Molecular patterns of resistance to immune checkpoint blockade in melanoma.

Immune checkpoint blockade (ICB) has improved outcome for patients with metastatic melanoma but not all benefit from treatment. Several immune- and tumor intrinsic features are associated with clinical response at baseline. However, we need to further understand the molecular changes occurring during development of ICB resistance.

Enhanced protein synthesis is a defining requirement for neonatal B cell development.

The LIN28B RNA binding protein exhibits an ontogenically restricted expression pattern and is a key molecular regulator of fetal and neonatal B lymphopoiesis. It enhances the positive selection of CD5+ immature B cells early in life through amplifying the CD19/PI3K/c-MYC pathway and is sufficient to reinitiate self-reactive B-1a cell output when ectopically expressed in the adult. In this study, interactome analysis in primary B cell precursors showed direct binding by LIN28B to numerous ribosomal protein transcripts, consistent with a regulatory role in cellular protein synthesis.

A fetal tumor suppressor axis abrogates MLL-fusion-driven acute myeloid leukemia.

MLL-rearrangements (MLL-r) are recurrent genetic events in acute myeloid leukemia (AML) and frequently associate with poor prognosis. In infants, MLL-r can be sufficient to drive transformation. However, despite the prenatal origin of MLL-r in these patients, congenital leukemia is very rare with transformation usually occurring postnatally.

A self-sustaining layer of early-life-origin B cells drives steady-state IgA responses in the adult gut.

The adult immune system consists of cells that emerged at various times during ontogeny. We aimed to define the relationship between developmental origin and composition of the adult B cell pool during unperturbed hematopoiesis. Lineage tracing stratified murine adult B cells based on the timing of output, revealing that a substantial portion originated within a restricted neonatal window.

Developmental cues license megakaryocyte priming in murine hematopoietic stem cells.

The fetal-to-adult switch in hematopoietic stem cell (HSC) behavior is characterized by alterations in lineage output and entry into deep quiescence. Here we identify the emergence of megakaryocyte (Mk)-biased HSCs as an event coinciding with this developmental switch. Single-cell chromatin accessibility analysis reveals a ubiquitous acquisition of Mk lineage priming signatures in HSCs during the fetal-to-adult transition.

Developmental changes in the rules for B cell selection.

The autoimmune checkpoint during B cell maturation eliminates self-antigen reactive specificities from the mature B cell repertoire. However, an exception to this rule is illustrated by B-1 cells, an innate-like self-reactive B cell subset that is positively selected into the mature B cell pool in a self-antigen-driven fashion. The mechanisms by which B-1 cells escape central tolerance have puzzled the field for decades.

B-1 Cells Carry the Memory of Neonatal Immune Imprinting.

Group A Streptococcus is a common pathogen that elicits a protective humoral response against the cell wall component GlcNAc. In this issue of Immunity, New et al. demonstrate the ability of long-lived B-1 cells to be programmed by microbial colonization and early life immunization to uniquely incorporate GlcNAc reactivity in mice, establishing their critical role in mediating neonatal immune imprinting.

Lin28b controls a neonatal to adult switch in B cell positive selection.

The ability of B-1 cells to become positively selected into the mature B cell pool, despite being weakly self-reactive, has puzzled the field since its initial discovery. Here, we explore changes in B cell positive selection as a function of developmental time by exploiting a link between CD5 surface levels and the natural occurrence of self-reactive B cell receptors (BCRs) in BCR wild-type mice. We show that the heterochronic RNA binding protein Lin28b potentiates a neonatal mode of B cell selection characterized by enhanced overall positive selection in general and the developmental progression of CD5 immature B cells in particular.

Direct role of FLT3 in regulation of early lymphoid progenitors.

