Exhaled NO levels and blood eosinophil counts independently associate with wheeze and asthma events

Eosinophils and exhaled Nitric Oxide (NO) levels are prominent features of asthma.  It is known that both exhaled NO and blood eosinophil counts (B-Eos) are markers of local and systemic eosinophil inflammation respectively, and are elevated in patients with the disease. However, little is known about the association of these markers with wheeze and asthma events.  Malinovschi et al [J Allergy Clin Immunol 2013; 132(4):821-827] examined subjects from National Health and Nutrition Examination Survey 2007-2008 and 2009-2010 to determine individual and independent B-Eos and Fraction of Exhaled Nitric Oxide (FENO) levels in relation to wheeze, asthma diagnosis, and asthma events.

From the cross sectional study, 12,408 subjects ranging from 6-80 years old were selected who had FENO measurements and blood differential counts.  The authors report that the prevalence of current asthma and wheeze increased progressively with FENO values and B-Eos values.  Furthermore, there was an increase in asthma attacks and asthma related ED visits which associated with an increase in both FENO and B-Eos respectively. While intermediate or high FENO and B-Eos levels were independently associated with having asthma, wheeze, and asthma attacks, only the B-Eos values were independently associated with asthma-related ED visits. 

Malinovschi explains that these 2 markers cannot be used interchangeably but rather in combination due to the finding that the correlation between the markers is weak. This indicates that they represent 2 different inflammatory pathways with separate trigger mechanisms, contrary to previous thought.  The B-Eos levels associated with asthma-related ED visits, which is in-line with recent findings that eosinophilic asthma patients receiving anti-IL-5 treatment have a reduction in severe asthma symptoms. Whereas FENO values appear to precede moderate but not severe asthma exacerbations and signal local IL-4/IL-13 mediated mechanisms in bronchial mucosa that are triggered by aeroallergen exposure. 

The authors conclude that both local and systemic Th2 cytokine-driven mechanisms, partly with different triggers, are involved in eosinophilic asthma, suggesting a double-hit mechanism for the development of respiratory symptoms and asthma.   The clinical significance of assessing both of these components for individualizing treatment warrants further study.

A major allergen of public health relevance in the inner city of Baltimore

The prevalence of asthma is ubiquitous across the United States, but the major allergens of public health relevance that contribute to the disease vary across geographical regions.  Within inner cities, the 2 most common allergens are mouse and cockroach. Ahluwalia et al [J Allergy Clin Immunol 2013; 132(4):830-835] sought to determine the relevant antigen(s) most highly associated with inner city asthma morbidity within Baltimore city. The motivation of their study was not only to aid in the management of asthma within the community but to assist in the reduction of levels of these antigens community wide. 

The authors selected 144 children between 7 and 10 years old that had been clinically identified with asthma at least one year before the start of the study.  At the start of the study, they underwent skin prick tests and had clinical data collected at baseline and again at 3, 6, 9, and 12 months.  At the same time points, settled house dust samples were collected to quantify indoor allergens.  The participants were grouped based on sensitization and exposure status of common allergens from the dust samples. 

Results indicated that mouse was the most relevant allergen with regard to asthma outcomes.  Both mouse and cockroach sensitization and exposure was significantly associated with an increased prevalence of heath care use for asthma, but only mouse sensitization and exposure was associated with higher levels of pulmonary inflammation.  Furthermore, the authors report that mouse IgE levels were also associated with poor asthma health whereas cockroach-specific IgE levels were not.   The authors went on to determine that the relationships between asthma outcomes and mouse antigen were independent of cockroach antigen. 

Ahluwalia points out that although cockroach antigen is prevalent and has some effect on outcomes, mouse antigen appears to be the strong driver of asthma morbidity among Baltimore City children.  Their data show that mouse allergen is strongly associated with a range of outcomes, including acute asthma visits, pulmonary inflammation, and lung function.  There is a profound clinical implication of these data by allowing for specific treatments for the patients and reduction of the mouse antigen at the community level.

Question for the authors: If individual urban communities used this type of study to determine the primary antigen(s) that cause community wide allergy and asthma exacerbation, what type of outcomes do you anticipate both clinically and financially both within the community and across the country? 

This question is terrific and gets to the heart of whether community-wide environmental interventions would be expected to have a broad public health benefit - by, for example, reducing asthma ED visits or hospitalizations - and what the associated costs would be.  The best means we have of estimating the potential public health impact is to calculate the proportion of asthma-related hospitalizations, for example, that can be attributed to sensitization and exposure to a particular allergen in a community.  We have done this using another Baltimore City population and estimated that between 20 and 25% of hospitalizations for asthma among Baltimore City children may be attributed to mouse allergen sensitization and exposure. In terms of costs, one multifaceted environmental intervention cost about $1500 per child [Morgan W et al NEJM 2004], which is currently the best estimate we have for the cost of an effective environmental intervention.  Whether the reduction in hospitalizations expected with, for example a mouse-targeted public health intervention in Baltimore, would be worth the cost remains to be seen.  However, a year's supply of controller medication typically costs more than $1500, so that if a mouse-targeted environmental intervention was at least as effective as controller medication, a strong case could be made to allocate more public health resources to target mouse infestation and for insurance to cover such an intervention.  Thus, the data accumulated to date suggest that a public health approach to environmental control has the potential to make a meaningful dent in asthma morbidity and asthma-related costs, not just in Baltimore City, but also in other communities with high asthma prevalence and morbidity.

