Eosinophil Plasticity and Diversity: Proceedings of the 2023 International Eosinophil Society Symposium.

This issue highlights and details the programme and scientific presentations at the International Eosinophil Society's 12th biennial Symposium, which was held in Hamilton, Ontario, Canada in July 2023. The meeting included sessions on regulation of eosinophil development; cell death, stress, and autophagy in eosinophils; local immunity interactions of eosinophils with multiple cell types; eosinophils in host defense; eosinophils and mast cells in gastrointestinal disorders; reciprocal interactions between eosinophils and the microbiome in homeostasis and dysbiosis; and, eosinophils in tissue injury and repair, in tumor biology and cancer therapy. There was a mixture of special invited lectures and cutting-edge abstracts on specific aspects of eosinophil science, as well as enlivened pro-con debates on targeting eosinophils with biologics. A major thrust and overarching theme was that eosinophils exhibit remarkable plasticity and heterogeneity in executing their functions both in homeostasis and in pathobiology; there is a new "Eo-verse" to understand. We trust that this special volume of the Journal of Leukocyte Biology will be of interest across many disciplines and medical sub-specialties in biomedical sciences and demonstrate both the complexity and versatility of the eosinophil in biology and medicine.

Comparison of three dosing intervals for the primary vaccination of the SARS-CoV-2 mRNA Vaccine (BNT162b2) on magnitude, neutralization capacity and durability of the humoral immune response in health care workers: A prospective cohort study.

OBJECTIVES: The dosing interval of a primary vaccination series can significantly impact on vaccine immunogenicity and efficacy. The current study compared 3 dosing intervals for the primary vaccination series of the BNT162b2 mRNA COVID-19 vaccine, on humoral immune response and durability against SARS-CoV-2 ancestral and Beta variants up to 9 months post immunization. METHODS: Three groups of age- and sex-matched healthcare workers (HCW) who received 2 primary doses of BNT162b2 separated by 35-days, 35-42 days or >42-days were enrolled. Vaccine induced antibody titers at 3 weeks, 3 and 6-9 months post-second dose were assessed. RESULTS: There were 309 age- and sex-matched HCW (mean age 43 [sd 13], 58% females) enrolled. Anti-SARS-CoV-2 binding (IgG, IgM, IgA) and neutralizing antibody titers showed significant waning in levels beyond 35 days post first dose. The second dose induced a significant rise in antibody titers, which peaked at 3 weeks and then declined at variable rates across groups. The magnitude, consistency and durability of response was greater for anti-Spike than anti-RBD antibodies; and for IgG than IgA or IgM. Compared to the shorter schedules, a longer interval of >42 days offered the highest binding and neutralizing antibody titers against SARS-CoV-2 ancestral and Beta (B1.351) variants beyond 3 months post-vaccination. CONCLUSIONS: This is the first comprehensive study to compare 3 dosing intervals for the primary vaccination of BNT162b2 mRNA COVID-19 vaccine implemented in the real world. These findings suggest that delaying the second dose beyond 42 days can potentiate and prolong the humoral response against ancestral and Beta variants of SARS-CoV-2 up to 9 months post-vaccination.

Cytomegalovirus Seropositivity in Older Adults Changes the T Cell Repertoire but Does Not Prevent Antibody or Cellular Responses to SARS-CoV-2 Vaccination.

Chronic infection with human CMV may contribute to poor vaccine efficacy in older adults. We assessed the effects of CMV serostatus on Ab quantity and quality, as well as cellular memory recall responses, after two and three SARS-CoV-2 mRNA vaccine doses, in older adults in assisted living facilities. CMV serostatus did not affect anti-Spike and anti-receptor-binding domain IgG Ab levels, nor neutralization capacity against wild-type or ß variants of SARS-CoV-2 several months after vaccination. CMV seropositivity altered T cell expression of senescence-associated markers and increased effector memory re-expressing CD45RA T cell numbers, as has been previously reported; however, this did not impact Spike-specific CD4+ T cell memory recall responses. CMV-seropositive individuals did not have a higher incidence of COVID-19, although prior infection influenced humoral immunity. Therefore, CMV seropositivity may alter T cell composition but does not impede the durability of humoral protection or cellular memory responses after SARS-CoV-2 mRNA vaccination in older adults.

