Mitochondrial respiration in B lymphocytes is essential for humoral immunity by controlling the flux of the TCA cycle

To elucidate the function of oxidative phosphorylation (OxPhos) during B cell differentiation, we employ CD23Cre-driven expression of the dominant-negative K320E mutant of the mitochondrial helicase Twinkle (DNT). DNT-expression depletes mitochondrial DNA during B cell maturation, reduces the abundance of respiratory chain protein subunits encoded by mitochondrial DNA, and, consequently, respiratory chain super-complexes in activated B cells. Whereas B cell development in DNT mice is normal, B cell proliferation, germinal centers, class switch to IgG, plasma cell maturation, and T cell-dependent as well as T cell-independent humoral immunity are diminished. DNT expression dampens OxPhos but increases glycolysis in lipopolysaccharide and B cell receptor-activated cells. Lipopolysaccharide-activated DNT-B cells exhibit altered metabolites of glycolysis, the pentose phosphate pathway, and the tricarboxylic acid cycle and a lower amount of phosphatidic acid. Consequently, mTORC1 activity and BLIMP1 induction are curtailed, whereas HIF1α is stabilized. Hence, mitochondrial DNA controls the metabolism of activated B cells via OxPhos to foster humoral immunity.

Metabolic impairment of non-small cell lung cancers by mitochondrial HSPD1 targeting

Background

The identification of novel targets is of paramount importance to develop more effective drugs and improve the treatment of non-small cell lung cancer (NSCLC), the leading cause of cancer-related deaths worldwide. Since cells alter their metabolic rewiring during tumorigenesis and along cancer progression, targeting key metabolic players and metabolism-associated proteins represents a valuable approach with a high therapeutic potential. Metabolic fitness relies on the functionality of heat shock proteins (HSPs), molecular chaperones that facilitate the correct folding of metabolism enzymes and their assembly in macromolecular structures.

Methods

Gene fitness was determined by bioinformatics analysis from available datasets from genetic screenings. HSPD1 expression was evaluated by immunohistochemistry from formalin-fixed paraffin-embedded tissues from NSCLC patients. Real-time proliferation assays with and without cytotoxicity reagents, colony formation assays and cell cycle analyses were used to monitor growth and drug sensitivity of different NSCLC cells in vitro. In vivo growth was monitored with subcutaneous injections in immune-deficient mice. Cell metabolic activity was analyzed through extracellular metabolic flux analysis. Specific knockouts were introduced by CRISPR/Cas9.

Results

We show heat shock protein family D member 1 (HSPD1 or HSP60) as a survival gene ubiquitously expressed in NSCLC and associated with poor patients’ prognosis. HSPD1 knockdown or its chemical disruption by the small molecule KHS101 induces a drastic breakdown of oxidative phosphorylation, and suppresses cell proliferation both in vitro and in vivo. By combining drug profiling with transcriptomics and through a whole-genome CRISPR/Cas9 screen, we demonstrate that HSPD1-targeted anti-cancer effects are dependent on oxidative phosphorylation and validated molecular determinants of KHS101 sensitivity, in particular, the creatine-transporter SLC6A8 and the subunit of the cytochrome c oxidase complex COX5B.

Conclusions

These results highlight mitochondrial metabolism as an attractive target and HSPD1 as a potential theranostic marker for developing therapies to combat NSCLC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-021-02049-8.

B Cell Speed and B-FDC Contacts in Germinal Centers Determine Plasma Cell Output via Swiprosin-1/EFhd2

Plasma cells secreting affinity-matured antibodies develop in germinal centers (GCs), where B cells migrate persistently and directionally over defined periods of time. How modes of GC B cell migration influence plasma cell development remained unclear. Through genetic deletion of the F-actin bundling protein Swiprosin-1/EF-hand domain family member 2 (EFhd2) and by two-photon microscopy, we show that EFhd2 restrains B cell speed in GCs and hapten-specific plasma cell output. Modeling the GC reaction reveals that increasing GC B cell speed promotes plasma cell generation. Lack of EFhd2 also reduces contacts of GC B cells with follicular dendritic cells in vivo. Computational modeling uncovers that both GC output and antibody affinity depend quantitatively on contacts of GC B cells with follicular dendritic cells when B cells migrate more persistently. Collectively, our data explain how GC B cells integrate speed and persistence of cell migration with B cell receptor affinity.

