Promiscuous gene expression patterns in single medullary thymic epithelial cells argue for a stochastic mechanism.

Promiscuous expression of tissue-restricted autoantigens in medullary thymic epithelial cells (mTECs) imposes central T cell tolerance. The molecular regulation of this unusual gene expression is not understood, in particular its delineation from cell lineage-specific gene expression control remains unclear. Here, we compared the expression profile of the casein gene locus in mTECs and mammary gland epithelial cells by single cell PCR. Mammary gland cells showed highly correlated intra- and interchromosomal coexpression of milk proteins (the casein genes, lactalbumin-alpha and whey acidic protein) and one of its transcriptional regulators (Elf5). In contrast, coexpression of these genes in mature CD80(hi) mTECs was rarely observed and no pattern of gene expression in individual mTECs was discernible. The apparent stochastic expression pattern of genes within the casein locus, the lower mRNA levels compared with mammary gland cells in conjunction with frequent coexpression of insulin in single mTECs clearly delineates the molecular mechanism(s) of promiscuous gene expression from cell lineage-specific gene control.

Common clonal origin of conventional T cells and induced regulatory T cells in breast cancer patients.

Regulatory CD4+ T cells (Treg) prevent tumor clearance by conventional T cells (Tconv) comprising a major obstacle of cancer immune-surveillance. Hitherto, the mechanisms of Treg repertoire formation in human cancers remain largely unclear. Here, we analyze Treg clonal origin in breast cancer patients using T-Cell Receptor and single-cell transcriptome sequencing. While Treg in peripheral blood and breast tumors are clonally distinct, Tconv clones, including tumor-antigen reactive effectors (Teff), are detected in both compartments. Tumor-infiltrating CD4+ cells accumulate into distinct transcriptome clusters, including early activated Tconv, uncommitted Teff, Th1 Teff, suppressive Treg and pro-tumorigenic Treg. Trajectory analysis suggests early activated Tconv differentiation either into Th1 Teff or into suppressive and pro-tumorigenic Treg. Importantly, Tconv, activated Tconv and Treg share highly-expanded clones contributing up to 65% of intratumoral Treg. Here we show that Treg in human breast cancer may considerably stem from antigen-experienced Tconv converting into secondary induced Treg through intratumoral activation.

Inflammation-Induced Emergency Megakaryopoiesis Driven by Hematopoietic Stem Cell-like Megakaryocyte Progenitors.

Infections are associated with extensive platelet consumption, representing a high risk for health. However, the mechanism coordinating the rapid regeneration of the platelet pool during such stress conditions remains unclear. Here, we report that the phenotypic hematopoietic stem cell (HSC) compartment contains stem-like megakaryocyte-committed progenitors (SL-MkPs), a cell population that shares many features with multipotent HSCs and serves as a lineage-restricted emergency pool for inflammatory insults. During homeostasis, SL-MkPs are maintained in a primed but quiescent state, thus contributing little to steady-state megakaryopoiesis. Even though lineage-specific megakaryocyte transcripts are expressed, protein synthesis is suppressed. In response to acute inflammation, SL-MkPs become activated, resulting in megakaryocyte protein production from pre-existing transcripts and a maturation of SL-MkPs and other megakaryocyte progenitors. This results in an efficient replenishment of platelets that are lost during inflammatory insult. Thus, our study reveals an emergency machinery that counteracts life-threatening platelet depletions during acute inflammation.

Molecular mechanisms of L-selectin-induced co-localization in rafts and shedding [corrected].

Leukocyte recruitment to lymph nodes or inflammatory sites is regulated by adhesion and activation. L-selectin (CD62L) is expressed on leukocytes and mediates tethering and rolling of leukocytes on endothelial cells. Upon stimulation L-selectin is down-regulated by proteolytic cleavage but the molecular mechanisms regulating this shedding step are poorly defined. To study intracellular mechanisms, we induced shedding of L-selectin by cross-linking with an immobilized L-selectin antibody (Dreg56) in Jurkat cells. The loss of surface expression was quantitated by flow cytometry and the increase of soluble L-selectin was determined by Western blot analysis. We find that Jurkat and p56(lck)-deficient JCaM1.6 cells released L-selectin to similar extent (18+/-4% and 17+/-3%, respectively) and revealed comparable inhibition with the src-tyrosine kinase inhibitor PP2. Glutathione (GSH), an inhibitor of the neutral sphingomyelinase, PD98059, a MAP-kinase (MAP-K) inhibitor and metalloprotease inhibitors (MPI) (TAPI, Ro 31-9790, and BB-3103) reduced significantly L-selectin-induced shedding by 60-80%. In Jurkat cells, L-selectin was present in Triton X-100 insoluble membrane rafts and was constitutively tyr-phosphorylated. Dreg56 cross-linking enhanced phosphorylation and recruitment of L-selectin into rafts which was significantly decreased by pretreatment of cells with PD98059. We conclude, that the metalloproteinase-mediated cleavage of L-selectin from cell surface is triggered by intracellular signaling pathways that are independent of p56(lck) tyrosine kinase activity, but require other tyrosine kinases and the neutral sphingomyelinase. The cleavage of L-selectin might involve membrane rafts as signaling platform.