Given that FLT3 expression is highly restricted on lymphoid progenitors, it is possible that the established role of FLT3 in the regulation of B and T lymphopoiesis reflects its high expression and role in regulation of lymphoid-primed multipotent progenitors (LMPPs) or common lymphoid progenitors (CLPs). We generated a Flt3 conditional knock-out (Flt3) mouse model to address the direct role of FLT3 in regulation of lymphoid-restricted progenitors, subsequent to turning on Rag1 expression, as well as potentially ontogeny-specific roles in B and T lymphopoiesis. Our studies establish a prominent and direct role of FLT3, independently of the established role of FLT3 in regulation of LMPPs and CLPs, in regulation of fetal as well as adult early B cell progenitors, and the early thymic progenitors (ETPs) in adult mice but not in the fetus.

Murine HSCs contribute actively to native hematopoiesis but with reduced differentiation capacity upon aging.

A hallmark of adult hematopoiesis is the continuous replacement of blood cells with limited lifespans. While active hematopoietic stem cell (HSC) contribution to multilineage hematopoiesis is the foundation of clinical HSC transplantation, recent reports have questioned the physiological contribution of HSCs to normal/steady-state adult hematopoiesis. Here, we use inducible lineage tracing from genetically marked adult HSCs and reveal robust HSC-derived multilineage hematopoiesis.

Comprehensive Proteomic Characterization of Ontogenic Changes in Hematopoietic Stem and Progenitor Cells.

Hematopoietic stem and progenitor cells (HSPCs) in the fetus and adult possess distinct molecular landscapes that regulate cell fate and change their susceptibility to initiation and progression of hematopoietic malignancies. Here, we applied in-depth quantitative proteomics to comprehensively describe and compare the proteome of fetal and adult HSPCs. Our data uncover a striking difference in complexity of the cellular proteomes, with more diverse adult-specific HSPC proteomic signatures.

PTIP chromatin regulator controls development and activation of B cell subsets to license humoral immunity in mice.

B cell receptor signaling and downstream NF-κB activity are crucial for the maturation and functionality of all major B cell subsets, yet the molecular players in these signaling events are not fully understood. Here we use several genetically modified mouse models to demonstrate that expression of the multifunctional BRCT (BRCA1 C-terminal) domain-containing PTIP (Pax transactivation domain-interacting protein) chromatin regulator is controlled by B cell activation and potentiates steady-state and postimmune antibody production in vivo. By examining the effects of PTIP deficiency in mice at various ages during ontogeny, we demonstrate that PTIP promotes bone marrow B cell development as well as the neonatal establishment and subsequent long-term maintenance of self-reactive B-1 B cells.

Lentiviral Barcode Labeling and Transplantation of Fetal Liver Hematopoietic Stem and Progenitor Cells.

Cellular barcoding enables the dissection of clonal dynamics in heterogeneous cell populations through single cell lineage tracing. The labeling of hematopoietic stem and progenitor cells (HSPCs) with unique and heritable DNA barcodes, makes it possible to resolve donor cell heterogeneity in terms of differentiation potential and lineage bias at the single cell level, through subsequent transplantation and high-throughput sequencing. Furthermore, cellular barcoding allows for bona fide hematopoietic stem cells (HSCs) to be defined based on functional rather than immunophenotypic parameters.

Clonal reversal of ageing-associated stem cell lineage bias via a pluripotent intermediate.

Ageing associates with significant alterations in somatic/adult stem cells and therapies to counteract these might have profound benefits for health. In the blood, haematopoietic stem cell (HSC) ageing is linked to several functional shortcomings. However, besides the recent realization that individual HSCs might be preset differentially already from young age, HSCs might also age asynchronously.

Fetal to Adult Hematopoiesis with the "Flk" of a Switch.

Blood development relies on discrete stem and progenitor cell populations with unclear lineage relationships and distinct functional characteristics that change during ontogeny. In this issue of Cell Stem Cell, Beaudin et al. (2016) identify a hematopoietic stem cell population with fetal characteristics that is developmentally restricted yet capable of long-term multi-lineage reconstitution upon transplantation into adult recipients.

Cellular Barcoding Links B-1a B Cell Potential to a Fetal Hematopoietic Stem Cell State at the Single-Cell Level.

