Serine enrichment in tumors promotes regulatory T cell accumulation through sphinganine-mediated regulation of c-Fos.

CD4+ regulatory T (Treg) cells accumulate in the tumor microenvironment (TME) and suppress the immune system. Whether and how metabolite availability in the TME influences Treg cell differentiation is not understood. Here, we measured 630 metabolites in the TME and found that serine and palmitic acid, substrates required for the synthesis of sphingolipids, were enriched. A serine-free diet or a deficiency in Sptlc2, the rate-limiting enzyme catalyzing sphingolipid synthesis, suppressed Treg cell accumulation and inhibited tumor growth. Sphinganine, an intermediate metabolite in sphingolipid synthesis, physically interacted with the transcription factor c-Fos. Sphinganine c-Fos interactions enhanced the genome-wide recruitment of c-Fos to regions near the transcription start sites of target genes including Pdcd1 (encoding PD-1), which promoted Pdcd1 transcription and increased inducible Treg cell differentiation in vitro in a PD-1-dependent manner. Thus, Sptlc2-mediated sphingolipid synthesis translates the extracellular information of metabolite availability into nuclear signals for Treg cell differentiation and limits antitumor immunity.

Mast cells link immune sensing to antigen-avoidance behaviour.

The physiological functions of mast cells remain largely an enigma. In the context of barrier damage, mast cells are integrated in type 2 immunity and, together with immunoglobulin E (IgE), promote allergic diseases. Allergic symptoms may, however, facilitate expulsion of allergens, toxins and parasites and trigger future antigen avoidance1-3. Here, we show that antigen-specific avoidance behaviour in inbred mice4,5 is critically dependent on mast cells; hence, we identify the immunological sensor cell linking antigen recognition to avoidance behaviour. Avoidance prevented antigen-driven adaptive, innate and mucosal immune activation and inflammation in the stomach and small intestine. Avoidance was IgE dependent, promoted by Th2 cytokines in the immunization phase and by IgE in the execution phase. Mucosal mast cells lining the stomach and small intestine rapidly sensed antigen ingestion. We interrogated potential signalling routes between mast cells and the brain using mutant mice, pharmacological inhibition, neural activity recordings and vagotomy. Inhibition of leukotriene synthesis impaired avoidance, but overall no single pathway interruption completely abrogated avoidance, indicating complex regulation. Collectively, the stage for antigen avoidance is set when adaptive immunity equips mast cells with IgE as a telltale of past immune responses. On subsequent antigen ingestion, mast cells signal termination of antigen intake. Prevention of immunopathology-causing, continuous and futile responses against per se innocuous antigens or of repeated ingestion of toxins through mast-cell-mediated antigen-avoidance behaviour may be an important arm of immunity.

Flt3- and Tie2-Cre tracing identifies regeneration in sepsis from multipotent progenitors but not hematopoietic stem cells.

In response to infections and stress, hematopoiesis rapidly enhances blood and immune cell production. The stage within the hematopoietic hierarchy that accounts for this regeneration is unclear under natural conditions in vivo. We analyzed by differentiation tracing, using inducible Tie2- or Flt3-driven Cre recombinase, the roles of mouse hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs). During polymicrobial sepsis, HSCs responded transcriptionally and increased their proliferation and cell death, yet HSC differentiation rates remained at steady-state levels. HSC differentiation was also independent from the ablation of various cellular compartments-bleeding, the antibody-mediated ablation of granulocytes or B lymphocytes, and genetic lymphocyte deficiency. By marked contrast, the fate mapping of MPPs in polymicrobial sepsis identified these cells as a major source for accelerated myeloid cell production. The regulation of blood and immune cell homeostasis by progenitors rather than stem cells may ensure a rapid response while preserving the integrity of the HSC population.

Usp22 is an intracellular regulator of systemic emergency hematopoiesis.

Emergency hematopoiesis is a concerted response aimed toward enhanced protection from infection, involving multiple cell types and developmental stages across the immune system. Despite its importance, the underlying molecular regulation remains poorly understood. The deubiquitinase USP22 regulates the levels of monoubiquitinated histone H2B (H2Bub1), which is associated with activation of interferon responses upon viral infection. Here, we show that in the absence of infection or inflammation, mice lacking Usp22 in all hematopoietic cells display profound systemic emergency hematopoiesis, evident by increased hematopoietic stem cell proliferation, myeloid bias, and extramedullary hematopoiesis. Functionally, loss of Usp22 results in elevated phagocytosis by neutrophilic granulocytes and enhanced innate protection against Listeria monocytogenes infection. At the molecular level, we found this state of emergency hematopoiesis associated with transcriptional signatures of myeloid priming, enhanced mitochondrial respiration, and innate and adaptive immunity and inflammation. Augmented expression of many inflammatory genes was linked to elevated locus-specific H2Bub1 levels. Collectively, these results demonstrate the existence of a tunable epigenetic state that promotes systemic emergency hematopoiesis in a cell-autonomous manner to enhance innate protection, identifying potential paths toward immune enhancement.

Toward the dissection of hematopoietic stem cell fates and their determinants.

Hematopoietic stem cell (HSC) functions have long been difficult to study under physiological conditions. Recently, genetic in vivo approaches have been developed for lineage tracing of differentiating progeny emerging from HSC over time (output), and for high-resolution, endogenous barcoding to uncover the lineages that HSC contribute to (fate). Such fate measurements have in principle led to the recognition of three major fate groups of HSC: multilineage, myelo-erythroid-restricted, and inactive, that is, no or no known progeny, in addition to a minor group of megakaryocyte-restricted HSC. The most recent RNA-barcoding experiments have begun to directly link fate measurements with transcriptome reading in HSC clones and single HSC, which yielded insights into transcriptional signatures associated with fate patterns. Here, we discuss these findings in light of the structure of the hematopoietic differentiation hierarchy, and we provide an outlook on strategies to dissect molecular determinants of HSC fates.

Resolving Fates and Single-Cell Transcriptomes of Hematopoietic Stem Cell Clones by PolyloxExpress Barcoding.

Lineage tracing reveals hematopoietic stem cell (HSC) fates, while single-cell RNA sequencing identifies snapshots of HSC transcriptomes. To obtain information on fate plus transcriptome in the same cell, we developed the PolyloxExpress allele, enabling Cre-recombinase-dependent RNA barcoding in situ. Linking fates to single HSC transcriptomes provided the information required to identify transcriptional signatures of HSC fates, which were not apparent in single-HSC transcriptomes alone. We find that differentiation-inactive, multilineage, and lineage-restricted HSC clones reside in distinct regions of the transcriptional landscape of hematopoiesis. Differentiation-inactive HSC clones are closer to the origin of the transcriptional trajectory, yet they are not characterized by a quiescent gene signature. Fate-specific gene signatures imply coherence of clonal HSC fates, and HSC output toward short-lived lineage progenitors indicates stability of HSC fates over time. These combined analyses of fate and transcriptome under physiological conditions may pave the way toward identifying molecular determinants of HSC fates.