Primary NK immunodeficiencies

Keeping on the topic of natural killer cells, Jordan Orange, MD, PhD contributes a review on immunodeficiencies associated with NK cell dysfunction [J Allergy Clin Immunol 2013; 132(3):515-525].  The author provides a concise review of NK cell biology, covering their intrinsic activities of cytotoxicity, tumor surveillance, and co-stimulation and signaling.  Orange points out that NK cell deficiency [NKD] is a subset of primary immunodeficiency diseases [PID] that is difficult to diagnose and treat because of the limited clinical information and testing available. 

Like other PID, the author notes that NKD patients are characterized by a susceptibility to chronic and/or severe viral infections, especially herpes viruses.  Accurate diagnosis hinges on determining that the seminal deficiency is associated with NK cells, and that NKD is not secondary to other causes. The author provides an algorithm for identifying primary NKD. 

Orange discusses the current classification of NKD into two types:  classical NKD and functional NKD.  Classical NKD [CNKD] is characterized by severe depletion or absence of NK cells in peripheral blood, while functional NKD [FNKD] is typified by the presence of peripheral NK cells with impaired or abrogated activity.  He points out that there is some overlap in these phenotypes in the reported cases.  Orange further discusses the subtypes of CKND and their associated genetic abnormalities.  In his discussion of FNKD, the author reports on the first identified subtype, FNKD1, which involves a defect in the IgG receptor. 

Orange continues describing the clinical cases that have been reported and the availability and applicability of diagnostics for NKD.  The author also reviews briefly other PID that effect NK cell immunity, but that affect other components of the immune system in the majority.  Addressing the clinical treatment of NKD, he notes that intervention is focused on the herpetic infection susceptibility and employs approved antivirals such as gancyclovir.  Additionally, the author reports that severe presentations of NKD have been treated successfully with stem cell transplantation.  

Natural killer cell interactions in adaptive immunity

This month, Deniz et al. give an overview of current knowledge about natural killer cells [NK cells] and their interface with pathways and mechanisms of adaptive immunity, with attention to allergic disease processes [J Allergy Clin Immunol 2013; 132(3):527-535]. 

The authors cover fundamentals of NK biology, such as their surface marker characterization, IFN-γ secretion, MHC class I interactions, phenotypes, tissue prevalence, cytokine profiles, and cytotoxicity to target cells.  NK cells are characterized by their cytotoxic activity through release of perforin and granzymes that are targeted at tumor cells, virally infected cells, and IgG antibody expressing cells, cytokine and chemokine secretion and signaling of adaptive immune cells, and co-stimulatory interaction with antigen presenting cells [APC] via IL-10 and TGF-β.   Deniz et al. note that NK chemokine secretion is particularly important in the co-localization and mutual maturation of dendritic cells [DC] and NK cells in areas of inflammation. 

The authors discuss the interesting parallels between NK cell subsets and T cells, noting the overlaps in surface markers and cytokine expression.  NK cells subsets consist of NK1 and NK2 cells, analogous to Th1 and Th2 cells, NK regulatory cells, NK-17 cells and NK-22.  They point out that peripheral blood mononuclear cells [PBMC] from patients with asthma showed decreased NK1 and increased NK2 levels, suggesting a NK2 bias that shadows the Th2 bias.  Also discussed was the protective effect that has been associated with NK-17 cells in rheumatoid arthritis and NK-22 cells on epithelial cell response to contact sensitivity. 

The authors discuss the limited evidence that has been reported to date on NK cell interactions in allergic diseases.  They discuss skin NK cells are known orchestration of keratinocyte apoptosis through type I cytokine signaling.  Also covered is the critical role of DCs in the evolution of NK cells.  Deniz et al report that NK cell expression and cytotoxicity is increased in patients with allergic rhinitis.  NK1 cells, but not NK2 cells, are also known to have anti-IgE activity. 

The authors conclude commenting that, while findings are limited, there is growing evidence that NK cells, like many innate immune cells, have important interactions with adaptive immune cells.  The research should now be focused to understand the characteristics of these cells in different endotypes and phenotypes of asthma, atopic dermatitis and other chronic inflammatory diseases.

Is the human gut microbiota the critical mediator of health?

In a review this month, Greer and co-authors present interesting information on just how much our health depends on the well-being and communal balance of the microscopic symbionts in our gastrointestinal tract [Journal of Allergy and Clinical Immunology 2013; 132(2): 253-262].  They begin their review noting that, until fairly recently, immunity, metabolic functions and gut physiology had been studied as separate biological systems.  In light of growing evidence that the delineation between these is arbitrary, the authors point out that systems biology has developed new methods for investigating the interactions between the intestinal microbiota and immune and metabolic outcomes.

