Interferon and IL-27 antagonize the function of group 2 innate lymphoid cells and type 2 innate immune responses.

Group 2 innate lymphoid cells (ILC2 cells) are type 2 cytokine-producing cells of the innate immune system with important roles in helminth infection and allergic inflammation. Here we found that tissue-resident ILC2 cells proliferated in situ without migrating during inflammatory responses. Both type I and type II interferons and interleukin 27 (IL-27) suppressed ILC2 function in a manner dependent on the transcription factor STAT1. ILC2-mediated lung inflammation was enhanced in the absence of the interferon-γ (IFN-γ) receptor on ILC2 cells in vivo. IFN-γ effectively suppressed the function of tissue-resident ILC2 cells but not that of inflammatory ILC2 cells, and IL-27 suppressed tissue-resident ILC2 cells but not tissue-resident TH2 cells during lung inflammation induced by Alternaria alternata. Our results demonstrate that suppression mediated by interferon and IL-27 is a negative feedback mechanism for ILC2 function in vivo.

Cell-intrinsic lysosomal lipolysis is essential for alternative activation of macrophages.

Alternative (M2) activation of macrophages driven via the α-chain of the receptor for interleukin 4 (IL-4Rα) is important for immunity to parasites, wound healing, the prevention of atherosclerosis and metabolic homeostasis. M2 polarization is dependent on fatty acid oxidation (FAO), but the source of the fatty acids that support this metabolic program has not been clear. We found that the uptake of triacylglycerol substrates via the scavenger receptor CD36 and their subsequent lipolysis by lysosomal acid lipase (LAL) was important for the engagement of elevated oxidative phosphorylation, enhanced spare respiratory capacity (SRC), prolonged survival and expression of genes that together define M2 activation. Inhibition of lipolysis suppressed M2 activation during infection with a parasitic helminth and blocked protective responses to this pathogen. Our findings delineate a critical role for cell-intrinsic lysosomal lipolysis in M2 activation.

Genetic analysis of basophil function in vivo.

Contributions by basophils to allergic and helminth immunity remain incompletely defined. Using sensitive interleukin 4 (Il4) reporter alleles, we demonstrate here that basophil IL-4 production occurs by a CD4(+) T cell-dependent process restricted to the peripheral tissues affected. We genetically marked and achieved specific deletion of basophils and found that basophils did not mediate T helper type 2 (T(H)2) priming in vivo. Two-photon imaging confirmed that basophils did not interact with antigen-specific T cells in lymph nodes but engaged in prolonged serial interactions with T cells in lung tissues. Although targeted deletion of IL-4 and IL-13 in either CD4(+) T cells or basophils had a minimal effect on worm clearance, deletion from both lineages demonstrated a nonredundant role for basophil cytokines in primary helminth immunity.

The Foxp3+ regulatory T-cell population requires IL-4Rα signaling to control inflammation during helminth infections.