A thymic stromal lymphopoietin polymorphism may provide protection from asthma by altering gene expression.

BACKGROUND: Genome-wide association studies have identified associations of the single nucleotide polymorphism rs1837253 in the thymic stromal lymphopoietin (TSLP) gene with asthma, allergic disease and eosinophilia. The TSLP gene encodes two isoforms, long and short, and previous studies have indicated functional differences between these two isoforms. OBJECTIVE: We investigated the expression of these TSLP isoforms in response to a pro-inflammatory signal, and the role of the rs1837253 genotype in gene isoform regulation. METHODS: We cultured nasal epithelial cells of asthmatic and non-asthmatic subjects and evaluated poly(I:C)-induced TSLP protein secretion using multiplex protein assays and gene expression profiles of the TSLP isoforms, and related genes using real-time qPCR. We correlated these profiles with rs1837253 genotype. RESULTS: Asthmatic nasal epithelial cells exhibited increased TSLP protein secretion compared with nasal epithelial cells from healthy controls. The long TSLP isoform was more responsive to poly(I:C) stimulation. Additionally, the minor T allele of rs1837253 was less inducible than the major C allele, suggesting differential regulation; this may explain the "protective" effects of the T allele in asthma. CONCLUSION: Our results provide important insights into the differential regulation and function of TSLP isoforms, including the role of TSLP rs1837253 polymorphisms in allergic inflammatory processes. CLINICAL RELEVANCE: The key finding on the influence of TSLP genetic variation on disease expression/endotype could provide basis for investigation into targeted biologics for anti-TSLP therapies.

Primary Immune Deficiencies in the Adult: A Previously Underrecognized Common Condition.

The large majority of classified primary immune deficiency (PID) diseases present in childhood. Yet, most patients with PID are adults, with a large proportion experiencing onset of symptoms beyond their childhood years. Most of these are diagnosed predominantly with antibody defects, but cellular and other disorders are increasingly being identified in older patients as well. Moreover, advances in clinical immunology are allowing pediatric patients, even those with severe disease, to reach adulthood. Because of differences in the physiology and pathophysiology of children and adults, the presentation, diagnosis, and management of a complex chronic disease could differ significantly between these patient populations and therefore require modifications in approach.

The LINA Study: Higher Sensitivity of Infant Compared to Maternal Eosinophil/Basophil Progenitors to Indoor Chemical Exposures.

PURPOSE: Enhanced eosinophil/basophil (Eo/B) progenitor cell levels are known to be associated with allergic inflammation and atopy risk. The aim of the present study was to investigate the influence of different indoor exposures on the recruitment and differentiation of Eo/B progenitors in mother-child pairs. METHODS: In 68 mother-child pairs of the LINA study peripheral blood mononuclear cells were used to assess Eo/B colony forming units (CFUs). Information about disease outcomes and indoor exposures was obtained from questionnaires. Indoor concentrations of volatile organic compounds (VOCs) were measured by passive sampling. RESULTS: Infant's Eo/B CFUs were positively associated with exposure to tobacco smoke, disinfectants, or VOCs. In contrast, for maternal Eo/B CFUs, only a few associations were seen. Higher numbers of infant Eo/B CFUs were observed in children with wheezing symptoms within the second year of life. CONCLUSIONS: We demonstrate that infant's hematopoietic cells seem to respond with more sensitivity to environmental exposure compared to maternal cells. At least in infants, an activation of these hematopoietic cells by environmental exposure could contribute to an enhanced risk for the development of respiratory outcomes.

The Canadian Healthy Infant Longitudinal Development (CHILD) Study: examining developmental origins of allergy and asthma.

