The layered development of mouse B and T Cells

The search to understand the development of the chicken immune system: Differences in expression of MHC class I loci and waves of thymocytes as evolutionary relics?

Chickens are renowned as a model for embryogenesis but have also been responsible for crucial advances in virology, cancer research and immunology. However, chickens are best known as a major source of animal protein for human nutrition, with roughly 80 billion chickens alive each year supplying meat and eggs, the vast majority part of a global poultry industry. As a result, avian immunology been studied intensively for over 60 years, and it has become clear that a major genetic locus in chickens determining resistance to infectious disease and response to vaccines is the major histocompatibility complex (MHC). Compared to typical mammals, the chicken MHC is compact and simple, with only two classical class I genes. A dominantly-expressed class I gene, BF2, is the major ligand for cytotoxic T lymphocytes (CTLs), while the other locus, BF1, is much less well-expressed, lacking in some MHC haplotypes, and is a ligand for natural killer (NK) cells. Cell surface class I expression in neonatal chicks is far less than in adults, and one possibility is that BF2 is not well-expressed early in ontogeny. A precedent is found for amphibians: the single classical class I molecule is not expressed in tadpoles of Xenopus frogs, although non-polymorphic (and thus non-classical) class I molecules from the XNC locus are expressed, which are recognised for immune defence by non-canonical NKT lymphocytes. Indeed, three waves of different T cells are produced by the Xenopus thymus: in tadpoles, during metamorphosis and finally as adults. Three waves of thymic emigrants are also found for chickens, and reasoning by analogy, it may be that the waves of thymocytes and the expression of class I molecules during ontogeny of chickens are evolutionary relics. As well as scientific interest in the ontogeny of MHC class I expression and appearance of peripheral T cells, there are potential practical implications, given the importance of vaccination in ovo and in day-old chicks for the poultry industry.

Distinct T-cell receptor (TCR) gene segment usage and MHC-restriction between foetal and adult thymus

Here, we sequenced rearranged TCRβ and TCRα chain sequences in CD4+CD8+ double positive (DP), CD4+CD8- single positive (SP4) and CD4-CD8+ (SP8) thymocyte populations from the foetus and young adult mouse. We found that life-stage had a greater impact on TCRβ and TCRα gene segment usage than cell-type. Foetal repertoires showed bias towards 3'TRAV and 5'TRAJ rearrangements in all populations, whereas adult repertoires used more 5'TRAV gene segments, suggesting that progressive TCRα rearrangements occur less frequently in foetal DP cells. When we synchronised young adult DP thymocyte differentiation by hydrocortisone treatment the new recovering DP thymocyte population showed more foetal-like 3'TRAV and 5'TRAJ gene segment usage. In foetus we identified less influence of MHC-restriction on α-chain and β-chain combinatorial VxJ usage and CDR1xCDR2 (V region) usage in SP compared to adult, indicating weaker impact of MHC-restriction on the foetal TCR repertoire. The foetal TCRβ repertoire was less diverse, less evenly distributed, with fewer non-template insertions, and all foetal populations contained more clonotypic expansions than adult. The differences between the foetal and adult thymus TCR repertoires are consistent with the foetal thymus producing αβT-cells with properties and functions that are distinct from adult T-cells: their repertoire is less governed by MHC-restriction, with preference for particular gene segment usage, less diverse with more clonotypic expansions, and more closely encoded by genomic sequence.

