Turnover of naive- and memory-phenotype T cells

The dynamics and longevity of circulating CD4+ memory T cells depend on cell age and not the chronological age of the host

Quantifying the kinetics with which memory T cell populations are generated and maintained is essential for identifying the determinants of the duration of immunity. The quality and persistence of circulating CD4 effector memory (TEM) and central memory (TCM) T cells in mice appear to shift with age, but it is unclear whether these changes are driven by the aging host environment, by cell age effects, or both. Here, we address these issues by combining DNA labelling methods, established fate-mapping systems, a novel reporter mouse strain, and mathematical models. Together, these allow us to quantify the dynamics of both young and established circulating memory CD4 T cell subsets, within both young and old mice. We show that that these cells and their descendents become more persistent the longer they reside within the TCM and TEM pools. This behaviour may limit memory CD4 T cell diversity by skewing TCR repertoires towards clones generated early in life, but may also compensate for functional defects in new memory cells generated in old age.

The dynamics and longevity of circulating CD4+ memory T cells depend on cell age and not the chronological age of the host

Quantifying the kinetics with which memory T cell populations are generated and maintained is essential for identifying the determinants of the duration of immunity. The quality and persistence of circulating CD4+ effector memory (TEM) and central memory (TCM) T cells in mice appear to shift with age, but it is unclear whether these changes are driven by the aging host environment, by cell age effects, or both. Here we address these issues by combining DNA labelling methods, established fate-mapping systems, a novel reporter mouse strain, and mathematical models. Together, these allow us to quantify the dynamics of both young and established circulating memory CD4+ T cell subsets, within both young and old mice. We show that that these cells and their descendents become more persistent the longer they reside within the TCM and TEM pools. This behaviour may limit memory CD4 T cell diversity by skewing TCR repertoires towards clones generated early in life, but may also compensate for functional defects in new memory cells generated in old age.

Astragaloside IV targeting autophagy of T cells improves inflammation of asthma

Astragaloside IV (AST) has been confirmed to have antiasthmatic effects. However, the underline mechanism is unclear. The study aimed to explore the treatment mechanism of AST based on autophagy of memory T cells. AST treatment significantly decreased the number of T effector cells in asthma mice blood and the nude mice that received AST-treated TCMs had relieved inflammation compared with the untreated group; meanwhile, we found that AST significantly decreased the autophagy level and inhibited OX40/OX40L signal pathway of lymphocytes. The results highlighted that AST regulated autophagy to inhibit differentiation of effector T-cell phenotype.

Optimising IL-2 for Cancer Immunotherapy

The key role of T cells in cancer immunotherapy is well established and is highlighted by the remarkable capacity of Ab-mediated checkpoint blockade to overcome T-cell exhaustion and amplify anti-tumor responses. However, total or partial tumor remission following checkpoint blockade is still limited to only a few types of tumors. Hence, concerted attempts are being made to devise new methods for improving tumor immunity. Currently, much attention is being focused on therapy with IL-2. This cytokine is a powerful growth factor for T cells and optimises their effector functions. When used at therapeutic doses for cancer treatment, however, IL-2 is highly toxic. Nevertheless, recent work has shown that modifying the structure or presentation of IL-2 can reduce toxicity and lead to effective anti-tumor responses in synergy with checkpoint blockade. Here, we review the complex interaction of IL-2 with T cells: first during normal homeostasis, then during responses to pathogens, and finally in anti-tumor responses.

Immunological Phenotyping of Mice with a Point Mutation in Cdk4

Cyclin-dependent kinases (CDKs) play a crucial role in regulation of the mammalian cell cycle. CDK4 and CDK6 control the G1/S restriction checkpoint through their ability to associate with cyclin D proteins in response to growth factor signals. CDK4 deficiency in mice gives rise to a range of endocrine-specific phenotypes including diabetes, infertility, dwarfism, and atrophy of the anterior pituitary. Although CDK6 deficiency can cause thymic atrophy due to a block in the double-negative (DN) to double-positive (DP) stage of T cell development, there are no overt defects in immune cell development reported for CDK4-deficient mice. Here, we examined the impact of a novel N-ethyl-N-nitrosourea-induced point mutation in the gene encoding CDK4 on immune cell development. Mutant mice (Cdk4wnch/wnch) showed normal development and differentiation of major immune cell subsets in the thymus and spleen. Moreover, T cells from Cdk4wnch/wnch mice exhibited normal cytokine production in response to in vitro stimulation. However, analysis of the mixed bone marrow chimeras revealed that Cdk4wnch/wnch-derived T cell subsets and NK cells are at a competitive disadvantage compared to Cdk4+/+-derived cells in the thymus and periphery of recipients. These results suggest a possible role for the CDK4wnch mutation in the development of some immune cells, which only becomes apparent when the Cdk4wnch/wnch mutant cells are in direct competition with wild-type immune cells in the mixed bone marrow chimera.

Homeostasis and immunological function of self-driven memory-phenotype CD4+ T lymphocytes

CD4⁺ T lymphocytes play an essential role in adaptive immune responses. In pathogen infection, naïve CD4⁺ T cells that strongly respond to foreign antigens robustly proliferate to differentiate into effector/memory cells, contributing to elimination of the pathogen concerned. In addition to this conventional T cell activation pathway, naïve T cells can also weakly respond to self antigens in the periphery to spontaneously acquire a memory phenotype through homeostatic proliferation in steady state. Such 'memory-phenotype' (MP) CD4⁺ T lymphocytes are distinguishable from foreign antigen-specific memory cells in terms of marker expression. Once generated, MP cells are maintained by rapid proliferation while differentiating into the T-bet⁺ 'MP1' subset, with the latter response promoted by IL-12 homeostatically produced by type 1 dendritic cells. Importantly, MP1 cells possess innate immune function; they can produce IFN-γ in response to IL-12 and IL-18 to contribute to host defense against pathogens. Similarly, the presence of RORγt⁺ 'MP17' and Gata3^hi 'MP2' cells as well as their potential immune functions have been proposed. In this review, I will discuss our current understanding on the unique mechanisms of generation, maintenance, and differentiation of MP CD4⁺ T lymphocytes as well as their functional significance in various disease conditions.

Redefining the Foreign Antigen and Self-Driven Memory CD4+ T-Cell Compartments via Transcriptomic, Phenotypic, and Functional Analyses

Under steady-state conditions, conventional CD4⁺ T lymphocytes are classically divided into naïve (CD44^lo CD62L^hi) and memory (CD44^hi CD62L^lo) cell compartments. While the latter population is presumed to comprise a mixture of distinct subpopulations of explicit foreign antigen (Ag)-specific “authentic” memory and foreign Ag-independent memory-phenotype (MP) cells, phenotypic markers differentially expressed in these two cell types have yet to be identified. Moreover, while MP cells themselves have been previously described as heterogeneous, it is unknown whether they consist of distinct subsets defined by marker expression. In this study, we demonstrate using combined single-cell RNA sequencing and flow cytometric approaches that self-driven MP CD4⁺ T lymphocytes are divided into CD127^hi Sca1^lo, CD127^hi Sca1^hi, CD127^lo Sca1^hi, and CD127^lo Sca1^lo subpopulations that are Bcl2^lo, while foreign Ag-specific memory cells are CD127^hi Sca1^hi Bcl2^hi. We further show that among the four MP subsets, CD127^hi Sca1^hi lymphocytes represent the most mature and cell division-experienced subpopulation derived from peripheral naïve precursors. Finally, we provide evidence arguing that this MP subpopulation exerts the highest responsiveness to Th1-differentiating cytokines and can induce colitis. Together, our findings define MP CD4⁺ T lymphocytes as a unique, self-driven population consisting of distinct subsets that differ from conventional foreign Ag-specific memory cells in marker expression and establish functional relevance for the mature subset of CD127^hi Sca1^hi MP cells.

