Rita B. Effros

Workshop on HIV Infection and Aging: What Is Known and Future Research Directions

Highly active antiretroviral treatment has resulted in dramatically increased life expectancy among patients with HIV infection who are now aging while receiving treatment and are at risk of developing chronic diseases associated with advanced age. Similarities between aging and the courses of human immunodeficiency virus (HIV) infection and acquired immunodeficiency syndrome suggest that HIV infection compresses the aging process, perhaps accelerating comorbidities and frailty. In a workshop organized by the Association of Specialty Professors, the Infectious Diseases Society of America, the HIV Medical Association, the National Institute on Aging, and the National Institute on Allergy and Infectious Diseases, researchers in infectious diseases, geriatrics, immunology, and gerontology met to review what is known about HIV infection and aging, to identify research gaps, and to suggest high priority topics for future research. Answers to the questions posed are likely to help prioritize and balance strategies to slow the progression of HIV infection, to address comorbidities and drug toxicity, and to enhance understanding about both HIV infection and aging.

Shortened telomeres in the expanded CD28- CD8+ cell subset in HIV disease implicate replicative senescence in HIV pathogenesis

OBJECTIVE To test the hypothesis that the expanded population of non-proliferative CD28-CD8+ T cells in HIV disease have shortened telomeres, thereby providing evidence that increased rounds of CD8+ cell division occur during HIV disease, possibly leading to replicative senescence and exhaustion of CD8+ T-cell responses. DESIGN CD8+ cells play a central role in control of HIV infection. In late HIV disease, an expanded population of CD28-CD8+ cells with reduced proliferative potential has been documented. A similar population of CD28-CD8+ cells has been identified in ageing humans, where telomere length measurements have suggested that these cells have reached the irreversible state of replicative senescence. METHODS CD8+ cells from HIV-infected and control subjects were sorted by flow cytometry into CD28+ and CD28- fractions. Telomere lengths were determined as mean terminal restriction fragment (TRF) lengths by Southern hybridization. RESULTS The TRF lengths of sorted CD28-CD8+ cells in HIV-infected subjects ranged between 5 and 7 kilobases (kb) and were significantly shorter than TRF lengths of CD28-CD8+ cells in uninfected subjects (P = 0.003). The TRF length in CD28-CD8+ cells from HIV-infected subjects was the same as that observed for centenarian peripheral blood mononuclear cells and is compatible with a state of replicative senescence. CONCLUSIONS The shortened telomeres in the CD28-CD8+ cells in HIV-infected subjects and the poor proliferative potential of these cells identifies CD8+ cell replicative senescence as a newly described feature of HIV disease. Our results provide a mechanism for the loss of CD8+ cell control of viral replication that accompanies advanced HIV disease. Replicative senescence may contribute to exhaustion of the T-cell response as a result of chronic HIV disease. Whether this phenomenon occurs in other chronic viral infections is unknown.

Telomere shortening in T cells correlates with Alzheimer’s disease status

Telomeres, the repeated sequences that cap chromosome ends, undergo shortening with each cell division, and therefore serve as markers of a cells replicative history. In vivo, clonal expansion of T cells during immune responses to both foreign and autoantigens is associated with telomere shortening. To investigate possible immune alterations in Alzheimers disease (AD) that might impact current vaccine-based therapeutic strategies, we analyzed telomere lengths in immune cell populations from AD patients. Our data show a significant telomere shortening in PBMC from AD versus controls (P=0.04). Importantly, telomere length of T cells, but not of B cells or monocytes, correlated with AD disease status, measured by Mini Mental Status Exam (MMSE) scores (P=0.025). T cell telomere length also inversely correlated with serum levels of the proinflammatory cytokine TNFalpha (a clinical marker of disease status), with the proportion of CD8+ T cells lacking expression of the CD28 costimulatory molecule, and with apoptosis. These findings suggest an immune involvement in AD pathogenesis.

Replicative senescence of T cells: does the Hayflick Limit lead to immune exhaustion?

Extensive in vitro research on fibroblasts has defined numerous genetic and phenotypic changes associated with replicative senescence. Identification of T-cell replicative senescence as a feature of human immunodeficiency virus (HIV) disease and ageing suggests this phenomenon merits more careful consideration by immunologists, especially with regard to chronic infection, memory and adoptive immunotherapy.

Ageing of lymphocytes and lymphocytes in the aged

Abstract Immunosenescence is a complex process involving multiple reorganizational and developmentally regulated changes, rather than simple unidirectional decline in all functions. Here, these compensatory modulations are reviewed with reference to the importance of integrating information on cellular alterations identified in vitro with data on global physiological changes in the aged environment with which these cells interact in vivo .

