Brant Ward

Human mast cells present antigen to autologous CD4+ T Cells

&NA; Figure. No caption available. Background: Mast cells (MCs), the primary effector cell of the atopic response, participate in immune defense at host/environment interfaces, yet the mechanisms by which they interact with CD4+ T cells has been controversial. Objective: We used in situ–matured primary human MCs and matched CD4+ T cells to diligently assess the ability of MCs to act as antigen‐presenting cells. Methods: We examined mature human skin‐derived MCs using flow cytometry for expression of antigen‐presenting molecules, for their ability to stimulate CD4+ T cells to express CD25 and proliferate when exposed to superantigen or to cytomegalovirus (CMV) antigen using matched T cells and MCs from CMV‐seropositive or CMV‐seronegative donors, and for antigen uptake. Subcellular localization of antigen, HLA molecules, and tryptase was analyzed by using structured illumination microscopy. Results: Our data show that IFN‐&ggr; induces HLA class II, HLA‐DM, CD80, and CD40 expression on MCs, whereas MCs take up soluble and particulate antigens in an IFN‐&ggr;–independent manner. IFN‐&ggr;–primed MCs guide activation of T cells by Staphylococcus aureus superantigen and, when preincubated with CMV antigens, induce a recall CD4+ TH1 proliferation response only in CMV‐seropositive donors. MCs co‐opt their secretory granules for antigen processing and presentation. Consequently, MC degranulation increases surface delivery of HLA class II/peptide, further enhancing stimulation of T‐cell proliferation. Conclusions: IFN‐&ggr; primes human MCs to activate T cells through superantigen and to present CMV antigen to TH1 cells, co‐opting MC secretory granules for antigen processing and presentation and creating a feed‐forward loop of T‐cell–MC cross‐activation.

Obesity is not linked to increased whole-body mast cell burden in children.