Memory-phenotype CD4+ T cells: a naturally arising T lymphocyte population possessing innate immune function

In conventional adaptive immune responses, upon recognition of foreign antigens, naive CD4+ T lymphocytes are activated to differentiate into effector/memory cells. In addition, emerging evidence suggests that in the steady state, naive CD4+ T cells spontaneously proliferate in response to self-antigens to acquire a memory phenotype (MP) through homeostatic proliferation. This expansion is particularly profound in lymphopenic environments but also occurs in lymphoreplete, normal conditions. The 'MP T lymphocytes' generated in this manner are maintained by rapid proliferation in the periphery and they tonically differentiate into T-bet-expressing 'MP1' cells. Such MP1 CD4+ T lymphocytes can exert innate effector function, producing IFN-γ in response to IL-12 in the absence of antigen recognition, thereby contributing to host defense. In this review, we will discuss our current understanding of how MP T lymphocytes are generated and persist in steady-state conditions, their populational heterogeneity as well as the evidence for their effector function. We will also compare these properties with those of a similar population of innate memory cells previously identified in the CD8+ T lymphocyte lineage.

Pharmacologic control of homeostatic and antigen-driven proliferation to target HIV-1 persistence

The presence of latent human immunodeficiency virus 1 (HIV-1) in quiescent memory CD4 + T cells represents a major barrier to viral eradication. Proliferation of memory CD4 + T cells is the primary mechanism that leads to persistence of the latent reservoir, despite effective antiretroviral therapy (ART). Memory CD4 + T cells are long-lived and can proliferate through two mechanisms: homeostatic proliferation via γc-cytokine stimulation or antigen-driven proliferation. Therefore, therapeutic modalities that perturb homeostatic and antigen-driven proliferation, combined with ART, represent promising strategies to reduce the latent reservoir. In this study, we investigated a library of FDA-approved oncology drugs to determine their ability to inhibit homeostatic and/or antigen-driven proliferation. We confirmed potential hits by evaluating their effects on proliferation in memory CD4 + T cells from people living with HIV-1 on ART (PLWH) and interrogated downstream signaling of γc-cytokine stimulation. We found that dasatinib and ponatinib, tyrosine kinase inhibitors, and trametinib, a MEK inhibitor, reduced both homeostatic and antigen-driven proliferationby >65%, with a reduction in viability <45%, ex vivo. In memory CD4 + T cells from PLWH, only dasatinib restricted both homeostatic and antigen-driven proliferation and prevented spontaneous rebound, consistent with promoting a smaller reservoir size. We show that dasatinib restricts IL-7 induced proliferation through STAT5 phosphorylation inhibition. Our results establish that the anti-cancer agent dasatinib is an exciting candidate to be used as an anti-proliferative drug in a clinical trial, since it efficiently blocks proliferation and iswell tolerated in patients with chronic myeloid leukemia (CML).

OMIP-079: Cell cycle of CD4+ and CD8+ naïve/memory T cell subsets, and of Treg cells from mouse spleen

A multicolor flow cytometry panel was designed and optimized to define the following nine mouse T cell subsets: Treg (CD3⁺ CD4⁺ CD8⁻ FoxP3⁺), CD4⁺ T naïve (CD3⁺ CD4⁺ CD8⁻FoxP3⁻ CD44^int/low CD62L⁺), CD4⁺ T central memory (CD3⁺ CD4⁺ CD8⁻ FoxP3⁻ CD44^high CD62L⁺), CD4⁺ T effector memory (CD3⁺ CD4⁺ CD8⁻ FoxP3⁻ CD44^high CD62L⁻), CD4⁺ T EMRA (CD3⁺ CD4⁺ CD8⁻ FoxP3⁻ CD44^int/low CD62L⁻), CD8⁺ T naïve (CD3⁺ CD8⁺ CD4⁻ CD44^int/low CD62L⁺), CD8⁺ T central memory (CD3⁺ CD8⁺ CD4⁻ CD44^high CD62L⁺), CD8⁺ T effector memory (CD3⁺ CD8⁺ CD4⁻ CD44^high CD62L⁻), and CD8⁺ T EMRA (CD3⁺ CD8⁺ CD4⁻ CD44^int/low CD62L⁻). In each T cell subset, a dual staining for Ki‐67 expression and DNA content was employed to distinguish the following cell cycle phases: G₀ (Ki67⁻, with 2n DNA), G₁ (Ki67⁺, with 2n DNA), and S‐G₂/M (Ki67⁺, with 2n < DNA ≤ 4n). This panel was established for the analysis of mouse (C57BL/6J) spleen.

Dynamics and turnover of memory CD8 T cell responses following yellow fever vaccination

Understanding how immunological memory lasts a lifetime requires quantifying changes in the number of memory cells as well as how their division and death rates change over time. We address these questions by using a statistically powerful mixed-effects differential equations framework to analyze data from two human studies that follow CD8 T cell responses to the yellow fever vaccine (YFV-17D). Models were first fit to the frequency of YFV-specific memory CD8 T cells and deuterium enrichment in those cells 42 days to 1 year post-vaccination. A different dataset, on the loss of YFV-specific CD8 T cells over three decades, was used to assess out of sample predictions of our models. The commonly used exponential and bi-exponential decline models performed relatively poorly. Models with the cell loss following a power law (exactly or approximately) were most predictive. Notably, using only the first year of data, these models accurately predicted T cell frequencies up to 30 years post-vaccination. Our analyses suggest that division rates of these cells drop and plateau at a low level (0.1% per day, ∼ double the estimated values for naive T cells) within one year following vaccination, whereas death rates continue to decline for much longer. Our results show that power laws can be predictive for T cell memory, a finding that may be useful for vaccine evaluation and epidemiological modeling. Moreover, since power laws asymptotically decline more slowly than any exponential decline, our results help explain the longevity of immune memory phenomenologically.

Immunological memory, generated in response to infection or vaccination, may provide complete or partial protection from antigenically similar infections for the lifetime. Memory CD8 T cells are important players in protection from secondary viral infections but quantitative understanding of their dynamics in humans is limited. We analyze data from two studies where immunization with the yellow fever virus vaccine (YFV-17D) generates a mild acute infection and long-term memory. We find that: (i) the division rate of YFV-17D-specific CD8 T cells drops and stabilizes at ∼ 0.1% per day during the first year following vaccination whereas the death rate declines more gradually, and (ii) the number of these cells declines approximately in accordance with a power law (∝ time^−0.75) for at least several decades following vaccination.

Homeostasis of Naive and Memory T Lymphocytes

Conventional CD4+ and CD8+ T lymphocytes comprise a mixture of naive and memory cells. Generation and survival of these T-cell subsets is under strict homeostatic control and reflects contact with self-major histocompatibility complex (MHC) and certain cytokines. Naive T cells arise in the thymus via T-cell receptor (TCR)-dependent positive selection to self-peptide/MHC complexes and are then maintained in the periphery through self-MHC interaction plus stimulation via interleukin-7 (IL-7). By contrast, memory T cells are largely MHC-independent for their survival but depend strongly on stimulation via cytokines. Whereas typical memory T cells are generated in response to foreign antigens, some arise spontaneously through contact of naive precursors with self-MHC ligands; we refer to these cells as memory-phenotype (MP) T cells. In this review, we discuss the generation and homeostasis of naive T cells and these two types of memory T cells, focusing on their relative interaction with MHC ligands and cytokines.

The role of the thymus in COVID-19 disease severity: implications for antibody treatment and immunization

The thymus is a largely neglected organ but plays a significant role in the regulation of adaptive immune responses. The effect of aging on the thymus and immune senescence is well established, and the resulting inflammaging is found to be implicated in the development of many chronic diseases including atherosclerosis, hypertension and type 2 diabetes. Both aging and diseases of inflammaging are associated with severe COVID-19 disease, and a dysfunctional thymus may be a predisposing factor. In addition, insults on the thymus during childhood may lead to abnormal thymic function and may explain severe COVID-19 disease among younger individuals; therefore, measurement of thymic function may assist COVID-19 care. Those with poor thymic function may be treated prophylactically with convalescent serum or recombinant antibodies, and they may respond better to high-dose or adjuvanted COVID-19 vaccines. Treatments inducing thymic regeneration may improve patients' overall health and may be incorporated in COVID-19 management.

