Sara Wernersson

Mast cell secretory granules: armed for battle

Mast cells are important effector cells of the immune system and recent studies show that they have immunomodulatory roles in diverse processes in both health and disease. Mast cells are distinguished by their high content of electron-dense secretory granules, which are filled with large amounts of preformed and pre-activated immunomodulatory compounds. When appropriately activated, mast cells undergo degranulation, a process by which these preformed granule compounds are rapidly released into the surroundings. In many cases, the effects that mast cells have on an immune response are closely associated with the biological actions of the granule compounds that they release, as exemplified by the recent studies showing that mast cell granule proteases account for many of the protective and detrimental effects of mast cells in various inflammatory settings. In this Review, we discuss the current knowledge of mast cell secretory granules.

Mast cell proteases: multifaceted regulators of inflammatory disease.

Mast cells (MCs) are currently receiving increased attention among the scientific community, largely because of the recent identification of crucial functions for MCs in a variety of disorders. However, it is in many cases not clear exactly how MCs contribute in the respective settings. MCs express extraordinarily high levels of a number of proteases of chymase, tryptase, and carboxypeptidase A type, and these are stored in high amounts as active enzymes in the MC secretory granules. Hence, MC degranulation leads to the massive release of fully active MC proteases, which probably have a major impact on any condition in which MC degranulation occurs. Indeed, the recent generation and evaluation of mouse strains lacking individual MC proteases have indicated crucial contributions of these to a number of different disorders. MC proteases may thus account for many of the effects ascribed to MCs and are currently emerging as promising candidates for treatment of MC-driven disease. In this review, we discuss these findings.

IgG2a-Mediated Enhancement of Antibody and T Cell Responses and Its Relation to Inhibitory and Activating Fcγ Receptors

A number of studies in experimental animal models point to an important role of FcγRs in autoimmunity and allergy. In this study, we investigate how the production of IgG, an early step in the chain of events leading to inflammation, is regulated by activating and inhibitory FcγRs. IgG Abs are known to feedback-enhance Ab responses to soluble Ags, and this effect requires activating FcγRs. To test proliferation of Th cells, mice were adoptively transferred with CD4+ T cells expressing a transgenic OVA-specific TCR before immunization with IgG2a anti-2,4,6-trinitrophenyl (TNP) plus OVA-TNP or with OVA-TNP alone. IgG2a induced a significant increase in OVA-specific T cell numbers, which preceded the OVA-specific Ab response and was dependent on the FcRγ chain. The role of the inhibitory FcγRIIB in Ab responses was studied in mice lacking this receptor. Although IgG2a enhanced primary Ab responses, development of germinal centers, and immunological memory in wild-type mice, enhancement was markedly stronger in FcγRIIB−/− mice. The presented data are compatible with the hypothesis that the mechanism behind IgG2a-mediated up-regulation of Ab responses involves increased Ag presentation to CD4+ T cells by FcγR+ APCs. Our observations also illustrate the intricate immunoregulatory role of IgG Abs. On the one hand, they enhance Ab responses via activating FcγRs, and on the other hand, they set an upper limit for the same Ab response via FcγRIIB.

Serotonin and histamine storage in mast cell secretory granules is dependent on serglycin proteoglycan

BACKGROUND Serotonin and histamine are components of human and rodent mast cell secretory granules. OBJECTIVE Serotonin and histamine are stored in the same compartment as serglycin proteoglycan. Here we addressed the possibility that serglycin may be involved in their storage and/or release. METHODS The storage and release of histamine and serotonin was studied in bone marrow-derived mast cells (BMMCs) and in peritoneal mast cells from wild-type or serglycin-/- mice. RESULTS Both serotonin and histamine storage in BMMCs was positively correlated with the degree of mast cell differentiation, and the amount of stored amine was reduced in serglycin-/- BMMCs compared with wild-type controls. The amounts of histamine/serotonin stored were reflected by the expression levels of histidine decarboxylase and tryptophan hydroxylase 1, respectively. Calcium ionophore activation resulted in serotonin/histamine release both from wild-type and serglycin-/- BMMCs. Interestingly, serotonin release was induced in cells lacking intracellular stores of serotonin, suggesting de novo synthesis. The knockout of serglycin affected the levels of stored and released mast cell serotonin and histamine to an even larger extent in in vivo-derived mast cells than in BMMCs. CONCLUSION These results establish a previously assumed, but not proven, role of serglycin in storage of histamine and, further, establish for the first time that serotonin storage in mast cells is dependent on serglycin proteoglycan.