Active immunization against gonadotropin-releasing hormone enhances the generation of B cells but does not affect their colonization in peripheral immune organs in male rats

Active immunization against gonadotropin-releasing hormone (GnRH) affects the immune system by inhibiting testosterone production. Our previous study investigated the effects of GnRH immunization on thymic T-cell generation, migration, and colonization in peripheral immune organs. However, the mechanisms by which GnRH immunization influences B cell generation and the characteristics of B cell colonization in peripheral immune organs remain unclear. Herein, GnRH immunization enhanced B cell generation by reducing apoptosis. GnRH immunization did not markedly affect the cell cycle of bone marrow cells, B cell development-related signaling molecules, or the percentage of B cells in the blood, spleen, or inguinal lymph nodes. After testosterone supplementation in GnRH-immunocastrated rats, the generation of B cells in the bone marrow was significantly reduced, and the apoptosis of B cells was remarkably increased. Testosterone did not significantly affect the cell cycle of bone marrow cells or the proportion of B cells in the blood, spleen, or inguinal lymph nodes of the GnRH-immunocastrated rats. Overall, these results clarify the mechanisms related to B cell expansion in the bone marrow and the settlement characteristics of B cells in peripheral immune organs after GnRH immunization.

Developmental Vitamin D Deficiency and the Vitamin D Receptor Control Hematopoiesis

Vitamin D status, the vitamin D receptor (VDR), and the ability to produce active vitamin D [1,25(OH)2D, regulated by Cyp27b1] regulate fetal and adult hematopoiesis. Transgenic reporter mice that express the tdTomato RFP as an indication of Vdr expression were used to identify immune cells that express the Vdr. Vdr/tdTomato+ hematopoietic progenitors were identified as early as embryonic day (E)15.5, establishing that these cells have expressed the Vdr and are vitamin D targets. Maternal vitamin D deficiency [D-; serum 25(OH)D < 20 ng/ml] or Vdr knockout or Cyp27b1 knockout resulted in embryos with fewer fetal progenitors. Vdr/tdTomato+ expression was found to increase with age in CD8+ T cells and innate lymphoid cells (ILCs)1 and ILC3, suggesting that initial Vdr expression in these cells is dependent on environmental factors immediately postbirth. In adult tissues, the frequencies of mature T cells and ILCs as well as Vdr/tdTomato expression were reduced by D-. Maternal D- resulted in fewer progenitors that expressed Vdr/tdTomato+ at E15.5 and fewer Vdr/tdTomato+ immune cells in the adult spleen than offspring from D+ mice. We challenged D- mice with H1N1 influenza infection and found that D- mice were more susceptible than D+ mice. Treating D- mice with vitamin D restored Vdr/tdTomato+ expression in splenic T cells and partially restored resistance to H1N1 infection, which shows that developmental D- results in lingering effects on Vdr expression in the adult immune system that compromise the immune response to H1N1 infection. Vitamin D and the Vdr regulate hematopoiesis in both fetal and postnatal phases of immune cell development that impact the immune response to a viral infection.

The Janus (dual) model of immunoglobulin isotype evolution: Conservation and plasticity are the defining paradigms

The study of antibodies in jawed vertebrates (gnathostomes) provides every immunologist with a bird's eye view of how human immunoglobulins (Igs) came into existence and subsequently evolved into their present forms. It is a fascinating Darwinian history of conservation on the one hand and flexibility on the other, exemplified by the Ig heavy chain (H) isotypes IgM and IgD/W, respectively. The cartilaginous fish (e.g., sharks) Igs provide a glimpse of "how everything got off the ground," while the amphibians (e.g., the model Xenopus) reveal how the adaptive immune system made an about face with the emergence of Ig isotype switching and IgG-like structure/function. The evolution of mucosal Igs is a captivating account of malleability, convergence, and conservation, and a call to arms for future study! In between there are spellbinding chronicles of antibody evolution in each class of vertebrates and rather incredible stories of how antibodies can adapt to occupy niches, for example, single-domain variable regions, cold-adapted Igs, convergent mechanisms to dampen antibody function, provision of mucosal defense, and many more. The purpose here is not to provide an encyclopedic examination of antibody evolution, but rather to hit the high points and entice readers to appreciate how things "came to be."

