Effect of homeostatic T-cell proliferation in the vaccine responsiveness against influenza in elderly people

Defining Human Regulatory T Cells beyond FOXP3: The Need to Combine Phenotype with Function

Regulatory T cells (Tregs) are essential to maintain immune homeostasis by promoting self-tolerance. Reduced Treg numbers or functionality can lead to a loss of tolerance, increasing the risk of developing autoimmune diseases. An overwhelming variety of human Tregs has been described, based on either specific phenotype, tissue compartment, or pathological condition, yet the bulk of the literature only addresses CD25-positive and CD127-negative cells, coined by naturally occurring Tregs (nTregs), most of which express the transcription factor Forkhead box protein 3 (FOXP3). While the discovery of FOXP3 was seminal to understanding the origin and biology of nTregs, there is evidence in humans that not all T cells expressing FOXP3 are regulatory, and that not all Tregs express FOXP3. Namely, the activation of human T cells induces the transient expression of FOXP3, irrespective of whether they are regulatory or inflammatory effectors, while some induced T cells that may be broadly defined as Tregs (e.g., Tr1 cells) typically lack demethylation and do not express FOXP3. Furthermore, it is unknown whether and how many nTregs exist without FOXP3 expression. Several other candidate regulatory molecules, such as GITR, Lag-3, GARP, GPA33, Helios, and Neuropilin, have been identified but subsequently discarded as Treg-specific markers. Multiparametric analyses have uncovered a plethora of Treg phenotypes, and neither single markers nor combinations thereof can define all and only Tregs. To date, only the functional capacity to inhibit immune responses defines a Treg and distinguishes Tregs from inflammatory T cells (Teffs) in humans. This review revisits current knowledge of the Treg universe with respect to their heterogeneity in phenotype and function. We propose that it is unavoidable to characterize human Tregs by their phenotype in combination with their function, since phenotype alone does not unambiguously define Tregs. There is an unmet need to align the expression of specific markers or combinations thereof with a particular suppressive function to coin functional Treg entities and categorize Treg diversity.

Higher plasma levels of thymosin-α1 are associated with a lower waning of humoral response after COVID-19 vaccination: an eight months follow-up study in a nursing home

BACKGROUND

Older people achieve lower levels of antibody titers than younger populations after Covid-19 vaccination and show a marked waning humoral immunity over time, likely due to the senescence of the immune system. Nevertheless, age-related predictive factors of the waning humoral immune response to the vaccine have been scarcely explored. In a cohort of residents and healthcare workers from a nursing home that had received two doses of the BNT162b2 vaccine, we measured specific anti-S antibodies one (T1), four (T4), and eight (T8) months after receiving the second dose. Thymic-related functional markers, including thymic output, relative telomere length, and plasma thymosin-α1 levels, as well as immune cellular subsets, and biochemical and inflammatory biomarkers, were determined at T1, and tested for their associations with the magnitude of the vaccine response (T1) and the durability of such response both, at the short- (T1-T4) and the long-term (T1-T8). We aimed to identify age-related factors potentially associated with the magnitude and persistence of specific anti-S immunoglobulin G (IgG)-antibodies after COVID-19 vaccination in older people.

RESULTS

Participants (100% men, n = 98), were subdivided into three groups: young (< 50 years-old), middle-age (50-65 years-old), and older (≥65 years-old). Older participants achieved lower antibody titers at T1 and experienced higher decreases in both the short- and long-term. In the entire cohort, while the magnitude of the initial response was mainly associated with the levels of homocysteine [β (95% CI); - 0.155 (- 0.241 to - 0.068); p = 0.001], the durability of such response at both, the short-term and the long-term were predicted by the levels of thymosin-α1 [- 0.168 (- 0.305 to - 0.031); p = 0.017, and - 0.123 (- 0.212 to - 0.034); p = 0.008, respectively].

CONCLUSIONS

Higher plasma levels of thymosin-α1 were associated with a lower waning of anti-S IgG antibodies along the time. Our results suggest that plasma levels of thymosin-α1 could be used as a biomarker for predicting the durability of the responses after COVID-19 vaccination, possibly allowing to personalize the administration of vaccine boosters.

