Memory T-Cell Homeostasis and Senescence during Aging

Senescent T cells: Beneficial and detrimental roles

As the thymus involutes during aging, the T-cell pool has to be maintained by the periodic expansion of preexisting T cells during adulthood. A conundrum is that repeated episodes of activation and proliferation drive the differentiation of T cells toward replicative senescence, due to telomere erosion. This review discusses mechanisms that regulate the end-stage differentiation (senescence) of T cells. Although these cells, within both CD4 and CD8 compartments, lose proliferative activity after antigen-specific challenge, they acquire innate-like immune function. While this may confer broad immune protection during aging, these senescent T cells may also cause immunopathology, especially in the context of excessive inflammation in tissue microenvironments.

Dynamic changes in thymic sub-populations during acute and long-term infections with virulent and virulence-attenuated Salmonella Typhimurium strains in C57BL/6 and autoimmune-prone lpr mice

SALMONELLA

Typhimurium infection in mice results in drastic loss of immature CD4^- CD8^- double negative (DN) and CD4⁺ CD8⁺ double positive (DP) thymic subsets compared to mature single positive (SP) subsets. We investigated changes in thymocyte sub-populations post infection with a wild type (WT) virulent strain and ΔrpoS, a virulence-attenuated strain, of Salmonella Typhimurium in C57BL/6 (B6) and Fas-deficient autoimmune-prone lpr mice. The WT strain caused acute thymic atrophy with greater loss of thymocytes in lpr mice compared to B6 mice. Infection with ΔrpoS caused progressive thymic atrophy in B6 and lpr mice. Analysis of thymocyte subsets revealed that immature thymocytes including the DN, immature single positive (ISP), and DP thymocytes underwent extensive loss. SP thymocytes were more resistant to loss in WT-infected B6 mice, whereas WT-infected lpr and ΔrpoS-infected mice exhibited depletion of SP thymocytes. Overall, thymocyte sub-populations exhibited differential susceptibilities depending on bacterial virulence and the host background.

A subset of gut leukocytes has telomerase-dependent "hyper-long" telomeres and require telomerase for function in zebrafish

BACKGROUND

Telomerase, the enzyme capable of elongating telomeres, is usually restricted in human somatic cells, which contributes to progressive telomere shortening with cell-division and ageing. T and B-cells cells are somatic cells that can break this rule and can modulate telomerase expression in a homeostatic manner. Whereas it seems intuitive that an immune cell type that depends on regular proliferation outbursts for function may have evolved to modulate telomerase expression it is less obvious why others may also do so, as has been suggested for macrophages and neutrophils in some chronic inflammation disease settings. The gut has been highlighted as a key modulator of systemic ageing and is a key tissue where inflammation must be carefully controlled to prevent dysfunction. How telomerase may play a role in innate immune subtypes in the context of natural ageing in the gut, however, remains to be determined.

RESULTS

Using the zebrafish model, we show that subsets of gut immune cells have telomerase-dependent"hyper-long" telomeres, which we identified as being predominantly macrophages and dendritics (mpeg1.1+ and cd45+mhcII+). Notably, mpeg1.1+ macrophages have much longer telomeres in the gut than in their haematopoietic tissue of origin, suggesting that there is modulation of telomerase in these cells, in the gut. Moreover, we show that a subset of gut mpeg1.1+ cells express telomerase (tert) in young WT zebrafish, but that the relative proportion of these cells decreases with ageing. Importantly, this is accompanied by telomere shortening and DNA damage responses with ageing and a telomerase-dependent decrease in expression of autophagy and immune activation markers. Finally, these telomerase-dependent molecular alterations are accompanied by impaired phagocytosis of E. coli and increased gut permeability in vivo.

CONCLUSIONS

Our data show that limiting levels of telomerase lead to alterations in gut immunity, impacting on the ability to clear pathogens in vivo. These are accompanied by increased gut permeability, which, together, are likely contributors to local and systemic tissue degeneration and increased susceptibility to infection with ageing.