Proteasomal degradation induced by DPP9-mediated processing competes with mitochondrial protein import

Plasticity of the proteome is critical to adapt to varying conditions. Control of mitochondrial protein import contributes to this plasticity. Here, we identified a pathway that regulates mitochondrial protein import by regulated N-terminal processing. We demonstrate that dipeptidyl peptidases 8/9 (DPP8/9) mediate the N-terminal processing of adenylate kinase 2 (AK2) en route to mitochondria. We show that AK2 is a substrate of the mitochondrial disulfide relay, thus lacking an N-terminal mitochondrial targeting sequence and undergoing comparatively slow import. DPP9-mediated processing of AK2 induces its rapid proteasomal degradation and prevents cytosolic accumulation of enzymatically active AK2. Besides AK2, we identify more than 100 mitochondrial proteins with putative DPP8/9 recognition sites and demonstrate that DPP8/9 influence the cellular levels of a number of these proteins. Collectively, we provide in this study a conceptual framework on how regulated cytosolic processing controls levels of mitochondrial proteins as well as their dual localization to mitochondria and other compartments.

TFG is upregulated during plasma cell differentiation and regulates apoptosis, ER stress and autophagy in a B cell line

Long-term survival and antibody production of plasma cells depend on an intact endoplasmic reticulum (ER) quality control and autophagy. The cytosolic adaptor protein TRK-fused gene (TFG) has been shown to function in ER-Golgi transport, secretion and proteasome regulation. We show here that TFG becomes upregulated by LPS and CpG in primary B1 and B2 cells, and the highest expression of TFG was found in plasma cells.

To address the function of TFG we disrupted the tfg gene in B1-like CH12 lymphoma cells and analyzed mice with heterozygous TFG deletion. TFG KO CH12 cells had higher spontaneous apoptosis rates and were more sensitive to ER stress induced by Tunicamycin, MG132 or an ER-trapped μ heavy chain. Autophagy flux as determined by LC3 turnover, electron microscopy and RFP-GFP-LC3 turnover assays indicated a late suppression of the autophagy pathway in TFG KO CH12 cells. While homozygous deletion of TFG was embryonically lethal, TFG+/− mice had normal B development and germinal center B cells, but significantly less CD138+/TACI+ plasmablast/plasma cells in Peyer’s patches, concomitant with reduced serum IgA.

Based on these data we suggest that TFG is a critical factor of ER homeostasis and autophagy in B cell lines. We propose that TFG counters ER stress by enhancing autophagy and sustain cell survival. Especially plasma cells in Peyer’s patches may critically depend on these TFG-mediated mechanisms.

This work is supported in part by grants TRR130 and GRK1660 from the German Research Foundation to D.M and H.-M.J

Essential respiratory chain subunits encoded by mitochondrial DNA control plasma cell development

The contribution of mitochondrial metabolism to plasma cell development is largely unknown. To define the function of the mitochondrial respiratory chain during B cell activation and differentiation, we used a mouse model in which CD23-Cre drives the expression of a dominant-negative mutant of the mitochondrial helicase Twinkle (DNTwinkle) and the reporter GFP in mature B cells. DNTwinkle expression induces mitochondrial DNA (mtDNA) deletions in dividing cells and results in impaired respiratory chain activity. Flow cytometry in non-immunized DNTwinkle mice did not reveal a difference in either total cell numbers or frequencies of mature B cell subsets. However, only about 8% of the plasma cells in the bone marrow were positive for DNTwinkle/GFP, indicating that these cells have a significant disadvantage in the presence of wildtype plasma cells. T-dependent immunization showed a decreased primary immune response, whereas the secondary immune response was not affected. Strikingly, in immunized mice, all antigen-specific plasma cells in the bone marrow were GFP-negative, again highlighting the advantage of escaper cells. Comparing to in vitro stimulated wild-type B cells, DNTwinkle-positive splenic B cells showed comparable survival, but proliferation, Ig class switch, and plasmablast differentiation were impaired. Also, OxPhos was reduced, but glycolysis and lactate secretion were increased. Finally, AMPK activation was enhanced along with reduced mTOR activity, which could explain the observed impaired Blimp-1 expression and plasmablast differentiation of DNTwinkle B cells. Our results place mitochondrial DNA replication and respiratory chain activity upstream of mTOR and Blimp-1 via AMPK.

Guidelines for the use of flow cytometry and cell sorting in immunological studies (second edition)

These guidelines are a consensus work of a considerable number of members of the immunology and flow cytometry community. They provide the theory and key practical aspects of flow cytometry enabling immunologists to avoid the common errors that often undermine immunological data. Notably, there are comprehensive sections of all major immune cell types with helpful Tables detailing phenotypes in murine and human cells. The latest flow cytometry techniques and applications are also described, featuring examples of the data that can be generated and, importantly, how the data can be analysed. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid, all written and peer-reviewed by leading experts in the field, making this an essential research companion.

