Apoptotic cell fragments locally activate tingible body macrophages in the germinal center.

Germinal centers (GCs) that form within lymphoid follicles during antibody responses are sites of massive cell death. Tingible body macrophages (TBMs) are tasked with apoptotic cell clearance to prevent secondary necrosis and autoimmune activation by intracellular self antigens. We show by multiple redundant and complementary methods that TBMs derive from a lymph node-resident, CD169-lineage, CSF1R-blockade-resistant precursor that is prepositioned in the follicle. Non-migratory TBMs use cytoplasmic processes to chase and capture migrating dead cell fragments using a "lazy" search strategy. Follicular macrophages activated by the presence of nearby apoptotic cells can mature into TBMs in the absence of GCs. Single-cell transcriptomics identified a TBM cell cluster in immunized lymph nodes which upregulated genes involved in apoptotic cell clearance. Thus, apoptotic B cells in early GCs trigger activation and maturation of follicular macrophages into classical TBMs to clear apoptotic debris and prevent antibody-mediated autoimmune diseases.

B cells in the balance: Offsetting self-reactivity avoidance with protection against foreign.

Antibodies are theoretically limitless in their diversity and specificity to foreign antigens; however they are constrained by the need to avoid binding to self. Germinal centers (GC) allow diversification and maturation of the antibody response towards the foreign antigen. While self-tolerance mechanisms controlling self-reactivity during B cell maturation are well recognized, the mechanisms by which GCs balance self-tolerance and foreign binding especially in the face of cross-reactivity between self and foreign, remain much less well defined. In this review we explore the extent to which GC self-tolerance restricts affinity maturation. We present studies suggesting that the outcome is situationally dependent, affected by affinity and avidity to self-antigen, and the extent to which self-binding and foreign-binding are interdependent. While auto-reactive GC B cells can mutate away from self while maturing towards the foreign antigen, if no mutational trajectories allow for self-reactive redemption, self-tolerance prevails and GC responses to the foreign pathogen are restricted, except when self-tolerance checkpoints are relaxed. Finally, we consider whether polyreactivity is subject to the same level of restriction in GC responses, especially if polyreactivity is linked to an increase in foreign protection, as occurs in certain broadly neutralizing antibodies. Overall, the outcomes for GC B cells that bind self-antigen can range from redemption, transient relaxation in self-tolerance or restriction of the antibody response to the foreign pathogen.

Positive selection of IgG+ over IgM+ B cells in the germinal center reaction.

Positive selection of high-affinity B cells within germinal centers (GCs) drives affinity maturation of antibody responses. Here, we examined the mechanism underlying the parallel transition from immunoglobulin M (IgM) to IgG. Early GCs contained mostly unswitched IgM+ B cells; IgG+ B cells subsequently increased in frequency, dominating GC responses 14-21 days after antigen challenge. Somatic hypermutation and generation of high-affinity clones occurred with equal efficiency among IgM+ and IgG+ GC B cells, and inactivation of Ig class-switch recombination did not prevent depletion of IgM+ GC B cells. Instead, high-affinity IgG+ GC B cells outcompeted high-affinity IgM+ GC B cells via a selective advantage associated with IgG antigen receptor structure but independent of the extended cytoplasmic tail. Thus, two parallel forms of GC B-cell-positive selection, based on antigen receptor variable and constant regions, respectively, operate in tandem to ensure high-affinity IgG antibodies predominate in mature serum antibody responses.

BAFFR controls early memory B cell responses but is dispensable for germinal center function.

The TNF superfamily ligand BAFF maintains the survival of naive B cells by signaling through its surface receptor, BAFFR. Activated B cells maintain expression of BAFFR after they differentiate into germinal center (GC) or memory B cells (MBCs). However, the functions of BAFFR in these antigen-experienced B cell populations remain unclear. Here, we show that B cell-intrinsic BAFFR does not play a significant role in the survival or function of GC B cells or in the generation of the somatically mutated MBCs derived from them. Instead, BAFF/BAFFR signaling was required to generate the unmutated, GC-independent MBCs that differentiate directly from activated B cell blasts early in the response. Furthermore, amplification of BAFFR signaling in responding B cells did not affect GCs or the generation of GC-derived MBCs but greatly expanded the GC-independent MBC response. Although BAFF/BAFFR signaling specifically controlled the formation of the GC-independent MBC response, both types of MBCs required input from this pathway for optimal long-term survival.

Selection in the germinal center.

Germinal centers (GCs) are well known for their important role in shaping the secondary B cell repertoire to generate antibodies capable of binding with high-affinity and specificity to foreign antigens. Somatic hypermutation of the Ig variable region genes in GC B cells represents a highly efficient mechanism for generating new antibody variants with increased antigen affinity. To be effective, however, this process needs to be intimately linked with equally efficient processes that positively select high-affinity clones for perpetuation in the GC and, ultimately, for differentiation into plasma cell and memory B cell effector populations. Just as important is the need for mechanisms of negative selection that remove GC B cell clones with unwanted specificities, particularly those that have gained reactivity with self-components. Here, we discuss recent advances in our understanding of the various selective processes that occur within the GC and identify the major questions in this field that remain to be answered.

Measurement of fluorescence in a rhodamine-123 doped self-assembled "giant" mesostructured silica sphere using a smartphone as optical hardware.

The blue OLED emission from a mobile phone was characterised, revealing a sharp emission band centred at λ = 445 nm with a 3dB bandwidth Δλ ∼ 20 nm. It was used to excite Rhodamine 123 doped within a "giant" mesostructured silica sphere during fabrication through evaporative self-assembly of silica nanoparticles. Fluorescence was able to be detected using a standard optical microscope fitted with a green transmission pass filter and cooled CCD and with 1 ms exposure time demonstrating the potential of mobile platforms as the basis for portable diagnostics in the field.