Obesity is clearly associated with insulin resistance and chronic low-grade inflammation.1,2 Macrophages appear to be especially important in this relationship, as they infiltrate adipose tissue and produce a variety of inflammatory cytokines.1 Exploring the contribution of other immune cells to the development of obesity, Liu, et al., described a role for mast cells in the development of obesity and diabetes in mice.3 Using genetically modified mice and pharmacologic stabilizers of mast cells, they demonstrated that mast cells and mast cell-mediated protease expression may promote the growth of white adipose tissue (WAT). Importantly, the idea that mast cells may function in a similar manner in human obesity was suggested by the finding of increased numbers of mast cells in human WAT from obese compared to lean subjects in their study. Furthermore, mean serum tryptase levels were higher in obese (13.1 ng/ml) than lean (7.7 ng/ml) individuals using an in-house tryptase assay. We attempted to replicate this data by comparing serum tryptase levels in obese, overweight, and lean individuals from a pediatric population. As the serum total tryptase level, comprised primarily of α and β protryptases, seems to correlate with the total-body burden of mast cells,4 we measured the level of this protein in individuals 8–18 years old, recruited through newspaper advertisement for research participation. The cohort contained a diverse group of children and adolescents with and without obesity, impaired glucose tolerance, and/or diabetes mellitus (see Figure 1A and Tables E1 and E2). As the body mass index (BMI) is age- and gender-specific in children and teens,5 we divided the subjects based on the BMI percentiles by age and sex into those of healthy weight (BMI ≥5th– <85th percentile), overweight (≥85th – <95th percentile), and obese (≥95th percentile). Comparisons were made using the non-parametric Kruskal-Wallis test, as the data were not normally distributed after standardization to percentiles. No statistical difference in the median serum tryptase levels was seen among the groups (3.1, 3.5, and 2.7 ng/ml, respectively; P = 0.068) (see Figure 1B). While the BMI varies with age and sex in pediatric patients, the raw values still accurately reflect total-body fatness5 and thus should directly correlate with the serum tryptase levels according to the data presented by Liu, et al. Therefore, to directly compare our results to those of Liu, et al., lean individuals (BMI<26) were compared to overweight (BMI ≥ 26, but <32) and obese (BMI ≥32) individuals; these data were normally distributed. Again, no statistical difference among the groups was observed by one-way analysis of variance (ANOVA, P = 0.449; see Figure E1A). Because some of the individuals in the cohort demonstrated impaired glucose tolerance or overt type 2 diabetes mellitus, the above comparisons were repeated with data from only those individuals with normal glucose tolerance (NGT). Once again, no statistical difference was seen between lean, overweight, and obese individuals, as defined either by BMI percentile (Kruskal-Wallis, P = 0.138; see Figure 1C) or by BMI alone (ANOVA, P = 0.386; see Figure E1B). Finally, multiple linear regression analysis was performed on the entire cohort to determine the association of multiple different obesity-related parameters (BMI, BMI percentile, age, race, height, weight, fat mass, percent body fat, waist circumference, hip circumference, waist:hip ratio, visceral adipose tissue, percent visceral adipose tissue, subcutaneous adipose tissue, percent subcutaneous adipose tissue, and total adipose tissue) to serum tryptase levels. The combined model was unable to significantly predict serum tryptase levels (R2 = 0.043, P = 0.997, for all individuals; R2 = 0.110, P = 0.974, for only NGT individuals). Moreover, none of the individual parameters were found to significantly correlate with serum tryptase levels. Figure 1 Serum total tryptase levels in healthy weight, overweight, and obese pediatric patients. A, Serum total tryptase levels were measured in a cohort of pediatric patients with varying degrees of body fat, approximated here by body mass index (BMI). Individuals ... We did not perform histologic examination of the adipose tissue, and thus we cannot comment on the numbers of mast cells within the adipose tissue. It is quite possible that they are indeed elevated as in the Liu, et al., report. However, despite any possible increase in mast cells in the adipose tissue, our data indicate that obese youth do not have a significantly increased whole body mast cell burden that would manifest as an increased serum total tryptase. There are several potential reasons for the conflicting results, the most obvious being the age difference of the subjects— in our study, ages ranged from 8 to 18 years, whereas in the Liu, et al., report, ages ranged from 20 to 66 years.3 Another recent study involving subjects aged 15–69 showed a positive correlation between BMI and serum tryptase levels (median tryptase=4.7 ng/ml [BMI 30 kg/m2]),6 albeit with a more modest effect than in Liu, et al. In that same study, age was also found to correlate with serum tryptase levels, with greater than three times the standardized effect of BMI. However, the link between obesity and chronic low-grade systemic inflammation has been established in children as well as adults, with similar immunologic mechanisms and cytokine profiles found in both populations.7,8 One should therefore expect that similar mechanisms for mast cell recruitment and activation should be present in both age groups, though this does not seem to be the case. Another potential cause for the difference in findings is unintentional selection bias. While Liu, et al., attempted to exclude those with evidence of infection or inflammatory disease, the significantly elevated serum tryptase levels seen in the obese group (up to 73 ng/ml) suggest that one or more of the subjects may have had an undiagnosed clonal mast cell disorder, such as systemic mastocytosis.9 We would also note that the tryptase assay used by Liu, et al., has been utilized in a limited number of studies, and thus may not be well characterized or validated in a clinical setting. Indeed, the mean serum tryptase for lean individuals in their study appears to be substantially higher than the mean level in normal individuals as established by other groups (3.8 ng/ml) using the commercially available assay (see http://www.phadia.com/en/Health-Care-Providers/Allergy/Products1/ImmunoCAp-Tryptase/). Finally, while the differences in median tryptase levels among patients in our cohort grouped by BMI percentile did not meet statistical significance, they indeed approached it, and thus one could argue that our study lacks sufficient power to pick up obesity-related differences in serum tryptase. However, the median tryptase level for the obese group was lower than that of both the healthy weight and overweight groups, not higher. Thus, additional power would be unlikely to support a positive correlation between tryptase levels and obesity in our study population. In their report, Liu, et al. provided an elegant argument for the role of mast cells in mouse obesity and diabetes. As inflammatory mechanisms are known to be associated with obesity and insulin resistance in humans, the stated implication of their study was that mast cells may have a similar roles in human and mouse obesity. We agree that the potential role for mast cells in human obesity and diabetes is intriguing, but using serum tryptase to show an increase in total-body mast cell burden in children with increased body fat is not supported by our data. Clearly additional studies are needed to fully establish and characterize the potential relationship between mast cells and obesity in humans.

Future Prospects of Biologic Therapies for Immunologic Diseases

This article presents an overview of future uses for biologic therapies in the treatment of immunologic and allergic conditions. Discussion is centered on the use of existing therapies outside of their current indication or on new therapies that are close to approval. This information may help familiarize practicing allergists and immunologists with therapies they may soon encounter in their practice as well as help identify conditions and treatments that will require further study in the near future.

Anaphylaxis and Its Management

Although mast cells and anaphylaxis were discovered near the turn of the nineteenth century, the relationship between this cell type to anaphylaxis only occurred decades later. Anaphylaxis is a clinical event occurring when mast cells and/or basophils are activated to secrete mediators with vasoactive, smooth muscle spasmogenic activities, often through a pathway involving allergen, IgE, and high-affinity IgE receptors on the surfaces of these cells, but sometimes through non-immunologic direct activators of these cells or because of genetic disorders that increase the activatability of these cells. Clinical and laboratory parameters are used to increase the precision with which anaphylaxis is diagnosed, and an increasing variety of interventions are designed to mitigate acute anaphylaxis or to prevent future episodes of anaphylaxis. However, unmet needs remain, particularly with better diagnostic tests and treatments of this condition, as well as a more complete understanding of what determines anaphylactic severity.