| Abstract: | The mechanisms involved in the maintenance of memory IgE responses are poorly understood, and the role played by germinal center (GC) IgE+ cells in memory responses is particularly unclear. IgE+ B cell differentiation is characterized by a transient GC phase, a bias toward the plasma cell (PC) fate, and dependence on sequential switching for the production of high-affinity IgE. We show here that IgE+ GC B cells are unfit to undergo the conventional GC differentiation program due to impaired B cell receptor function and increased apoptosis. IgE+ GC cells fail to populate the GC light zone and are unable to contribute to the memory and long-lived PC compartments. Furthermore, we demonstrate that direct and sequential switching are linked to distinct B cell differentiation fates: direct switching generates IgE+ GC cells, whereas sequential switching gives rise to IgE+ PCs. We propose a comprehensive model for the generation and memory of IgE responses.IgE antibodies are critical mediators of allergic reactions (Gould and Sutton, 2008). Cross-linking of IgE molecules bound to high affinity FcεRI receptors on mast cells and basophils leads to the rapid release of potent proinflammatory molecules (Kinet, 1999; Galli and Tsai, 2012). In spite of its pathological potential, IgE exhibits the lowest serum concentration and the shortest half-life of all the antibody isotypes (Vieira and Rajewsky, 1988; Gould and Sutton, 2008). The low frequency of IgE-producing cells makes their study particularly challenging. Using mouse models of high IgE responses (Katona et al., 1988; Curotto de Lafaille et al., 2001), we discovered that IgE-producing cells develop via a unique differentiation pathway that occurs during the germinal center (GC) phase of T cell–dependent responses and yet favors the production of plasma cells (PCs; Erazo et al., 2007; Yang et al., 2012). In our early studies a GC IgE+ population was not clearly detectable, but the IgE antibodies produced were observed to have undergone affinity maturation, indicating a GC history for IgE+ PC. We proposed at the time that high affinity IgE originated from the sequential switching of high affinity IgG1 cells, and hence we speculated that classical IgE+ memory cells may be absent in mice (Erazo et al., 2007; Curotto de Lafaille and Lafaille, 2010).Sequential switching of IgG cells to IgE was first discovered by the identification of switch(S)γ region footprints in the Sμ-Sε DNA region of IgE genes (Matsuoka et al., 1990; Yoshida et al., 1990; Jabara et al., 1993; Mandler et al., 1993; Zhang et al., 1994; Baskin et al., 1997), but the biological significance of this finding was at that time unknown. Sequential switching in mice entails two recombination events, Sμ→Sγ1 and SμSγ1→Sε, that may be either continuous or temporally separate events. The latter scenario allows for the existence of an intermediate IgG1 cellular phase in which affinity maturation can occur in GCs. Indeed, stimulation of IgG1 cells in the presence of IL-4 either in vivo or in vitro resulted in the production of IgE antibodies (Erazo et al., 2007; Wesemann et al., 2012). Importantly, mice deficient in class switching to IgG1 due to a mutation in the Iγ1 exon (Lorenz et al., 1995) were unable to produce high affinity IgE antibodies (Xiong et al., 2012a,b), indicating that sequential switching is essential for the formation of high affinity IgE.The recent development of fluorescent reporter mice for IgE has facilitated the identification of IgE GC cells (Talay et al., 2012; Yang et al., 2012). However, the in vivo phenotype and role of IgE GC cells in supporting IgE responses and its relationship with the sequential switching process remain unclear (Lafaille et al., 2012; Xiong et al., 2012a).In the current study, we used a new reporter mouse for class switch recombination (CSR) to IgE, improved methods to functionally study IgE B cells ex vivo and in vivo, and in silico modeling to analyze the origin, functional properties, and population dynamics of IgE GC cells and PC. We show that IgE GC cells are unfit to undergo the conventional GC differentiation program and instead undergo apoptosis at a high rate. This “failure to thrive” of IgE GC cells greatly limits their contribution to the memory pool and high affinity PC compartment. Furthermore, we show that the two types of rearrangement to IgE are associated with distinct B cell differentiation fates. Direct Sμ-Sε rearrangements generate IgE GC cells, whereas sequential switching of IgG1 cells gives rise to IgE PC. |
