Chemical reprogramming takes the fast lane.

Small molecule-induced cell fate transitions are characterized by low efficiency and slow kinetics. An optimized chemical reprogramming approach now facilitates the robust and rapid conversion of somatic cells to pluripotent stem cells, unlocking exciting avenues to study and manipulate human cell identity.

Active DNA demethylation promotes cell fate specification and the DNA damage response.

Neurons harbor high levels of single-strand DNA breaks (SSBs) that are targeted to neuronal enhancers, but the source of this endogenous damage remains unclear. Using two systems of postmitotic lineage specification-induced pluripotent stem cell-derived neurons and transdifferentiated macrophages-we show that thymidine DNA glycosylase (TDG)-driven excision of methylcytosines oxidized with ten-eleven translocation enzymes (TET) is a source of SSBs. Although macrophage differentiation favors short-patch base excision repair to fill in single-nucleotide gaps, neurons also frequently use the long-patch subpathway. Disrupting this gap-filling process using anti-neoplastic cytosine analogs triggers a DNA damage response and neuronal cell death, which is dependent on TDG. Thus, TET-mediated active DNA demethylation promotes endogenous DNA damage, a process that normally safeguards cell identity but can also provoke neurotoxicity after anticancer treatments.

Protection of Quiescence and Longevity of IgG Memory B Cells by Mitochondrial Autophagy.

The development of long-lived immune memory cells against pathogens is critical for the success of vaccines to establish protection against future infections. However, the mechanisms governing the long-term survival of immune memory cells remain to be elucidated. In this article, we show that the maintenance mitochondrial homeostasis by autophagy is critical for restricting metabolic functions to protect IgG memory B cell survival. Knockout of mitochondrial autophagy genes, Nix and Bnip3, leads to mitochondrial accumulation and increases in oxidative phosphorylation and fatty acid synthesis, resulting in the loss of IgG+ memory B cells in mice. Inhibiting fatty acid synthesis or silencing necroptosis gene Ripk3 rescued Nix-/-Bnip3-/- IgG memory B cells, indicating that mitochondrial autophagy is important for limiting metabolic functions to prevent cell death. Our results suggest a critical role for mitochondrial autophagy in the maintenance of immunological memory by protecting the metabolic quiescence and longevity of memory B cells.

Maintenance of Germinal Center B Cells by Caspase-9 through Promotion of Apoptosis and Inhibition of Necroptosis.

In response to T cell-dependent Ag encounter, naive B cells develop into germinal center (GC) B cells, which can further differentiate into Ab-secreting plasma cells or memory B cells. GC B cells are short lived and are prone to caspase-mediated apoptosis. However, how apoptotic caspases regulate GC B cell fate has not been fully characterized. In this study, we show that mice with B cell-specific knockout of caspase-9 had decreases in GC B cells and Ab production after immunization. Caspase-9-deficient B cells displayed defects in caspase-dependent apoptosis but increases in necroptosis signaling. Additional deletion of Ripk3 restored GC B cells and Ab production in mice with B cell-specific knockout of caspase-9. Our results indicate that caspase-9 plays an important role in the maintenance of Ab responses by promoting apoptosis and inhibiting necroptosis in B cells.

Resistance to natural killer cell immunosurveillance confers a selective advantage to polyclonal metastasis.

Polyclonal metastases frequently arise from clusters of circulating tumor cells (CTCs). CTC clusters metastasize better than single CTCs, but the underlying molecular mechanisms are poorly understood. Here, we show that polyclonal metastatic seeds exhibit higher resistance to natural killer (NK) cell killing. Using breast cancer models, we observed higher proportions of polyclonal lung metastasis in immunocompetent mice compared with mice lacking NK cells. Depleting NK cells selectively increased monoclonal but not polyclonal metastases, suggesting that CTC clusters are less sensitive to NK-mediated suppression. Transcriptional analyses revealed that clusters have elevated expression of cell-cell adhesion and epithelial genes, which is associated with decreased expression of NK cell activating ligands. Furthermore, perturbing tumor cell epithelial status altered NK ligand expression and sensitivity to NK-mediated killing. Collectively, our findings show that NK cells can determine the fate of CTCs of different epithelial and mesenchymal states, and impact metastatic clonal evolution by favoring polyclonal seeding.

Regulation of B cell fate, survival, and function by mitochondria and autophagy.

B cells are responsible for protective antibody production after differentiation into antibody-secreting cells during humoral immune responses. From early B cell development in the bone marrow, to their maturation in the periphery, activation in the germinal center, and differentiation into plasma cells or memory B cells, B cells display ever-changing functions and properties. Autophagy and mitochondria play important roles in B cell development, activation, and differentiation to accommodate the phenotypic and environmental changes encountered over the lifetime of the cell. Among their many functions, mitochondria and autophagy generate energy, mediate cell survival, and produce/eliminate reactive oxygen species that can serve as signal molecules to regulate differentiation. As B cells mature and differentiate into plasma or memory cells, both autophagic and mitochondrial functions undergo significant changes. In this review, we aim to provide an overview of the role of the autophagosome and mitochondria in regulating B cell fate, survival, and function. Moreover, we will discuss the interplay between these two highly metabolic organelles during B cell development, maturation, and differentiation.

Requirement for autophagy in the long-term persistence but not initial formation of memory B cells.

Autophagy is required for the long-term maintenance of Ag-specific memory B cells. However, whether autophagy is also important for the initial formation of memory B cells remains unclear. In this study, we show that newly generated memory B cells do not display active autophagy but are capable of forming Ab-secreting cells after rechallenge with Ags. Increases in autophagy took place over time after the initial formation of memory B cells. The expression of transcription factors involved in autophagy, but not changes in epigenetic regulation by DNA methylation, was required for autophagy gene expression and the development of active autophagy in memory B cells. This indicates that autophagy is not critical for the initial generation of memory B cells but is required for their long-term persistence. Our results suggest that promoting autophagy to improve Ab-dependent immunological memory is more effective during memory B cell maintenance stage.