The Canadian Healthy Infant Longitudinal Development (CHILD) birth cohort study recruited 3624 pregnant women, most partners and 3542 eligible offspring. We hypothesise that early life physical and psychosocial environments, immunological, physiological, nutritional, hormonal and metabolic influences interact with genetics influencing allergic diseases, including asthma. Environmental and biological sampling, innate and adaptive immune responses, gene expression, DNA methylation, gut microbiome and nutrition studies complement repeated environmental and clinical assessments to age 5. This rich data set, linking prenatal and postnatal environments, diverse biological samples and rigorous phenotyping, will inform early developmental pathways to allergy, asthma and other chronic inflammatory diseases.

Human mast cell and basophil/eosinophil progenitors.

Mast cell, basophil, and eosinophil lineages all derive from CD34(+) hemopoietic stem cells; however, mast cells are derived from a distinct, nonmyeloid progenitor, while eosinophils and basophils share a common myeloid progenitor. These progenitors likely evolved from an ancestral leukocyte population involved in innate immunity and currently play a central role in the pathology of allergic disease. Advances in isolation and analysis of mast cell and basophil/eosinophil progenitor populations have been critical to understanding lineage commitment, differentiation, function, and transcriptional regulation of these cells and have provided a way of monitoring the effect of novel investigational therapies on these cell populations in samples of blood, bone marrow, and airway secretions.

The effects of thymic stromal lymphopoietin and IL-3 on human eosinophil-basophil lineage commitment: Relevance to atopic sensitization.

An important immunopathological hallmark of allergic disease is tissue eosinophilic and basophilic inflammation, a phenomenon which originates from hemopoietic progenitors (HP). The fate of HP is determined by local inflammatory cytokines that permit "in situ hemopoiesis," which leads to the accumulation of eosinophils and basophils (Eo/B). Given that recent evidence supports a critical immunomodulatory role for thymic stromal lymphopoietin (TSLP) in allergic inflammation, as well as TSLP effects on CD34+ progenitor cytokine and chemokine secretion, we investigated the role of TSLP in mediating eosinophilo- and basophilopoiesis, the mechanisms involved, and the association of these processes with atopic sensitisation. In the studies presented herein, we demonstrate a direct role for TSLP in Eo/B differentiation from human peripheral blood CD34+ cells. In the presence of IL-3, TSLP significantly promoted the formation of Eo/B colony forming units (CFU) (including both eosinophils and basophils) from human HP (HHP), which was dependent on TSLP-TSLPR interactions. IL-3/TSLP-stimulated HHP actively secreted an array of cytokines/chemokines, key among which was TNFα, which, together with IL-3, enhanced surface expression of TSLPR. Moreover, pre-stimulation of HHP with IL-3/TNFα further promoted TSLP-dependent Eo/B CFU formation. HHP isolated from atopic individuals were functionally and phenotypically more responsive to TSLP than those from nonatopic individuals. This is the first study to demonstrate enhanced TSLP-mediated hemopoiesis ex vivo in relation to clinical atopic status. The capacity of HHP to participate in TSLP-driven allergic inflammation points to the potential importance of "in situ hemopoiesis" in allergic inflammation initiated at the epithelial surface.

IL-4 and IL-13 differentially regulate TLR-induced eosinophil-basophil differentiation of cord blood CD34+ progenitor cells.