Forkhead box P3 (Foxp3+) regulatory T (Treg)-cell function is controlled by environmental cues of which cytokine-mediated signaling is a dominant component. In vivo, interleukin-4 (IL-4)-mediated signaling via IL-4 receptor alpha (IL-4Rα) mediates Treg cell transdifferentiation into ex-Foxp3 T helper 2 (Th2) or T helper 17 (Th17) cells. However, IL-4-mediated signaling also reinforces the Foxp3 Treg compartment in vitro. We generated Foxp3-specific IL-4Rα-deficient mice and demonstrated differential efficiency of IL-4Rα deletion in male (approximately 90%) and female (approximately 40%) animals, because of cyclic recombinase (Cre)-mediated X-linked foxp3 inactivation. Irrespective of the degree of IL-4Rα deletion within the Foxp3+ Treg cell population, mice showed exacerbation of immune effector responses with aggravated tissue pathology in tissue-dwelling helminth infections (Schistosoma mansoni or Nippostrongylus brasiliensis). Mechanistically, IL-4Rα deletion in males and females led to a reduced expression of Foxp3 and subsequently an impaired accumulation of Foxp3+ Treg cells to inflamed tissues. In-depth cellular typing by flow cytometry revealed that the impairment of IL-4Rα-mediated signaling during helminth infections decreased the ability of central Treg cells to convert into effector Treg (eTreg) cells and caused a significant down-regulation of markers associated with Treg cell migration (C-X-C motif chemokine receptor 3 [CXCR3]) and accumulation in inflamed tissues (GATA binding protein 3 [GATA3]) as well as survival (B cell lymphoma 2 [Bcl-2]). These findings unprecedentedly, to our knowledge, uncover a role for IL-4Rα signaling in the positive regulation of Foxp3+ Treg cell function in vivo. Complementing our past knowledge on a widely reported role for IL-4Rα signaling in the negative regulation and transdifferentiation of Foxp3+ Treg cells in vivo, our present findings reveal the host requirement for an intact, but not reduced or potentiated, IL-4Rα-mediated signaling on Foxp3+ Treg cells to optimally control inflammation during helminth infections.

Non-hierarchical cluster analysis for determination of resistance to worm infection in meat sheep.

The aim of this study was to determine the resistance to worm infection in Santa Inês sheep by combining different sets of gastrointestinal parasite resistance indicator traits, using the k-means algorithm. Records from 221 animals reared in the Mid-North sub-region of Brazil were used. The following phenotypes were used: hematocrit (HCT); white blood cell count; red blood cell count (RBC); hemoglobin (HGB); platelets; mean corpuscular hemoglobin; mean corpuscular volume; mean corpuscular hemoglobin concentration; fecal egg count (FEC); coloration of the ocular mucosa (FAMACHA score); body condition score (BCS); withers height; and rump height. Two files with phenotypic information of animals were edited: complete, including all traits, and reduced, in which only FAMACHA score, HCT, FEC, and BCS were used. For determination of worm resistance, three groups were formed using the k-means non-hierarchical clustering by combining the traits of the complete and reduced analyses. The animals of the group in which individuals had the lowest values for FEC and FAMACHA score, as well as the highest values for HCT, RBC, HGB, and BCS were classified as resistant. In the group with opposite values for the aforementioned traits, the animals were classified as sensitive. The animals of the group with values between the other two groups were classified as moderately resistant. The results obtained in complete and reduced analyses were equivalent. Thus, it is possible to identify animals of the Santa Inês sheep breed according to their status of resistance to worm infection based on a reduced trait set.

Endogenous Calcitriol Synthesis Controls the Humoral IgE Response in Mice.

The vitamin D receptor participates in the control of IgE class-switch recombination in B cells. The physiologic vitamin D receptor agonist, 1,25(OH)2D3 (calcitriol), is synthesized by the essential enzyme 25-hydroxyvitamin D3-1α-hydroxylase (CYP27B1), which can be expressed by activated immune cells. The role of endogenous calcitriol synthesis for the regulation of IgE has not been proven. In this study, we investigated IgE-responses in Cyp27b1-knockout (KO) mice following sensitization to OVA or intestinal infection with Heligmosomoides polygyrus Specific Igs and plasmablasts were determined by ELISA and ELISpot, Cyp27b1 expression was measured by quantitative PCR. The data show elevated specific IgE and IgG1 concentrations in the blood of OVA-sensitized Cyp27b1-KO mice compared with wild-type littermates (+898 and +219%). Accordingly, more OVA-specific IgG1-secreting cells are present in spleen and fewer in the bone marrow of Cyp27b1-KO mice. Ag-specific mechanisms are suggested as the leucopoiesis is in general unchanged and activated murine B and T lymphocytes express Cyp27b1 Accordingly, elevated specific IgE concentrations in the blood of sensitized T cell-specific Cyp27b1-KO mice support a lymphocyte-driven mechanism. In an independent IgE-inducing model, i.e., intestinal infection with H. polygyrus, we validated the increase of total and specific IgE concentrations of Cyp27b1-KO compared with wild-type mice, but not those of IgG1 or IgA. We conclude that endogenous calcitriol has an impact on the regulation of IgE in vivo. Our data provide genetic evidence supporting previous preclinical and clinical findings and suggest that vitamin D deficiency not only promotes bone diseases but also type I sensitization.