Mouse Mast Cell Protease 4 Is the Major Chymase in Murine Airways and Has a Protective Role in Allergic Airway Inflammation

It is widely established that mast cells (MCs) have a harmful role in asthma, for example by secreting various proinflammatory substances stored within their secretory granule. However, in this study, we show that one of the substances stored within MC granule, chymase, in fact has a protective role in allergic airway inflammation, indicating that MCs may possess both harmful and protective activities in connection with this type of disease. Wild-type (WT) mice and mice lacking mouse MC protease 4 (mMCP-4), a chymase that is functionally homologous to human chymase, were sensitized and challenged with OVA, followed by the assessment of airway physiology and inflammatory parameters. Our results show that the airway hyperresponsiveness was significantly higher in mMCP-4−/− as compared with WT mice. Moreover, the degree of lung tissue inflammation was markedly higher in mice lacking mMCP-4 than in WT controls. Histological analysis revealed that OVA sensitization/challenge resulted in a marked increased in the thickness of the smooth muscle cell (SMC) layer and, notably, that the degree of SMC layer thickening was more pronounced in mMCP-4−/− animals than in WT controls, thus indicating that chymase may have an effect on airway SMCs. In support of this, mMCP-4-positive MCs were located in the close vicinity of the SMC layer, mainly in the upper airways, and mMCP-4 was shown to be the major chymase expressed in these MCs. Taken together, our results indicate that chymase present in the upper airways protects against allergic airway responses, possibly by regulating SMCs.

Mast cell chymase modulates IL-33 levels and controls allergic sensitization in dust-mite induced airway inflammation

Mast cells (MCs) are major effector cells contributing to allergic conditions. When activated, they can release large amounts of active proteases, including chymase from their secretory granules. Here we assessed the role of the chymase mouse mast cell protease 4 (mMCP-4) in allergic airway inflammation induced by house-dust mite (HDM) extract. mMCP-4−/− mice demonstrated elevated airway reactivity and eosinophilia compared with wild-type (WT) animals, suggesting a protective role for mMCP-4 during the late inflammatory phase of the disease. However, mMCP-4 also contributed to the sensitization phase, as indicated by higher levels of serum immunoglobulin E in mMCP-4−/− vs. WT mice and higher levels of cytokines secreted by HDM-restimulated mMCP-4−/− vs. WT splenocytes. In line with a contribution of mMCP-4 in the early stages of disease, HDM extract directly induced chymase secretion from MCs. The elevated airway and inflammatory responses of mMCP-4−/− mice were associated with a profound increase in the levels of interleukin (IL)-33 in the lung tissue. Moreover, WT MCs degraded IL-33 more efficiently than did MCs lacking mMCP-4. Together, our findings identify a protective role of a MC chymase in a physiologically relevant model for airway inflammation and suggest that chymase-mediated regulation of IL-33 can account for this protective function.

Novel insights into the biological function of mast cell carboxypeptidase A

When mast cells are activated they can respond by releasing their secretory granule compounds, including mast cell-specific proteases of chymase, tryptase and carboxypeptidase A (MC-CPA) type. MC-CPA is a dominant protein component of the mast cell granule and the MC-CPA gene is extremely highly expressed. Despite this, relatively little has been known of its biological function. However, the recent generation of mouse strains lacking MC-CPA has opened up new possibilities for investigations related to this protease. This recent development has revealed a role for MC-CPA in regulating innate immunity responses, including the degradation of harmful substances such as the vasoconstrictive factor endothelin 1 and snake venom toxins. Here, we summarize the current knowledge of MC-CPA.