Intrauterine environmental exposures have been shown to influence neonatal immunity and subsequent allergic disease development. We have previously shown that fewer lipopolysaccharide (LPS)-stimulated eosinophil-basophil (Eo/B) colonies grow from cord blood (CB) of high-atopic risk infants, compared to low-atopic risk infants. In the present study, we investigated whether a surrogate ex vivo TH2 milieu (i.e., either IL-4 or IL-13) could represent an underlying mechanism to explain our previous findings. CB CD34+ cells from healthy donors were cultured with IL-4 or IL-13 (in combination with LPS) and assessed for Eo/B differentiation using methylcellulose cultures and flow cytometry for related intracellular signalling pathways. Pharmacological inhibitors were added to the methylcellulose cultures to determine the effect of blocking intracellular signalling in CB CD34+ cells in relation to Eo/B colony forming unit (CFU) formation. Stimulation of CD34+ cells with IL-4, but not IL-13, reduced Eo/B CFU formation in the presence of LPS; this was found to be dependent on IL-4Rα and not IL-13Rα1. Additionally, IL-4 reduced the expression of ERK 1/2 after LPS stimulation, which was recovered by inhibition of IL-4Rα. While IL-13 did not have an inhibitory effect on ERK 1/2 expression, inhibition of ERK 1/2 significantly reduced Eo/B CFU formation. Thus, the responsiveness of CB CD34+ progenitor cells to LPS is differentially regulated by the TH2 cytokines, IL-4 and IL-13. This may have implications for in utero interactions between placental-derived pro-allergic cytokines and neonatal progenitor cells influencing Eo/B-mediated inflammatory responses in early life.

The LINA cohort: Cord blood eosinophil/basophil progenitors predict respiratory outcomes in early infancy.

RATIONALE: Cord blood eosinophil/basophil progenitor cells (Eo/B) of high risk infants have been shown to predict respiratory illnesses in infancy. Here we investigated this association in a population-based cohort. Furthermore, we analysed whether newborns Th1/Th2 balance and prenatal environmental exposure impact Eo/B recruitment. METHODS: In a sub-cohort of the LINA study cord blood mononuclear cells were used for methylcellulose assays to assess Eo/B differentiation. Questionnaires were recorded during pregnancy and annually thereafter. Volatile organic compounds were measured during pregnancy and cord blood cytokines after ex vivo stimulation. RESULTS: Cord blood IL-4 and IL-13 positively correlated with Eo/B. Tobacco smoke related benzene was also positively associated with Eo/B. Enhanced Eo/B numbers increased the risk for wheezing within the first 24 months. CONCLUSIONS: The association between cord blood Eo/B and respiratory illnesses is not restricted to high-risk children. Prenatal environmental exposure and a Th2 milieu at birth contribute to Eo/B recruitment.

Interleukin 13 and serotonin: linking the immune and endocrine systems in murine models of intestinal inflammation.

OBJECTIVE: Infiltration of activated immune cells and increased cytokine production define the immunophenotype of gastrointestinal (GI) inflammation. In addition, intestinal inflammation is accompanied by alteration in the numbers of serotonin (5-hydroxytryptamine; 5-HT) synthesizing enterochromaffin (EC) cells and in 5-HT amount. It has been established that EC cells express interleukin (IL)-13 receptor, additionally IL-13 has been implicated in the pathogenesis of ulcerative colitis. In this study, we investigated the role of IL-13 mediated 5-HT signaling in pathogenesis of colitis. METHODOLOGY: Colitis was induced in IL-13 deficient (IL-13-/-) and wild-type (WT) mice with dextran sulfate sodium (DSS) and dinitrobenzene sulfonic acid (DNBS), as well as in IL-13-/- mice given recombinant mouse IL-13 (rmIL-13) and 5-hydroxytryptamine (5-HTP), the direct precursor of 5-HT. PRINCIPAL FINDINGS AND CONCLUSION: Elevated colonic IL-13 levels were observed in WT mice receiving DSS in comparison to control. IL-13-/- mice administered DSS exhibited significantly reduced severity of colitis compared to WT mice as reflected by macroscopic and histological damage assessments. Following DSS administration, significantly lower pro-inflammatory cytokine production and fewer infiltrating macrophages were observed in IL-13-/- mice compared to WT. The reduced severity of colitis observed in IL-13-/- mice was also accompanied by down-regulation of EC cell numbers and colonic 5-HT content. In addition, increasing colonic 5-HT content by administration of rmIL-13 or 5-HTP exacerbated severity of DSS colitis in IL-13-/- mice. IL-13-/- mice also exhibited reduced severity of DNBS-induced colitis. These results demonstrate that IL-13 plays a critical role in the pathogenesis of experimental colitis and 5-HT is an important mediator of IL-13 driven intestinal inflammation. This study revealed important information on immune-endocrine axis in gut in relation to inflammation which may ultimately lead to better strategy in managing various intestinal inflammatory conditions including inflammatory bowel disease.