Host immunity shapes the impact of climate changes on the dynamics of parasite infections.

Global climate change is predicted to alter the distribution and dynamics of soil-transmitted helminth infections, and yet host immunity can also influence the impact of warming on host-parasite interactions and mitigate the long-term effects. We used time-series data from two helminth species of a natural herbivore and investigated the contribution of climate change and immunity on the long-term and seasonal dynamics of infection. We provide evidence that climate warming increases the availability of infective stages of both helminth species and the proportional increase in the intensity of infection for the helminth not regulated by immunity. In contrast, there is no significant long-term positive trend in the intensity for the immune-controlled helminth, as immunity reduces the net outcome of climate on parasite dynamics. Even so, hosts experienced higher infections of this helminth at an earlier age during critical months in the warmer years. Immunity can alleviate the expected long-term effect of climate on parasite infections but can also shift the seasonal peak of infection toward the younger individuals.

Globule Leukocytes and Other Mast Cells in the Mouse Intestine.

Only 2 major mast cell (MC) subtypes are commonly recognized in the mouse: the large connective tissue mast cells (CTMCs) and the mucosal mast cells (MMCs). Interepithelial mucosal inflammatory cells, most commonly identified as globule leukocytes (GLs), represent a third MC subtype in mice, which we term interepithelial mucosal mast cells (ieMMCs). This term clearly distinguishes ieMMCs from lamina proprial MMCs (lpMMCs) while clearly communicating their common MC lineage. Both lpMMCs and ieMMCs are rare in normal mouse intestinal mucosa, but increased numbers of ieMMCs are seen as part of type 2 immune responses to intestinal helminth infections and in food allergies. Interestingly, we found that increased ieMMCs were consistently associated with decreased mucosal inflammation and damage, suggesting that they might have a role in controlling helminth-induced immunopathology. We also found that ieMMC hyperplasia can develop in the absence of helminth infections, for example, in Treg-deficient mice, Arf null mice, some nude mice, and certain graft-vs-host responses. Since tuft cell hyperplasia plays a critical role in type 2 immune responses to intestinal helminths, we looked for (but did not find) any direct relationship between ieMMC and tuft cell numbers in the intestinal mucosa. Much remains to be learned about the differing functions of ieMMCs and lpMMCs in the intestinal mucosa, but an essential step in deciphering their roles in mucosal immune responses will be to apply immunohistochemistry methods to consistently and accurately identify them in tissue sections.

Eosinophil-specific deletion of IκBα in mice reveals a critical role of NF-κB-induced Bcl-xL for inhibition of apoptosis.

Eosinophils are associated with type 2 immune responses to allergens and helminths. They release various proinflammatory mediators and toxic proteins on activation and are therefore considered proinflammatory effector cells. Eosinophilia is promoted by the cytokines interleukin (IL)-3, IL-5, and granulocyte macrophage-colony-stimulating factor (GM-CSF) and can result from enhanced de novo production or reduced apoptosis. In this study, we show that only IL-5 induces differentiation of eosinophils from bone marrow precursors, whereas IL-5, GM-CSF, and to a lesser extent IL-3 promote survival of mature eosinophils. The receptors for these cytokines use the common ß chain, which serves as the main signaling unit linked to signal transducer and activator of transcription 5, p38 mitogen-activated protein kinase, and nuclear factor (NF)-κB pathways. Inhibition of NF-κB induced apoptosis of in vitro cultured eosinophils. Selective deletion of IκBα in vivo resulted in enhanced expression of Bcl-xL and reduced apoptosis during helminth infection. Retroviral overexpression of Bcl-xL promoted survival, whereas pharmacologic inhibition of Bcl-xL in murine or human eosinophils induced rapid apoptosis. These results suggest that therapeutic strategies targeting Bcl-xL in eosinophils could improve health conditions in allergic inflammatory diseases.

