Basophils function as antigen-presenting cells for an allergen-induced T helper type 2 response

Innate cells and T helper 2 cell immunity in airway inflammation

Activated mast cells, eosinophils, and basophils infiltrate the airways of asthmatics as a result of an overexuberant T helper 2 (Th2) cell immune response that drives the production of IgE, primes mast cells and basophils, and promotes tissue eosinophilia and mast cell hyperplasia. Recent evidence demonstrates that these innate effectors can be activated outside of this classical Th2 cell paradigm and that they have additional roles in promoting the development of innate and adaptive pulmonary inflammation. There is also an appreciation for the role of airway epithelial cells in orchestrating allergic pulmonary inflammation. Emerging data from basic research highlight the involvement of many unique pathways in the inflammation triggered by complex native allergens and microbes at the airway mucosal surface. Here, we review the role of effector cells and airway epithelial cells in augmenting and, at times, bypassing traditional Th2 cell-mediated allergic inflammation.

The role of basophils in helminth infection

Protective immunity against gastrointestinal and tissue dwelling helminths is coordinated by interaction of different effector cells of the innate and adaptive immune system. Helminths induce a strong type 2 immune response which is characterized by high levels of IgE and increased numbers of Th2 cells, eosinophils, mast cells and basophils. Basophils are rapidly mobilized after helminth infection and can be efficiently recruited into lymphoid and peripheral tissues where they execute their effector functions. Recent work demonstrated that basophils contribute to initiation and execution of type 2 immunity. This review discusses the potential role of basophils for protective immunity against helminths.

The emergence of basophils as antigen-presenting cells in Th2 inflammatory responses

Basophils gain prominence in Th2 inflammatory responses with the discovery that they function as antigen-presenting cells and are sufficient to drive Th2 cell differentiation.

Chronic inflammation and asthma

Allergic asthma is a complex and chronic inflammatory disorder which is associated with airway hyper-responsiveness and tissue remodelling of the airway structure. Although originally thought to be a Th2-driven inflammatory response to inhaled innocuous allergen, the immune response in asthma is now considered highly heterogeneous. There are now various in vivo systems which have been designed to examine the pathways leading to the development of this chronic immune response and reflect, in part this heterogeneity. Furthermore, the emergence of endogenous immunoregulatory pathways and active pro-resolving mediators hold great potential for future therapeutic intervention. In this review, the key cellular and molecular mediators relating to chronic allergic airway disease are discussed, as well as emerging players in the regulation of chronic allergic inflammation.

Alum induces innate immune responses through macrophage and mast cell sensors, but these sensors are not required for alum to act as an adjuvant for specific immunity

To understand more about how the body recognizes alum we characterized the early innate and adaptive responses in mice injected with the adjuvant. Within hours of exposure, alum induces a type 2 innate response characterized by an influx of eosinophils, monocytes, neutrophils, DCs, NK cells and NKT cells. In addition, at least 13 cytokines and chemokines are produced within 4 h of injection including IL-1beta and IL-5. Optimal production of some of these, including IL-1beta, depends upon both macrophages and mast cells, whereas production of others, such as IL-5, depends on mast cells only, suggesting that both of these cell types can detect alum. Alum induces eosinophil accumulation partly through the production of mast cell derived IL-5 and histamine. Alum greatly enhances priming of endogenous CD4 and CD8 T cells independently of mast cells, macrophages, and of eosinophils. In addition, Ab levels and Th2 bias was similar in the absence of these cells. We found that the inflammation induced by alum was unchanged in caspase-1-deficient mice, which cannot produce IL-1beta. Furthermore, endogenous CD4 and CD8 T cell responses, Ab responses and the Th2 bias were also not impacted by the absence of caspase-1 or NLRP3. These data suggest that activation of the inflammasome and the type 2 innate response orchestrated by macrophages and mast cells in vivo are not required for adjuvant effect of alum on endogenous T and B cell responses.

The role of T helper subsets in autoimmunity and allergy

The classification of T helper (T(H)) cells in subsets has progressively expanded and more effector subsets, besides T(H)1 and T(H)2, have been documented. These include follicular helper T cells (T(FH)), and the more recent T(H)17, 'T(H)9', and 'T(H)22'. In addition, T(H) are no longer thought of as terminally committed effector cells, with plasticity now recognized. Identification of the molecular mechanisms that drive differentiation of T(H) cells has established a link between environmental factors and T(H) subsets, with regard to both the initiation and severity of immune disorders. The role of T(H) in autoimmune-disorders and allergic-disorders is now re-evaluated, with current data suggesting a central role for T(H)17 in orchestrating adaptive-immune responses, while T(FH) are instrumental in coordinating B cell immunity.