Toll-like receptor-mediated eosinophil-basophil differentiation: autocrine signalling by granulocyte-macrophage colony-stimulating factor in cord blood haematopoietic progenitors.

Eosinophils are multi-functional leucocytes that play a role in inflammatory processes including allergy and infection. Although bone marrow (BM) inflammatory cells are the main source of eosinophil-basophil (Eo/B) differentiation-inducing cytokines, a recent role has been demonstrated for cytokine induction through Toll-like receptor (TLR)-mediated signalling in BM progenitors. Having previously demonstrated that cord blood (CB) progenitors induce Eo/B colony-forming units (CFU) after lipopolysaccharide (LPS) stimulation, we sought to investigate the intracellular mechanisms by which LPS induces Eo/B differentiation. Freshly isolated CD34-enriched human CB cells were stimulated with LPS (and/or pharmacological inhibitors) and assessed for alterations in haematopoietic cytokine receptor expression and signalling pathways by flow cytometry, Eo/B CFU in methylcellulose cultures, and cytokine secretion using Luminex assays. The LPS stimulation resulted in a significant increase in granulocyte-macrophage colony-stimulating factor (GM-CSF)-responsive, as opposed to interleukin-5-responsive, Eo/B CFU, which also correlated with significant increases in CD34(+) cell GM-CSFRα expression. Functionally, CB CD34(+) cells secrete abundant amounts of GM-CSF following LPS stimulation, via a p38 mitogen-activated protein kinase (MAPK)-dependent mechanism; this secretion was responsible for Eo/B CFU formation ex vivo, as shown by antibody blockade. We show for the first time that LPS stimulation of CB progenitor cells results in autocrine activation of p38 MAPK-dependent GM-CSF secretion facilitating Eo/B differentiation ex vivo. This work provides evidence that early life exposure to products of bacterial agents can modulate Eo/B differentiation, representing a novel mechanism by which progenitor cells can respond to microbial stimuli and so affect immune and inflammatory responses.

Long-term safety of mepolizumab for the treatment of hypereosinophilic syndromes.

BACKGROUND: Hypereosinophilic syndromes (HESs) are chronic disorders that require long-term therapy to suppress eosinophilia and clinical manifestations. Corticosteroids are usually effective, yet many patients become corticosteroid refractory or develop corticosteroid toxicity. Mepolizumab, a humanized monoclonal anti-IL-5 antibody, showed corticosteroid-sparing effects in a double-blind, placebo-controlled study of FIP1L1/PDGFRA-negative, corticosteroid-responsive subjects with HESs. OBJECTIVE: We evaluated long-term safety and efficacy of mepolizumab (750 mg) in HES. METHODS: MHE100901 is an open-label extension study. The primary end point was the frequency of adverse events (AEs). Optimal dosing frequency, corticosteroid-sparing effect of mepolizumab, and development of antimepolizumab antibodies were also explored. RESULTS: Seventy-eight subjects received 1 to 66 mepolizumab infusions each (including mepolizumab infusions received in the placebo-controlled trial). Mean exposure was 251 weeks (range, 4-302 weeks). The most common dosing interval was 9 to 12 weeks. The incidence of AEs was 932 events per 100 subject-years in the first year, declining to 461 events per 100 subject-years after 48 months. Serious AEs, including 1 death, were reported by the investigator as possibly due to mepolizumab in 3 subjects. The median daily prednisone dose decreased from 20.0 to 0 mg in the first 24 weeks. The median average daily dose for all subjects over the course of the study was 1.8 mg. Sixty-two percent of subjects were prednisone free without other HES medications for ≥ 12 consecutive weeks. No neutralizing antibodies were detected. Twenty-four subjects withdrew before study completion for death (n = 4), lack of efficacy (n = 6), or other reasons. CONCLUSION: Mepolizumab was well tolerated and effective as a long-term corticosteroid-sparing agent in PDGFRA-negative HES.