NK- and T-cell granzyme B and K expression correlates with age, CMV infection and influenza vaccine-induced antibody titres in older adults

Granzymes are a family of serine-proteases that act as critical mediators in the cytolytic and immunomodulatory activities of immune cells such as CD8⁺ T-cells and natural killer (NK) cells. Previous work indicates that both granzyme B (GZB) and K (GZK) are increased with age in CD8⁺ T-cells, and in the case of GZB, contribute to dysfunctional immune processes observed in older adults. Here, we sought to determine how GZB and GZK expression in NK-cells, and CD4⁺, CD8⁺, and gamma-delta T-cells, quantified in terms of positive cell frequency and mean fluorescence intensity (MFI), differed with age, age-related health-traits and the antibody response to high-dose influenza vaccine. We found that the frequency and MFI of GZB-expressing NK-cells, and CD8⁺ and Vδ1+ T-cells, and GZK-expressing CD8⁺ T-cells was significantly higher in older (66-97 years old; n = 75) vs. younger (24-37 years old; n = 10) adults by up to 5-fold. There were no significant associations of GZB/GZK expression with sex, frailty or plasma levels of TNF or IL-6 in older adults, but those who were seropositive for cytomegalovirus (CMV) exhibited significantly higher frequencies of GZB+ NK-cells, and CD4⁺, CD8⁺ and Vδ1+ T-cells, and GZK+ CD8⁺ T-cells (Cohen's d = .5-1.5). Pre-vaccination frequencies of GZB+ NK-cells were positively correlated with vaccine antibody responses against A/H3N2 (d = .17), while the frequencies of GZK+ NK and CD8⁺ T-cells were inversely associated with A/H1N1 (d = -0.18 to -0.20). Interestingly, GZK+ NK-cell frequency was inversely correlated with pre-vaccination A/H1N1 antibody titres, as well as those measured over the previous 4 years, further supporting a role for this subset in influencing vaccine antibody-responses. These findings further our understanding of how granzyme expression in different lymphoid cell-types may change with age, while suggesting that they influence vaccine responsiveness in older adults.

Decreased resistance to bacterial cold-water disease and excessive inflammatory response in ayu (Plecoglossus altivelis) reared at high water temperature

Introduction

Temporal elevation of water temperature positively affects immune activity and disease resistance in poikilothermic teleost fish. The ayu, Plecoglossus altivelis, an important fish species for Japanese freshwater fisheries, is usually produced under higher water temperatures than the natural conditions to facilitate rapid growth. However, it has been reported that rearing fish at higher water temperatures inhibits the development of the thymus, suggesting that resistance to infectious diseases is reduced in ayu reared at higher water temperatures. Here, we show that decreased resistance to bacterial cold-water disease and excessive inflammatory responses occurred in ayu reared at 22°C compared with those reared at lower temperatures.

Methods

Ayu larvae were reared at 12°C, 15°C and 22°C for 77 days and fed 3% of their body weight. Thymus index and condition factor was calculated after the fish rearing. Then, ayu reared at the different temperatures were challenged with Flavobacterium psychrophilum and the fish were sampled for histopathology and gene expression analyses. Further, the fish were vaccinated with formalin-killed F. psychrophilum and continuously reared at the three different water temperatures. Serum antibody titer was determined by ELISA and cumulative mortality in each group was recorded after the bacterial challenge.

Results

Ayu reared at 22°C showed a significantly lower thymus index and higher condition factor than those reared at lower temperatures. Infiltrated leukocytes and many melanin pigments were frequently observed in the adipose tissues and spleens of ayu reared at 22°C, respectively, but not in those reared at 12°C. The gene expression levels of inflammatory cytokines such as IL-1β, IL-8 and TNFα in the spleen were significantly higher in the 22°C group than in the 12°C group. The cumulative survival rate after challenge with Flavobacterium psychrophilum was 51.7%, 40.0% and 13.3% in the 12°C, 15°C and 22°C groups, respectively. The relative percent survival values of vaccinated fish reared at 15°C and 22°C groups were lower than those reared at 12°C. Moreover, the specific antibody titer of the vaccinated fish was the lowest in the 22°C group and the highest in the 12°C group.

Discussion

These results suggest that rearing the fish under high water temperature causes excessive inflammatory responses similar to metabolic inflammation in human obesity, resulting in a decrease of disease resistance. In addition, thymic involution induced by higher water temperature probably leads the poor response to vaccination. The present study provides insights into the physiological and immunological changes of fish under global warming.