Antibody-mediated trapping of helminth larvae requires CD11b and Fcγ receptor I.

Infections with intestinal helminths severely impact on human and veterinary health, particularly through the damage that these large parasites inflict when migrating through host tissues. Host immunity often targets the motility of tissue-migrating helminth larvae, which ideally should be mimicked by anti-helminth vaccines. However, the mechanisms of larval trapping are still poorly defined. We have recently reported an important role for Abs in the rapid trapping of tissue-migrating larvae of the murine parasite Heligmosomoides polygyrus bakeri. Trapping was mediated by macrophages (MΦ) and involved complement, activating FcRs, and Arginase-1 (Arg1) activity. However, the receptors and Ab isotypes responsible for MΦ adherence and Arg1 induction remained unclear. Using an in vitro coculture assay of H. polygyrus bakeri larvae and bone marrow-derived MΦ, we now identify CD11b as the major complement receptor mediating MΦ adherence to the larval surface. However, larval immobilization was largely independent of CD11b and instead required the activating IgG receptor FcγRI (CD64) both in vitro and during challenge H. polygyrus bakeri infection in vivo. FcγRI signaling also contributed to the upregulation of MΦ Arg1 expression in vitro and in vivo. Finally, IgG2a/c was the major IgG subtype from early immune serum bound by FcγRI on the MΦ surface, and purified IgG2c could trigger larval immobilization and Arg1 expression in MΦ in vitro. Our findings reveal a novel role for IgG2a/c-FcγRI-driven MΦ activation in the efficient trapping of tissue-migrating helminth larvae and thus provide important mechanistic insights vital for anti-helminth vaccine development.

Intestinal helminths regulate lethal acute graft-versus-host disease and preserve the graft-versus-tumor effect in mice.

Donor T lymphocyte transfer with hematopoietic stem cells suppresses residual tumor growth (graft-versus-tumor [GVT]) in cancer patients undergoing bone marrow transplantation (BMT). However, donor T cell reactivity to host organs causes severe and potentially lethal inflammation called graft-versus-host disease (GVHD). High-dose steroids or other immunosuppressive drugs are used to treat GVHD that have limited ability to control the inflammation while incurring long-term toxicity. Novel strategies are needed to modulate GVHD, preserve GVT, and improve the outcome of BMT. Regulatory T cells (Tregs) control alloantigen-sensitized inflammation of GVHD, sustain GVT, and prevent mortality in BMT. Helminths colonizing the alimentary tract dramatically increase the Treg activity, thereby modulating intestinal or systemic inflammatory responses. These observations led us to hypothesize that helminths can regulate GVHD and maintain GVT in mice. Acute GVHD was induced in helminth (Heligmosomoides polygyrus)-infected or uninfected BALB/c recipients of C57BL/6 donor grafts. Helminth infection suppressed donor T cell inflammatory cytokine generation and reduced GVHD-related mortality, but maintained GVT. H. polygyrus colonization promoted the survival of TGF-ß-generating recipient Tregs after a conditioning regimen with total body irradiation and led to a TGF-ß-dependent in vivo expansion/maturation of donor Tregs after BMT. Helminths did not control GVHD when T cells unresponsive to TGF-ß-mediated immune regulation were used as donor T lymphocytes. These results suggest that helminths suppress acute GVHD using Tregs and TGF-ß-dependent pathways in mice. Helminthic regulation of GVHD and GVT through intestinal immune conditioning may improve the outcome of BMT.