A biphenotypic lymphocyte subset displays both T- and B-cell functionalities

T cell/B cell mixed phenotypic lymphocytes have been observed in different disease contexts, yet their presence and function in physiological conditions remain elusive. Here, we provide evidence for the existence of a lymphocyte subset endogenously expressing both T- and B-cell lineage markers in mice. The majority of these T/B phenotypic lymphocytes (CD3+CD19+) show an origin of pro/pre B cells and distribute widely in mouse bone marrow, lymph nodes, spleen, and peripheral blood. Functional assays show that these biphenotypic lymphocytes can be activated through stimulating TCR or BCR signaling pathways. Moreover, we show that these cells actively participate both the humoral and cellular immune responses elicited by vaccination. Compared to conventional T cells, these biphenotypic lymphocytes can secrete a higher level of IL-2 but a lower level of TNF-α upon antigen specific stimulation. An equivalent lymphocyte subset is found in freshly isolated human PBMCs and exhibits similar functionality, albeit at a lower frequency than in mice.

Disorganization of secondary lymphoid organs and dyscoordination of chemokine secretion as key contributors to immune aging

Aging is characterized by progressive loss of organ and tissue function, and the immune system is no exception to that inevitable principle. Of all the age-related changes in the body, reduction of the size of, and naïve T (Tn) cell output from, the thymus occurs earliest, being prominent already before or by the time of puberty. Therefore, to preserve immunity against new infections, over much of their lives, vertebrates dominantly rely on peripheral maintenance of the Tn cell pool in the secondary lymphoid organs (SLO). However, SLO structure and function subsequently also deteriorate with aging. Several recent studies have made a convincing case that this deterioration is of major importance to the erosion of protective immunity in the last third of life. Specifically, the SLO were found to accumulate multiple degenerative changes with aging. Importantly, the results from adoptive transfer and parabiosis studies teach us that the old microenvironment is the limiting factor for protective immunity in old mice. In this review, we discuss the extent, mechanisms, and potential role of stromal cell aging in the age-related alteration of T cell homeostatic maintenance and immune function decline. We use that discussion to frame the potential strategies to correct the SLO stromal aging defects - in the context of other immune rejuvenation approaches, - to improve functional immune responses and protective immunity in older adults.

The miR-142 miRNAs: Shaping the naïve immune system

Immunity in a naïve organism is tightly controlled. Adequate proportions of the many immune cell subsets must be produced to mount efficient responses to eventual challenges. In addition, a functioning immune system is highly dynamic at steady state. Mature immune cells must be positioned properly and/or circulate to facilitate the detection of dangers. They must also be poised to promptly react to unusual encounters, while ignoring innocuous germs and self. Numerous regulatory mechanisms act at the molecular level to generate such an exquisite structure, including miRNA-mediated repression of protein synthesis. Notably, the miRNAs from the miR-142 locus are preferentially expressed in hematopoietic cells. Their importance is underscored by the deeply disturbed immune system seen upon inactivation of the locus in mice. In this review, we explore reported roles for the miR-142 miRNAs in the shaping of immunity in vertebrates, discussing in particular their contributions to the generation, migration and survival of hematopoietic cells.

Engineering T Cell Development for the Next Generation of Stem Cell-Derived Immunotherapies

Engineered T cells are at the leading edge of clinical cell therapy. T cell therapies have had a remarkable impact on patient care for a subset of hematological malignancies. This foundation has motivated the development of off-the-shelf engineered cell therapies for a broad range of devastating indications. Achieving this vision will require cost-effective manufacturing of precision cell products capable of addressing multiple process and clinical-design challenges. Pluripotent stem cell (PSC)-derived engineered T cells are emerging as a solution of choice. To unleash the full potential of PSC-derived T cell therapies, the field will require technologies capable of robustly orchestrating the complex series of time- and dose-dependent signaling events needed to recreate functional T cell development in the laboratory. In this article, we review the current state of allogenic T cell therapies, focusing on strategies to generate engineered lymphoid cells from PSCs. We highlight exciting recent progress in this field and outline timely opportunities for advancement with an emphasis on niche engineering and synthetic biology.