Michael Meyer-Hermann

Germinal Center Dynamics Revealed by Multiphoton Microscopy with a Photoactivatable Fluorescent Reporter

The germinal center (GC) reaction produces high-affinity antibodies by random mutation and selective clonal expansion of B cells with high-affinity receptors. The mechanism by which B cells are selected remains unclear, as does the role of the two anatomically defined areas of the GC, light zone (LZ) and dark zone (DZ). We combined a transgenic photoactivatable fluorescent protein tracer with multiphoton laser-scanning microscopy and flow cytometry to examine anatomically defined LZ and DZ B cells and GC selection. We find that B cell division is restricted to the DZ, with a net vector of B cell movement from the DZ to the LZ. The decision to return to the DZ and undergo clonal expansion is controlled by T helper cells in the GC LZ, which discern between LZ B cells based on the amount of antigen captured and presented. Thus, T cell help, and not direct competition for antigen, is the limiting factor in GC selection.

Multicellular tumor spheroid in an off-lattice Voronoi-Delaunay cell model

We study multicellular tumor spheroids by introducing a new three-dimensional agent-based Voronoi-Delaunay hybrid model. In this model, the cell shape varies from spherical in thin solution to convex polyhedral in dense tissues. The next neighbors of the cells are provided by a weighted Delaunay triangulation with on average linear computational complexity. The cellular interactions include direct elastic forces and cell-cell as well as cell-matrix adhesion. The spatiotemporal distribution of two nutrients--oxygen and glucose--is described by reaction-diffusion equations. Viable cells consume the nutrients, which are converted into biomass by increasing the cell size during the G1 phase. We test hypotheses on the functional dependence of the uptake rates and use computer simulations to find suitable mechanisms for the induction of necrosis. This is done by comparing the outcome with experimental growth curves, where the best fit leads to an unexpected ratio of oxygen and glucose uptake rates. The model relies on physical quantities and can easily be generalized towards tissues involving different cell types. In addition, it provides many features that can be directly compared with the experiment.

Germinal center B cells govern their own fate via antibody feedback

High-affinity antibodies reenter germinal centers (GCs) and limit antigen access, thus causing sustained directional evolution in GCs toward higher-affinity antibody production.

Visualizing antibody affinity maturation in germinal centers

Diversity reigns in antibody responses During the course of an immune response, B cells specific for an invading pathogen divide. The antibodies they produce increase in affinity via somatic mutation in specialized lymph node structures called germinal centers. Tas et al. used multiphoton microscopy and sequencing to determine how different B cell clones compete with one another within mouse germinal centers. Multiple B cell clones can seed individual germinal centers, and germinal centers lose diversity at disparate rates. Such heterogeneity suggests that manipulating minor clonal populations to gain an advantage during vaccination may one day be possible. Science, this issue p. 1048 Germinal center B cells are clonally diverse, and the antibodies these cells express mature at different rates. Antibodies somatically mutate to attain high affinity in germinal centers (GCs). There, competition between B cell clones and among somatic mutants of each clone drives an increase in average affinity across the population. The extent to which higher-affinity cells eliminating competitors restricts clonal diversity is unknown. By combining multiphoton microscopy and sequencing, we show that tens to hundreds of distinct B cell clones seed each GC and that GCs lose clonal diversity at widely disparate rates. Furthermore, efficient affinity maturation can occur in the absence of homogenizing selection, ensuring that many clones can mature in parallel within the same GC. Our findings have implications for development of vaccines in which antibodies with nonimmunodominant specificities must be elicited, as is the case for HIV-1 and influenza.

From useful algorithms for slowly convergent series to physical predictions based on divergent perturbative expansions

This review is focused on the borderline region of theoretical physics and mathematics. First, we describe numerical methods for the acceleration of the convergence of series. These provide a useful toolbox for theoretical physics which has hitherto not received the attention it actually deserves. The unifying concept for convergence acceleration methods is that in many cases, one can reach much faster convergence than by adding a particular series term by term. In some cases, it is even possible to use a divergent input series, together with a suitable sequence transformation, for the construction of numerical methods that can be applied to the calculation of special functions. This review both aims to provide some practical guidance as well as a groundwork for the study of specialized literature. As a second topic, we review some recent developments in the field of Borel resummation, which is generally recognized as one of the most versatile methods for the summation of factorially divergent (perturbation) series. Here, the focus is on algorithms which make optimal use of all information contained in a finite set of perturbative coefficients. The unifying concept for the various aspects of the Borel method investigated here is given by the singularities of the Borel transform, which introduce ambiguities from a mathematical point of view and lead to different possible physical interpretations. The two most important cases are: (i) the residues at the singularities correspond to the decay width of a resonance, and (ii) the presence of the singularities indicates the existence of nonperturbative contributions which cannot be accounted for on the basis of a Borel resummation and require generalizations toward resurgent expansions. Both of these cases are illustrated by examples.

