-by Naftali Horwitz, Scientific Venture Consultant and Jyothi Devakumar, Chief Scientific Officer
COVID impacted all aspects of life including the world economy. One aspect which COVID, as a condition, highlighted was its ability to differentially impact the older populations, especially those with comorbidities. The vulnerability of older adults was brought to fore, and made clear that there exist key differences in the immune system across the lifespan. As we age, our immune cells weaken in their ability to fight infections and allow for opportunistic infections to ravage the elderly. In recent years we have come to better understand the reasons for this deficiency and T cells are a major player in this dysfunction.
Higher order organisms require an immune system by which to protect themselves from invading pathogens. In humans, the innate immune system, which we’re born with and which we share with most vertebrates, is the body’s first line of defense for fast-acting and non-specific responses. The adaptive immune system, which relies on T-cells and B-cells, is a more refined system that utilizes surface markers to specifically target pathogens and ensure appropriate immune activation. This system also ensures that a “memory” exists of previous pathogen infections such that recurring infections can be addressed more succinctly.
T cells in particular serve as critical mediators of adaptive immunity to choreograph the maintenance of immune response, homeostasis, and memory throughout an organism’s lifetime. To do so, T cells splinter into three different subtypes: Naïve T cells which respond to new antigens, memory T cells which arise after antigen exposure and serve to maintain lifelong immunity, and regulatory T cells which tightly regulate the magnitude of immune responses. Naïve and memory T cells are further split between CD4+ cells (CDs represent unique cell surface topology) which serve to signal to and activate other immune cells to mount an immune response, and CD8+ cells which directly induce cell death either through lysis or apoptosis.
Early in life, lymphoid progenitors migrate from the bone marrow to the thymus where they undergo T cell receptor rearrangement and several selection processes, finally emerging as mostly naïve T cells. Additionally, 9-10% of emerging T cells at this point become regulatory T cells. Soon after development and during childhood, new T cells spread across the body and encounter a large variety of antigens. During this period, Naïve T cells fend off pathogens and a portion of them establish pools of memory T cells that last beyond childhood and plateau in adulthood. Since fewer new antigens are encountered in adulthood, T cell function shifts from memory formation into immunoregulation especially as it relates to repeat infections and tumor monitoring.
As we age, T cell functionality decreases and this leads to immune dysregulation and a range of pathologies.
One contributing factor to this decrease in T cell function in older individuals stems from the process of thymic involution. From as early as the first year of human life, the thymus gradually declines in tissue mass and cellularity and steadily accumulates adipocytes. This process results in a stark reduction of naïve T cell output which in turn reduces the diversity of T cell antigen receptor (TCR) repertoire and culminates in disrupted T cell homeostasis.
Given this issue of thymic involution, T cell homeostasis in adults largely depends on the peripheral proliferation of naïve T cells. In young adults the thymus generates only 16% of total T cells and this declines further to < 1% in older individuals. As such, to maintain T cell homeostasis, naïve T cell populations must function as their own stem cells. In doing so, these cells open themselves up to the same mechanisms that affect stem cells as they age, namely loss of quiescence, increased differentiation and senescence.
Loss of quiescence and increased differentiation is especially evident in the fact that naïve CD8+ T cells experience accelerated turnover, and they also exhibit a 5 to 10-fold decrease in TCR diversity compared to their CD4+ counterparts. Explanations for this phenomenon vary and include: growth advantages to fractions of T cells via clonal hematopoiesis (selective expansion of specific clones), persistent selection by peripheral antigens causing ‘memory inflation’, epigenetic alterations, or impaired access to secondary lymphoid organs. The idea that lifelong antigen stimulation drives the naïve-memory imbalance is challenged however by the existence of cytokine activated T cells, otherwise known as “virtual memory T cells”, which present similar aging associated phenotypes to antigen-experienced memory cells but are in fact antigen inexperienced. Separately, there are distinct populations of T cells that exhibit several criteria of cellular senescence (zombie-like non functional state). These cells arise due to chronic viral exposure with viruses like human Cytomegalovirus (CMV) and Epstein-Barr virus (EBV). The longer an individual lives, the higher the likelihood of re-infection and the bigger the pool of senescent T cells becomes. Like their senescent cell counterparts these cells are implicated in several chronic disease states as well as poor vaccine responses.
Unsurprisingly, all of these aging-associated defects in T cells translates to a lapse of immunosurveillance towards cancer. Under normal conditions CD4+ T cells interact with antigen-presenting cells that cross-present tumor associated antigens and signal to CD8+ cells to kill the tumor cells. Diminished T-cell receptor repertoires as well as skewed ratios of CD4+ and CD8+ T cells compromise the response to novel cancer antigens. Likewise the production of inflammatory SASP-like secretomes in certain aged T cell populations contribute to a chronic inflammatory state that may promote tumor growth. Furthermore, recent mathematical models suggest a direct link between declining T cell production in the thymus and the incidence of several types of cancers along with infectious disease in the elderly. In terms of longevity, members of long-lived families have been found to be protected from the emergence of senescent T cells and the reduction of naïve T cells following viral infection. Similarly, inverse correlations have been found between the number of new T-cells exported from the thymus, and the incidence of coronary artery disease. Likewise, thymic output has been associated with an increased risk of complications following kidney transplants.