The dark side of Tregs during aging

In the last century, we have seen a dramatic rise in the number of older persons globally, a trend known as the grey (or silver) tsunami. People live markedly longer than their predecessors worldwide, due to remarkable changes in their lifestyle and in progresses made by modern medicine. However, the older we become, the more susceptible we are to a series of age-related pathologies, including infections, cancers, autoimmune diseases, and multi-morbidities. Therefore, a key challenge for our modern societies is how to cope with this fragile portion of the population, so that everybody could have the opportunity to live a long and healthy life. From a holistic point of view, aging results from the progressive decline of various systems. Among them, the distinctive age-dependent changes in the immune system contribute to the enhanced frailty of the elderly. One of these affects a population of lymphocytes, known as regulatory T cells (Tregs), as accumulating evidence suggest that there is a significant increase in the frequency of these cells in secondary lymphoid organs (SLOs) of aged animals. Although there are still discrepancies in the literature about modifications to their functional properties during aging, mounting evidence suggests a detrimental role for Tregs in the elderly in the context of bacterial and viral infections by suppressing immune responses against non-self-antigens. Interestingly, Tregs seem to also contribute to the reduced effectiveness of immunizations against many pathogens by limiting the production of vaccine-induced protective antibodies. In this review, we will analyze the current state of understandings about the role of Tregs in acute and chronic infections as well as in vaccination response in both humans and mice. Lastly, we provide an overview of current strategies for Treg modulation with potential future applications to improve the effectiveness of vaccines in older individuals.

Gal3 Plays a Deleterious Role in a Mouse Model of Endotoxemia

Lipopolysaccharide (LPS)-induced endotoxemia induces an acute systemic inflammatory response that mimics some important features of sepsis, the disease with the highest mortality rate worldwide. In this work, we have analyzed a murine model of endotoxemia based on a single intraperitoneal injection of 5 mg/kg of LPS. We took advantage of galectin-3 (Gal3) knockout mice and found that the absence of Gal3 decreased the mortality rate oflethal endotoxemia in the first 80 h after the administration of LPS, along with a reduction in the tissular damage in several organs measured by electron microscopy. Using flow cytometry, we demonstrated that, in control conditions, peripheral immune cells, especially monocytes, exhibited high levels of Gal3, which were early depleted in response to LPS injection, thus suggesting Gal3 release under endotoxemia conditions. However, serum levels of Gal3 early decreased in response to LPS challenge (1 h), an indication that Gal3 may be extravasated to peripheral organs. Indeed, analysis of Gal3 in peripheral organs revealed a robust up-regulation of Gal3 36 h after LPS injection. Taken together, these results demonstrate the important role that Gal3 could play in the development of systemic inflammation, a well-established feature of sepsis, thus opening new and promising therapeutic options for these harmful conditions.

Regulatory T cells and vaccine effectiveness in older adults. Challenges and prospects

Since the discovery of lymphocytes with immunosuppressive activity, increasing interest has arisen in their possible influence on the immune response induced by vaccines. Regulatory T cells (Tregs) are essential for maintaining peripheral tolerance, preventing autoimmune diseases, and limiting chronic inflammatory diseases. However, they also limit beneficial immune responses by suppressing anti-infectious and anti-tumor immunity. Mounting evidence suggests that Tregs are involved, at least in part, in the low effectiveness of immunization against various diseases where it has been difficult to obtain protective vaccines. Interestingly, increased activity of Tregs is associated with aging, suggesting a key role for these cells in the lower vaccine effectiveness observed in older people. In this review, we analyze the impact of Tregs on vaccination, with a focus on older adults. Finally, we address an overview of current strategies for Tregs modulation with potential application to improve the effectiveness of future vaccines targeting older populations.

Coronavirus-specific antibody production in middle-aged mice requires phospholipase A2G2D