A Theory of Germinal Center B Cell Selection, Division, and Exit

High-affinity antibodies are generated in germinal centers in a process involving mutation and selection of B cells. Information processing in germinal center reactions has been investigated in a number of recent experiments. These have revealed cell migration patterns, asymmetric cell divisions, and cell-cell interaction characteristics, used here to develop a theory of germinal center B cell selection, division, and exit (the LEDA model). According to this model, B cells selected by T follicular helper cells on the basis of successful antigen processing always return to the dark zone for asymmetric division, and acquired antigen is inherited by one daughter cell only. Antigen-retaining B cells differentiate to plasma cells and leave the germinal center through the dark zone. This theory has implications for the functioning of germinal centers because compared to previous models, high-affinity antibodies appear one day earlier and the amount of derived plasma cells is considerably larger.

Toll-like Receptor 4 Signaling by Follicular Dendritic Cells Is Pivotal for Germinal Center Onset and Affinity Maturation

Germinal 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.

The distinctive germinal center phase of IgE+ B lymphocytes limits their contribution to the classical memory response

Direct class switching to IgE generates IgE+ GC cells that are highly apoptotic and do not contribute to the memory compartment, while sequential switching through an IgG+ intermediate results in the generation of long-lived IgE+ plasma cells.

Deceleration of fusion-fission cycles improves mitochondrial quality control during aging.

Mitochondrial dynamics and mitophagy play a key role in ensuring mitochondrial quality control. Impairment thereof was proposed to be causative to neurodegenerative diseases, diabetes, and cancer. Accumulation of mitochondrial dysfunction was further linked to aging. Here we applied a probabilistic modeling approach integrating our current knowledge on mitochondrial biology allowing us to simulate mitochondrial function and quality control during aging in silico. We demonstrate that cycles of fusion and fission and mitophagy indeed are essential for ensuring a high average quality of mitochondria, even under conditions in which random molecular damage is present. Prompted by earlier observations that mitochondrial fission itself can cause a partial drop in mitochondrial membrane potential, we tested the consequences of mitochondrial dynamics being harmful on its own. Next to directly impairing mitochondrial function, pre-existing molecular damage may be propagated and enhanced across the mitochondrial population by content mixing. In this situation, such an infection-like phenomenon impairs mitochondrial quality control progressively. However, when imposing an age-dependent deceleration of cycles of fusion and fission, we observe a delay in the loss of average quality of mitochondria. This provides a rational why fusion and fission rates are reduced during aging and why loss of a mitochondrial fission factor can extend life span in fungi. We propose the ‘mitochondrial infectious damage adaptation’ (MIDA) model according to which a deceleration of fusion–fission cycles reflects a systemic adaptation increasing life span.

In Vivo Killing Capacity of Cytotoxic T Cells Is Limited and Involves Dynamic Interactions and T Cell Cooperativity

Summary According to in vitro assays, T cells are thought to kill rapidly and efficiently, but the efficacy and dynamics of cytotoxic T lymphocyte (CTL)-mediated killing of virus-infected cells in vivo remains elusive. We used two-photon microscopy to quantify CTL-mediated killing in mice infected with herpesviruses or poxviruses. On average, one CTL killed 2–16 virus-infected cells per day as determined by real-time imaging and by mathematical modeling. In contrast, upon virus-induced MHC class I downmodulation, CTLs failed to destroy their targets. During killing, CTLs remained migratory and formed motile kinapses rather than static synapses with targets. Viruses encoding the calcium sensor GCaMP6s revealed strong heterogeneity in individual CTL functional capacity. Furthermore, the probability of death of infected cells increased for those contacted by more than two CTLs, indicative of CTL cooperation. Thus, direct visualization of CTLs during killing of virus-infected cells reveals crucial parameters of CD8+ T cell immunity.