Sugarcane (Saccharum officinarum L.) Top Extract Ameliorates Cognitive Decline in Senescence Model SAMP8 Mice: Modulation of Neural Development and Energy Metabolism

Age-related biological alterations in brain function increase the risk of mild cognitive impairment and dementia, a global problem exacerbated by aging populations in developed nations. Limited pharmacological therapies have resulted in attention turning to the promising role of medicinal plants and dietary supplements in the treatment and prevention of dementia. Sugarcane (Saccharum officinarum L.) top, largely considered as a by-product because of its low sugar content, in fact contains the most abundant amounts of antioxidant polyphenols relative to the rest of the plant. Given the numerous epidemiological studies on the effects of polyphenols on cognitive function, in this study, we analyzed polyphenolic constituents of sugarcane top and examined the effect of sugarcane top ethanolic extract (STEE) on a range of central nervous system functions in vitro and in vivo. Orally administrated STEE rescued spatial learning and memory deficit in the senescence-accelerated mouse prone 8 (SAMP8) mice, a non-transgenic strain that spontaneously develops a multisystemic aging phenotype including pathological features of Alzheimer's disease. This could be correlated with an increased number of hippocampal newborn neurons and restoration of cortical monoamine levels in STEE-fed SAMP8 mice. Global genomic analysis by microarray in cerebral cortices showed multiple potential mechanisms for the cognitive improvement. Gene set enrichment analysis (GSEA) revealed biological processes such as neurogenesis, neuron differentiation, and neuron development were significantly enriched in STEE-fed mice brain compared to non-treated SAMP8 mice. Furthermore, STEE treatment significantly regulated genes involved in neurotrophin signaling, glucose metabolism, and neural development in mice brain. Our in vitro results suggest that STEE treatment enhances the metabolic activity of neuronal cells promoting glucose metabolism with significant upregulation of genes, namely PGK1, PGAM1, PKM, and PC. STEE also stimulated proliferation of human neural stem cells (hNSCs), regulated bHLH factor expression and induced neuronal differentiation and astrocytic process lengthening. Altogether, our findings suggest the potential of STEE as a dietary intervention, with promising implications as a novel nutraceutical for cognitive health.

Tyrosine Kinase Inhibition: a New Perspective in the Fight against HIV

PURPOSE OF REVIEW

HIV-1 infection is incurable due to the existence of latent reservoirs that persist in the face of cART. In this review, we describe the existence of multiple HIV-1 reservoirs, the mechanisms that support their persistence, and the potential use of tyrosine kinase inhibitors (TKIs) to block several pathogenic processes secondary to HIV-1 infection.

RECENT FINDINGS

Dasatinib interferes in vitro with HIV-1 persistence by two independent mechanisms. First, dasatinib blocks infection and potential expansion of the latent reservoir by interfering with the inactivating phosphorylation of SAMHD1. Secondly, dasatinib inhibits the homeostatic proliferation induced by γc-cytokines. Since homeostatic proliferation is thought to be the main mechanism behind the maintenance of the latent reservoir, we propose that blocking this process will gradually reduce the size of the reservoir. TKIs together with cART will interfere with HIV-1 latent reservoir persistence, favoring the prospect for viral eradication.

Memory CD4+ T Cells in Immunity and Autoimmune Diseases

CD4⁺ T helper (Th) cells play central roles in immunity in health and disease. While much is known about the effector function of Th cells in combating pathogens and promoting autoimmune diseases, the roles and biology of memory CD4⁺ Th cells are complex and less well understood. In human autoimmune diseases such as multiple sclerosis (MS), there is a critical need to better understand the function and biology of memory T cells. In this review article we summarize current concepts in the field of CD4⁺ T cell memory, including natural history, developmental pathways, subsets, and functions. Furthermore, we discuss advancements in the field of the newly-described CD4⁺ tissue-resident memory T cells and of CD4⁺ memory T cells in autoimmune diseases, two major areas of important unresolved questions in need of answering to advance new vaccine design and development of novel treatments for CD4⁺ T cell-mediated autoimmune diseases.

Studying Peripheral T Cell Homeostasis in Mice: A Concise Technical Review

For several years, it was believed that the thymus was entirely responsible for maintaining T cell homeostasis. Today, it is well-known that homeostatic peripheral mechanisms are essential in order to maintain T cell numbers and diversity constant in the periphery. Naïve and memory T cells require continual access to self-peptide MHC class I and II molecules and/or cytokines to survive in the periphery. Under normal conditions, homeostatic resources are low, and lymphocytes undergo very slow proliferation and survive. Following T cell depletion, the bioavailability of homeostatic resources is significantly increased, and T cell proliferation is dramatically augmented. The development of lymphopenic mouse models has helped our current understanding of factors involved in the regulation of peripheral T cell homeostasis. In this minireview, we will give a brief overview about basic techniques used to study peripheral T cell homeostasis in mice.

CD22 and CD72 contribute to the development of scleroderma in a murine model

BACKGROUND

Systemic sclerosis (SSc) is a systemic autoimmune disease that is characterized by excessive fibrosis. CD22 and CD72 are B cell-specific cell surface molecules that negatively regulate B cell function.

OBJECTIVE

The aim of the present study was to investigate the roles of CD22 and CD72 in a murine scleroderma model.

METHODS

The experimental fibrosis model was generated by subcutaneous injection of bleomycin or hypochlorous acid (HOCL) into wild-type (WT), CD22-deficient (CD22^-/-), CD72-deficient (CD72^-/-) and CD22 and CD72 double-deficient (CD22^-/-/CD72^-/-) mice. We histologically assessed skin fibrosis and inflammatory cell infiltration. Cytokine and chemokine expression levels were measured by real-time polymerase chain reaction.

RESULTS

The severity of fibrosis in the skin and lung was significantly less in CD22^-/-, CD72^-/-, and CD22^-/-/CD72^-/- mice than in WT mice in the bleomycin-induced model. In the skin of bleomycin-treated mice, the numbers of CD3⁺ T cells, CD8⁺ T cells, and F4/80⁺ macrophages were significantly lower in CD22^-/-, CD72^-/-, and CD22^-/-/CD72^-/- mice than in WT mice. The expression levels of mRNAs for IL-6, TNF-α, TGF-β, CTGF, IL-1β, IL-13, CXCL2, and ICAM-1 were significantly lower in CD22^-/-, CD72^-/-, and CD22^-/-/CD72^-/- mice than in WT mice. In the HOCL-induced model, both skin and lung fibrosis were ameliorated in CD22^-/-, CD72^-/- and CD22^-/-/CD72^-/- mice compared to WT mice.

CONCLUSION

These results indicate that CD22 and CD72 likely play crucial roles in skin and lung fibrosis. Moreover, the inhibition of CD22 and CD72 function has potential as a therapeutic approach to SSc.

Hematopoietic Stem Cell Transplantation Restores Naïve T-Cell Populations in Atm-Deficient Mice and in Preemptively Treated Patients With Ataxia-Telangiectasia

Background: Ataxia-telangiectasia (A-T) is a multisystem disorder with progressive cerebellar ataxia, immunodeficiency, chromosomal instability, and increased cancer susceptibility. Cellular immunodeficiency is based on naïve CD4⁺ and CD8⁺ T-cell lymphopenia. Hematopoietic stem cell transplantation (HSCT) offers a potential to cure immunodeficiency and cancer due to restoration of the lymphopoietic system. The aim of this investigation was to analyze the effect of HSCT on naïve CD4⁺ as well as CD8⁺ T-cell numbers in A-T.

