B7-1/2 (CD80/CD86) direct signaling to B cells enhances IgG secretion.

B cell responses are regulated by Ag recognition, costimulatory signals provided by interaction with helper T cells, and by innate signals. We recently provided evidence for a link between the effects of innate and costimulatory signals on B cells during influenza virus infection, by demonstrating that most B cells in the regional lymph nodes of the respiratory tract enhance surface expression of the costimulator B7-2 (CD86) within 24-48 h following infection via a type I IFNR-dependent mechanisms, a finding we are confirming here. While the role of B7-1/2 for helper T cell activation is well documented, its role in direct B cell regulation is poorly understood. Here, our in vivo studies with mixed bone marrow irradiation chimeric mice, lacking B7-1/2 only on B cells, demonstrated that B7-1/2 expression is crucial for induction of maximal local, but to a lesser extent systemic, IgG Ab responses following influenza virus infection. In contrast to mice that completely lack B7-1/2 expression, loss of B7-1/2 on B cells alone did not significantly affect germinal center formation or the extent of CD4(+) T cell activation and IFN-gamma secretion. Instead, our in vitro studies identify a dramatic effect of B7-2 engagement on IgG, but not IgM secretion by already class-switched B cells. Concomitantly, B7-2 engagement induced expression of X-box binding protein 1 (XBP-1) and spliced XBP1, evidence for increased protein synthesis by these cells. Taken together, these results identify direct signaling through B7-1/2 as a potent regulator of IgG secretion by previously activated B cells.

Influenza virus infection causes global respiratory tract B cell response modulation via innate immune signals.

Induction of primary B cell responses requires the presence of Ag and costimulatory signals by T cells. Innate signals further enhance B cell activation. The precise nature and kinetics of such innate immune signals and their functional effects are unknown. This study demonstrates that influenza virus-induced type I IFN is the main innate stimulus affecting local B cells within 48 h of infection. It alters the transcriptional profile of B cells and selectively traps them in the regional lymph nodes, presumably via up-regulation of CD69. Somewhat paradoxically, innate B cell stimulation inhibited the ability of regional lymph node B cells to clonally expand following BCR-mediated stimulation. This inhibition was due to IFNR-signaling independent B cell intrinsic, as well as IFNR-dependent B cell extrinsic, regulation induced following influenza infection. IFNR-mediated signals also reduced B cell migration to various chemotactic agents. Consistent with the lack of responsiveness to CCR7 ligands, unaltered or reduced expression of MHC class II and genes associated with MHC class II Ag processing/presentation and CD40, B cells were unable to induce proliferation of naive CD4 T cells. Instead, they showed increased expression of a subset of nonclassical MHC molecules that facilitate interaction with gammadelta T cells and NK T cells. We conclude that type I IFN is the main "third" B cell signal following influenza infection causing early trapping of B cells in regional lymph nodes and, at a time when cognate T cell help is rare, enhancing their propensity to interact with innate immune cells for noncognate stimulation.

O-(2-[(18)F]Fluoroethyl)- L-tyrosine (FET): a tracer for differentiation of tumour from inflammation in murine lymph nodes.

High uptake of [(18)F]fluoro-2-deoxy- D-glucose (FDG) by inflammatory cells is a frequent cause of false positive results in lymph node (LN) staging by positron emission tomography. Previous studies suggest that radiolabelled amino acids may be more specific markers for viable tumour tissue than FDG. The aim of this study was to investigate quantitatively the uptake of FDG, [(3)H]methyl- L-methionine (MET) and O-2-([(18)F]fluoroethyl)- L-tyrosine (FET) in tumour-infiltrated and immunologically stimulated LNs. Popliteal LNs of Balb/c and DBA/2 mice were stimulated by injection into the right posterior foot pad of mice of either streptozotocin (STZ), causing chronic lymphadenitis, or concanavalin A (Con A), resulting in acute lymphadenitis. Tumour-infiltrated popliteal LNs were induced by inoculation of 2x10(5) lacZ-tagged T cell mouse lymphoma cells into the right posterior foot pad of syngeneic mice. Twenty-one days post inoculation of tumour cells or at various time points after STZ or Con A injection, mice were simultaneously injected intravenously with MET and FDG or MET and FET. After 30 min, mice were sacrificed and tracer uptake was determined in popliteal LNs. Contralateral LNs and LNs of untreated mice served as controls. Histopathological and immunohistochemical analysis demonstrated typical signs of chronic inflammation (non-specific sinus hyperplasia with macrophages) in STZ-treated animals and acute inflammatory changes (accumulation of neutrophilic granulocytes, vascular dilation, follicular hyperplasia) in Con A-treated animals. X-Gal staining confirmed the presence of tumour cells in the LNs of the injected side of tumour-inoculated mice. In the chronic lymphadenitis model, FDG uptake increased 3.0+/-0.1 fold [from 2.7+/-0.2 to 8.2+/-1.2 percent of injected dose per gram tissue (%ID/g)] and MET uptake 2.0+/-0.01 fold (from 4.5+/-0.6 to 9.2+/-1.1 %ID/g). In the acute lymphadenitis model, FDG uptake increased 3.9+/-0.3 fold (from 2.7+/-0.2 to 10.6+/-2.4 %ID/g) and MET uptake 1.9+/-0.1 fold (from 4.5+/-0.6 to 8.5+/-1.4 %ID/g). In contrast, FET uptake in both lymphadenitis models (1.0+/-0.03 and 1.2+/-0.04 fold) was not significantly different from that in controls (from 4.2+/-0.3 to 4.7+/-0.7 and to 5.1+/-0.4 %ID/g, respectively). Uptake of all three tracers in tumour-infiltrated LNs was significantly higher than that in control LNs. FDG uptake increased 2.8+/-0.15 fold (from 2.7+/-0.2 to 7.6+/-1.3%ID/g), MET uptake 1.7+/-0.11 fold (from 4.5+/-0.6 to 7.5+/-1.3 %ID/g) and FET uptake 2.4+/-0.15 fold (from 4.2+/-0.3 to 10.0+/-1.8 %ID/g). MET and FDG uptake was similar or higher in inflammatory than in tumour-infiltrated LNs ( P=0.01 and P<0.01, respectively). In contrast, uptake of FET showed no overlap between tumour-infiltrated and inflammatory LNs ( P<0.00001). In conclusion, tumour-infiltrated and inflammatory LNs could not be differentiated by means of FDG and MET uptake. FET, in contrast, proved to be a specific tracer for differentiating between tumour-infiltrated and inflammatory LNs in the murine models studied.