Restoration of the Antibody Response to IgE/Antigen Complexes in CD23-Deficient Mice by CD23+ Spleen or Bone Marrow Cells

Mice immunized with IgE/Ag complexes produce significantly more Ag-specific Abs than mice immunized with Ag alone. The enhancement is mediated via the low-affinity receptor for IgE (FcεRII or CD23), as shown by its complete absence in mice pretreated with mAbs specific for CD23 and in CD23-deficient mice. Because the constitutive expression of murine CD23 is limited to B cells and follicular dendritic cells (FDCs), one of these cell types is likely to be involved. One of the suggested modes of action of IgE/CD23 is to increase the ability of B cells to present Ag to T cells, as demonstrated to take place in vitro. Another possibility is that FDCs capture the IgE/Ag complexes and present these directly to B cells. The purpose of the present study was to determine whether CD23+ B cells or FDCs are responsible for the IgE/CD23-mediated enhancement of specific Ab responses in vivo. We show that the enhancement is completely restored in irradiated CD23-deficient mice reconstituted with CD23+ spleen or bone marrow cells. In these mice, the B cells are CD23+ and the FDCs are presumably CD23− because the FDCs are radiation resistant and are reported not to be replaced by donor cells after this type of cell transfer. In contrast, enhancement was not restored in irradiated wild-type mice reconstituted with CD23− cells. These results indicate that CD23+ B cells, and not FDCs, are the cells that capture IgE/Ag complexes and induce enhancement of Ab responses in vivo.

Serglycin proteoglycan: Regulating the storage and activities of hematopoietic proteases

Serglycin (SG), like all other proteoglycans, consists of a protein “core” to which sulfated and thereby negatively charged polysaccharide chains of glycosaminoglycan type are attached. The recent generation of mice lacking a functional SG gene has revealed a number of biological functions of SG. In particular, it has been shown that SG has a key role in promoting the storage and in regulating the activities of a number of proteases expressed in hematopoietic cell types, most notably various mast cell proteases. In this review, we summarize the recent development in our understanding of the biological function of SG, in particular by focusing on the novel insight provided through analysis of the SG‐deficient mouse strain.

Cytokines as Immunological Markers for Systemic Inflammation in Dogs with Pyometra

Pyometra is a disease in dogs caused by bacterial infection of the uterus and resulting in SIRS (systemic inflammatory response syndrome) in nearly 6 of 10 cases. Clinical diagnostic criteria for SIRS are relatively unspecific, and biomarkers for the diagnosis of pyometra and SIRS in dogs are needed. Serum samples from 32 dogs were used in this study and grouped into dogs with pyometra and SIRS, dogs with pyometra without SIRS and healthy controls. The serum concentrations of IFN-γ, IL-4, IL-6, IL-7, IL-8, IL-10, IL-15, IL-18 and TNF-α were measured using multiplex analyses. The serum concentrations of CRP (C-reactive protein) were determined using sandwich ELISA. IL-7, IL-8, IL-15, IL-18 and TNF-α were detected in >94% of samples. IL-10 was detected in 28% of samples, and IL-4, IL-6 and IFN-γ were undetectable. Higher serum concentrations of IL-7 (p < 0.05) were detected in SIRS-positive dogs with pyometra (n = 13) as compared with healthy controls (n = 11). The concentrations of IL-8 were higher in SIRS-positive dogs with pyometra compared to the SIRS-negative group (n = 8; p < 0.05). Positive correlations of IL-15 with IL-18 (p < 0.0001) and with the concentrations of IL-7 (p < 0.0001 for both) were found, although there was no significant difference between groups. Furthermore, IL-15 correlated with concentrations of CRP (p < 0.05), which were higher in dogs with pyometra compared to controls (p < 0.0001). Our data suggest a role of several cytokines in the development of a systemic disease in dogs with pyometra and a possible diagnostic value for serum CRP, IL-7, IL-15 and IL-18 in canine SIRS caused by pyometra.