Effect of Pregnancy Specific β1-Glycoprotein on the Replicative Potential of Naïve T Cells and Immune Memory T Cells

We studied the effects of pregnancy-specific β1-glycoprotein (PSG) on the replicative potential of naïve T cells (CD45RA⁺) and immune memory T cells (CD45R0⁺) in vitro by evaluating the expression of the hTERT gene in combination with the proliferative activity of cells. Human PSG was obtained by the author's patented method of immunopurification using a biospecific sorbent with subsequent removal of immunoglobulin contamination on a HiTrap Protein G HP column. We used monocultures of CD45RA⁺ and CD45R0⁺ lymphocytes isolated from peripheral blood mononuclear cells of reproductive-age women. It was found that PSG in physiological concentrations inhibited the expression of the hTERT gene mRNA in naïve T cells and immune memory T cells and simultaneously reduced the number of proliferating T cells estimated by the differential gating method. At the same time, PSG reduced CD71 expression only on naïve T cells without affecting this molecule on immune memory T cells. Thus, PSG decreased the replication potential and suppressed the proliferation of T cells and immune memory T cells, which in the context of pregnancy can contribute to the formation of immune tolerance to the semi-allogeneic embryo.

Immunosenescence is both functional/adaptive and dysfunctional/maladaptive

Alterations in the immune system with aging are considered to underlie many age-related diseases. However, many elderly individuals remain healthy until even a very advanced age. There is also an increase in numbers of centenarians and their apparent fitness. We should therefore change our unilaterally detrimental consideration of age-related immune changes. Recent data taking into consideration the immunobiography concept may allow for meaningful distinctions among various aging trajectories. This implies that the aging immune system has a homeodynamic characteristic balanced between adaptive and maladaptive aspects. The survival and health of an individual depends from the equilibrium of this balance. In this article, we highlight which parts of the aging of the immune system may be considered adaptive in contrast to those that may be maladaptive.

Allo-priming as a universal anti-viral vaccine: protecting elderly from current COVID-19 and any future unknown viral outbreak

BACKGROUND

We present the rationale for a novel allo-priming approach to serve the elderly as a universal anti-virus vaccine, as well serving to remodel the aging immune system in order to reverse immunosenescence and inflammaging. This approach has the potential to protect the most vulnerable from disease and provide society an incalculable economic benefit. Allo-priming healthy elderly adults is proposed to provide universal protection from progression of any type of viral infection, including protection against progression of the current outbreak of COVID-19 infection, and any future variants of the causative SARS-CoV-2 virus or the next 'Disease X'. Allo-priming is an alternative approach for the COVID-19 pandemic that provides a back-up in case vaccination strategies to elicit neutralizing antibody protection fails or fails to protect the vulnerable elderly population. The allo-priming is performed using activated, intentionally mismatched, ex vivo differentiated and expanded living Th1-like cells (AlloStim^®) derived from healthy donors currently in clinical use as an experimental cancer vaccine. Multiple intradermal injections of AlloStim^® creates a dominate titer of allo-specific Th1/CTL memory cells in circulation, replacing the dominance of exhausted memory cells of the aged immune system. Upon viral encounter, by-stander activation of the allo-specific memory cells causes an immediate release of IFN-ϒ, leading to development of an "anti-viral state", by-stander activation of innate cellular effector cells and activation of cross-reactive allo-specific CTL. In this manner, the non-specific activation of allo-specific Th1/CTL initiates a cascade of spatial and temporal immune events which act to limit the early viral titer. The release of endogenous heat shock proteins (HSP) and DAMP from lysed viral-infected cells, in the context of IFN-ϒ, creates of conditions for in situ vaccination leading to viral-specific Th1/CTL immunity. These viral-specific Th1/CTL provide sterilizing immunity and memory for protection from disease recurrence, while increasing the pool of Th1/CTL in circulation capable of responding to the next viral encounter.

CONCLUSION

Allo-priming has potential to provide universal protection from viral disease and is a strategy to reverse immunosenescence and counter-regulate chronic inflammation (inflammaging). Allo-priming can be used as an adjuvant for anti-viral vaccines and as a counter-measure for unknown biological threats and bio-economic terrorism.

