Yoseph A. Mekori

Mast cells in innate immunity

Summary: Mast cells are known to be the main effector cells in the elicitation of the IgE‐mediated allergic response. The specific location of mast cells within tissues that interface the external environment, and the extent of their functional capacity, including the ability to phagocytose and to produce and secrete a wide spectrum of mediators, have led investigators to propose a potential role for mast cells in innate immune responses. Certain microorganisms have been found to interact either directly or indirectly with mast cells. This interaction results in mast cell activation and mediator release which elicit an inflammatory response or direct killing leading to bacterial clearance. The in vivo relevance of these in vitro observations has been demonstrated by the use of complement‐deficient and/or mast cell‐deficient and mast cell‐reconstituted mice. It thus has been shown that both C3 and mast cell‐ and tumor necrosis factor‐a‐dependent recruitment of circulating leukocytes with bactericidal properties are crucial to a full response in certain models of acute infection. Modulation of mast cell numbers in vivo was also found to affect the host response against bacterial infection. Thus, mast cells do have a role in innate immunity in defined animal models of bacterial infection. Whether mast cells participate in innate immune responses in the protection of the human host against bacteria remains to be determined.

Mast cell-T cell interactions.

In addition to being a major effector cell in the elicitation of allergic inflammation, mast cells have been found to be activated in various T cell-mediated inflammatory processes and to reside in close physical proximity to T cells. Such observations and the wide spectrum of mediators produced and secreted by mast cells have led investigators to propose a functional relationship between these 2 cell populations. Indeed, mast cell activation has been reported to induce T-cell migration either directly by the release of chemotactic factors, such as lymphotactin or IL-16, or indirectly by the induction of adhesion molecule expression on endothelial cells. Mast cells are also able to present antigens to T cells, resulting in their activation in either an MHC class I- or class II-restricted and costimulatory molecule-dependent fashion. Adhesion molecule-dependent intercellular contact or MHC class II cognate interactions between T cells and mast cells result in the release of both granule-associated mediators and cytokines from the latter. Also, T cell-derived mediators, such as beta-chemokines, directly induce mast cell degranulation. On the other hand, mast cell-derived cytokines, such as IL-4, have been found to polarize T cells to preferentially differentiate into the T(H2) subset. Thus T cell-mast cell interactions are bidirectional, fulfilling regulatory and/or modulatory roles affecting various aspects of the immune response.

Human mast cells release metalloproteinase-9 on contact with activated T cells: juxtacrine regulation by TNF-alpha.

Mast cells, essential effector cells in allergic inflammation, have been found to be activated in T cell-mediated inflammatory processes in accordance with their residence in close physical proximity to T cells. We have recently reported that mast cells release granule-associated mediators and TNF-α upon direct contact with activated T cells. This data suggested an unrecognized activation pathway, where mast cells may be activated during T cell-mediated inflammation. Herein, we show that this cell-cell contact results in the release of matrix metalloproteinase (MMP)-9 and the MMP inhibitor tissue inhibitor of metalloproteinase 1 from HMC-1 human mast cells or from mature peripheral blood-derived human mast cells. The expression and release of these mediators, as well as of β-hexosaminidase and several cytokines, were also induced when mast cells were incubated with cell membranes isolated from activated, but not resting, T cells. Subcellular fractionation revealed that the mature form of MMP-9 cofractionated with histamine and tryptase, indicating its localization within the secretory granules. MMP-9 release was first detected at 6 h and peaked at 22 h of incubation with activated T cell membranes, while TNF-α release peaked after only 6 h. Anti-TNF-α mAb inhibited the T cell membrane-induced MMP-9 release, indicating a possible autocrine regulation of MMP release by mast cell TNF-α. This cascade of events, whereby mast cells are activated by T cells to release cytokines and MMP-9, which are known to be essential for leukocyte extravasation and recruitment to affected sites, points to an important immunoregulatory function of mast cells within the context of T cell-mediated inflammatory processes.

Glucocorticoids decrease tissue mast cell number by reducing the production of the c-kit ligand, stem cell factor, by resident cells: in vitro and in vivo evidence in murine systems.