Comparative responses to nasal allergen challenge in allergic rhinitic subjects with or without asthma.

BACKGROUND: Nasal allergen challenge (NAC) is useful to study the pathophysiology of rhinitis, and multiple challenges may more adequately approximate natural exposure. OBJECTIVE: To determine the effect of 4 consecutive daily NAC, on clinical and inflammatory parameters in rhinitics with or without asthma. METHODS: Rhinitic subjects were recruited: 19 with mild asthma and 13 without asthma. Subjects underwent a control challenge (normal saline) followed by 4 consecutive daily NAC. Allergen challenge consisted of spraying the chosen allergen extract into each nostril until a positive nasal response occurred. Symptoms were recorded on a Likert scale, and oral peak expiratory and nasal peak inspiratory flows allowed assessment of a nasal blockage index (NBI), for a period of 7 hours. Induced sputum and nasal lavage were performed on control day and after 1 and 4 days of NAC. RESULTS: Compared with the control day, there was a significant increase in symptom scores and NBI 10 minutes after each last daily NAC in both groups (p < 0.05). Symptom scores and NBI were similar for the 2 groups, except for nasal obstruction and rhinorrhea, which were more marked in subjects with asthma and rhinitis, respectively. Nasal lavage eosinophils were increased after 4 days of challenges in both groups, but there was no change in sputum eosinophils. No cumulative effect or any late response were observed in any of the groups over the challenge period. CONCLUSION: Multiple NAC may be a useful tool to study the pathophysiology of allergic rhinitis or its relationships with asthma. TRIAL REGISTRATION: ClinicalTrials.gov NCT01286129.

Maternal allergy modulates cord blood hematopoietic progenitor Toll-like receptor expression and function.

BACKGROUND: Little is known regarding the prenatal determinants of innate immune responses in relation to infant allergic risk. Environmental exposures, including microbial stimuli, might predispose susceptible subjects to atopy and asthma in early infancy or even in utero. OBJECTIVE: Because Toll-like receptors (TLRs) recognize microbial products and because cord blood (CB) progenitor alterations have been observed in neonates at risk for atopy, we investigated the expression and function of TLRs on CB hematopoietic progenitors in relation to atopic risk, as defined by maternal allergic sensitization. METHODS: Thirty-two (15 with low and 17 with high atopic risk) infant CB samples were assessed for phenotypic and functional alterations in CD34(+) cells by means of flow cytometry and methylcellulose culture, respectively. CD34(+) hematopoietic progenitors were stained for TLR-2, TLR-4, TLR-9, GM-CSF receptor α, IL-5 receptor α, and IL-3 receptor α or cultured in methylcellulose assays for hematopoietic cytokine-stimulated eosinophil-basophil (Eo/B) colony-forming units (CFUs) with or without LPS. RESULTS: High-atopic-risk infants had significantly lower CB CD34(+) cell TLR-2, TLR-4, and TLR-9 expression (P = .009). High-risk infant progenitors gave rise to significantly more Eo/B CFUs (P = .002) with hematopoietic cytokine (IL-3, IL-5, or GM-CSF) stimulation ex vivo. Although LPS costimulation induced Eo/B CFUs from both low- and high-risk infant CB CD34(+) cells, this response was significantly (P = .020) muted in the high-risk CB progenitors. CONCLUSIONS: Neonatal CB CD34(+) hematopoietic progenitor cell TLR expression and functional responsiveness are altered in CB from atopic at-risk infants. Maternal allergic sensitization might modulate hematopoietic progenitor TLR expression and function in utero; specifically, Eo/B "lineage priming" at birth might be circumvented through engagement of TLR pathways in early life.

