Angioedema: differential diagnosis and acute management.

A clinical vignette illustrates a typical presentation of a patient seeking help for acute angioedema. Despite the risks of SARS-CoV-2 (COVID-19) exposure, it is critical to evaluate patients with acute angioedema in person, because there is always the potential for angioedema to progress to the head, neck, or lungs, which can rapidly compromise the airways and require immediate intervention to avoid potential asphyxiation. There are three mediators of angioedema, histamine, leukotriene, or bradykinin, each requiring different management. This article provides clinicians essential information for differentiating between these types of angioedema, including an overview of the underlying pathogenies of angioedema, and the subjective and objective findings that are useful in differentiating between angioedema types. The article ends with the appropriate management for each type of acute angioedema, including the medications approved by the FDA for on-demand treatment of an HAE attack.

Histidine decarboxylase deficiency inhibits NBP-induced extramedullary hematopoiesis by modifying bone marrow and spleen microenvironments.

Histidine decarboxylase (HDC), a histamine synthase, is expressed in various hematopoietic cells and is induced by hematopoietic cytokines such as granulocyte colony-stimulating factor (G-CSF). We previously showed that nitrogen-containing bisphosphonate (NBP)-treatment induces extramedullary hematopoiesis via G-CSF stimulation. However, the function of HDC in NBP-induced medullary and extramedullary hematopoiesis remains unclear. Here, we investigated changes in hematopoiesis in wild-type and HDC-deficient (HDC-KO) mice. NBP treatment did not induce anemia in wild-type or HDC-KO mice, but did produce a gradual increase in serum G-CSF levels in wild-type mice. NBP treatment also enhanced Hdc mRNA expression and erythropoiesis in the spleen and reduced erythropoiesis in bone marrow and the number of vascular adhesion molecule 1 (VCAM-1)-positive macrophages in wild-type mice, as well as increased the levels of hematopoietic progenitor cells and proliferating cells in the spleen and enhanced expression of bone morphogenetic protein 4 (Bmp4), CXC chemokine ligand 12 (Cxcl12), and hypoxia inducible factor 1 (Hif1) in the spleen. However, such changes were not observed in HDC-KO mice. These results suggest that histamine may affect hematopoietic microenvironments of the bone marrow and spleen by changing hematopoiesis-related factors in NBP-induced extramedullary hematopoiesis.

DM-induced Hypermethylation of IR and IGF1R attenuates mast cell activation and airway responsiveness in rats.

Diabetes has been reported to modulate the airway smooth muscle reactivity and lead to attenuation of allergic inflammatory response in the lungs. In this study, we aimed to study the effect of insulin on cell activation and airway responsiveness in patients with diabetes mellitus (DM). The airway contraction in rat model groups including a non-DM group, a non-DM+INDUCTION group, a DM+INDUCTION group and a DM+INDUCTION+INSULIN group was measured to observe the effect of insulin on airway responsiveness. Radioenzymatic assay was conducted to measure the levels of histamine, and ELISA assay was conducted to measure bronchial levels of interleukin (IL)-1b, tumour necrosis factor (TNF)-a, cytokine-induced neutrophil chemoattractant (CINC)-1, P-selectin and ß-hexosaminidase. The tension in the main and intrapulmonary bronchi of DM+INDUCTION rats was lower than that of the non-DM+INDUCTION rats, whereas the treatment of insulin partly restored the normal airway responsiveness to OA in DM rats. The release of histamine was remarkably suppressed in DM+INDUCTION rats but was recovered by the insulin treatment. Also, OA significantly increased the levels of IL-1b, TNF-a, CINC-1 and P-selectin in non-DM rats, whereas insulin treatment in DM+INDUCTION rats partly restored the normal levels of IL-1b, TNF-a, CINC-1 and P-selectin in DM rats. Moreover, the expression of IR and IGF1R was evidently suppressed in DM rats, with the methylation of both IR and IGF1R promoters was aggravated in DM rats. Therefore, we demonstrated that DM-induced hypermethylation inhibited mast cell activation and airway responsiveness, which could be reversed by insulin treatment.

Immunohistochemical relationships of huntingtin-associated protein 1 with enteroendocrine cells in the pyloric mucosa of the rat stomach.