The local delivery of glucocorticoids to tissues significantly decreases mast cell number. This pharmacologic effect of glucocorticoids is believed to be one of the mechanisms by which glucocorticoids regulate allergic inflammation. To determine the mechanism by which glucocorticoids are able to exert this effect, we first applied the glucocorticoid fluocinonide to mouse dermis and observed that the decrease in mast cell number was associated with an increase in mast cell apoptosis. This did not appear to be due to a direct effect of the glucocorticoid on mast cells, as the addition of 0.01-1.0 microM of the glucocorticoid dexamethasone into stem cell factor (SCF)-dependent mast cell cultures did not enhance mast cell death. However, addition of dexamethasone to cultured fibroblasts did result in a downregulation of SCF mRNA and a significant decrease in SCF protein production. Similarly, immunohistochemistry performed on fluocinonide-treated mouse dermis revealed a decrease in immunoreactive SCF. Administration of SCF at sites of fluocinonide administration to the dermis abolished the mast cell-depleting effect of this glucocorticoid. Thus, glucocorticoids decrease tissue mast cell number by downregulating tissue SCF production required for the survival of local mast cells. This observation may be applicable to the design of improved strategies to treat mast cell-mediated disorders.

Soluble intercellular adhesion molecule-1 and long-term risk of acute coronary events in patients with chronic coronary heart disease. Data from the Bezafibrate Infarction Prevention (BIP) Study.

OBJECTIVES The goal of this study was to assess soluble intercellular adhesion molecule-1 (sICAM-1) level as a predictor of future acute coronary events in patients with chronic coronary heart disease (CHD). BACKGROUND Increased sICAM-1 concentration has been shown to be associated with the incidence of CHD in healthy persons. Its significance in patients with CHD has been scarcely investigated. METHODS We designed a prospective, nested case-control study. Sera were collected from patients with CHD enrolled in a secondary prevention trial that evaluated the efficacy of bezafibrate in reducing coronary events. We measured baseline sICAM-1 concentration in the sera of patients who developed subsequent cardiovascular events (cases: n = 136) during follow-up (mean: 6.2 years) and in age- and gender-matched controls (without events: n = 136). RESULTS Baseline serum concentrations of sICAM-1 were significantly higher in cases versus controls (375 vs. 350 ng/ml; p < 0.05). Each 100 ng/ml increase in sICAM-1 concentration was associated with 1.27 (95% confidence interval [CI]: 1.00 to 1.63) higher relative odds of coronary events. Soluble ICAM-1 concentration in the highest quartile (>394 ng/ml) was associated with significantly higher odds of coronary events (compared with the lowest quartile), even after multivariate adjustment (2.31, 95% CI: 1.02 to 5.50). After adding fibrinogen and total white blood cell count to the multivariate model, the relative odds were 2.12 (95% CI: 0.88 to 5.35) and 2.70 (95% CI: 1.10 to 7.05), respectively. CONCLUSIONS Elevated sICAM-1 concentration in CHD patients is associated with increased risk of future coronary events independent of other traditional risk factors.

Activated T lymphocytes induce degranulation and cytokine production by human mast cells following cell-to-cell contact.

Activated mast cells reside in close apposition to T cells in some inflammatory processes. In this study, we analyzed whether this close physical proximity affects human mast cell degranulation and cytokine release. Thus HMC‐1 human mast cells or primary bone marrow‐derived human mast cells were cocultured with activated and with resting T cells. Mast cells cocultured with activated T cells released histamine and β‐hexosaminidase and produced tumor necrosis factor α (TNF‐α), an effect that peaked at 20 h. Kinetics of histamine release paralleled the formation of heterotypic aggregates. Separation of the two cell populations with a porous membrane prevented mediator release and TNF‐α production. Addition of the PI3‐kinase inhibitor, wortmannin, inhibited the heterotypic adhesion‐associated degranulation but not TNF‐α production. These data thus indicate a novel pathway through which human mast cells are activated to both release granule‐associated mediators and to produce cytokines in association with heterotypic adhesion to activated human T cells. J. Leukoc. Biol. 63: 337–341; 1998.

