Germinal centers (GCs) are inducible lymphoid microenvironments that support the generation of memory B cells, affinity maturation, and isotype switching. Previously, phenotypic transitions following in vivo B cell activation have been exploited to discriminate GC from non-GC B cells in the mouse and to delineate as many as seven distinct human peripheral B cell subsets. To better understand the differentiative processes occurring within murine GCs, we sought to identify subpopulations of GC B cells corresponding to discrete stages of GC B cell ontogeny. We performed multiparameter flow-cytometric analyses of GC B cells at consecutive time points following immunization of BALB/c mice with SRBC. We resolved the murine GC compartment into subsets based on the differential expression of activation markers, surface Ig isotypes, and differentiation Ags. Class-switched and nonswitched GC B cells emerged contemporaneously, and their relative frequencies remained nearly constant throughout the GC reaction, perhaps reflecting the establishment of a steady state. A significant percentage of the nonswitched B cells with a GC phenotype exhibited surface markers associated with naive B cells, including CD23, surface IgD, and high levels of CD38 consistent with either prolonged recruitment into the GC reaction or protracted expression of these markers during differentiation within the GC. Expression of the activation marker BLA-1 was dynamic over time, with all GC B cells being positive early after immunization, followed by progressive loss as the GC reaction matured into the second and third week. Implications of these results concerning GC evolution are discussed.

The germinal center reaction is the foundation of T-cell-dependent humoral responses. Antigen-specific B cells recruited into germinal centers undergo a complex cellular program that allows for extensive expansion, isotype switching, somatic hypermutation, and differentiation into antibody-forming cells and memory cells. Importantly, the germinal center environment filters the repertoire of differentiating B cells such that high affinity variants are preferentially selected while low affinity or self-reactive clones are eliminated by apoptosis. The present article reviews the many processes that govern germinal center B-cell differentiation, as well as the cellular and molecular elements necessary to initiate and sustain a germinal center response. The major histologic features of the germinal center are also discussed, as well as the current dominant models of the germinal center reaction in humans and mice. Finally, a new model of murine B-cell differentiation is described on the basis of a multiparameter flow cytometric kinetic analysis of germinal center B cells.

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IntroductionWe report data from adult and pediatric patients with severe sepsis from studies evaluating drotrecogin alfa (activated) (DrotAA) and presenting with purpura fulminans (PF), meningitis (MEN), or meningococcal disease (MD) (PF/MEN/MD). Such conditions may be associated with an increased bleeding risk but occur in a relatively small proportion of patients presenting with severe sepsis; pooling data across clinical trials provides an opportunity for improving the characterization of outcomes.MethodsA retrospective analysis of placebo-controlled, open-label, and compassionate-use trials was conducted. Adult patients received infusions of either DrotAA or placebo. All pediatric patients (<18 years old) received DrotAA. 189 adult and 121 pediatric patients presented with PF/MEN/MD.ResultsFewer adult patients with PF/MEN/MD met cardiovascular (68.3% versus 78.8%) or respiratory (57.8% versus 80.5%) organ dysfunction entry criteria than those without. DrotAA-treated adult patients with PF/MEN/MD (n = 163) had an observed 28-day mortality rate of 19.0%, a 28-day serious bleeding event (SBE) rate of 6.1%, and an intracranial hemorrhage (ICH) rate of 4.3%. Six of the seven ICHs occurred in patients with MEN (three of whom were more than 65 years old with a history of hypertension). DrotAA-treated adult patients without PF/MEN/MD (n = 3,088) had an observed 28-day mortality rate of 25.5%, a 28-day SBE rate of 5.8%, and an ICH rate of 1.0%. In contrast, a greater number of pediatric patients with PF/MEN/MD met the cardiovascular organ dysfunction entry criterion (93.5% versus 82.5%) than those without. DrotAA-treated PF/MEN/MD pediatric patients (n = 119) had a 14-day mortality rate of 10.1%, an SBE rate of 5.9%, and an ICH rate of 2.5%. DrotAA-treated pediatric patients without PF/MEN/MD (n = 142) had a 14-day mortality rate of 14.1%, an SBE rate of 9.2%, and an ICH rate of 3.5%.ConclusionDrotAA-treated adult patients with severe sepsis presenting with PF/MEN/MD had a similar SBE rate, a lower observed 28-day mortality rate, and a higher observed rate of ICH than DrotAA-treated patients without PF/MEN/MD. DrotAA-treated pediatric patients with severe sepsis with PF/MEN/MD may differ from adults, because all three outcome rates (SBE, mortality, and ICH) were lower in pediatric patients with PF/MEN/MD.

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