Mitochondrial control of lymphocyte homeostasis.

Mitochondria play a multitude of essential roles within mammalian cells, and understanding how they control immunity is an emerging area of study. Lymphocytes, as integral cellular components of the adaptive immune system, rely on mitochondria for their function, and mitochondria can dynamically instruct their differentiation and activation by undergoing rapid and profound remodelling. Energy homeostasis and ATP production are often considered the primary functions of mitochondria in immune cells; however, their importance extends across a spectrum of other molecular processes, including regulation of redox balance, signalling pathways, and biosynthesis. In this review, we explore the dynamic landscape of mitochondrial homeostasis in T and B cells, and discuss how mitochondrial disorders compromise adaptive immunity.

Dynamic mitochondrial transcription and translation in B cells control germinal center entry and lymphomagenesis.

Germinal center (GC) B cells undergo proliferation at very high rates in a hypoxic microenvironment but the cellular processes driving this are incompletely understood. Here we show that the mitochondria of GC B cells are highly dynamic, with significantly upregulated transcription and translation rates associated with the activity of transcription factor A, mitochondrial (TFAM). TFAM, while also necessary for normal B cell development, is required for entry of activated GC precursor B cells into the germinal center reaction; deletion of Tfam significantly impairs GC formation, function and output. Loss of TFAM in B cells compromises the actin cytoskeleton and impairs cellular motility of GC B cells in response to chemokine signaling, leading to their spatial disorganization. We show that B cell lymphoma substantially increases mitochondrial translation and that deletion of Tfam in B cells is protective against the development of lymphoma in a c-Myc transgenic mouse model. Finally, we show that pharmacological inhibition of mitochondrial transcription and translation inhibits growth of GC-derived human lymphoma cells and induces similar defects in the actin cytoskeleton.

Plasma iron controls neutrophil production and function.

Low plasma iron (hypoferremia) induced by hepcidin is a conserved inflammatory response that protects against infections but inhibits erythropoiesis. How hypoferremia influences leukocytogenesis is unclear. Using proteomic data, we predicted that neutrophil production would be profoundly more iron-demanding than generation of other white blood cell types. Accordingly in mice, hepcidin-mediated hypoferremia substantially reduced numbers of granulocytes but not monocytes, lymphocytes, or dendritic cells. Neutrophil rebound after anti-Gr-1-induced neutropenia was blunted during hypoferremia but was rescued by supplemental iron. Similarly, hypoferremia markedly inhibited pharmacologically stimulated granulopoiesis mediated by granulocyte colony-stimulating factor and inflammation-induced accumulation of neutrophils in the spleen and peritoneal cavity. Furthermore, hypoferremia specifically altered neutrophil effector functions, suppressing antibacterial mechanisms but enhancing mitochondrial reactive oxygen species-dependent NETosis associated with chronic inflammation. Notably, antagonizing endogenous hepcidin during acute inflammation enhanced production of neutrophils. We propose plasma iron modulates the profile of innate immunity by controlling monocyte-to-neutrophil ratio and neutrophil activity in a therapeutically targetable system.

Structural, biochemical, and functional properties of the Rap1-Interacting Adaptor Molecule (RIAM).

Leukocytes, the leading players of immune system, are involved in innate and adaptive immune responses. Leukocyte adhesion to endothelial cells during transmigration or to antigen presenting cells during T cell activation, requires integrin activation through a process termed inside-out integrin signaling. In hematopoietic cells, Rap1 and its downstream effector RIAM (Rap1-interacting adaptor molecule) form a cornerstone for inside-out integrin activation. The Rap1/RIAM pathway is involved in signal integration for activation, actin remodeling and cytoskeletal reorganization in T cells, as well as in myeloid cell differentiation and function. RIAM is instrumental for phagocytosis, a process requiring particle recognition, cytoskeletal remodeling and membrane protrusion for engulfment and digestion. In the present review, we discuss the structural and molecular properties of RIAM and the recent discoveries regarding the functional role of the Rap1/RIAM module in hematopoietic cells.

Loss of hnRNPLL-dependent splicing of Ptprc has no impact on B-cell development, activation and terminal differentiation into antibody-secreting cells.

The RNA-binding protein heterogeneous nuclear ribonucleoprotein L-like (hnRNPLL) controls alternative splicing of protein tyrosine phosphatase receptor type C (Ptprc) which encodes CD45. hnRNPLL deficiency leads to a failure in silencing Ptprc exons 4-6 causing aberrant expression of the corresponding CD45 isoforms, namely, CD45RA, RB and RC. While an N-ethyl-N-nitrosourea-induced point mutation in murine Hnrnpll results in loss of peripheral naïve T cells, its role in B-cell biology remains unclear. Here, we demonstrate that B-cell development in the bone marrow of Hnrnpllthu/thu mice is normal and the number of mature B-cell subsets in the spleen and peritoneal cavity is comparable to control littermates. In response to in vivo immunization, Hnrnpllthu/thu mice were deficient in generating germinal center (GC) B cells, and analysis of mixed bone marrow chimeras revealed that the GC B-cell deficiency was a B-cell extrinsic effect of the hnRNPLL mutation. Mature Hnrnpllthu/thu B cells proliferated normally in response to various B-cell receptor- and Toll-like receptor-mediated stimuli. Similarly, in vitro stimulation of mutant B cells led to normal generation of plasmablasts, but mutant plasmablasts failed to downregulate B220 expression because of the inability of cells to undergo proper CD45 pre-messenger RNA alternative splicing. These findings collectively suggest that, like in T and natural killer T cells, the mutation disrupts hnRNPLL-mediated alternative splicing of the Ptprc gene in plasmablasts, but this dysregulation of Ptprc alternative splicing does not affect the development and function of B cells.

Unraveling Key Players of Humoral Immunity: Advanced and Optimized Lymphocyte Isolation Protocol from Murine Peyer's Patches.

In the gut mucosa, immune cells constitute a unique immunological entity, which promotes immune tolerance while concurrently conferring immune defense against pathogens. It is well established that Peyer's patches (PPs) have an essential role in the mucosal immune network by hosting several effector T and B cell subsets. A certain fraction of these effector cells, follicular T helper (TFH) and germinal center (GC) B cells are professionalized in the regulation of humoral immunity. Hence, the characterization of these cell subsets within PPs in terms of their differentiation program and functional properties can provide important information about mucosal immunity. To this end, an easily applicable, efficient and reproducible method of lymphocyte isolation from PPs would be valuable to researchers. In this study, we aimed to generate an effective method to isolate lymphocytes from mouse PPs with high cell yield. Our approach revealed that initial tissue processing such as the use of digestive reagents and tissue agitation, as well as cell staining conditions and selection of antibody panels, have great influence on the quality and identity of the isolated lymphocytes and on experimental outcomes. Here, we describe a protocol enabling researchers to efficiently isolate lymphocyte populations from PPs allowing reproducible flow cytometry-based assessment of T and B cell subsets primarily focusing on TFH and GC B cell subsets.