The role of CD8+ T-cell replicative senescence in human aging

Summary:  The strict limit in proliferative potential of normal human somatic cells – a process known as replicative senescence – is highly relevant to the immune system, because clonal expansion is fundamental to adaptive immunity. CD8+ T cells that undergo extensive rounds of antigen‐driven proliferation in cell culture invariably reach the end stage of replicative senescence, characterized by irreversible cell‐cycle arrest and a critically short telomere length. Cultures of senescent CD8+ T cells also show resistance to apoptosis, permanent loss of CD28 expression, altered cytokine profiles, reduced ability to respond to stress, and various functional changes. Cells with similar characteristics accumulate during normal aging as well as in younger persons infected with human immunodeficiency virus, suggesting that the process of replicative senescence is not an artifact of cell culture but is also occurring in vivo. Interestingly, in elderly persons, the presence of high proportions of CD8+ T cells with characteristics of replicative senescence is correlated with reduced antibody responses to vaccines as well as with osteoporotic fractures. CD8+CD28– T cells also accumulate in patients with certain types of cancer. The emerging picture is that senescent CD8+ T cells may modulate both immune and non‐immune functions, contributing not only to reduced anti‐viral immunity but also to diverse age‐related pathologies.

Immunosenescence: what does it mean to health outcomes in older adults?

The most profound consequences of immune senescence with respect to human health are the increased susceptibility to infectious diseases and decreased vaccine efficacy. Changes in both innate and adaptive immune function converge in the reduced response to vaccination and protection against infection and related diseases. The decline in thymic output of naïve T cells diminishes responses to novel antigens, such as West Nile Virus, while clonal expansions leading to defects in the T cell repertoire are associated with blunted responses of memory T cells to conserved epitopes of the influenza virus. Recent studies on how immunologic mechanisms of protection change during aging have led to novel strategies for improving vaccine responsiveness and outcomes of infectious diseases in older adults.

Calorie restriction inhibits the age-related dysregulation of the cytokines TNF-α and IL-6 in C3B10RF1 mice

TNF-alpha and IL-6 are generally increased in the sera of aged humans and mice. The dysregulation of these cytokines may be critical in autoreactivity and immune dysfunction. In earlier studies we demonstrated that production of TNF-alpha and IL-6 following in vitro stimulation of peritoneal macrophages by LPS was reduced in old compared to young mice, and that dietary caloric restriction (CR) had no effect on the induction of TNF-alpha in this system. In the present study we examined the effects of age and calorie restriction on the constitutive production of both TNF-alpha and IL-6. Serum levels of both cytokines were significantly higher in old versus young mice. However, in old mice subjected to long term CR the serum levels were comparable to those of young mice. The potential involvement of normalization of TNF-alpha and IL-6 levels in the life extension effect of CR are discussed.

Reduced telomerase activity in human T lymphocytes exposed to cortisol

Accelerated telomere shortening in lymphocytes has been associated with a variety of human pathologies, including HIV disease, Down syndrome, and cardiovascular disease. Recent findings indicate that reduced telomere length is also associated with chronic psychological stress and mood disorders. Telomerase, which prevents telomere shortening, can be upregulated in T lymphocytes in concert with activation, thereby retarding telomere shortening. Here, we demonstrate that exposure of human T lymphocytes to cortisol is associated with a significant reduction in telomerase activity both during primary stimulation of resting cells and secondary stimulation of previously activated cells. The effect is observed in both CD4 and CD8 T lymphocytes, and is associated with reduced transcription of hTERT, the telomerase catalytic component. These findings provide a potential mechanism for stress-associated telomere length attrition, and suggest that strategies to enhance T lymphocyte telomerase activity may provide beneficial effects on immune function in situations of chronic emotional stress.

CD28 Expression in T Cell Aging and Human Longevity

Functional decrements of the immune system have a major contribution to aging and age-related diseases. Here, we further characterize the decline in proportion of CD28-positive T cells previously identified in centenarians. Cohorts of 97 centenarians, 40 subjects aged 70-90 (ELD group), and 40 young adults (under age 40) were phenotyped for T cell surface expression of CD28, CD4, and CD8 antigens. The significant decline in T cells expressing CD28 (p < 10(-4) for comparisons between adults and either ELD or centenarians) affects preferentially the CD8+ subset of T cells. This decline accounts largely for the age-related diminution of T cell responsiveness to mitogenic signals. CD28 expression is modulated in T cell cultures in a growth-related fashion and this modulation is dampened in cultures from centenarians. We propose that the decrease in CD28 expression reflects a compensatory adaptation of the immune system during aging in the face of chronic stimulation.