Methods: We analyzed total numbers of peripheral naïve (CD45RA⁺CD62L⁺) and memory (CD45RO⁺CD62L⁻) CD4⁺ and CD8⁺ T-cells of 32 A-T patients. Naïve (CD62L^highCD44^low) and memory (CD62L^lowCD44^high) T-cells were also measured in Atm-deficient mice before and after HSCT with GFP-expressing bone marrow derived hematopoietic stem cells. In addition, we analyzed T-cells in the peripheral blood of two A-T patients after HLA-identic allogeneic HSCT.

Results: Like in humans, naïve CD4⁺ as well as naïve CD8⁺ lymphocytes were decreased in Atm-deficient mice. HSCT significantly inhibited thymic lymphomas and increased survival time in these animals. Donor cell chimerism increased up to more than 50% 6 months after HSCT accompanied by a significant increase of naïve CD4 and CD8 T-cell subpopulations, but not of memory T-cells. This finding was also identified in the blood of the A-T patients after HSCT.

Conclusion: HSCT seems to be a feasible strategy to overcome immunodeficiency and might be a conceivable strategy to avoid T-cell driven cancer in A-T at higher risk for malignancy. Naïve CD4 and CD8 T-cells counts are suitable markers for monitoring immune reconstitution post-HSCT. However, risks and benefits of HSCT in A-T have to be properly weighted.

Anti-Leishmanial Vaccines: Assumptions, Approaches, and Annulments

Leishmaniasis is a neglected protozoan parasitic disease that occurs in 88 countries but a vaccine is unavailable. Vaccination with live, killed, attenuated (physically or genetically) Leishmania have met with limited success, while peptide-, protein-, or DNA-based vaccines showed promise only in animal models. Here, we critically assess several technical issues in vaccination and expectation of a host-protective immune response. Several studies showed that antigen presentation during priming and triggering of the same cells in infected condition are not comparable. Altered proteolytic processing, antigen presentation, protease-susceptible sites, and intracellular expression of pathogenic proteins during Leishmania infection may vary dominant epitope selection, MHC-II/peptide affinity, and may deter the reactivation of desired antigen-specific T cells generated during priming. The robustness of the memory T cells and their functions remains a concern. Presentation of the antigens by Leishmania-infected macrophages to antigen-specific memory T cells may lead to change in the T cells' functional phenotype or anergy or apoptosis. Although cells may be activated, the peptides generated during infection may be different and cross-reactive to the priming peptides. Such altered peptide ligands may lead to suppression of otherwise active antigen-specific T cells. We critically assess these different immunological issues that led to the non-availability of a vaccine for human use.

Extracorporeal Shockwave Therapy Accelerates the Healing of a Meniscal Tear in the Avascular Region in a Rat Model

BACKGROUND

The treatment of meniscal tears in the avascular region remains a clinical challenge. Extracorporeal shockwave therapy (ESWT) is a minimally invasive, safe, and effective therapy for various orthopaedic disorders. However, the therapeutic effect of ESWT on meniscal tears has not been reported.

PURPOSE

To evaluate the therapeutic effect of ESWT in the treatment of meniscal tears.

STUDY DESIGN

Controlled laboratory study.

METHODS

Twelve-week-old male Wistar rats were divided into 3 groups (normal, ESWT-, and ESWT+). The authors made a full-thickness 2-mm longitudinal tear in the avascular region of the anterior horn in the latter 2 groups. At 1 week after surgery, the ESWT+ group received 800 impulses of shockwave at 0.22-mJ/mm² energy flux density in a single session. The authors performed a histological examination to evaluate meniscal healing (n = 10 for each group) and immunohistochemistry to analyze the expression of bromodeoxyuridine (BrdU; n = 5 for each group) and CCN family member 2/connective tissue growth factor (CCN2/CTGF; n = 5 for each group) at 2, 4, and 8 weeks after ESWT. The mRNA levels of CCN2, SOX 9, VEGF-a, aggrecan, Col1a2, and Col2a1 at the site of the meniscal tear at 4 weeks after ESWT were quantitatively evaluated by a real-time polymerase chain reaction (n = 5 for each group).

RESULTS

The meniscus healing scores in the ESWT+ group were significantly higher than those in the ESWT- group at 4 weeks and 8 weeks. The ratio of BrdU-positive cells was the highest in the ESWT+ group at all observation periods. The ratio of CCN2-positive cells was highest in the ESWT+ group at 4 and 8 weeks. In the ESWT+ group, real-time polymerase chain reaction revealed that the levels of CCN2, SOX9, aggrecan, and Col2a1 were upregulated (All significant data were P < .05).

CONCLUSION

ESWT promoted the healing of meniscal tears in the avascular area. ESWT stimulated proliferation of meniscal cells and the upregulation of cartilage-repairing factors such as CCN2, with the upregulation of cartilage-specific extracellular matrix expression.

CLINICAL RELEVANCE

ESWT may be an effective therapeutic option that promotes meniscal healing in the avascular region.

Insights into the HIV Latency and the Role of Cytokines

Human immunodeficiency virus-1 (HIV-1) has the ability to infect latently at the level of individual CD4+ cells. Latent HIV-1 proviruses are transcriptionally silent and immunologically inert, but are still capable of reactivating productive lytic infection following cellular activation. These latent viruses are the main obstacle in the eradication of HIV-1, because current HIV-1 treatment regimens are ineffective against them. Normal immunological response against an antigen activates CD4+ naïve T cells. The activated CD4+ naïve T cells undergo cell cycle, resulting in further transformation and profound proliferation to form effector CD4+ T-cells. Notably, in HIV-1 infected individuals, some of the effector CD4+ T cells get infected with HIV-1. Upon fulfillment of their effector functions, almost all activated CD4+ T cells are committed to apoptosis or programmed cell death, but a miniscule fraction revert to quiescence and become resting memory CD4+ T cells to mediate a rapid immunological response against the same antigen in the future. However, due to the quiescent nature of the resting memory T cells, the integrated HIV-1 becomes transcriptionally silent and acquires a latent phenotype. Following re-exposure to the same antigen, memory cells and integrated HIV-1 are stimulated. The reactivated latent HIV provirus subsequently proceeds through its life cycle and eventually leads to the production of new viral progeny. Recently, many strategies against HIV-1 latency have been developed and some of them have even matured to the clinical level, but none can yet effectively eliminate the latent HIV reservoir, which remains a barrier to HIV-1 cure. Therefore, alternative strategies to eradicate latent HIV need to be considered. This review provides vital knowledge on HIV latency and on strategies to supplement highly active anti-retroviral therapy (HAART) with cytokine-mediated therapeutics for dislodging the latent HIV reservoirs in order to open up new avenues for curing HIV.

Neurogenesis changes and the fate of progenitor cells after subarachnoid hemorrhage in rats

BACKGROUND

Subarachnoid hemorrhage (SAH) is a devastating cerebrovascular disease that leads to poor outcomes. Neurogenesis, an essential recovery mechanism after brain injury, has not been fully elucidated after SAH.

METHODS

A total of 122 SD rats were used in this study. For experiment one, the rats were randomly divided into six groups: sham and SAH with different time points (1,3,5,7,14 days) (n = 12/group). An endovascular perforation method was conducted for SAH model. Rats were injected with 5-Bromo-2'-deoxyuridine (BrdU, 50 mg/kg) 24 h before euthanasia at different time points after SAH. The BrdU labeled cells were detected by immunohistochemistry; Doublecortin (DCX) and glial fibrillary acidic protein (GFAP) were measured by western blot and immunohistochemistry. For experiment two, rats were randomly divided into five groups: sham and SAH with different time points (1, 2, 4, 8 weeks) (n = 6/group). Rats received BrdU (50 mg/kg) once daily for 7 days after the induction of SAH. Double immunofluorescence staining was used to verify proliferation, differentiation and migration of progenitor cells. Rotarod test and water maze used to test the neurobehavioral recovery.