Huntingtin-associated protein 1 (HAP1) is a neuronal cytoplasmic protein that is predominantly expressed in the brain and spinal cord. In addition to the central nervous system, HAP1 is also expressed in the peripheral organs including endocrine system. Different types of enteroendocrine cells (EEC) are present in the digestive organs. To date, the characterization of HAP1-immunoreactive (ir) cells remains unreported there. In the present study, the expression of HAP1 in pyloric stomach in adult male rats and its relationships with different chemical markers for EEC [gastrin, marker of gastrin (G) cells; somatostatin, marker of delta (D) cells; 5-HT, marker of enterochromaffin (EC) cells; histamine, marker of enterochromaffin-like (ECL) cells] were examined employing single- or double-labelled immunohistochemistry and with light-, fluorescence- or electron-microscopy. HAP1-ir cells were abundantly expressed in the glandular mucosa but were very few or none in the surface epithelium. Double-labelled immunofluorescence staining for HAP1 and markers for EECs showed that almost all the G-cells expressed HAP1. In contrast, HAP1 was completely lacking in D-cells, EC-cells or ECL-cells. Our current study is the first to clarify that HAP1 is selectively expressed in G-cells in rat pyloric stomach, which probably reflects HAP1's involvement in regulation of the secretion of gastrin.

Characterization of a novel enzyme from Photobacterium phosphoreum with histidine decarboxylase activity.

Histamine or scombrotoxin fish poisoning is caused by ingestion of bacterially produced histamine in fish. Histamine-producing bacteria generally contain the histidine decarboxylase gene (hdc). However, some strains of Photobacterium phosphoreum are known to produce significant levels of histamine, although the hdc gene in these strains has not been recognized. The objective of this study was to investigate a previously unidentified mechanism of histamine production by P. phosphoreum. We identified a protein with histidine decarboxylase (HDC) activity comparable to activity of the pyridoxal-5-phosphate (PLP) dependent HDC from P. kishitanii and M. morganii. The newly identified protein (HDC2) in P. phosphoreum and P. kishitanii strains, was approximately 2× longer than the HDC protein from other Gram-negative bacteria and had 12% similarity to previously identified HDCs. In addition, the hdc2 gene cluster in P. phosphoreum was identical to the hdc gene cluster in P. kishitanii. HDC2 had optimal activity at 20-35 °C, at pH 4, and was not affected by 0-8% NaCl concentrations. Compared to the hdc gene from P. kishitanii, expression of the hdc2 gene was constitutive and not affected by pH or excess histidine. This newly identified protein explains possible mechanisms of histamine production in P. phosphoreum. Characterization of this protein will help in designing control measures to prevent or reduce histamine production in fish.

Protein and Antibody Engineering: Suppressing Degranulation of the Mast Cells and Type I Hypersensitivity Reaction.

With an increase in atopic cases and owing to a significant role of mast cells in type I hypersensitivity, a therapeutic need to inhibit degranulation of mast cells has risen. Mast cells are notorious for IgE-mediated allergic response. Advancements have allowed researchers to improve clinical outcomes of already available therapies. Engineered peptides and antibodies can be easily manipulated to attain desired characteristics as per the biological environment. A number of these molecules are designed to target mast cells in order to regulate the release of histamine and other mediators, thereby controlling type I hypersensitivity response. The aim of this review paper is to highlight some of the significant molecules designed for the purpose.

Histamine production in Lactobacillus vaginalis improves cell survival at low pH by counteracting the acidification of the cytosol.

Histamine, one of the most toxic and commonly encountered biogenic amines (BA) in food, is produced by the microbial decarboxylation of histidine. It may accumulate at high concentrations in fish and fermented food. Cheese has some of the highest histamine concentrations, the result of the histidine-decarboxylase activity of certain lactic acid bacteria (LAB). The present work describes the nucleotide sequence and transcriptional organization of the gene cluster responsible for histamine biosynthesis (the HDC cluster) in Lactobacillus vaginalis IPLA 11064 isolated from cheese. The influence of histidine availability and pH on histamine production and the expression of the HDC cluster genes is also examined. As expected, the results suggest that the production of histamine under acidic conditions improves cell survival by maintaining the cytosol at an appropriate pH. However, the transcriptional regulation of the HDC cluster is quite different from that described in other dairy histamine-producing LAB, probably due to the lack of a termination-antitermination system in the histidyl-tRNA synthetase gene (hisS).

KLF4 is required for suppression of histamine synthesis by polyamines during bone marrow-derived mast cell differentiation.