Transcriptomics identified a critical role for Th2 cell-intrinsic miR-155 in mediating allergy and antihelminth immunity.

Allergic diseases, orchestrated by hyperactive CD4(+) Th2 cells, are some of the most common global chronic diseases. Therapeutic intervention relies upon broad-scale corticosteroids with indiscriminate impact. To identify targets in pathogenic Th2 cells, we took a comprehensive approach to identify the microRNA (miRNA) and mRNA transcriptome of highly purified cytokine-expressing Th1, Th2, Th9, Th17, and Treg cells both generated in vitro and isolated ex vivo from allergy, infection, and autoimmune disease models. We report here that distinct regulatory miRNA networks operate to regulate Th2 cells in house dust mite-allergic or helminth-infected animals and in vitro Th2 cells, which are distinguishable from other T cells. We validated several miRNA (miR) candidates (miR-15a, miR-20b, miR-146a, miR-155, and miR-200c), which targeted a suite of dynamically regulated genes in Th2 cells. Through in-depth studies using miR-155(-/-) or miR-146a(-/-) T cells, we identified that T-cell-intrinsic miR-155 was required for type-2 immunity, in part through regulation of S1pr1, whereas T-cell-intrinsic miR-146a was required to prevent overt Th1/Th17 skewing. These data identify miR-155, but not miR-146a, as a potential therapeutic target to alleviate Th2-medited inflammation and allergy.

Protective responses of intestinal mucous cells in a range of fish-helminth systems.

Histopathological, immunofluorescence and ultrastructural studies were conducted on the intestines of four fish species infected with different taxa of enteric helminths. Brown trout (Salmo trutta trutta), eel (Anguilla anguilla) and tench (Tinca tinca) obtained from Lake Piediluco (central Italy) were examined. Brown trout and eel were infected with two species of acanthocephalans, and tench was parasitized with a tapeworm species. In addition to the above site, specimens of chub (Squalius cephalus) and brown trout infected with an acanthocephalan were examined from the River Brenta (north Italy). Moreover, eels were examined from a brackish water, Comacchio lagoons (north Italy), where one digenean species was the predominant enteric worm. All the helminths species induced a similar response, the hyperplasia of the intestinal mucous cells, particularly of those secreting acid mucins. Local endocrine signals seemed to affect the production and secretion of mucus in the parasitized fish, as worms often were surrounded by an adherent mucus layer or blanket. This is the first quantitative report of enteric worm effects on the density of various mucous cell types and on the mucus composition in intestine of infected/uninfected conspecifics. We provide a global comparison between the several fish-helminth systems examined.

Fish innate immunity against intestinal helminths.

Most individual fish in farmed and wild populations are infected with parasites. Upon dissection of fish, helminths from gut are often easily visible. Enteric helminths include several species of digeneans, cestodes, acanthocephalans and nematodes. Some insights into biology, morphology and histopathological effects of the main fish enteric helminths taxa will be described here. The immune system of fish, as that of other vertebrates, can be subdivided into specific and aspecific types, which in vivo act in concert with each other and indeed are interdependent in many ways. Beyond the small number of well-described models that exist, research focusing on innate immunity in fish against parasitic infections is lacking. Enteric helminths frequently cause inflammation of the digestive tract, resulting in a series of chemical and morphological changes in the affected tissues and inducing leukocyte migration to the site of infection. This review provides an overview on the aspecific defence mechanisms of fish intestine against helminths. Emphasis will be placed on the immune cellular response involving mast cells, neutrophils, macrophages, rodlet cells and mucous cells against enteric helminths. Given the relative importance of innate immunity in fish, and the magnitude of economic loss in aquaculture as a consequence of disease, this area deserves considerable attention and support.

Relationships between host body condition and immunocompetence, not host sex, best predict parasite burden in a bat-helminth system.

