Cracking the BAFF code.

The tumour necrosis factor (TNF) family members B cell activating factor (BAFF) and APRIL (a proliferation-inducing ligand) are crucial survival factors for peripheral B cells. An excess of BAFF leads to the development of autoimmune disorders in animal models, and high levels of BAFF have been detected in the serum of patients with various autoimmune conditions. In this Review, we consider the possibility that in mice autoimmunity induced by BAFF is linked to T cell-independent B cell activation rather than to a severe breakdown of B cell tolerance. We also outline the mechanisms of BAFF signalling, the impact of ligand oligomerization on receptor activation and the progress of BAFF-depleting agents in the clinical setting

Dispersion and mixing in quasigeostrophic turbulence

The dynamics of passive Lagrangian tracers in three-dimensional quasigeostrophic turbulence is studied numerically and compared with the behavior of two-dimensional barotropic turbulence. Despite the different Eulerian properties of the two flows, the Lagrangian dynamics of passively advected tracers in three-dimensional quasigeostrophic turbulence is very similar to that of barotropic turbulence. In both systems, coherent vortices play a major role in determining the mixing and dispersion properties. This work indicates that recent results on particle dynamics in barotropic, two-dimensional turbulence carry over to more realistic baroclinic flows, such as those encountered in the large-scale dynamics of the atmosphere and ocean

Vortex merging in quasi-geostrophic flows

We study symmetric vortex merger in quasi-geostrophic hows using numerical simulations with high vertical resolution. We analyse the effect of varying the vertical aspect ratio of the vortices and compare with the barotropic case. During the merging of potential vorticity cores with small aspect ratio, we observe the birth of secondary instabilities on the filaments. This is a new phenomenon not seen in baroclinic simulations having low vertical resolution. Passive Lagrangian tracers are used to explore the transport of fluid parcels during vortex merger, to provide a detailed view of the flow evolution, and to determine the value of the critical merging distance for baroclinic vortices