RESULTS

Our results showed that BrdU positive cells in hippocampus changed overtime after SAH. BrdU positive cells decreased as early as 1 day reaching lowest levels at 3 days after SAH, after which it gradually recovered. Similar change patterns were observed with DCX, which was reversed with GFAP. In addition, BrdU did not co-localize with cleaved caspase-3. The BrdU positive cells mainly differentiated into immature neurons for short-term fate, whereas they differentiated into mature neurons for long-term fate but not astrocytes, which facilitated neurobehavioral recovery after SAH.

CONCLUSION

Neurogenesis in the hippocampus changes overtime after SAH. The neuronal progenitor cells may play an essential role in the neurobehavioral recovery after brain injury induced by SAH, since short-term progenitors helped with the recovery of immature neurons in the hippocampus, whereas long-term progenitors differentiated into mature neurons.

Age, but not short-term intensive swimming, affects chondrocyte turnover in zebrafish vertebral cartilage

Both age and intensive exercise are generally considered critical risk factors for osteoarthritis. In this work, we intend to establish zebrafish models to assess the role of these two factors on cartilage homeostasis. We designed a swimming device for zebrafish intensive exercise. The body measurements, bone mineral density (BMD) and the histology of spinal cartilages of 4- and 12-month-old zebrafish, as well the 12-month-old zebrafish before and after a 2-week exercise were compared. Our results indicate that both age and exercise affect the body length and body weight, and the micro-computed tomography reveals that both age and exercise affect the spinal BMD. However, quantitative analysis of immunohistochemistry and histochemistry indicate that short-term intensive exercise does not affect the extracellular matrix (ECM) of spinal cartilage. On the other hand, the cartilage ECM significantly grew from 4 to 12 months of age with an increase in total chondrocytes. dUTP nick end labeling staining shows that the percentages of apoptotic cells significantly increase as the zebrafish grows, whereas the BrdU labeling shows that proliferative cells dramatically decrease from 4 to 12 months of age. A 30-day chase of BrdU labeling shows some retention of labeling in cells in 4-month-old spinal cartilage but not in cartilage from 12-month-old zebrafish. Taken together, our results suggest that zebrafish chondrocytes are actively turned over, and indicate that aging is a critical factor that alters cartilage homeostasis. Zebrafish vertebral cartilage may serve as a good model to study the maturation and homeostasis of articular cartilage.

Spontaneous Proliferation of CD4+ T Cells in RAG-Deficient Hosts Promotes Antigen-Independent but IL-2-Dependent Strong Proliferative Response of Naïve CD8+ T Cells

The fast and intense proliferative responses have been well documented for naïve T cells adoptively transferred into chronic lymphopenic hosts. This response known as spontaneous proliferation (SP), unlike antigen-independent lymphopenia-induced proliferation (LIP), is driven in a manner dependent on antigens derived from commensal microbiota. However, the precise nature of the SP response and its impact on homeostasis and function for T cells rapidly responding under this lymphopenic condition are still unclear. Here we demonstrate that, when naïve T cells were adoptively transferred into specific pathogen-free (SPF) but not germ-free (GF) RAG^-/- hosts, the SP response of these cells substantially affects the intensity and tempo of the responding T cells undergoing LIP. Therefore, the resulting response of these cells in SPF RAG^-/- hosts was faster and stronger than the typical LIP response observed in irradiated B6 hosts. Although the intensity and tempo of such augmented LIP in SPF RAG^-/- hosts were analogous to those of antigen-dependent SP, the former was independent of antigenic stimulation but most importantly, dependent on IL-2. Similar observations were also apparent in other acute lymphopenic settings where antigen-dependent T cell activation can strongly occur and induce sufficient levels of IL-2 production. Consequently, the resulting T cells undergoing IL-2-driven strong proliferative responses showed the ability to differentiate into functional effector and memory cells that can control infectious pathogens. These findings therefore reveal previously unappreciated role of IL-2 in driving the intense form of T cell proliferative responses in chronic lymphopenic hosts.

CXCR3high CD8+ T cells with naïve phenotype and high capacity for IFN-γ production are generated during homeostatic T-cell proliferation

Naïve phenotype (NP) T cells spontaneously initiate homeostatic proliferation (HP) as T-cell output is reduced because of physiologic thymic involution with age. However, the effects of sustained HP on overall immune function are poorly understood. We demonstrated that the NP CD8⁺ T cell population in adult thymectomized mice showing accelerated HP has an increased capacity for TCR-mediated interferon-γ and tumor necrosis factor α production, which is attributed to an increase in CXCR3⁺ cells in the NP CD8⁺ T cell population. The CXCR3⁺ NP CD8⁺ T cells developed during persistent HP with a slow cell division rate, but rarely during robust antigen-driven proliferation with a fast cell division rate. In ontogeny, the proportions of CXCR3⁺ cells in the NP CD8⁺ T cell population showed a biphasic profile, which was high at the newborn and aged stages. Upon transfer, CXCR3⁺ NP CD8⁺ T cells, but not CXCR3^- NP CD8⁺ T cells, potently enhanced Th17-mediated inflammatory tissue reactions in vivo. Furthermore, CXCR3^high NP CD8⁺ T cells with similar features were also detected at variable levels in healthy human blood. These results suggest that CXCR3⁺ NP CD8⁺ T cells generated during physiological HP significantly impact overall immunity at the immunologically vulnerable neonatal and aged stages.

Lymphatic cannulation models in sheep: Recent advances for immunological and biomedical research

Lymphatic cannulation models are useful tools for studying the immunobiology of the lymphatic system and the immunopathology of specific tissues in diseases. Sheep cannulations have been used extensively, as models for human physiology, fetal and neonatal development, human diseases, and for studies of ruminant pathobiology. The development of new and improved cannulation techniques in recent years has meant that difficult to access sites, such as mucosal associated tissues, are now more readily available to researchers. This review highlights the new approaches to cannulation and how these, in combination with advanced omics technologies, will direct future research using the sheep model.

Clinical use of lentiviral vectors

Viral vectors provide an efficient means for modification of eukaryotic cells, and their use is now commonplace in academic laboratories and industry for both research and clinical gene therapy applications. Lentiviral vectors, derived from the human immunodeficiency virus, have been extensively investigated and optimized over the past two decades. Third-generation, self-inactivating lentiviral vectors have recently been used in multiple clinical trials to introduce genes into hematopoietic stem cells to correct primary immunodeficiencies and hemoglobinopathies. These vectors have also been used to introduce genes into mature T cells to generate immunity to cancer through the delivery of chimeric antigen receptors (CARs) or cloned T-cell receptors. CAR T-cell therapies engineered using lentiviral vectors have demonstrated noteworthy clinical success in patients with B-cell malignancies leading to regulatory approval of the first genetically engineered cellular therapy using lentiviral vectors. In this review, we discuss several aspects of lentiviral vectors that will be of interest to clinicians, including an overview of lentiviral vector development, the current uses of viral vectors as therapy for primary immunodeficiencies and cancers, large-scale manufacturing of lentiviral vectors, and long-term follow-up of patients treated with gene therapy products.

Mechanisms regulating expansion of CD8+ T cells during HIV-1 infection

Abnormal immune activation and expansion of CD8+ T cells, especially of memory and effector phenotypes, take place during HIV-1 infection, and these abnormal features persist during administration of antiretroviral therapy (ART) to infected patients. The molecular mechanisms for CD8+ T-cell expansion remain poorly characterized. In this article, we review the literature addressing features of CD8+ T-cell immune pathology and present an integrated view on the mechanisms leading to abnormal CD8+ T-cell expansion during HIV-1 infection. The expression of molecules important for directing the homing of CD8+ T cells between the circulation and lymphoid tissues, in particular CCR5 and CXCR3, is increased in CD8+ T cells in circulation and in inflamed tissues during HIV-1 infection; these disturbances in the homing capacity of CD8+ T cells have been linked to increased CD8+ T-cell proliferation. The production of IL-15, a cytokine responsible for physiological proliferation of CD8+ T cells, is increased in lymphoid tissues during HIV-1 infection as result of microbial translocation and severe inflammation. IL-15, and additional inflammatory cytokines, may lead to deregulated proliferation of CD8+ T cells and explain the accumulation of CD8+ T cells in circulation. The decreased capacity of CD8+ T cells to localize to gut-associated lymphoid tissue also contributes to the accumulation of these cells in blood. Control of inflammation, through ART administration during primary HIV-1 infection or therapies aimed at controlling inflammation during HIV-1 infection, is pivotal to prevent abnormal expansion of CD8+ T cells during HIV-1 infection.