[The influence of chorionic gonadotropin on phenotype conversion and hTERT gene expression by T-lymphocytes of different degrees of differentiation]

The effects of chorionic gonadotropin (hCG) on the expression of the hTERT gene in combination with the conversion of the phenotype of naive T-cells and T-cells of immune memory in vitro were studied. hCG inhibited expression of hTERT mRNA in naive T-cells (CD45RA+) and immune memory T cells (CD45RO+), causing a decrease in the replicative potential of the cells. The presence of hCG in the culture led to the conversion of the phenotype of T-lymphocytes. hCG reduced the number of proliferating T-cells of immune memory, estimated by phenotypic signs by differential gating. hCG (10 IU/ml and 100 IU/ml) inhibited expression of CD25 by the studied populations, but did not modulate expression of the CD71 proliferation marker. Thus, hCG inhibited the functional activity of naive T-cells and T-cells of immune memory, which, in the context of pregnancy, can contribute to the formation of immune tolerance to the semi-allogenic fetus.

How do PrPSc Prions Spread between Host Species, and within Hosts?

Prion diseases are sub-acute neurodegenerative diseases that affect humans and some domestic and free-ranging animals. Infectious prion agents are considered to comprise solely of abnormally folded isoforms of the cellular prion protein known as PrP^Sc. Pathology during prion disease is restricted to the central nervous system where it causes extensive neurodegeneration and ultimately leads to the death of the host. The first half of this review provides a thorough account of our understanding of the various ways in which PrP^Sc prions may spread between individuals within a population, both horizontally and vertically. Many natural prion diseases are acquired peripherally, such as by oral exposure, lesions to skin or mucous membranes, and possibly also via the nasal cavity. Following peripheral exposure, some prions accumulate to high levels within the secondary lymphoid organs as they make their journey from the site of infection to the brain, a process termed neuroinvasion. The replication of PrP^Sc prions within secondary lymphoid organs is important for their efficient spread to the brain. The second half of this review describes the key tissues, cells and molecules which are involved in the propagation of PrP^Sc prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain. This section also considers how additional factors such as inflammation and aging might influence prion disease susceptibility.

The Cellular and Molecular Basis of Translational Immunometabolism

The immune response requires major changes to metabolic processes, and indeed, energy metabolism and functional activation are fully integrated in immune cells to determine their ability to divide, differentiate, and carry out effector functions. Immune cell metabolism has therefore become an attractive target area for therapeutic purposes. A neglected aspect in the translation of immunometabolism is the critical connection between systemic and cellular metabolism. Here, we discuss the importance of understanding and manipulating the integration of systemic and immune cell metabolism through in-depth analysis of immune cell phenotype and function in human metabolic diseases and, in parallel, of the effects of conventional metabolic drugs on immune cell differentiation and function. We examine how the recent identification of selective metabolic programs operating in distinct immune cell subsets and functions has the potential to deliver tools for cell- and function-specific immunometabolic targeting.

Vaccination in Older Adults in Singapore: A Summary of Recent Literature

Singaporeans are living longer and not having enough babies to replace themselves, with projected 20% of the population aged 65 years and above by 2030. These worrying trends can exert significant pressure on Singapore's economy and society. Health-resource utilisations are increasing as our population ages, with increase in hospitalisations, emergency room visits and consumption of pharmaceutical drugs. Interventions such as vaccinations in older people are well-studied approaches to prevent infections and the related complications. The Advisory Committee on Immunization Practice (ACIP) by the US Centers for Disease Control and Prevention (CDC) recommends routine immunisation against influenza, tetanus, pneumococcal pneumonia as well as herpes zoster infections in all people aged 65 years and above. The administrations of other vaccines (meningococcal, hepatitis A and B, varicella and Haemophilus influenzae type b) are recommended only in older people with certain risk factors. Despite the benefits of vaccination in older people, the data in the West shows that the rate of vaccination in this age group remains lower than targets recommended by the World Health Organisation (WHO). The main barriers to vaccinations exist in the healthcare system, health care providers, patients and caregivers. Interventions are warranted to increase awareness among physicians, to engage the involvement of the pharmacist/nurse to provide standing orders and reduction in cost of vaccination; this potentially improves vaccination uptake to protect our increasing elderly population.