Hematopoietic stem cells (HSCs) undergo a functional switch in neonatal mice hallmarked by a decrease in self-renewing divisions and entry into quiescence. Here, we investigated whether the developmental attenuation of B-1a cell output is a consequence of a shift in stem cell state during ontogeny. Using cellular barcoding for in vivo single-cell fate analyses, we found that fetal liver definitive HSCs gave rise to both B-1a and B-2 cells.

A PTIP-PA1 subcomplex promotes transcription for IgH class switching independently from the associated MLL3/MLL4 methyltransferase complex.

Class switch recombination (CSR) diversifies antibodies for productive immune responses while maintaining stability of the B-cell genome. Transcription at the immunoglobulin heavy chain (Igh) locus targets CSR-associated DNA damage and is promoted by the BRCT domain-containing PTIP (Pax transactivation domain-interacting protein). Although PTIP is a unique component of the mixed-lineage leukemia 3 (MLL3)/MLL4 chromatin-modifying complex, the mechanisms for how PTIP promotes transcription remain unclear.

LIN28B-mediated expression of fetal hemoglobin and production of fetal-like erythrocytes from adult human erythroblasts ex vivo.

Reactivation of fetal hemoglobin (HbF) holds therapeutic potential for sickle cell disease and β-thalassemias. In human erythroid cells and hematopoietic organs, LIN28B and its targeted let-7 microRNA family, demonstrate regulated expression during the fetal-to-adult developmental transition. To explore the effects of LIN28B in human erythroid cell development, lentiviral transduction was used to knockdown LIN28B expression in erythroblasts cultured from human umbilical cord CD34+ cells.

Exploring the RNA world in hematopoietic cells through the lens of RNA-binding proteins.

The discovery of microRNAs has renewed interest in posttranscriptional modes of regulation, fueling an emerging view of a rich RNA world within our cells that deserves further exploration. Much work has gone into elucidating genetic regulatory networks that orchestrate gene expression programs and direct cell fate decisions in the hematopoietic system. However, the focus has been to elucidate signaling pathways and transcriptional programs.

Lin28b reprograms adult bone marrow hematopoietic progenitors to mediate fetal-like lymphopoiesis.

The immune system develops in waves during ontogeny; it is initially populated by cells generated from fetal hematopoietic stem cells (HSCs) and later by cells derived from adult HSCs. Remarkably, the genetic programs that control these two distinct stem cell fates remain poorly understood. We report that Lin28b is specifically expressed in mouse and human fetal liver and thymus, but not in adult bone marrow or thymus.

The transcription factor c-Myb primes CD4+CD8+ immature thymocytes for selection into the iNKT lineage.

Type I invariant NKT cells (iNKT cells) are a subset of alphabeta T cells characterized by the expression of an invariant alpha-chain variable region 14-alpha-chain joining region 18 (V(alpha)14J(alpha)18) T cell antigen receptor (TCR) alpha-chain. The iNKT cells derive from CD4(+)CD8(+) double-positive (DP) thymocytes, and their generation requires a long half-life of DP thymocytes to allow V(alpha)14-J(alpha)18 rearrangements, expression of glycolipid-loaded CD1d on DP thymocytes, and signaling through the signaling-activation molecule SLAM-adaptor SAP pathway. Here we show that the transcription factor c-Myb has a central role in priming DP thymocytes to enter the iNKT lineage by simultaneously regulating CD1d expression, the half-life of DP cells and expression of SLAMF1, SLAMF6 and SAP.

c-Myb promotes the survival of CD4+CD8+ double-positive thymocytes through upregulation of Bcl-xL.

Mechanisms that regulate the lifespan of CD4(+)CD8(+) double-positive (DP) thymocytes help shape the peripheral T cell repertoire. However, the molecular mechanisms controlling DP thymocyte survival remain poorly understood. The Myb proto-oncogene encodes a transcription factor required during multiple stages of T cell development.