Sex-biased parasitism highlights potentially divergent approaches to parasite resistance resulting in differing energetic trade-offs for males and females; however, trade-offs between immunity and self-maintenance could also depend on host body condition. We investigated these relationships in the big brown bat, Eptesicus fuscus, to determine if host sex or body condition better predicted parasite resistance, if testosterone levels predicted male parasite burdens, and if immune parameters could predict male testosterone levels. We found that male and female hosts had similar parasite burdens and female bats scored higher than males in only one immunological measure. Top models of helminth burden revealed interactions between body condition index and agglutination score as well as between agglutination score and host sex. Additionally, the strength of the relationships between sex, agglutination, and helminth burden is affected by body condition. Models of male parasite burden provided no support for testosterone predicting helminthiasis. Models that best predicted testosterone levels did not include parasite burden but instead consistently included month of capture and agglutination score. Thus, in our system, body condition was a more important predictor of immunity and worm burden than host sex.

Biomphalysin, a new ß pore-forming toxin involved in Biomphalaria glabrata immune defense against Schistosoma mansoni.

Aerolysins are virulence factors belonging to the ß pore-forming toxin (ß-PFT) superfamily that are abundantly distributed in bacteria. More rarely, ß-PFTs have been described in eukaryotic organisms. Recently, we identified a putative cytolytic protein in the snail, Biomphalaria glabrata, whose primary structural features suggest that it could belong to this ß-PFT superfamily. In the present paper, we report the molecular cloning and functional characterization of this protein, which we call Biomphalysin, and demonstrate that it is indeed a new eukaryotic ß-PFT. We show that, despite weak sequence similarities with aerolysins, Biomphalysin shares a common architecture with proteins belonging to this superfamily. A phylogenetic approach revealed that the gene encoding Biomphalysin could have resulted from horizontal transfer. Its expression is restricted to immune-competent cells and is not induced by parasite challenge. Recombinant Biomphalysin showed hemolytic activity that was greatly enhanced by the plasma compartment of B. glabrata. We further demonstrated that Biomphalysin with plasma is highly toxic toward Schistosoma mansoni sporocysts. Using in vitro binding assays in conjunction with Western blot and immunocytochemistry analyses, we also showed that Biomphalysin binds to parasite membranes. Finally, we showed that, in contrast to what has been reported for most other members of the family, lytic activity of Biomphalysin is not dependent on proteolytic processing. These results provide the first functional description of a mollusk immune effector protein involved in killing S. mansoni.

Occurrence of immune cells in the intestinal wall of Squalius cephalus infected with Pomphorhynchus laevis.

A sub-population of 34 specimens of chub, Squalius cephalus, was sampled from the River Brenta (Northern Italy) and examined for ecto- and endo-parasites. Pomphorhynchus laevis (Acanthocephala) was the only enteric helminth encountered. Immunofluorescence and ultrastructural studies were conducted on the intestines of chub. Near the site of parasite's attachment, mucous cells, mast cells (MCs), neutrophils and rodlet cells (RCs) were found to co-occur within the intestinal epithelium. The numbers of mucous cells, MCs and neutrophils were significantly higher in infected fish (Mann-Whitney U test, p < 0.05). Dual immunofluorescence staining with the lectin Dolichos Biflorus Agglutinin (DBA) and the macrophage-specific MAC387 monoclonal antibody, with parallel transmission electron microscopy, revealed that epithelial MCs often made intimate contact with the mucous cells. Degranulation of a large number of MCs around the site of the acanthocephalan's attachment and in proximity to mucous cells was also documented. MCs and neutrophils were abundant in the submucosa. Immune cells of the intestinal epithelium have been described at the ultrastructural level and their possible functions and interactions are discussed.

Heligmosomoides polygyrus bakeri infection activates colonic Foxp3+ T cells enhancing their capacity to prevent colitis.

