Katrin Lehmann

Homogeneous and Inhomogeneous Mixing in Cumulus Clouds: Dependence on Local Turbulence Structure

Abstract The helicopter-borne instrument payload known as the Airborne Cloud Turbulence Observation System (ACTOS) was used to study the entrainment and mixing processes in shallow warm cumulus clouds. The characteristics of the mixing process are determined by the Damkohler number, defined as the ratio of the mixing and a thermodynamic reaction time scale. The definition of the reaction time scale is refined by investigating the relationship between the droplet evaporation time and the phase relaxation time. Following arguments of classical turbulence theory, it is concluded that the description of the mixing process through a single Damkohler number is not sufficient and instead the concept of a transition length scale is introduced. The transition length scale separates the inertial subrange into a range of length scales for which mixing between ambient dry and cloudy air is inhomogeneous, and a range for which the mixing is homogeneous. The new concept is tested on the ACTOS dataset. The effect of ent...

Probing Finescale Dynamics and Microphysics of Clouds with Helicopter-Borne Measurements

Abstract Helicopter-based measurements provide an opportunity for probing the finescale dynamics and microphysics of clouds simultaneously in space and time. Due to the low true air speed compared with research aircraft, a helicopter allows for measurements with much higher spatial resolution. To circumvent the influence of the helicopter downwash the autonomous measurement pay-load Airborne Cloud Turbulence Observation System (ACTOS) is carried as an external cargo 140 m below the helicopter. ACTOS allows for collocated measurements of the dynamical and cloud microphysical parameters with a spatial resolution of better than 10 cm. The interaction between turbulence and cloud microphysical processes is demonstrated using the following two cloud cases from recent helicopter measurements: i) a cumulus cloud with a low degree of turbulence and without strong vertical dynamics, and, in contrast, ii) an actively growing cloud with increased turbulence and stronger updrafts. The turbulence and microphysical mea...

miR-148a is upregulated by Twist1 and T-bet and promotes Th1-cell survival by regulating the proapoptotic gene Bim

Repeatedly activated T helper 1 (Th1) cells present during chronic inflammation can efficiently adapt to the inflammatory milieu, for example, by expressing the transcription factor Twist1, which limits the immunopathology caused by Th1 cells. Here, we show that in repeatedly activated murine Th1 cells, Twist1 and T‐bet induce expression of microRNA‐148a (miR‐148a). miR‐148a regulates expression of the proapoptotic gene Bim, resulting in a decreased Bim/Bcl2 ratio. Inhibition of miR‐148a by antagomirs in repeatedly activated Th1 cells increases the expression of Bim, leading to enhanced apoptosis. Knockdown of Bim expression by siRNA in miR‐148a antagomir‐treated cells restores viability of the Th1 cells, demonstrating that miR‐148a controls survival by regulating Bim expression. Thus, Twist1 and T‐bet not only control the differentiation and function of Th1 cells, but also their persistence in chronic inflammation.

Minimizing Instrumental Broadening of the Drop Size Distribution with the M-Fast-FSSP

Abstract A modified version of the Fast-FSSP (the so-called M-Fast-FSSP) is introduced. It allows minimization of the instrumental broadening of measured cloud drop size distributions caused by laser beam inhomogeneities. This is achieved by applying a new technique based on a postexperiment stepwise reduction of the probes sampling volume. For monodisperse glass bead samples it is shown that the width of the measured size distribution is considerably reduced when applying this technique, especially for large glass bead diameters. The instrumental broadening may exceed a factor of about 4 for a mean glass bead diameter of 30 μm. The M-Fast-FSSP was applied in two cloud measurement campaigns. For two specific cloud cases, the profile of the width of the measured drop size distribution changes significantly when applying the method.

On the Use of Hot-Wire Anemometers for Turbulence Measurements in Clouds

Abstract The use of a hot-wire anemometer for high-resolution turbulence measurements in a two-phase flow (e.g., atmospheric clouds) is discussed. Experiments in a small wind tunnel (diameter of 0.2 and 2 m in length) with a mean flow velocity in the range between 5 and 16 m s−1 are performed. In the wind tunnel a spray with a liquid water content of 0.5 and 2.5 g m−3 is generated. After applying a simple despiking algorithm, power spectral analysis shows the same results as spectra observed without spray under similar flow conditions. The flattening of the spectrum at higher frequencies due to impacting droplets could be reduced significantly. The time of the signal response of the hot wire to impacting droplets is theoretically estimated and compared with observations. Estimating the fraction of time during which the velocity signal is influenced by droplet spikes, it turns out that the product of liquid water content and mean flow velocity should be minimized. This implies that for turbulence measureme...

Nuclear factor of activated T cells regulates the expression of interleukin-4 in Th2 cells in an all-or-none fashion.