Mast cells have secretory granules containing chemical mediators such as histamine and play important roles in the immune system. Polyamines are essential factors for cellular processes such as gene expression and translation. It has been reported that secretory granules contain both histamine and polyamines, which have similar chemical structures and are produced from the metabolism of cationic amino acids. We investigated the effect of polyamine depletion on mast cells using bone marrow-derived mast cells (BMMCs). Polyamine depletion was induced using α-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase. DFMO treatment resulted in a significant reduction of cell number and abnormal secretory granules in BMMCs. Moreover, the cells showed a 2.3-fold increase in intracellular histamine and up-regulation of histidine decarboxylase (HDC) at the transcriptional level during BMMC differentiation. Levels of the transcription factor kruppel-like factor 4 (KLF4) greatly decreased upon DFMO treatment; however, Klf4 mRNA was expressed at levels similar to controls. We determined the translational regulation of KLF4 using reporter genes encoding Klf4-luc2 fusion mRNA, for transfecting NIH3T3 cells, and performed in vitro translation. We found that the efficiency of KLF4 synthesis in response to DFMO treatment was enhanced by the existence of a GC-rich 5'-untranslated region (5'-UTR) on Klf4 mRNA, regardless of the recognition of the initiation codon. Taken together, these results indicate that the enhancement of histamine synthesis by DFMO depends on the up-regulation of Hdc expression, achieved by removal of transcriptional suppression of KLF4, during differentiation.

[Elucidation of New Function in Endothelial Cells for Efficient Delivery Strategy of Drug to Tissues].

The author has described two new functions of endothelial cells for efficient delivery of drugs to tissues. First, it was indicated that tight junction (TJ)-associated protein, claudin-1, exerts potent paracellular barrier function in cultured mouse lung microvascular endothelial cells (LMECs). This barrier was instantly and reversibly opened by reduction of TJ proteins expression via histamine H1 and H2 receptors. Histamine was biosynthesized by l-histidine decarboxylase from uptaken l-histidine, and biotransformed by type B of monoamine oxidase, suggesting that histamine concentration is controlled in rat brain MECs (BMECs) and LMECs. Moreover, uptake of l-histidine into BMECs and LMECs markedly increased with addition of ZnSO4. Second, it was suggested that drug-metabolizing enzymes such as CYP and flavin-containing monooxygenase exist in vascular endothelial cells exposed to blood and to aerobic conditions. These cells have the same ability to metabolize drugs as hepatocytes, demonstrating that vascular endothelial cells are a metabolic barrier against tissue transfer of drugs. From these results, it was suggested that reversible opening of TJ and selective inhibition of drug metabolism in vascular endothelial cells may be efficient delivery strategies of drugs to tissues. Finally, I hope that this research will lead to development of new drugs and possible re-evaluation of discontinued drugs.

Histamine Production Behaviors of a Psychrotolerant Histamine-Producer, Morganella psychrotolerans, in Various Environmental Conditions.

Histamine food poisoning is a major safety concern related to seafood consumption worldwide. Morganella psychrotolerans is a novel psychrotolerant histamine-producer. In this study, the histamine production behaviors of M. psychrotolerans and two other major histamine-producers, mesophilic Morganella morganii and psychrotrophic Photobacterium phosphoreum, were compared in seafood products, and histamine accumulation by M. psychrotolerans was characterized at various pH and temperature levels in culture broth. The growth of M. psychrotolerans and P. phosphoreum increased similarly at 4 °C in canned tuna, but M. psychrotolerans produced much higher levels of histamine than P. phosphoreum. Histamine accumulation by M. psychrotolerans was induced at lower environmental pH condition at 4 and 20 °C. The optimal temperature and pH for producing histamine by crude histidine decarboxylase of M. psychrotolerans were 30 °C and pH 7, respectively. The activity of the crude HDC extracted from M. psychrotolerans cells at 10 °C retained 45% of the activity at 30 °C. Histidine decarboxylase gene expression of M. psychrotolerans was induced by low pH conditions. These results suggest that M. psychrotolerans are also a very important histamine-producer leading to histamine poisoning associated with seafood below the refrigeration temperature.

Assessing the quality of fresh Whitemouth croaker (Micropogonias furnieri) meat based on micro-organism and histamine analysis using NGS, qPCR and HPLC-DAD.

