TH2 mediated immune responses are induced upon infection with multicellular parasites and can be triggered by a variety of allergens. Mechanisms of induction and the antigen-presenting cells involved in activation of TH2 responses remain poorly defined and the innate immune sensing pathways activated by parasites and allergens are largely unknown. Basophils are required for the in vivo induction of TH2 responses by protease allergens. Here we show that basophils also function as antigen presenting cells. We show that, while dendritic cells were dispensable, antigen presentation by basophils was necessary and sufficient for allergen-induced activation of TH2 responses in vitro and in vivo. Thus, basophils function as antigen presenting cells for TH2 differentiation in response to protease allergens.

SUMMARY CD4+ T cell differentiation is regulated by specialized antigen-presenting cells. Dendritic cells (DCs) produce cytokines that promote naive CD4+ T cell differentiation into T helper 1 (Th1), Th17, and inducible T regulatory (iTreg) cells. However, the initiation of Th2 cell responses remains poorly understood, although it is likely that more than one mechanism might be involved. Here we have defined a specific DC subset that is involved in Th2 cell differentiation in vivo in response to a protease allergen, as well as infection with Nippostrongylus brasiliensis. We have demonstrated that this subset is controlled by the transcription factor interferon regulatory factor 4 (IRF4), which is required for their differentiation and Th2 cell-inducing function. IRF4 is known to control Th2 cell differentiation and Th2 cell-associated suppressing function in Treg cells. Our finding suggests that IRF4 also plays a role in DCs where it controls the initiation of Th2 cell responses.

Autoreactive B cells that appear to be inactivated can be found in healthy individuals. In this study, we examined the potential of these anergic cells to become activated. We show that anergy of anti-double-stranded DNA (dsDNA) B cells in BALB/c mice is readily reversed, requiring only the provision of T cell help. We further show that spontaneous loss of anergy among anti-dsDNA B cells in autoimmune lpr/lpr mice occurs in two phases: an abortive initial response to T help followed by full loss of tolerance. Strikingly, the abortive response can be reproduced in nonautoimmune mice when CD4+CD25+ T regulatory cells are administered in conjunction with CD4+ T helper cells, suggesting that loss of B cell tolerance may require both the production of T cell help and the overcoming of T suppression.

SUMMARY Metazoan sibling cells often diverge in activity and identity, suggesting links between growth signals and cell fate. We show that unequal transduction of nutrient-sensitive PI3K/AKT/mTOR signaling during cell division bifurcates transcriptional networks and fates of kindred cells. A sibling B lymphocyte with stronger signaling, indexed by FoxO1 inactivation and IRF4 induction, undergoes PI3K-driven Pax5 repression and plasma cell determination, while its sibling with weaker PI3K activity renews a memory or germinal center B cell fate. PI3K-driven effector T cell determination silences TCF1 in one sibling cell, while its PI3K-attenuated sibling self-renews in tandem. Prior to bifurcations achieving irreversible plasma or effector cell fate determination, asymmetric signaling during initial divisions specifies a more proliferative, differentiation-prone lymphocyte in tandem with a more quiescent memory cell sibling. By triggering cell division but transmitting unequal intensity between sibling cells, nutrient-sensitive signaling may be a frequent arbiter of cell fate bifurcations during development and repair.