Basophils can directly present or cross-present antigen to CD8 lymphocytes and alter CD8 T cell differentiation into IL-10-producing phenotypes

There is increasing evidence suggesting that basophils play a critical role in developing Th2-type immunity both in vitro and in vivo. We previously reported that basophils cocultured with naive CD4 T cells stimulated with Ag promote the differentiation of the T cells into IL-4-producing Th2 cells. In the present study, we examined the roles of basophils during CD8 T cell activation. Although stimulating OVA-specific OT-I CD8 T cells with OVA peptide-pulsed splenic dendritic cells primarily induced the production of IFN-gamma, adding basophils into the coculture induced IL-10 production. Surprisingly, basophils were capable of directly presenting peptide Ag or of cross-presenting protein Ag to CD8 T cells. CD28-mediated costimulation dramatically enhanced T cell IL-10 production, yet neither ICOS nor CD86 was involved in IL-10 production. Basophil-mediated IL-10 induction was greatly diminished without IL-4 or IL-6, indicating that these cytokines are necessary for programming CD8 T cell IL-10 production. Adding IL-4 or IL-6 into CD8/APC coculture was not sufficient to induce IL-10 production; however, the presence of both cytokines significantly induced IL-10 production without basophils. Finally, CD8 T cells producing IL-10 induced by basophils did not display regulatory cell functions. Collectively, these results suggest a novel function of basophils that act as professional APCs to present Ag to CD8 T cells, thus inducing IL-10 production.

Helminth products bypass the need for TSLP in Th2 immune responses by directly modulating dendritic cell function

Thymic stromal lymphopoietin (TSLP) is an interleukin (IL)-7-like cytokine, mainly expressed by epithelial cells, and key to the development of allergic responses. The well-documented involvement of TSLP in allergy has led to the conviction that TSLP promotes the development of inflammatory Th2 cell responses. However, we now report that the interaction of TSLP with its receptor (TSLPR) has no functional impact on the development of protective Th2 immune responses after infection with 2 helminth pathogens, Heligmosomoides polygyrus and Nippostrongylus brasiliensis. Mice deficient in the TSLP binding chain of the TSLPR (TSLPR(-/-)) exhibited normal Th2 cell differentiation, protective immunity and memory responses against these two distinct rodent helminths. In contrast TSLP was found to be necessary for the development of protective Th2 responses upon infection with the helminth Trichuris muris (T. muris). TSLP inhibited IL-12p40 production in response to T. muris infection, and treatment of TSLPR(-/-) animals with neutralizing anti-IL-12p40 monoclonal antibody (mAb) was able to reverse susceptibility and attenuate IFN-gamma production. We additionally demonstrated that excretory-secretory (ES) products from H. polygyrus and N. brasiliensis, but not T. muris, were capable of directly suppressing dendritic cell (DC) production of IL-12p40, thus bypassing the need for TSLP. Taken together, our data show that the primary function of TSLP is to directly suppress IL-12 secretion, thus supporting Th2 immune responses.

Omega-1, a glycoprotein secreted by Schistosoma mansoni eggs, drives Th2 responses

Soluble egg antigens of the parasitic helminth Schistosoma mansoni (S. mansoni egg antigen [SEA]) induce strong Th2 responses both in vitro and in vivo. However, the specific molecules that prime the development of Th2 responses have not been identified. We report that omega-1, a glycoprotein which is secreted from S. mansoni eggs and present in SEA, is capable of conditioning human monocyte-derived dendritic cells in vitro to drive T helper 2 (Th2) polarization with similar characteristics as whole SEA. Furthermore, using IL-4 dual reporter mice, we show that both natural and recombinant omega-1 alone are sufficient to generate Th2 responses in vivo, even in the absence of IL-4R signaling. Finally, omega-1-depleted SEA displays an impaired capacity for Th2 priming in vitro, but not in vivo, suggesting the existence of additional factors within SEA that can compensate for the omega-1-mediated effects. Collectively, we identify omega-1, a single component of SEA, as a potent inducer of Th2 responses.

Cutting edge: Limiting MHC class II expression to dendritic cells alters the ability to develop Th2- dependent allergic airway inflammation

In allergic airway inflammation, dendritic cells (DCs) are required for Th2 generation, recruitment, and activation in the respiratory tract. DCs have been shown to be necessary and sufficient for the induction of Th1 immune responses. In Th2 immunity and allergic airway inflammation, the ability of a DC to function as the sole APC has not been tested. We show that CD11c/A(beta)(b) mice with MHC class II expression restricted to CD11c-expressing DCs develop airway neutrophilia rather than allergic airway inflammation. Although CD11c/A(beta)(b) mice are capable of Th2 recruitment and activation in the lung, Th2 priming in CD11c/A(beta)(b) mice results in IFN-gamma production. Effective Th2 generation and allergic airway inflammation was achieved in CD11c/A(beta)(b) mice after treatment with anti-IFN-gamma. These studies show that DCs alone cannot drive the development of Th2 cells but require an additional MHC class II signal to stimulate effective Th2 immunity.

Basophils trump dendritic cells as APCs for T(H)2 responses

Dendritic cells are best known as antigen-presenting cells that initiate adaptive immune responses. Three new papers suggest that basophils initiate allergen- and helminth-driven CD4+ T helper type 2 responses by functioning as antigen-presenting cells in draining lymph nodes.