AIMS: Quality evaluation of fresh whitemouth croaker (Micropogonias furnieri) by histamine determination using the HPLC-DAD method and quantification of histamine-forming bacteria using NGS and qPCR. METHODS AND RESULTS: The histamine content of fresh whitemouth croaker was detected by high performance liquid chromatography with diode array detector with a concentration ranging from 258·52 to 604·62 mg kg-1 being observed. The number of histidine decarboxylase (hdc gene) copies from Gram-negative bacteria and the bacteria Morganella morganii and Enterobacter aerogenes were quantified by quantitative polymerase chain reaction. All samples were positive, with copy numbers of the hdc gene ranging from 4·67 to 12·01 log10 per g. The microbial community was determined by sequencing the V4 region of the 16S rRNA gene using the Ion Torrent platform. The bioinformatics data generated by frog software showed that the phylum Proteobacteria was the most abundant, with the family Moraxellaceae being more prevalent in samples collected in the summer, whereas the Pseudomonadaceae was more present in the winter. CONCLUSIONS: All fish muscle samples analysed in this study presented histamine values higher than those allowed by CODEX Alimentarius. Additionally, a wide variety of spoilage micro-organisms capable of expressing the enzyme histidine decarboxylase were detected. Thus, improvements in handling and processing are required to minimize the prevalence of histamine-producing bacteria in fish. SIGNIFICANCE AND IMPACT OF THE STUDY: Global fish production in 2016 was 171 million tons, with the largest consumer being China, followed by Indonesia and the USA. In Brazil, 1·3 million tons of fish are consumed per year, with whitemouth croaker being the main fish landed. Notably, cases associated with histamine poisoning are quite common. According to the European Food Safety Authority and European Centre for Disease Prevention and Control, a total of 599 HFP outbreaks were identified in the European Union during the period 2010-2017. In the USA, there were 333 outbreaks with 1383 people involved between 1998 and 2008.

Lipopolysaccharide-induced expansion of histidine decarboxylase-expressing Ly6G+ myeloid cells identified by exploiting histidine decarboxylase BAC-GFP transgenic mice.

Histamine is a biogenic amine that is chiefly produced in mast cells and basophils and elicits an allergic response upon stimulation. Histidine decarboxylase (HDC) is a unique enzyme that catalyzes the synthesis of histamine. Therefore, the spatiotemporally specific Hdc gene expression profile could represent the localization of histamine-producing cells under various pathophysiological conditions. Although the bioactivity of histamine is well defined, the regulatory mechanism of Hdc gene expression and the distribution of histamine-producing cell populations in various disease contexts remains unexplored. To address these issues, we generated a histidine decarboxylase BAC (bacterial artificial chromosome) DNA-directed GFP reporter transgenic mouse employing a 293-kb BAC clone containing the entire Hdc gene locus and extended flanking sequences (Hdc-GFP). We found that the GFP expression pattern in the Hdc-GFP mice faithfully recapitulated that of conventional histamine-producing cells and that the GFP expression level mirrored the increased Hdc expression in lipopolysaccharide (LPS)-induced septic lungs. Notably, a CD11b+Ly6G+Ly6Clow myeloid cell population accumulated in the lung during sepsis, and most of these cells expressed high levels of GFP and indeed contain histamine. This study reveals the accumulation of a histamine-producing myeloid cell population during sepsis, which likely participates in the immune process of sepsis.

Adipose Tissue Mast Cells Promote Human Adipose Beiging in Response to Cold.

In a recent study, repeated cold application induced beiging in subcutaneous white adipose tissue (SC WAT) of humans independent of body mass index. To identify factors that promote or inhibit beiging, we performed multiplex analysis of gene expression with the Nanostring nCounter system (the probe set contained genes for specific immune cell markers, cytokines, and chemokines) on the SC WAT from lean subjects. Multiple correlations analysis identified mast cell tryptase and CCL26, a chemokine for mast cells, as genes whose change correlated positively with the change in UCP1 in SC WAT, leading to the hypothesis that mast cells promote SC WAT beiging in response to cold. We quantified mast cell recruitment into SC WAT and degranulation. Mast cells increased in number in SC WAT in lean subjects, and there was an increase in the number of degranulated mast cells in both lean subjects and subjects with obesity. We determined that norepinephrine stimulated mast cell degranulation and histamine release in vitro. In conclusion, cold stimulated adipose tissue mast cell recruitment in lean subjects and mast cell degranulation in SC WAT of all research participants independent of baseline body mass index, suggesting that mast cells promote adipose beiging through the release of histamine or other products.

Identification and biosynthesis of 2-(1H-imidazol-5-yl) ethan-1-ol (histaminol) in methanogenic archaea.