Worse outcomes occur in aged compared with young populations after infections with respiratory viruses, including pathogenic coronaviruses (SARS-CoV, MERS-CoV, and SARS-CoV-2), and are associated with a suboptimal lung milieu ("inflammaging"). We previously showed that a single inducible phospholipase, PLA2G2D, is associated with a proresolving/antiinflammatory response in the lungs, and increases with age. Survival was increased in naive Pla2g2d-/- mice infected with SARS-CoV resulting from augmented respiratory dendritic cell (rDC) activation and enhanced priming of virus-specific T cells. Here, in contrast, we show that intranasal immunization provided no additional protection in middle-aged Pla2g2d-/- mice infected with any of the 3 pathogenic human coronaviruses because virtually no virus-specific antibodies or follicular helper CD4+ T (Tfh) cells were produced. Using MERS-CoV-infected mice, we found that these effects did not result from T or B cell intrinsic factors. Rather, they resulted from enhanced, and ultimately, pathogenic rDC activation, as manifested most prominently by enhanced IL-1β expression. Wild-type rDC transfer to Pla2g2d-/- mice in conjunction with partial IL-1β blockade reversed this defect and resulted in increased virus-specific antibody and Tfh responses. Together, these results indicate that PLA2G2D has an unexpected role in the lungs, serving as an important modulator of rDC activation, with protective and pathogenic effects in respiratory coronavirus infections and immunization, respectively.

Immunological features beyond CD4/CD8 ratio values in older individuals

The CD4/CD8 T-cell ratio is emerging as a relevant marker of evolution for many pathologies and therapies. We aimed to explore immunological features beyond CD4/CD8 ratio values in older subjects (>65 years old) who were classified as having lower (<1.4), intermediate (1.4-2), or higher (>2) ratio values. The lower group showed a lower thymic output (sj/β-TREC ratio) and frequency of naïve T-cells, concomitant with increased mature T-cells. In these subjects, the CD4 T-cell subset was enriched in CD95+ but depleted of CD98+ cells. The regulatory T-cell (Treg) compartment was enriched in CTLA-4+ cells. The CD8 T-cell pool exhibited increased frequencies of CD95+ cells but decreased frequencies of integrin-β7+ cells. Interestingly, in the intermediate group, the CD4 pool showed greater differences than the CD8 pool, mostly for cellular senescence. Regarding inflammation, only hsCRP was elevated in the lower group; however, negative correlations between the CD4/CD8 ratio and β2-microglobulin and sCD163 were detected. These subjects displayed trends of more comorbidities and less independence in daily activities. Altogether, our data reveal different thymic output and immune profiles for T-cells across CD4/CD8 ratio values that can define immune capabilities, affecting health status in older individuals. Thus, the CD4/CD8 ratio may be used as an integrative marker of biological age.

The Complex Role of Regulatory T Cells in Immunity and Aging

The immune system is a tightly regulated network which allows the development of defense mechanisms against foreign antigens and tolerance toward self-antigens. Regulatory T cells (Treg) contribute to immune homeostasis by maintaining unresponsiveness to self-antigens and suppressing exaggerated immune responses. Dysregulation of any of these processes can lead to serious consequences. Classically, Treg cell functions have been described in CD4⁺ T cells, but other immune cells also harbour the capacity to modulate immune responses. Regulatory functions have been described for different CD8⁺ T cell subsets, as well as other T cells such as γδT cells or NKT cells. In this review we describe the diverse populations of Treg cells and their role in different scenarios. Special attention is paid to the aging process, which is characterized by an altered composition of immune cells. Treg cells can contribute to the development of various age-related diseases but they are poorly characterized in aged individuals. The huge diversity of cells that display immune modulatory functions and the lack of universal markers to identify Treg make the expanding field of Treg research complex and challenging. There are still many open questions that need to be answered to solve the enigma of regulatory T cells.

Overcoming immune dysfunction in the elderly: trained immunity as a novel approach

People with advanced age have a higher susceptibility to infections and exhibit increased mortality and morbidity as the ability of the immune system to combat infections decreases with age. While innate immune cells display functional defects such as decreased phagocytosis, chemotaxis and cytokine production, adaptive immune cells exhibit reduced receptor diversity, defective antibody production and a sharp decline in naive cell populations. Successful responses to vaccination in the elderly are critical to prevent common infections such as influenza and pneumonia, but vaccine efficacy decreases in older individuals compared with young adults. Trained immunity is a newly emerging concept that showed that innate immune cells possess non-specific immunological memory established through epigenetic and metabolic reprogramming upon encountering certain pathogenic stimuli. Clinical studies suggest that trained immunity can be utilized to enhance immune responses against infections and improve the efficiency of vaccinations in adults; however, how trained immunity responses are shaped with advanced age is still an open question. In this review, we provide an overview of the age-related changes in the immune system with a focus on innate immunity, discuss current vaccination strategies for the elderly, present the concept of trained immunity and propose it as a novel approach to enhance responses against infections and vaccinations in the elderly population.