Persistent in vivo cytogenetic effects of radioiodine therapy: a 21-year follow-up study using multicolor FISH

Our previous studies demonstrated the cytogenetic effects in the peripheral blood lymphocytes of a 34-year-old male patient who received ablative radioactive 131iodine therapy (RIT) on two different occasions in 1992 and 1994. Assessment of RIT-induced chromosomal damage by the cytokinesis-blocked micronucleus assay (CBMN) showed the persistence of elevated micronucleus frequency in this patient for more than two decades since the first RIT. Subsequent cytogenetic analysis performed in 2012 revealed both stable and unstable aberrations, whose frequencies were higher than the baseline reported in the literature. Here, we report the findings of our recent cytogenetic analysis peformed in 2015 on this patient using the multicolor fluorescence in situ hybridization (mFISH) technique. Our results showed that both reciprocal and non-reciprocal translocations persisted at higher frequencies in the patient than those reported in 2012. Persistence of structural aberrations for more than two decades indicate that these aberrations might have originated from long-lived T-lymphocytes or hematopoietic stem cells. Our study suggests that the long-term persistence of chromosome translocations in circulating lymphocytes can be useful for monitoring the extent of RIT-induced chromosomal instability several years after exposure and for estimating the cumulative absorbed dose after multiple RITs for retrospective biodosimetry purposes. This is perhaps the first and longest follow-up study documenting the persistence of cytogenetic damage for 21 years after internal radiation exposure.

Analysing Temporal Dynamics of T Cell Division in vivo Using Ki67 and BrdU Co-labelling by Flow Cytometry

This protocol was developed to increase the richness of information available from in vivo T cell proliferation studies. DNA labelling techniques such as BrdU incorporation allow precise control of label administration and withdrawal, so that the division history of a population can be tracked in detail over long timeframes (days-weeks). Ki67 is expressed in the nucleus of dividing cells, and is retained for a short time (3-4 days) after division ( Gossel et al., 2017 ); therefore acting as a molecular clock to identify cells that have recently divided. Combining these two techniques allows the integration of current and historical proliferation information from individual cells within a population. This data can subsequently be used to probe population dynamics by fitting mathematical models of proliferation ( Gossel et al., 2017 ).

IL-7-dependent STAT1 activation limits homeostatic CD4+ T cell expansion

IL-7 regulates homeostatic mechanisms that maintain the overall size of the T cell pool throughout life. We show that, under steady-state conditions, IL-7 signaling is principally mediated by activation of signal transducers and activators of transcription 5 (STAT5). In contrast, under lymphopenic conditions, there is a modulation of STAT1 expression resulting in an IL-7-dependent STAT1 and STAT5 activation. Consequently, the IL-7-induced transcriptome is altered with enrichment of IFN-stimulated genes (ISGs). Moreover, STAT1 overexpression was associated with reduced survival in CD4+ T cells undergoing lymphopenia-induced proliferation (LIP). We propose a model in which T cells undergoing LIP upregulate STAT1 protein, "switching on" an alternate IL-7-dependent program. This mechanism could be a physiological process to regulate the expansion and size of the CD4+ T cell pool. During HIV infection, the virus could exploit this pathway, leading to the homeostatic dysregulation of the T cell pools observed in these patients.

More or less-On the influence of labelling strategies to infer cell population dynamics

The adoptive transfer of labelled cell populations has been an essential tool to determine and quantify cellular dynamics. The experimental methods to label and track cells over time range from fluorescent dyes over congenic markers towards single-cell labelling techniques, such as genetic barcodes. While these methods have been widely used to quantify cell differentiation and division dynamics, the extent to which the applied labelling strategy actually affects the quantification of the dynamics has not been determined so far. This is especially important in situations where measurements can only be obtained at a single time point, as e.g. due to organ harvest. To this end, we studied the appropriateness of various labelling strategies as characterised by the number of different labels and the initial number of cells per label to quantify cellular dynamics. We simulated adoptive transfer experiments in systems of various complexity that assumed either homoeostatic cellular turnover or cell expansion dynamics involving various steps of cell differentiation and proliferation. Re-sampling cells at a single time point, we determined the ability of different labelling strategies to recover the underlying kinetics. Our results indicate that cell transition and expansion rates are differently affected by experimental shortcomings, such as loss of cells during transfer or sampling, dependent on the labelling strategy used. Furthermore, uniformly distributed labels in the transferred population generally lead to more robust and less biased results than non-equal label sizes. In addition, our analysis indicates that certain labelling approaches incorporate a systematic bias for the identification of complex cell expansion dynamics.

IL-7 plays a critical role for the homeostasis of allergen-specific memory CD4 T cells in the lung and airways

Memory T cells respond rapidly to repeated antigen exposure and can maintain their population for extended periods through self-renewal. These characteristics of memory T cells have mainly been studied during viral infections, whereas their existence and functions in allergic diseases have been studied incompletely. Since allergic patients can suffer repeated relapses caused by intermittent allergen exposure, we hypothesized that allergen- specific memory Th2 cells are present and the factors necessary for the maintenance of these cells are provided by the lung and airways. Using a murine model of airway inflammation, we found that allergen-specific CD4 T cells survived longer than 70 days in the lung and airways in an IL-7 dependent fashion. These T cells showing homeostatic proliferation were largely found in the mediastinal lymph node (mLN), rather than the airways; however, cells residing in the lung and airways developed recall responses successfully. We also found that CD4 T cells exhibited differential phenotypes in the mLN and in the lung. Altogether, we believe that allergen-specific memory T cells reside and function in the lung and airways, while their numbers are replenished through homeostatic turnover in the mLNs. Furthermore, we determined that IL-7 signaling is important for the homeostasis of these cells.

Modelling T cell proliferation: Dynamics heterogeneity depending on cell differentiation, age, and genetic background

Cell proliferation is the common characteristic of all biological systems. The immune system insures the maintenance of body integrity on the basis of a continuous production of diversified T lymphocytes in the thymus. This involves processes of proliferation, differentiation, selection, death and migration of lymphocytes to peripheral tissues, where proliferation also occurs upon antigen recognition. Quantification of cell proliferation dynamics requires specific experimental methods and mathematical modelling. Here, we assess the impact of genetics and aging on the immune system by investigating the dynamics of proliferation of T lymphocytes across their differentiation through thymus and spleen in mice. Our investigation is based on single-cell multicolour flow cytometry analysis revealing the active incorporation of a thymidine analogue during S phase after pulse-chase-pulse experiments in vivo, versus cell DNA content. A generic mathematical model of state transition simulates through Ordinary Differential Equations (ODEs) the evolution of single cell behaviour during various durations of labelling. It allows us to fit our data, to deduce proliferation rates and estimate cell cycle durations in sub-populations. Our model is simple and flexible and is validated with other durations of pulse/chase experiments. Our results reveal that T cell proliferation is highly heterogeneous but with a specific “signature” that depends upon genetic origins, is specific to cell differentiation stages in thymus and spleen and is altered with age. In conclusion, our model allows us to infer proliferation rates and cell cycle phase durations from complex experimental 5-ethynyl-2'-deoxyuridine (EdU) data, revealing T cell proliferation heterogeneity and specific signatures.