Background: Not every T helper type 2 (Th2) lymphocyte imprinted to express interleukin-4 (IL-4) does so when activated. Results: Preventing nuclear translocation of the nuclear factor of activated T cells (NFAT) reduces the number of Th2 lymphocytes reexpressing IL-4. Conclusion: NFAT is the limiting factor determining digital IL-4 expression in Th2 lymphocytes. Significance: This might help us to understand the regulation of immunopathology in allergy and asthma. Th2 memory lymphocytes have imprinted their Il4 genes epigenetically for expression in dependence of T cell receptor restimulation. However, in a given restimulation, not all Th cells with a memory for IL-4 expression express IL-4. Here, we show that in reactivated Th2 cells, the transcription factors NFATc2, NF-kB p65, c-Maf, p300, Brg1, STAT6, and GATA-3 assemble at the Il4 promoter in Th2 cells expressing IL-4 but not in Th2 cells not expressing it. NFATc2 is critical for assembly of this transcription factor complex. Because NFATc2 translocation into the nucleus occurs in an all-or-none fashion, dependent on complete dephosphorylation by calcineurin, NFATc2 controls the frequencies of cells reexpressing Il4, translates analog differences in T cell receptor stimulation into a digital decision for Il4 reexpression, and instructs all reexpressing cells to express the same amount of IL-4. This analog-to-digital conversion may be critical for the immune system to respond to low concentrations of antigens.

Nonfollicular reactivation of bone marrow resident memory CD4 T cells in immune clusters of the bone marrow

Significance The bone marrow (BM) harbors critical components of the adaptive immune system able to provide long-lasting protection against pathogens. Among those, CD4 memory T cells are potent helpers of immune reactions in secondary lymphoid organs. Here we analyze their reactivation in the BM in secondary immune reactions. The CD4 memory T cells form clusters with antigen-presenting cells and proliferate vigorously. Although these clusters contain many B lymphocytes, their formation is not dependent on them and no germinal centers develop. Rather, antigen-specific CD4 memory T cells are significantly amplified and, after termination of the immune reaction, they remain in the BM as resting cells. The BM thus provides a dynamic reservoir of CD4 memory T cells, adapting quantitatively to antigenic challenges. The bone marrow maintains memory CD4 T cells, which provide memory to systemic antigens. Here we demonstrate that memory CD4 T cells are reactivated by antigen in the bone marrow. In a secondary immune response, antigen-specific T cells of the bone marrow mobilize and aggregate in immune clusters together with MHC class II-expressing cells, mostly B lymphocytes. They proliferate vigorously and express effector cytokines, but they do not develop into follicular T-helper cells. Neither do the B lymphocytes develop into germinal center B cells in the bone marrow. Within 10 days, the immune clusters disappear again. Within 30 days, the expanded antigen-specific memory CD4 T cells return to memory niches and are maintained again individually as resting cells. Thus, in secondary immune responses in the bone marrow T-cell memory is amplified, while in germinal center reactions of secondary lymphoid organs humoral memory is adapted by affinity maturation.

Selective targeting of pro-inflammatory Th1 cells by microRNA-148a-specific antagomirs in vivo

In T lymphocytes, expression of miR-148a is induced by T-bet and Twist1, and is specific for pro-inflammatory Th1 cells. In these cells, miR-148a inhibits the expression of the pro-apoptotic protein Bim and promotes their survival. Here we use sequence-specific cholesterol-modified oligonucleotides against miR-148a (antagomir-148a) for the selective elimination of pro-inflammatory Th1 cells in vivo. In the murine model of transfer colitis, antagomir-148a treatment reduced the number of pro-inflammatory Th1 cells in the colon of colitic mice by 50% and inhibited miR-148a expression by 71% in the remaining Th1 cells. Expression of Bim protein in colonic Th1 cells was increased. Antagomir-148a-mediated reduction of Th1 cells resulted in a significant amelioration of colitis. The effect of antagomir-148a was selective for chronic inflammation. Antigen-specific memory Th cells that were generated by an acute immune reaction to nitrophenylacetyl-coupled chicken gamma globulin (NP-CGG) were not affected by treatment with antagomir-148a, both during the effector and the memory phase. In addition, antibody titers to NP-CGG were not altered. Thus, antagomir-148a might qualify as an effective drug to selectively deplete pro-inflammatory Th1 cells of chronic inflammation without affecting the protective immunological memory.

The intestinal microbiota determines the colitis-inducing potential of T-bet-deficient Th cells in mice

Conflicting evidence has been provided as to whether induction of intestinal inflammation by adoptive transfer of naïve T cells into Rag−/− mice requires expression of the transcription factor T‑bet by the T cells. Here, we formally show that the intestinal microbiota composition of the Rag−/− recipient determines whether or not T‐bet‐deficient Th cells can induce colitis and we have resolved the differences of the two microbiomes, permissive or non‐permissive to T‐bet‐independent colitis. Our data highlight the dominance of the microbiota over particular T cell differentiation programs in the pathogenesis of chronic intestinal inflammation.