Aging and the mucosal immune system in the intestine

Bacterial and viral infections of the gastrointestinal tract are more common in the elderly and represent a major cause of morbidity and mortality. The mucosal immune system provides the first line of defence against pathogens acquired by ingestion and inhalation, but its function is adversely affected in the elderly. This aging-related decline in the immune function is termed immunosenescence and is associated with diminished abilities to generate protective immunity, reduced vaccine efficacy, increased incidence of cancer, inflammation and autoimmunity, and the impaired ability to generate tolerance to harmless antigens. In this review we describe our current understanding of the effects immunosenescence has on the innate and adaptive arms of the mucosal immune system in the intestine. Current estimates suggest that by the year 2050 up to 40% of the UK population will be over 65 years old, bringing with it important health challenges. A thorough understanding of the mechanisms that contribute to the development of immunosenescence is therefore crucial to help identify novel approaches to improve mucosal immunity in the elderly.

Immune defects in the risk of infection and response to vaccination in monoclonal gammopathy of undetermined significance and multiple myeloma

The plasma cell proliferative disorders monoclonal gammopathy of undetermined significance (MGUS) and malignant multiple myeloma (MM) are characterized by an accumulation of transformed clonal plasma cells in the bone marrow and production of monoclonal immunoglobulin. They typically affect an older population, with median age of diagnosis of approximately 70 years. In both disorders, there is an increased risk of infection due to the immunosuppressive effects of disease and conjointly of therapy in MM, and response to vaccination to counter infection is compromised. The underlying factors in a weakened immune response in MGUS and MM are as yet not fully understood. A confounding factor is the onset of normal aging, which quantitatively and qualitatively hampers humoral immunity to affect response to infection and vaccination. In this review, we examine the status of immune alterations in MGUS and MM and set these against normal aging immune responses. We focus primarily on quantitative and functional aspects of B-cell immunity. Furthermore, we review the current knowledge relating to susceptibility to infectious disease in MGUS and MM, and how efficacy of conventional vaccination is affected by proliferative disease-related and therapy-related factors.

Human virus-specific effector-type T cells accumulate in blood but not in lymph nodes

It is believed that the size of the CD8(+) T-cell pool is fixed and that with every new viral challenge, the size of the pre-existing memory-cell population shrinks to make way for the new virus-specific cells. CMV-seropositive individuals have high numbers of CMV-specific resting-effector type CD8(+) T cells in their peripheral blood (PB). This prompted us to investigate whether CMV infection limits immunologic space at sites where immune reactions are initiated, such as in the lymph nodes (LNs). LN and paired PB samples were analyzed for CMV-, EBV-, and influenza-specific CD8(+) T cells. In marked contrast to blood, LNs contained significantly lower numbers of CX3CR1-expressing effector-type CD8(+) T cells, whereas the CMV-specific cells that were found in the LNs resembled polyfunctional memory-type cells. In contrast, EBV- and influenza-specific CD8(+) T cells were highly similar between PB and LNs both in number and function. Therefore, it is unlikely that CMV-specific CD8(+) T cells in the LNs restrain the immunologic space of other virus-specific cells.

It's not all equal: a multiphasic theory of thymic involution

Regression of the thymus is a key feature of immunosenescence, which coincides with a decrease in T cell output and contributes to the restriction of the T cell repertoire in the elderly, leading to increased susceptibility to illness and disease. However, the mechanisms involved in thymic involution are still not fully known. Although, it is often believed that thymic involution occurs during the onset of puberty, increasing data suggests alterations to the thymus happen much earlier in life. Therefore, the changes in the thymus and subsequent thymic function may not just be an ageing phenomenon. In this article, we propose that there are several, non-linear, phases to thymic atrophy, which are regulated by different mechanisms, including the familiar age-dependent thymic involution and a much earlier growth-dependent thymic involution.