Several key strategies have been proposed to combat the aging phenotype of reduced thymopoiesis and T cell dysregulation. Chief among them has been the idea of reversing the process of thymic involution. Keratinocyte growth factor (KGF) has been found to stimulate the differentiation and proliferation of epithelial cells including those in the thymus. KGF treatment in aged mice resulted in enhanced thymic architecture and improved T cell frequency and function. Likewise, the pro-longevity hormone FGF21 has been found to reverse thymic involution by reducing lipid invasion and elevating the export of T cells in aging mice. In recent human clinical trials, administration of human growth hormone in men over the age of 50 led to a reduction in thymic fat content as well as an increase in naive CD4+ and CD8+ T cells and recent thymic emigrants. Besides the idea of reversing thymic involution, some groups have focused on bioengineering a new thymus using stem cells taken from patients. Such bioengineered thymi were recently transplanted into mice and were able to support the development of mature and functional T cells.
Given that thymic involution likely accounts for only a small part of T cell dysregulation in aging, other strategies have focused on activating the existing T cell pool. For example, administration of IL-7, a λ chain receptor cytokine, to people for as little as one week results in expansion of naïve CD4+ and CD8+ T cell repertoires and expands T cell receptor diversity in the process. Similarly, GDF15, a mitokine generated in response to mitochondrial stress and dysfunction, has been found to regulate the TReg-mediated suppression of conventional T cell activation and inflammatory cytokines characteristic of an aging inflammatory system. Recent evidence suggests that N-glycan branching negatively regulates TCR clustering, inhibits inflammatory signalling and suppresses development of autoimmunity. Treating T cells with mannosidase I inhibitor kifunensine (KIF) which blocks n-glycan branching rejuvenates aged T cell responses and suggests that such an approach could be used as a therapy. Additionally, evidence also suggests that administration of classic senolytic combinations like quercetin and dasatinib can restore the impaired differentiation of CD4+ T cells in the lungs of aged mice and restore their proper immune function in the context of influenza infection.
Since the resident pool of existing T cells may be depleted in older individuals, there have also been attempts to exogenously repopulate these populations. The simplest approach consists of collecting blood at early life stages that could be autologously transfused later in life to serve as a diverse source of T cells for elderly individuals. Similarly, adoptive T cell therapy, consisting of ex vivo selection and expansion of antigen specific T cell clones could be a powerful way to augment antigen specific immunity, assuming that the antigen specific clone can be found in the elderly individual. Finally, much like T cells engineered to express chimeric antigen receptors (CAR T) targeting tumor antigens have been developed to fight cancer, this same approach could be used to develop engineered T cells targeting an array of aging associated antigen targets in order to compensate for the dysfunctional native T cell pool.
While all of these strategies appear to have great potential as therapeutics, there remain key questions in terms of their translatability and efficacy. In terms of thymic regeneration, it remains unclear what stage of involution is irreversible and thus what would be an appropriate therapeutic window. Likewise, many of these protein supplementation strategies have been administered as one-time doses and it’s unclear whether such an approach would be sufficient to rescue thymic involution and maintain proper thymic function in the eldely or whether repeated dosing would be necessary. Repeat dosing could be an issue particularly for proteins like human growth hormone which has been inversely correlated with longevity across a wide range of animal models. Additionally, given that the majority of T cell pools in adults are maintained and amplified by naive T cells in adults, it remains unclear whether thymic regeneration would actually improve the conditions associated with T cell aging.
Trying to stimulate existing T cell populations in the elderly with exogenous proteins also encounters similar challenges in terms of dosing, administration and timing. Likewise, it’s unclear to what extent these strategies re-invigorate TCR diversity. While they may amplify existing clones, they may do little in the way of bringing back the magnitude of diversity available in younger individuals. Also, while classic senolytics may target “senescent-like” T cells, these therapies aren’t specific to these cell types and they could cause serious side effects like thrombocytopenia depending on their mechanism of action.
Re-introducing cells into the elderly also poses some challenges. While it may seem simple enough to harvest a patient's own T cells at a young age and re-introduce them when the patient is old (autologous transplantation), there is no guarantee that the aged microenvironment won’t negatively impact these young cells and impede them from functioning as they should. Conversely, taking T cells from a young individual and transplanting into a different aged individual (allogeneic transplant) risks immune rejection and graft versus host disease. Even bioengineering T cells ex vivo or fabricating bioengineered Thymi comes with its own set of complications especially if the starting material is induced pluripotent stem cell (iPSC)-derived. This technology is still in its early stages and we should be cautious about any strategy that requires dedifferentiation and redifferentiation as many of these approaches fail to fully recapture the functional outputs of native target cells and these re-differentiated cells may contain genetic and epigenetic disruptions that could induce malignancies. Overall, it may be that no one strategy emerges as the most effective in our efforts to revitalise T cell function and an elderly immune system. We may have to mix and match a variety of therapeutic avenues to arrive at the end goal of bolstered immunity.
This field of immune rejuvenation is considered one of the most promising and exciting fields of longevity. Immune aging clocks are also becoming a research hotbed and combining the rejuvenation strategies with precise measurement can pave the way for healthy aging or health span extension.