Helminthic infections protect mice from colitis in murine models of inflammatory bowel disease and also may protect people. Helminths like Heligmosomoides polygyrus bakeri can induce regulatory T cells (Treg). Experiments explored whether H. polygyrus bakeri infection could protect mice from colitis through activation of colonic Treg and examined mechanisms of action. We showed that H. polygyrus bakeri infection increased the number of T cells expressing Foxp3 in the colon. More importantly, Foxp3(+)/IL-10(-) and Foxp3(+)/IL-10(+) T cell subsets isolated from the colon of H. polygyrus bakeri-infected mice prevented colitis when adoptively transferred into a murine model of inflammatory bowel disease, whereas Treg from uninfected mice could not provide protection. Only the transferred colonic Foxp3(+)/IL-10(-) T cells from H. polygyrus bakeri-infected mice readily accumulated in the colon and mesenteric lymph nodes of recipient mice, and they reconstituted the Foxp3(+)/IL-10(-) and Foxp3(+)/IL-10(+) T cell subsets. However, transferred Foxp3(+)/IL-10(+) T cells disappeared. IL-10 expression by Foxp3(+) T cells was necessary for colitis prevention. Thus, H. polygyrus bakeri infection activates colonic Foxp3(+) T cells, making them highly regulatory. The Foxp3(+) T cells that fail to express IL-10 may be critical for populating the colon with the Foxp3(+)/IL-10(+) T cells, which are required to control colitis.

Does host immunity influence helminth egg hatchability in the environment?

Transmission success for helminths with free-living stages depends on the ability of eggs and larvae to develop and survive once in the environment. While environmental conditions are often suggested to influence egg phenology and hatching rate, immunity against parasite eggs might also play a role. We examined this hypothesis using the gastrointestinal helminths Trichostrongylus retortaeformis and Graphidium strigosum, two common infections of the European rabbit. Changes in egg hatching rate and volume were examined in relation to specific antibodies in the serum and bound to eggshells, using eggs shed in host faeces over a 15-week period. Hatching rate was consistently higher for T. retortaeformis than G. strigosum and no changes were observed between weeks. Egg volume increased for G. strigosum but decreased for T. retortaeformis. We did find evidence of egg-specific antibody responses and fewer antibodies were bound to eggs of T. retortaeformis compared to G. strigosum. Little to no association was found between antibodies and hatchability, or volume, for both helminths. We suggest that host antibodies play a relatively minor role in the egg hatching rate of these gastrointestinal helminths.

Critical role of P1-Runx1 in mouse basophil development.

Runx1(P1N/P1N) mice are deficient in the transcription factor distal promoter-derived Runt-related transcription factor 1 (P1-Runx1) and have a > 90% reduction in the numbers of basophils in the BM, spleen, and blood. In contrast, Runx1(P1N/P1N) mice have normal numbers of the other granulocytes (neutrophils and eosinophils). Although basophils and mast cells share some common features, Runx1(P1N/P1N) mice have normal numbers of mast cells in multiple tissues. Runx1(P1N/P1N) mice fail to develop a basophil-dependent reaction, IgE-mediated chronic allergic inflammation of the skin, but respond normally when tested for IgE- and mast cell-dependent passive cutaneous anaphylaxis in vivo or IgE-dependent mast cell degranulation in vitro. These results demonstrate that Runx1(P1N/P1N) mice exhibit markedly impaired function of basophils, but not mast cells. Infection with the parasite Strongyloides venezuelensis and injections of IL-3, each of which induces marked basophilia in wild-type mice, also induce modest expansions of the very small populations of basophils in Runx1(P1N/P1N) mice. Finally, Runx1(P1N/P1N) mice have normal numbers of the granulocyte progenitor cells, SN-Flk2(+/-), which can give rise to all granulocytes, but exhibit a > 95% reduction in basophil progenitors. The results of the present study suggest that P1-Runx1 is critical for a stage of basophil development between SN-Flk2(+/-) cells and basophil progenitors.