Regulation of Energy Metabolism during Early B Lymphocyte Development

The most important feature of humoral immunity is the adaptation of the diversity of newly generated B cell receptors, that is, the antigen receptor repertoire, to the body's own and foreign structures. This includes the transient propagation of B progenitor cells and B cells, which possess receptors that are positively selected via anabolic signalling pathways under highly competitive conditions. The metabolic regulation of early B-cell development thus has important consequences for the expansion of normal or malignant pre-B cell clones. In addition, cellular senescence programs based on the expression of B cell identity factors, such as Pax5, act to prevent excessive proliferation and cellular deviation. Here, we review the basic mechanisms underlying the regulation of glycolysis and oxidative phosphorylation during early B cell development in bone marrow. We focus on the regulation of glycolysis and mitochondrial oxidative phosphorylation at the transition from non-transformed pro- to pre-B cells and discuss some ongoing issues. We introduce Swiprosin-2/EFhd1 as a potential regulator of glycolysis in pro-B cells that has also been linked to Ca2+-mediated mitoflashes. Mitoflashes are bioenergetic mitochondrial events that control mitochondrial metabolism and signalling in both healthy and disease states. We discuss how Ca2+ fluctuations in pro- and pre-B cells may translate into mitoflashes in early B cells and speculate about the consequences of these changes.

A defined metabolic state in pre B cells governs B-cell development and is counterbalanced by Swiprosin-2/EFhd1

B-cell development in the bone marrow comprises proliferative and resting phases in different niches. We asked whether B-cell metabolism relates to these changes. Compared to pro B and small pre B cells, large pre B cells revealed the highest glucose uptake and ROS but not mitochondrial mass, whereas small pre B cells exhibited the lowest mitochondrial membrane potential. Small pre B cells from Rag1^-/-;33.C9 μ heavy chain knock-in mice revealed decreased glycolysis (ECAR) and mitochondrial spare capacity compared to pro B cells from Rag1^-/- mice. We were interested in the step regulating this metabolic switch from pro to pre B cells and uncovered that Swiprosin-2/EFhd1, a Ca²⁺-binding protein of the inner mitochondrial membrane involved in Ca²⁺-induced mitoflashes, is expressed in pro B cells, but downregulated by surface pre B-cell receptor expression. Knockdown and knockout of EFhd1 in 38B9 pro B cells decreased the oxidative phosphorylation/glycolysis (OCR/ECAR) ratio by increasing glycolysis, glycolytic capacity and reserve. Prolonged expression of EFhd1 in EFhd1 transgenic mice beyond the pro B cell stage increased expression of the mitochondrial co-activator PGC-1α in primary pre B cells, but reduced mitochondrial ATP production at the pro to pre B cell transition in IL-7 cultures. Transgenic EFhd1 expression caused a B-cell intrinsic developmental disadvantage for pro and pre B cells. Hence, coordinated expression of EFhd1 in pro B cells and by the pre BCR regulates metabolic changes and pro/pre B-cell development.

Assessing metabolic demands in B cells

Assessing metabolism during early B cell development we show here that large pre B cells are superior in their metabolic activity but small pre B cells down-regulate glycolysis significantly, both ex vivo and in IL-7 cultures. The mitochondrial Ca2+ binding protein Swiprosin-2/EFhd1 (EFhd1) has been proposed to balance mitochondrial Ca2+ and the mitochondrial membrane potential. We show here that EFhd1 is expressed in pro B cells but becomes downregulated by surface expression of the pre BCR. Conversely, EFhd1 becomes up-regulated again in plasma cells. A transgenic mouse model, where EFhd1 cannot be downregulated by the pre BCR anymore (EFhd1tg), revealed that the mitochondrial metabolic regulator PGC-1a is upregulated by EFhd1. This lead to mitochondrial dysfunction in EFhd1tg pre B cells, to a disadvantage for EFhd1tg pro and pre B cells in mixed bone marrow chimeras and to an increased mitochondrial proton leak in EFhd1tg LPS blasts. In accordance, EFhd1 transfected into 293 cells is enriched in the mitochondrial fraction and co-localizes with mitochondria. On the other hand, CRISPR-Cas9 mediated knock-out of EFhd1 in the pro B cell line 38B9 as a surrogate for the pre BCR signal increased glycolysis, glycolytic capacity and reserve, providing a mechanism for increased metabolic activity in large pre B cells. Thus, proper regulation of EFhd1 is important to equilibrate mitochondrial and glycolytic metabolism during B cell differentiation. To gain more insight into metabolic demands and mechanisms during B cell development, activation and differentiation we generated EFhd1KO mice. First experiments with EFhd1KO mice revealed an important function for EFhd1 during B cell development, differentiation and function.