We assess the impact of genetics and aging on immune system dynamics by investigating the dynamics of proliferation of T lymphocytes across their differentiation through thymus and spleen in mice. Understanding cell proliferation dynamics requires specific experimental methods and mathematical modelling. Our investigation is based upon single-cell multicolour flow cytometry analysis thereby revealing the active incorporation in DNA of a thymidine analogue during S phase after pulse-chase experiments in vivo, versus cell DNA content. A generic mathematical model that simulates the evolution of single cell behaviour during the experiment allows us to fit our data, to deduce proliferation rates and mean cell cycle phase durations in sub-populations. This reveals that T cell proliferation is constrained by genetic influences, declines with age, and is specific to cell differentiation stage, revealing a specific “signature” of cell proliferation. Our model is simple and flexible and can be used with other pulse/chase experiments.

K48-linked KLF4 ubiquitination by E3 ligase Mule controls T-cell proliferation and cell cycle progression

T-cell proliferation is regulated by ubiquitination but the underlying molecular mechanism remains obscure. Here we report that Lys-48-linked ubiquitination of the transcription factor KLF4 mediated by the E3 ligase Mule promotes T-cell entry into S phase. Mule is elevated in T cells upon TCR engagement, and Mule deficiency in T cells blocks proliferation because KLF4 accumulates and drives upregulation of its transcriptional targets E2F2 and the cyclin-dependent kinase inhibitors p21 and p27. T-cell-specific Mule knockout (TMKO) mice develop exacerbated experimental autoimmune encephalomyelitis (EAE), show impaired generation of antigen-specific CD8⁺ T cells with reduced cytokine production, and fail to clear LCMV infections. Thus, Mule-mediated ubiquitination of the novel substrate KLF4 regulates T-cell proliferation, autoimmunity and antiviral immune responses in vivo.

The E3 ligase Mule has been previously reported to be essential for B cell development and function by modulating p53 ubiquitination and degradation. Here Hao et al. identify KLF4 as a novel ubiquitination target of Mule and show it controls T cell proliferation and autoimmunity.

Memory CD4 T cell subsets are kinetically heterogeneous and replenished from naive T cells at high levels

Characterising the longevity of immunological memory requires establishing the rules underlying the renewal and death of peripheral T cells. However, we lack knowledge of the population structure and how self-renewal and de novo influx contribute to the maintenance of memory compartments. Here, we characterise the kinetics and structure of murine CD4 T cell memory subsets by measuring the rates of influx of new cells and using detailed timecourses of DNA labelling that also distinguish the behaviour of recently divided and quiescent cells. We find that both effector and central memory CD4 T cells comprise subpopulations with highly divergent rates of turnover, and show that inflows of new cells sourced from the naive pool strongly impact estimates of memory cell lifetimes and division rates. We also demonstrate that the maintenance of CD4 T cell memory subsets in healthy mice is unexpectedly and strikingly reliant on this replenishment.

Survival of mature T cells in the periphery is intrinsically dependent on GIMAP1 in mice

An effective immune system depends upon the survival of mature T cells in the periphery. Members of the GIMAP family of GTPases have been proposed to regulate this homeostasis, supported by the paucity of peripheral T cells in rodents deficient for either GIMAP1 or GIMAP5. It is unclear whether this lack of T cells is a consequence of an ontological defect, causing the thymus to generate and export T cells incapable of surviving in the periphery, or whether (alternatively or additionally) mature T cells intrinsically require GIMAP1 for survival. Using the ER^T2 Cre⁺ transgene, we conditionally deleted Gimap1 in C57BL/6 mice and demonstrate that GIMAP1 is intrinsically required for the survival of mature T cells in the periphery. We show that, in contrast to GIMAP5, this requirement is independent of the T-cells' activation status. We investigated the nature of the survival defect in GIMAP1-deficient CD4⁺ T cells and show that the death occurring after GIMAP1 ablation is accompanied by mitochondrial depolarization and activation of the extrinsic apoptotic pathway. This study shows that GIMAP1 is critical for maintaining the peripheral T-cell pool in mice and offers a potent target for the treatment of T-cell-mediated diseases.

The Effect of Cellular Stress on T and B Cell Memory Pathways in Immunized and Unimmunized BALB/c Mice

Immunological memory is a fundamental function of vaccination. The antigenic breakdown products of the vaccine may not persist, and undefined tonic stimulation has been proposed to maintain the specific memory. We have suggested that cellular stress agents to which the immune cells are constantly exposed may be responsible for tonic stimulation. Here we have studied four stress agents: sodium arsenite, an oxidative agent; Gramicidin, eliciting K⁺ efflux and calcium influx; dithiocarbamate, a metal ionophore; and aluminum hydroxide (alum), an immunological adjuvant. The aims of this study are to extend these investigations to T and B cell responses of unimmunized and ovalbumin (OVA)-immunized BALB/c mice, and furthermore, to ascertain whether stress is involved in optimal expression of memory B cells, as demonstrated in CD4⁺ T cells. Examination of the homeostatic pathway defined by IL-15/IL-15R (IL-15 receptor) interaction and the inflammasome pathway defined by the IL-1-IL-1R interaction between dendritic cells (DC) and CD4⁺ T cells suggests that both pathways are involved in the development of optimal expression of CD4⁺CD45RO⁺ memory T cells in unimmunized and OVA-immunized BALB/c mice. Furthermore, significant direct correlation was found between CD4⁺CD44⁺ memory T cells and both IL-15 of the homeostatic and IL-1β of the inflammasome pathways. However, CD19⁺CD27⁺ memory B cells in vivo seem to utilize only the IL-15/IL-15R homeostatic pathway, although the proliferative responses are enhanced by the stress agents. Altogether, stress agents may up-regulate unimmunized and OVA-immunized CD4⁺CD44⁺ memory T cells by the homeostatic and inflammasome pathways. However, the CD19⁺CD27⁺ memory B cells utilize only the homeostatic pathway.

The MicroRNA miR-191 Supports T Cell Survival Following Common γ Chain Signaling

To ensure lifelong immunocompetency, naïve and memory T cells must be adequately maintained in the peripheral lymphoid tissues. Homeostatic maintenance of T cells is controlled by tonic signaling through T cell antigen receptors and common γ chain cytokine receptors. In this study, we identify the highly expressed microRNA miR-191 as a key regulator of naïve, memory, and regulatory T cell homeostasis. Conditional deletion of miR-191 using LckCre resulted in preferential loss of peripheral CD4⁺ regulatory T cells as well as naïve and memory CD8⁺ T cells. This preferential loss stemmed from reduced survival following deficient cytokine signaling and STAT5 activation. Mechanistically, insulin receptor substrate 1 (Irs1) is a direct target of miR-191, and dysregulated IRS1 expression antagonizes STAT5 activation. Our study identifies a novel role for microRNAs in fine-tuning immune homeostasis and thereby maintaining the lymphocyte reservoir necessary to mount productive immune responses.

Self-reactivity as the necessary cost of maintaining a diverse memory T-cell repertoire

The adaptive immune system is expected to protect the host from infectious agents and malignancies, while avoiding robust activation against self-peptides. However, T cells are notoriously inept at protection whenever the pathogen or tumor is persistent in the body for longer periods of time. While this has been thought of as an adaptation to limit the immunopathology from continued effector T-cell responses, it is also likely an extension of the T cell's intrinsic mechanisms which evolved to tolerate self-peptides. Here we deliberate on how the need to tolerate self-peptides might stem from a paradoxical requirement-the utility of such molecules in maintaining a diverse repertoire of pathogen-specific memory T cells in the body. Understanding the mechanisms underlying this intriguing nexus, therefore, has the potential to reveal therapeutic strategies not only for improving immune responses to chronic infections and tumors but also the long-term efficacy of vaccines aimed at cellular immune responses.

Two Niches in the Bone Marrow: A Hypothesis on Life-long T Cell Memory

The concept is emerging that the bone marrow (BM) sustains life-long persistence of memory T cells, as it does for plasma cells. Recent studies revived the debate on how this is achieved: is the BM essentially a nest for the proliferation of recirculating memory T cells, or a storage depot for resting memory T cells? Learning from division of labor in hematopoietic stem cells, this article proposes that two distinct BM niches support memory T cell cycling and quiescence, thereby enabling memory T cells to maintain all their distinguishing features. This framework might be instrumental to interpret some puzzling findings and conceptualize the mechanisms preserving either stability of memory T cell numbers or the capacity to mount secondary responses.

