Mast cells (MC) are potent innate immune cells that accumulate in chronically inflamed tissues. MC express the IL-33 receptor IL-1 receptor-related protein ST2 at high level, and this IL-1 family cytokine both activates MC directly and primes them to respond to other proinflammatory signals. Whether IL-33 and ST2 play a role in MC survival remains to be defined. In skin-derived human MC, we found that IL-33 attenuated MC apoptosis without altering proliferation, an effect mediated principally through the antiapoptotic molecule B-cell lymphoma-X large (BCLXL). Murine MC demonstrated a similar mechanism, dependent entirely on ST2. In line with these observations, St2−/− mice exhibited reduced numbers of tissue MC in inflamed arthritic joints, in helminthinfected intestine, and in normal peritoneum. Together, these data reveal a cell-intrinsic role for the IL-33/ST2 axis in the regulation of apoptosis in MC, identifying thereby a previously unappreciated pathway supporting expansion of the MC population with inflammation.arthritis | helminth infection M ast cells (MC) are tissue-resident effector cells that contribute both to innate and adaptive immunity (1). MC help defend against bacterial and helminthic infection (2, 3) and play a role in tissue remodeling (4, 5). MC can also contribute to a variety of allergic and nonallergic inflammatory diseases, such as anaphylaxis, atopic dermatitis, asthma, inflammatory arthritis, psoriasis, and multiple sclerosis (6, 7). Under many of these conditions, the number of MC in affected tissues increases by tenfold or more, amplifying the contribution of MC-derived mediators to ongoing inflammation (6-8). Regulation of the MC population is poorly understood, and therapeutic intervention to limit MC accumulation potentially could attenuate injury associated with inflammatory diseases (9, 10).The survival of mature MC in tissues depends on signals from neighboring cells (11,12). The single most important mediator in this process is stem cell factor (SCF), a master regulator of MC proliferation, differentiation, survival, and activation. Mice with genetic mutations affecting SCF or its receptor Kit have few or no MC in healthy or inflamed tissues, whereas activating mutations of Kit give rise to pathologic mastocytosis (13-15). IL-3 is another well-recognized MC growth factor (16). SCF/Kit and IL-3 interface with several antiapoptotic pathways, up-regulating the antiapoptotic protein B-cell lymphoma-2 (BCL-2) and downregulating the proapoptotic protein Bim (14,(17)(18)(19). Aside from SCF/Kit and IL-3, other cytokines have been reported to support MC survival, but their effects appear to be restricted to specific MC subtypes. IL-4 promotes survival of intestinal MC via both BCL-2 and B-cell lymphoma-X large (BCLXL) but may suppress survival in human cord blood MC as well as murine bone marrowderived MC (BMMC) (20-23). IL-10 can either promote or impair survival depending on the type of MC studied (21,22).Recently, substantial attention has focused on the role of IL-33 in MC biolog...

Monocytes are derived from hematopoietic stem cells through a series of intermediate progenitor stages, but the factors that regulate this process are incompletely defined. Using a Ccr2/Cx3cr1 dual-reporter system to model murine monocyte ontogeny, we conducted a small-molecule screen that identified an essential role of mechanistic target of rapamycin complex 1 (mTORC1) in the development of monocytes and other myeloid cells. Confirmatory studies using mice with inducible deletion of the mTORC1 component Raptor demonstrated absence of mature circulating monocytes, as well as disruption in neutrophil and dendritic cell development, reflecting arrest of terminal differentiation at the granulocyte-monocyte progenitor stage. Conversely, excess activation of mTORC1 through deletion of the mTORC1 inhibitor tuberous sclerosis complex 2 promoted spontaneous myeloid cell development and maturation. Inhibitor studies and stage-specific expression profiling identified failure to down-regulate the transcription factor Myc by the mTORC1 target ribosomal S6 kinase 1 (S6K1) as the mechanistic basis for disrupted myelopoiesis. Together, these findings define the mTORC1-S6K1-Myc pathway as a key checkpoint in terminal myeloid development.

Background/Purpose: Traditionally, immunologic memory was thought to be maintained by populations of central (TCM) and effector (TEM) memory lymphocytes that circulate in the blood and lymphatics, only temporarily extravagating into peripheral tissue to execute immunologic responses. Recently, this theory of adaptive memory has been challenged by the discovery of long‐lived and stable populations of tissue‐resident memory T cells (TRM). While TRM have been implicated in the pathogenesis of skin, intestinal, and lung inflammatory diseases, little information is available about TRM in synovium. The study of TRM in arthritis has been impaired by the scarcity of synovium samples coupled with the meager yield of lymphocytes from this tissue using conventional tissue digestion protocols. We have employed novel culturing techniques, previously used to examine TRM in skin, to further characterize TRM in inflammatory arthritis. Methods: Discard synovium samples were collected from patients with rheumatoid arthritis (RA) who were undergoing joint replacement surgery. A 3‐dimensional explant method was used to isolated synovial T cells. Synovial tissue was cut into pieces with scissors, loaded on titanium matrices, and cultured in 24 well plates with T cell media enriched with IL‐2, 15. After 3 weeks, T cells were harvested. Cell surface markers were studied with flow cytometry. Intracellular cytokine expression was assessed by flow cytometry after T cells were stimulated, fixed, and permeabilized, while Foxp3 expression was evaluated following fixation and permeabilization. Results: Seven RA synovial samples were obtained from 5 women and 2 men. All patients were receiving immunomodulatory medications (prednisone n = 5, methotrexate n = 2, tumor necrosis factor inhibitors n = 1). Synovial samples were small, ranging from 0.5 mm2 to 2 mm2; yet, a substantial number of mononuclear cells were harvested (mean: 7.5 × 106). The isolated lymphocytes were phenotypically similar to prior published reports of synovial T cell populations. The majority of CD3+ lymphocytes were CD4+ (Mean ± SEM: 73.5 ± 7.1%) compared to CD8+ cells (25.5 ± 7.3%). In the CD4+ subset, interferon gamma expression was common (39.5 ± 9.0%), IL‐17+ expression was rare (range: 0.1 to 3.4%), and T regulatory cells (CD4+CD25+Foxp3+) were present (8.9 ± 3.2%). A memory phenotype characterized by CD45RO+ expression predominated in both CD4+ (94.5 ± 2.4%) and CD8+ (78.0 ± 6.8%) lymphocytes. A TRM population that lacked the TCM cell surface markers CCR7+ and CD62L+ was identified in CD4+ (47.5 + 10.6%) and CD8+ (36.8 ± 8.1%) cells. Conclusion: The 3‐dimensional explant culturing method is a novel tool, which can be employed to study lymphocytes that are resident in synovium. We confirmed that the T cells isolated through this technique were phenotypically similar to populations of RA synovial lymphocytes previously assessed by immunohistochemistry and tissue digestion. A population of TRM was identified in all synovial samples, suggesting these T cells may be import...