Histaminol is a relatively rare metabolite most commonly resulting from histidine metabolism. Here we describe histaminol production and secretion into the culture broth by the methanogen Methanococcus maripaludis S2 as well as a number of other methanogens. This work is the first identification of this compound as a natural product in methanogens. Its biosynthesis from histidine was confirmed by the incorporation of 2H3-histidine into histaminol by growing cells of M. maripaludis S2. Possible functions of this molecule could be cell signaling as observed with histamine in eukaryotes or uptake of metal ions.

Yin-Yang of IL-33 in Human Skin Mast Cells: Reduced Degranulation, but Augmented Histamine Synthesis through p38 Activation.

Mast cells (MCs) are the principal effector cells of IgE-mediated allergy. IL-33 is released by resident skin cells as alarmin upon tissue damage or allergen contact. Owing to their pronounced receptor expression, MCs are important targets of IL-33 action, but consequences for skin MCs are ill-defined, especially upon chronic exposure to IL-33. Mimicking the inflammatory milieu of skin disorders, we found that persistent exposure to IL-33 (over a 5-week period) strengthened skin MC numbers through accelerated cell-cycle progression and restriction of apoptosis. Conversely, IL-33 attenuated degranulation and FcεRI expression, potentially as a feedback to chronic "alarmin" exposure. Interestingly, the negative impact on histamine release was counterbalanced by amplified histamine production. Considering the clinical significance of histamine and scarce information on its regulation, we explored the molecular underpinnings. IL-33 induced swift phosphorylation of p38 and JNK (but not of ERK1/2 or AKT), and stimulated histidine decarboxylase expression. Combining pharmacological inhibition and kinase elimination by Accell-facilitated RNA interference in skin MCs revealed a p38-dependent, but JNK-independent mechanism. Collectively, IL-33 exerts multifaceted effects on cutaneous MCs at a post-maturation stage. The IL-33-skin MC axis may contribute to and balance inflammation in chronic skin disorders.

[Development of the Genus and Species Determination Method for Histamine Producing Bacteria Isolated from Fishery Product with High-Resolution Melting Analysis].

Histamine in foods with a high histidine content may be produced by bacteria with histidine decarboxylase activity. Consumption of food enriched in histamine can produce symptoms of histamine poisoning that include flushing, headache, and urticaria. The number of histamine poisoning cases in Japan has decreased with developments in food hygiene management technology. However, approximately 10 cases are still reported each year. In addition, there have been cases where histamine was detected in the end products, prompting large product recalls. To prevent and identify causes of histamine toxicity, manufacturers must identify the bacteria causing the illness. A simple method of identification is needed, since sequence-based identification is complicated to perform and the analysis takes a long time. High-Resolution Melting Analysis (HRMA) is a method that detects differences in the base sequences of PCR products manifested as varied melting temperatures of double-stranded DNA. The present study was intended to develop a rapid identification method for major histamine-producing bacteria using HRMA. Species-specific HRMA primers were designed that specifically targeted the hdcA gene of 20 Gram-negative histamine-producing bacterial strains. The designed primers were used for HRM analysis of the 20 histamine-producing bacterial strains. The strains were divided into three groups (A, B, and C) based on differences in melting temperature values obtained by Tm Calling analysis program. Group A comprised terrestrial bacteria, such as Morganella, Enterobacter, and Raoultella, while Groups B and C comprised marine bacteria, such as those belonging to the genera Vibrio and Photobacterium. The melting profiles obtained in Group A by HRMA were used to identify the aforementioned terrestrial bacteria. The findings indicated that HRMA can easily identify the major gram-negative histamine-producing bacteria. A flow chart was created to identify histamine-producing bacterial species. This method enables the identification of histamine-producing bacterial species more quickly and easily than conventional sequence-based methods. Therefore, the method could be valuable for food companies to screen raw materials and products and track the source of contamination, which will in turn contribute to the prevention of histamine-food poisoning and investigation of its causes.

Inherent allergic potential of α-dioxygenase fragment: A pathogenesis related protein.