Cord blood molecular biomarkers of eosinophilopoiesis: kinetic analysis of GATA-1, MBP1 and IL-5R alpha mRNA expression.

Eosinophil/basophil (Eo/B) progenitor phenotype and function in cord blood (CB) are associated with atopic risk at birth and infant clinical outcomes. Molecular analyses of eosinophil-basophil differentiation events could identify clinically predictive biomarkers. To determine CB kinetic patterns of Eo/B lineage-associated gene expression (GATA-1, MBP1 and IL-5R alpha) after IL-5 stimulation, CB non-adherent mononuclear cells were isolated from random fresh and frozen samples and incubated in the presence of recombinant human interleukin-5. Some underwent CD34+ positive selection using magnetic cell separation. At various time-points, mRNA expression of GATA-1, MBP1 and IL-5R alpha (total transcripts) was determined utilizing multiplex quantitative polymerase chain reaction (Q-PCR). Relative expression levels of the IL-5R alpha soluble vs. transmembrane isoforms were also analyzed. Stimulation of the non-adherent mononuclear cells with IL-5 resulted in early up-regulation of GATA-1, peaking at 48 h, followed by decreasing expression and down-regulation by 96 h. The CD34+ enriched population demonstrated an equivalent expression pattern (r = 0.963, p = 0.0349). MBP1 mRNA expression [non-adherent mononuclear cells (NAMNCs) and CD34+ alike; r = 0.988, p = 0.012] was slowly up-regulated in response to IL-5, maximal at 96 h. Total IL-5R alpha expression appeared stable over the time-course, mediated by differential expression of the soluble and transmembrane isoforms (i.e., initial increase in the transmembrane contribution followed by a predominance of the soluble isoform by 48-72 h). Multiplex Q-PCR analysis of mRNA from CB demonstrates expression of critical eosinophil-basophil lineage-specific events that are consistent with current understanding of eosinophil differentiation and maturation. The non-adherent mononuclear cell population provides a surrogate signal for the CD34+ progenitor population.

CD34+ hemopoietic progenitor cells are potent effectors of allergic inflammation.

BACKGROUND: In steady state, hemopoietic progenitors constantly egress from the bone marrow (BM) into the blood and circulate through the peripheral tissues. In allergic diseases, the BM releases increased numbers of CD34(+) progenitor cells that migrate to the site of allergic inflammation, where they differentiate into tissue-dwelling and classic effector cells of allergy, such as mast cells, eosinophils, and basophils. OBJECTIVE: To examine whether peripheral blood CD34(+) cells in addition to being progenitors may also directly function as inflammatory effector cells. METHODS: Highly purified neonatal or adult blood CD34(+) cells were examined for the expression of thymic stromal lymphopoietin (TSLP) and IL-33 receptors and for their response to these cytokines as well as to supernatants of primary small airway epithelial cells and nasal explants from rhinosinusitis and control subjects. Sputum of patients with asthma was examined before and after allergen inhalation for the presence of IL-5 and IL-13-containing CD34(+) cells. RESULTS: Circulating CD34(+) cells expressed receptors for TSLP and IL-33 and responded to these cytokines by rapidly releasing high levels of proinflammatory T(H)2-like cytokines and chemokines. These cells were activated in a TSLP-dependent manner by the supernatant fluids from activated primary human small airway epithelial cells and from nasal explants of patients with chronic rhinosinusitis. Moreover, activated CD34(+) cells containing IL-5 and IL-13 could be detected in the sputum of individuals with allergic asthma, with numbers increasing in response to specific allergen inhalation challenge. CONCLUSION: Blood CD34(+) cells, in addition to being progenitors, may act as proinflammatory effector cells by themselves and directly contribute to the allergic inflammation.