Greer et al covers two broad categories, small intestine enteropathies and obesity with metabolic syndrome.  They describe current animal models used to study immunodeficiency enteropathies, celiac disease, inflammatory bowel disease, obesity and lipid metabolism dysregulation.

The authors discuss notable findings from mouse models that have been employed to study enteropathies.  For example, B lymphocyte deficient mice are known have fat absorption issues, which correlate to IgA deficiency.  B cell secreted IgA is required for maintaining a proper balance between immunity, fat metabolism and gut microbes.  They note that B cell deficient mice have intestinal gene expression profiles that are very similar to those seen in HIV/CVID patients.

They note that gut microbiota contribute to dyslipidemia and insulin resistance in obese mice and induce intestinal inflammation in response to increased fat intake.  Greer et al discuss also the physical changes in the ileum that cause increased uptake of fats in the diet-induced obese mice.  Interesting, they comment that TLR5 knock-out mice have increased weight gain, pointing to innate immune interactions in fat metabolism.

Greer et al discuss evidence on short chain fatty acid balance and metabolism as critical to maintenance of a “core” microbiota.  Transplantation of microbes from diet-induced obese mice to control mice results in obesity in the control mice without increase in food intake.  The authors suggest that this points to persisting changes in gut microbiota that may be causally related to obesity and altered fat metabolism.  Concluding, Greer et al suggest that the gut microbiota is a cardinal mediator between the immune system and gastrointestinal epithelium. 

In answer to the question, "In your opinion, does manipulation of the gut microbiota present a therapeutic intervention for obesity and/or lipid metabolism disorders?", the authors responded, "Yes, we believe that manipulation of gut microbiota presents great potential for therapeutic interventions in a range of diseases, including obesity and metabolic syndrome, but we need first to understand which taxa or which microbial genes might be most beneficial and in each case."

A novel mode of cell death in active versus resting eosinophils: a potential pathway for treatment of asthma and allergic disease

Apoptosis was previously thought to be the only mode of regulated or programmed cell death in eosinophils, as necrosis was regarded as unregulated cell death.  However, Kano et al., have discovered that the activated eosinophils, found in abundance in asthma and allergic disease, can die by means of a type of regulated necrosis in response to Siglec-8 ligation [J Allergy Clin Immunol 2013; 132(2): 437-445]. Siglec-8 is a cell-surface receptor protein that is highly and selectively expressed by human eosinophils, as well as mast cells and basophils. The authors show that Siglec-8 ligation in the presence of IL-5 triggers necrosis in activated eosinophils in a reactive oxygen species (ROS)–dependent manner. Further, they explain why IL-5 promotes cell death in this system even though it is typically a pro-survival signal.  They demonstrate that ROS switch IL-5’s function from pro-survival to cell death enhancement by augmenting ERK phosphorylation and this serves as a decisive trigger of necrotic cell death. These discoveries indicate that necrotic eosinophil cell death can be regulated by signal transduction, suggesting that potential therapeutics targeting regulation of the mode of cell death could be beneficial in various eosinophilic diseases.

Pattern recognition receptors in obesity and metabolic disturbances

Highlighting the role of innate immunity in the evolution of obesity and associated metabolic disorders, Jin and Flavell deliver a concise review of mechanisms involving pattern recognition receptors that produce pathology in liver, pancreas, brain and intestinal microbiota [J Allergy Clin Immunol 2013; 132(2):287-294]. 

The authors provide a summary of the biology of the major types of pattern recognition receptors [PRR], with emphasis on toll-like receptors [TLR] and NOD-like receptors [NLR].  They discuss briefly their activity in response to pathogen infection and endogenous injury.  Several NLRs can coalesce into multiprotein complexes called inflammasomes which have proven to be importantly involved in the development of insulin resistance. 

Jin and Flavell review PRR mechanisms in five critical physiologic areas: brain, pancreatic islet, and vascular inflammation, induction of peripheral insulin resistance, and disruption of intestinal microbiota homeostasis.  They note that PRRs can directly mediate and sustain inflammation in response to excessive nutrient resulting in abnormal lipid metabolism and insulin resistance in multiple tissues, which accounts for comorbidities such as type 2 diabetes and atherosclerosis.

In the gut, the authors point out that PRRs are critical to sensing and regulating the microbiota as well as responding to pathogenic insult.  TLR/NLR deficiency-associated microbial imbalance has been associated with obesity risk, insulin resistance, and fatty liver.  Interestingly, transplantation of abnormal microbiota from obese mice into wild-type mice results in reproduction of the obese metabolic phenotype that can be corrected by antibiotic treatment.  This points to a causal relationship between disrupted intestinal microbiota and the development of metabolic syndrome. 

Regarding the recent AMA announcement that obesity is a disease as opposed to lifestyle that results in a disease state, I think this is supported by findings from basic research that the development of obesity is not simply due to a lifestyle exemplified by overeating and inactivity, but also profoundly impacted by intrinsic genetic factors in metabolic system, immune system and intestinal microbial ecosystem. Recognizing obesity as a disease will help the community pay more attention to this emerging health issue and also hopefully stimulate research to understand the complex pathophysiology of obesity.