Analysis of Cell Proliferation and Homeostasis Using EdU Labeling

Determination of cellular proliferation and population turnover is an important tool for research on lymphoid cell function. Historically this has been done using radiolabeled nucleotides or nucleoside analogs, such as BrdU (5-bromo-2-deoxyuridine), that are incorporated into nascent DNA during S-phase. Recently, a new procedure was developed to label nascent DNA using EdU (5-Ethynyl-2-deoxyuridine). This new method overcomes limitations imposed by the procedure used to detect BrdU because EdU detection is based on an easily performed chemical reaction that does not require DNA denaturation, is quick and reproducible, and has a superior signal-to-noise ratio. This technique offers a wide range of opportunities to analyze cellular proliferation, population homeostasis, and cell marking procedures.

Distinct IL-7 signaling in recent thymic emigrants versus mature naïve T cells controls T-cell homeostasis

IL-7 is essential for T-cell survival but its availability is limited in vivo. Consequently, all peripheral T cells, including recent thymic emigrants (RTEs) are constantly competing for IL-7 to survive. RTEs are required to replenish TCR diversity and rejuvenate the peripheral T-cell pool. However, it remains unknown how RTEs successfully compete with resident mature T cells for IL-7. Moreover, RTEs express low levels of IL-7 receptors, presumably rendering them even less competitive. Here, we show that, surprisingly, RTEs are more responsive to IL-7 than mature naïve T cells as demonstrated by markedly increased STAT5 phosphorylation upon IL-7 stimulation. Nonetheless, adoptive transfer of RTE cells into lymphopenic host mice resulted in slower IL-7-induced homeostatic proliferation and diminished expansion compared to naïve donor T cells. Mechanistically, we found that IL-7 signaling in RTEs preferentially upregulated expression of Bcl-2, which is anti-apoptotic but also anti-proliferative. In contrast, naïve T cells showed diminished Bcl-2 induction but greater proliferative response to IL-7. Collectively, these data indicate that IL-7 responsiveness in RTE is designed to maximize survival at the expense of reduced proliferation, consistent with RTE serving as a subpopulation of T cells rich in diversity but not in frequency.

Interleukin-21 administration to aged mice rejuvenates their peripheral T-cell pool by triggering de novo thymopoiesis

The vaccination efficacy in the elderly is significantly reduced compared to younger populations due to thymic involution and age‐related intrinsic changes affecting their naïve T‐cell compartment. Interleukin (IL)‐21 was recently shown to display thymostimulatory properties. Therefore, we hypothesized that its administration to ageing hosts may improve T‐cell output and thus restore a competent peripheral T‐cell compartment. Indeed, an increase in the production of recent thymic emigrants (RTEs) attributable to intrathymic expansion of early thymic progenitors (ETPs), double‐negative (DN), and double‐positive (DP) thymocytes as well as thymic epithelial cell (TEC) was observed in recombinant (r)IL‐21‐treated aged mice. In sharp contrast, no alterations in the frequency of bone marrow (BM)‐derived progenitors were detected following rIL‐21 administration. Enhanced production of naïve T cells improved the T‐cell receptor (TCR) repertoire diversity and re‐established a pool of T cells exhibiting higher levels of miR‐181a and diminished amounts of the TCR‐inhibiting phosphatases SHP‐2 and DUSP5/6. As a result, stimulation of T cells derived from rIL‐21‐treated aged mice displayed enhanced activation of Lck, ZAP‐70, and ERK, which ultimately boosted their IL‐2 production, CD25 expression, and proliferation capabilities in comparison with T cells derived from control aged mice. Consequently, aged rIL‐21‐treated mice vaccinated using a tyrosinase‐related protein 2 (Trp2)‐derived peptide exhibited a substantial delay in B16 tumor growth and improved survival. The results of this study highlight the immunorestorative function of rIL‐21 paving its use as a strategy for the re‐establishment of effective immunity in the elderly.

Strong Components of Epigenetic Memory in Cultured Human Fibroblasts Related to Site of Origin and Donor Age

Differentiating pluripotent cells from fibroblast progenitors is a potentially transformative tool in personalized medicine. We previously identified relatively greater success culturing dura-derived fibroblasts than scalp-derived fibroblasts from postmortem tissue. We hypothesized that these differences in culture success were related to epigenetic differences between the cultured fibroblasts by sampling location, and therefore generated genome-wide DNA methylation and transcriptome data on 11 intrinsically matched pairs of dural and scalp fibroblasts from donors across the lifespan (infant to 85 years). While these cultured fibroblasts were several generations removed from the primary tissue and morphologically indistinguishable, we found widespread epigenetic differences by sampling location at the single CpG (N = 101,989), region (N = 697), “block” (N = 243), and global spatial scales suggesting a strong epigenetic memory of original fibroblast location. Furthermore, many of these epigenetic differences manifested in the transcriptome, particularly at the region-level. We further identified 7,265 CpGs and 11 regions showing significant epigenetic memory related to the age of the donor, as well as an overall increased epigenetic variability, preferentially in scalp-derived fibroblasts—83% of loci were more variable in scalp, hypothesized to result from cumulative exposure to environmental stimuli in the primary tissue. By integrating publicly available DNA methylation datasets on individual cell populations in blood and brain, we identified significantly increased inter-individual variability in our scalp- and other skin-derived fibroblasts on a similar scale as epigenetic differences between different lineages of blood cells. Lastly, these epigenetic differences did not appear to be driven by somatic mutation—while we identified 64 probable de-novo variants across the 11 subjects, there was no association between mutation burden and age of the donor (p = 0.71). These results depict a strong component of epigenetic memory in cell culture from primary tissue, even after several generations of daughter cells, related to cell state and donor age.

Regenerative medicine specialists have been using a type of cell commonly found in the skin called the fibroblast because it is easily obtained from skin samples, grows well in culture, and can be manipulated in the laboratory to de-differentiate into a primordial state known as the induced pluripotent stem cell. These primitive stem cells can then be transformed into mature tissues, such as liver or pancreas cells. Here we show that fibroblasts, coming from different locations in the same individual, vary significantly in epigenetic marks called DNA methylation, which are involved in the regulation of gene expression. In addition to location-specific patterns of DNA methylation, we also find that fibroblasts from different anatomical locations respond differently in epigenetic patterns related to aging. As the field of regenerative medicine advances, our study demonstrates that deciding upon the source of fibroblasts from an individual to generate new tissues and organs may be an important consideration.

Bone Marrow T Cells and the Integrated Functions of Recirculating and Tissue-Resident Memory T Cells

Changes in T cell trafficking accompany the naive to memory T cell antigen-driven differentiation, which remains an incompletely defined developmental step. Upon priming, each naive T cell encounters essential signals – i.e., antigen, co-stimuli and cytokines – in a secondary lymphoid organ; nevertheless, its daughter effector and memory T cells recirculate and receive further signals during their migration through various lymphoid and non-lymphoid organs. These additional signals from tissue microenvironments have an impact on immune response features, including T cell effector function, expansion and contraction, memory differentiation, long-term maintenance, and recruitment upon antigenic rechallenge into local and/or systemic responses. The critical role of T cell trafficking in providing efficient T cell memory has long been a focus of interest. It is now well recognized that naive and memory T cells have different migratory pathways, and that memory T cells are heterogeneous with respect to their trafficking. We and others have observed that, long time after priming, memory T cells are preferentially found in certain niches such as the bone marrow (BM) or at the skin/mucosal site of pathogen entry, even in the absence of residual antigen. The different underlying mechanisms and peculiarities of resulting immunity are currently under study. In this review, we summarize key findings on BM and tissue-resident memory (TRM) T cells and revisit some issues in memory T cell maintenance within such niches. Moreover, we discuss BM seeding by memory T cells in the context of migration patterns and protective functions of either recirculating or TRM T cells.