Soluble Intercellular Adhesion Molecule-1 and Risk of Future Ischemic Stroke A Nested Case-Control Study From the Bezafibrate Infarction Prevention (BIP) Study Cohort

Background and Purpose— Inflammation is considered to be involved in the pathogenesis of ischemic stroke. Our purpose was to assess the role of soluble intercellular adhesion molecule-1 (sICAM-1) concentration, a marker of inflammation, in predicting future ischemic stroke among patients at risk because of chronic coronary heart disease. Methods— We obtained baseline serum samples from patients with chronic coronary heart disease enrolled in the Bezafibrate Infarction Prevention trial (n=3090), which assessed the efficacy of bezafibrate in secondary prevention. Using a prospective nested case-control design, we measured baseline sICAM-1 concentration in sera of patients who developed ischemic stroke during a mean follow-up of 8.2 years (cases, n=134) and in age- and sex-matched controls without any subsequent cardiovascular events (n=134). Results— Baseline serum concentrations of sICAM-1 were significantly higher in cases compared with controls (379 versus 350 ng/mL, P <0.05). sICAM-1 concentration at the highest quartile (>394 ng/mL) was associated with significantly higher relative odds of ischemic stroke compared with the lower concentrations after adjustment for potential confounding variables (relative odds, 2.1; 95% CI, 1.1 to 4.3). After fibrinogen and total white blood cell count were added to the multivariable model, the relative odds were 2.1 (95% CI, 1.1 to 4.2) and 2.2 (95% CI, 1.1 to 4.8), respectively. The risk associated with raised concentrations of sICAM-1 seemed to be highest for large disabling strokes of cardioembolic origin. Conclusions— Elevated concentrations of sICAM-1, a marker of inflammation, are associated with increased risk of ischemic stroke, independent of other traditional cerebrovascular risk factors and of plasma fibrinogen, among patients at increased risk because of manifest coronary heart disease.

Human Mast Cell Apoptosis Is Regulated Through Bcl-2 and Bcl-XL

It is well established that human mast cell proliferation and maturation are regulated by kit ligand (stem cell factor). Little is known, however, about how these two processes are negatively regulated and thus, how mast cell number is controlled in normal and pathologic conditions. We therefore first hypothesized that SCF-dependent human mast cells would undergo programmed cell death (apoptosis) on removal of SCF as has been shown for growth factor-dependent rodent mast cells. We then examined whether SCF acts as a survival factor through the regulation of the bcl-2 family of apoptosis-regulatory genes. As hypothesized, elimination of SCF from primary peripheral blood-derived human mast cell cultures resulted in a significant apoptotic process. During apoptosis, down-regulation of the two apoptosis-regulatory proteins Bcl-2 and Bcl-XLwas observed. Moreover, a deregulated expression of these two proteins was found in two human mast cell lines which are SCF-independent. Thus, SCF functions as a survival factor by repressing apoptosis of human mast cells through Bcl-2 and Bcl-XL. Deregulated expression of these antiapoptotic proteins may contribute to proliferation and accumulation of mast cells in certain forms of systemic mast cell disorders.

The role of c-Kit and its ligand, stem cell factor, in mast cell apoptosis.

The regulation of tissue mast cell number depends both on the rate of production of mast cell precursors and the length of survival of mature mast cells within tissues. Once mast cell precursors target to tissues, their survival may largely be dependent upon the local production of stem cell factor (SCF). Withdrawal of interleukin (IL)-3 results in mast cell apoptosis. The apoptotic changes following IL-3 deprivation are prevented by the addition of SCF which exerts its rescue effect upon interaction with its c-Kit tyrosine kinase receptor. Mast cells undergo apoptosis on withdrawal of IL-3 coincident with a decrease in endogenous bcl-2 mRNA; however, SCF does not induce expression of bcl-2 when added to these cells. When overexpressed, bcl-2 prolongs survival of bcl-2-transfected mast cells following IL-3 deprivation. Transforming growth factor-beta was found to specifically prevent this SCF-mediated rescue from apoptosis, probably by down-regulating the expression of c-Kit. Thus, microenvironmental factors play an important role in regulating mast cell numbers by effecting survival in the periphery.

MOR : a simulation-based assessment centre for evaluating the personal and interpersonal qualities of medical school candidates

Context  Medical school admissions traditionally rely heavily on cognitive variables, with non‐cognitive measures assessed through interviews only. In recognition of the unsatisfactory reliability and validity of traditional interviews, medical schools are increasingly exploring alternative approaches that can provide improved measures of candidates’ personal and interpersonal qualities.