In the course of analyzing amino acid sequence of an allergen (≈20 kDa), we found this protein has a homology with the amino acid sequence of putative α-Dioxygenase fragment (ADF). Allergy caused by many allergens having an enzymatic activity have been reported previously, but allergenicity to neither α-Dioxygenase enzyme nor to it's any constituents has been reported. We sought to purify an ADF (≈19.5 kDa) from chickpea to investigate it's inherent allergic potential in BALB/c mice. The ADF showed IgE-affinity in sera of sensitized BALB/c mice and allergic patients. Enhanced levels of histamine, specific IgE as well as IgG1, IL-4, IL-17, IL-6, IL-2 and IL-10 were observed in the sera of mice treated with ADF allergen. A positive skin Type 1 test and elevated number of mast cells were found in the treated mice. Apart from this, enhanced number of immune cells i.e. CD19+ and CD4+ were also noticed in the ADF treated group. Higher expressions of IL-4 as well as GATA-3 and prominent histological changes were observed in tissues of treated animals. Furthermore, expressions of Th2 cytokines, associated transcription factors and mast cell signaling proteins were also increased at mRNA and protein levels in the intestines of ADF treated mice. Conclusively, present study demonstrated that ADF with molecular weight of 19.5 kDa is a clinical relevant allergen which causes allergic immune responses in BALB/c mice and may play a pivotal role in allergy caused by food containing α-Dioxygenase enzyme in sensitive individuals.

Bacterial secretion of histamine within the gut influences immune responses within the lung.

BACKGROUND: Histamine is an important immunomodulator influencing both the innate and adaptive immune system. Certain host cells express the histidine decarboxylase enzyme (HDC), which is responsible for catalysing the decarboxylation of histidine to histamine. We and others have shown that bacterial strains can also express HDC and secrete histamine; however, the influence of bacterial-derived histamine on the host immune responses distant to the gut is unclear. METHODS: The Escherichia coli BL21 (E coli BL21) strain was genetically modified to express the Morganella morganii (M morganii)-derived HDC gene (E coli BL21_HTW). E coli BL21 and E coli BL21_HTW were gavaged to ovalbumin (OVA) sensitized and challenged mice to investigate the effect of bacterial-derived histamine on lung inflammatory responses. RESULTS: Oral administration of E coli BL21_HTW, which is able to secrete histamine, to wild-type mice reduced lung eosinophilia and suppressed ex vivo OVA-stimulated cytokine secretion from lung cells in the OVA respiratory inflammation mouse model. In histamine receptor 2 (H2R)-deficient mice, administration of histamine-secreting bacteria also reduced inflammatory cell numbers in bronchoalveolar lavage (BAL). However, the suppressive effect of bacterial-derived histamine on BAL inflammation was lost in HDC-deficient mice. This loss of activity was associated with increased expression of histamine degrading enzymes and reduced histamine receptor expression. CONCLUSION: Histamine secretion from bacteria within the gut can have immunological consequences at distant mucosal sites, such as within the lung. These effects are influenced by host histamine receptor expression and the expression of histamine degrading enzymes.

Respiratory Chlamydia Infection Induce Release of Hepoxilin A3 and Histamine Production by Airway Neutrophils.

Background: Hepoxilins are biologically active metabolites of arachidonic acid that are formed through the 12-lipoxygenase pathway. Hepoxilin A3 is now known to be an important regulator of mucosal inflammation in response to infection by bacterial pathogens and was recently identified as a potent neutrophil chemoattractant in the intestinal mucosa. Our goal in this study was to determine if airway infection with Chlamydia in a murine model of allergic airway disease (AAD) induces hepoxilin secretion along with airway neutrophilia. Methods: We utilized an AAD adult Balb/c mouse model to evaluate airway pathology and immune response by assaying bronchoalveolar lavage (BAL) fluid cytokine, cellularity, histidine decarboxylase (HDC) as well as histamine released in response to in-vivo chlamydial antigen stimulation of purified airway neutrophils. Hepoxilin A3 production was determined by Western blot identification of 12-lipoxygenase precursor (12-LO). Results: Chlamydial infection induced increased production of IL-2, IL-12, TNF-α, and IFN-γ in BAL fluid compared to uninfected animals. Chlamydia-infected mice responded with robust airway neutrophil infiltration and upon induction of AAD increased their production of IL-4, IL-5, and IL-13 by >3 fold compared to unsensitized groups. In addition, 12-LO mRNA was upregulated in infected, but not in uninfected AAD mice, suggesting the production of hepoxilin A3. mRNA expression of HDC was induced only in neutrophils from the airways of Chlamydia-infected mice, but was not seen in AAD only or uninfected controls. When purified neutrophils from infected animals were challenged with chlamydial antigen in vitro there was significant histamine release. Conclusions: Our data confirms the production and release of hepoxilin A3 in the murine airways concomitant with airway neutrophilia in response to chlamydial infection. We further confirmed that Chlamydia provokes the production and release of histamine by these neutrophils. These findings suggest that neutrophils, provoked by Chlamydia infection can synthesize and release histamine, thereby contributing directly to airway inflammation.