysis of A1.25/Sup-T1 or A1.25/HUT cells, while lysis of HUT cells infected with the WMJ1 or IIIB strains of HIV-1 was IFN-alpha independent. These results indicate considerable variability in the susceptibility of different HIV-1 infected T cell lines to NK cell-mediated lysis and suggest the existence of alternative mechanisms of activation of NK cells for lysis of HIV-1-infected T cell lines

Strain States in YSZ / RE2O3 (RE = Er, Y) Multilayers as a Function of Layer Thickness and Their Effect on Interface Conductivity and Diffusion

ABSTRACTIn this study the strain states in alternating multilayers of an extrinsic O2− ion conductor yttria stabilized zirconia (YSZ) and an insulator RE2O3 (RE = Er, Y) are investigated as a function of the layer thickness. Multilayers with narrow columnar crystallites and coherent phase boundaries were grown by pulsed laser deposition (PLD). A detailed strain analysis is performed by X-Ray Diffraction XRD, measuring distinct reflections in and perpendicular to the interface planes. Because of small columnar crystallites in the layers, the interfacial strain decays by shear with increasing distance from the interface. The extent of the strained interface regions in the YSZ layers is estimated from XRD data. By using a quantitative analytical model based on the pressure dependence of the free migration enthalpy for vacancies the results are compared to former published experimental data on O2− ion conductivity and diffusion.</jats:p

Experimental study of thermodynamic properties and phase equilibria in Na2CO3–K2CO3 system

Sodium and potassium carbonates and their mixtures are important for different applications, e.g. for latent thermal energy storage, die-casting processes and molten carbonate fuel cells. In this work the phase diagram and thermodynamic properties of Na2CO3–K2CO3 system were studied by differential thermal analysis, differential scanning calorimetry and high temperature X-ray diffraction. Three carbonate mixtures (56, 25 and 75 mol% of Na2CO3) have solid-solid transition in a wide temperature range between 648 K and 823 K. The high temperature XRD analysis has shown that this transition is a continuous process of changing of the unit cell volume without structural changing of the hexagonal lattice. This phenomenon has also been observed on the measured heat capacity curves. The obtained experimental results were compared with calculations performed using the previous thermodynamic datasets. The comparison of these results shows that further thermochemical assessment of this system needs to be performed to achieve better agreement with the available experimental data