After a few articles considering Alzheimer disease from several angles, I would like to switch gears this month and talk more generally about the interaction between the immune system and aging.

In his 2012 paper[1], Caleb E. Finch documents the evolution of life expectancy in the course of human history. The life expectancy at birth of our shared ape ancestor 6 millions years ago is hypothesized to approximate that of a chimpanzee, 15 years. The first Homo species appeared 1-2 million years ago and had a life expectancy of ~20 years, while H. sapiens came onto the scene ~100,000 years ago and could expect about 30 years of life. But starting around 200 years ago, concurrent with industrialization, human life expectancy jumped rapidly, to somewhere between 70 and 80 years today.

As many readers are likely aware, the huge recent increases in life expectancy are commonly attributed to improvements in hygiene, nutrition, and medicine during the nineteenth and twentieth centuries that reduced mortality from infections at all ages. Finch hypothesizes, generally, that early age mortality over the course of human history is primarily due to (acute) infection, while old age mortality is primarily due to (chronic) inflammation. Further analysis of mortality rates over the last several hundred years leads him to further hypothesize that aging has been slowed in proportion to the reduced exposure to infections in early life. These hypotheses are supported by twentieth century examples which strongly demonstrate influences of the early life environment on adult health, such as the effects of prenatal and postnatal developmental influences (e.g., nutrition, exposure to infection) on adult chronic metabolic and vascular disorders as well as physical traits and mental characteristics. This leads Finch to suggest “broadening the concept of ‘developmental origins’ to include three groups of factors: nutritional deficits, chronic stress from socioeconomic factors, and direct and indirect damage from infections.”

Finch also considers the effects of inflammation and diet on human evolution, proposing several environmental and foraging factors that may have been important in the genetic basis for evolving lower basal mortality through interactions with chronic inflammation, in particular: dietary fat and caloric content; infections from pathogens ingested from carrion and from exposure to excreta; and noninfectious inflammagens such as those in aerosols and in cooked foods. He hypothesizes that exposure to these proinflammatory factors, which one would expect to shorten life expectancy, actually resulted in humans evolving lower mortality and longer lifespans in response to highly inflammatory environments.

A means for this, he argues, was the development of the apoE4 genotype. Noting that the apoE4 allele favors advantageous fat accumulation and is also associated with enhanced inflammatory responses, Finch argues that heightened inflammatory response and more efficient fat storage would have been adaptive in a pro-inflammatory environment and during times of uncertain nutrition. As has been discussed in prior articles in Cooler Minds Prevail, the apoE alleles also influence diverse chronic non-infectious degenerative diseases and lifespan. “Thus,” Finch concludes, “the apoE allele system has multiple influences relevant to evolution of brain development, metabolic storage, host defense, and longevity.”

With the general relationship between inflammation and the evolution of human aging and life expectancy in mind, let us now consider immune system involvement in more detail, and the relationship between HIV and immunosenescence more specifically.

Immunosenescence refers to the ageassociated deterioration of the immune system. As an organism ages it gradually becomes deficient in its ability to respond to infections and experiences a decline in long-term immune memory. This is due to a number of specific biological changes such as diminished self-renewal capacity of hematopoietic stem cells, a decline in total number of phagocytes, impairment of Natural Killer (NK) and dendritic cells, and a reduction in B-cell population. There is also a decline in the production of new naïve lymphocytes and the functional competence of memory cell populations. As a result, advanced age is associated with increased frequency and severity of pathological health problems as well as an increase in morbidity due to impaired ability to respond to infections, diseases, and disorders.

It is not hard to imagine that an increased viral load leading to chronic inflammatory response may accelerate aging and immunosenescence. Evidence for this is accumulating rapidly since the advent of antiretroviral therapies for treatment of HIV infection. An unforeseen consequence of these successful therapies is that HIV patients are living longer but a striking number of them appear to be getting older faster, particularly showing early signs of dementia usually seen in the elderly. In one study, slightly more than 10% of older patients (avg = 56.7 years) with wellcontrolled HIV infection had cerebrospinal fluid (CSF) marker profiles consistent with Alzheimer disease[2] – more than 10 times the risk prevalence of the general population at the same age. HIV patients are also registering higher rates of insulin resistance and cholesterol imbalances, suffer elevated rates of melanoma and kidney cancers, and seven times the rate of other non-HIV-related cancers. And ultimately, long-term treated HIV-infected individuals also die at an earlier age than HIV-uninfected individuals[3].

Recent research is beginning to explore and unravel the interplay between HIV infection and other environmental factors (such as co-infection with other viruses) in the acceleration of the aging process of the immune system, leading to immunosenescence. In the setting of HIV infection, the immune response is associated with abnormally high levels of activation, leading to a cascade of continued viral spread and cell death, and accelerating the physiologic steps associated with immunosenescence. Despite clear improvements associated with effective antiretroviral therapy, some subjects show persistent alterations in T cell homeostasis, especially constraints on T cell recovery, which are further exacerbated in the setting of co-infection and increasing age.

Unsurprisingly, it has been observed that markers of immunosenescence might predict morbidity and mortality in HIV-infected adults as well as the general population. In both HIV infection and aging, immunosenescence is marked by an increased proportion of CD28- to CD57+, and memory CD8+ T cells with reduced capacity to produce interleukin 2 (IL-2), increased production of interleukin 6 (IL-6), resistance to apoptosis, and shortened telomeres. Levels of markers of inflammation are elevated in HIV infected patients, and elevations in markers such as high-sensitivity C-reactive protein, D-dimer, and interleukin 6 (IL-6) have been associated with increased risk for cardiovascular disease, opportunistic conditions, or all-cause mortality[4].

But even as we are beginning to identify markers that appear to be associated with risk of poor outcome in HIV infection, it is still unclear how patients should be treated on the basis of this information. To that end, several trials are underway to evaluate the effects of modulation of immune activation and inflammation in HIV infection. At the same time, clinicians at the forefront of advancing knowledge and clinical care are performing research aimed at optimizing care for aging HIV patients.

The implications for such research may be far-reaching. In fact, many HIV clinicians and researchers think that HIV may be key to understanding aging in general. Dr. Eric Verdin states, “I think in treated, HIV-infected patients the primary driver of disease is immunological. The study of individuals who are HIV-positive is likely to teach us things that are really new and important, not only about HIV infection, but also about normal aging.”

Dr. Steven Deeks stresses the collaborative efforts of experts across fields. “I think there is a high potential for tremendous progress in understanding HIV if we can assemble a team of experts from the world of HIV immunology and the world of gerontology,” he says. “Each field can dramatically inform the other. I believe HIV is a well described, well studied, distinct disease that can be used as
a model by the larger community to look at issues of aging.”

References

[1] Finch, C (2012). Evolution of the Human Lifespan, Past, Present, and Future: Phases in the Evolution of Human Life Expectancy in Relation to the Inflammatory Load. Proceedings of the American Philosophical Society, 156:1, 9-44.

[2] Mascolini, M (2013). Over 10% in Older HIV Group Fit Alzheimer’s Biomarker Risk Profile. Conference Reports for NATAP: 20th Conference on Retroviruses and Opportunistic Infections, March 3-6, 2013.

[3] Aberg, X (2012). Aging, Inflammation, and HIV Infection. Topics in Antiviral Medicine, 20:3, 101-105.

[4] Deeks, S, Verdin, S. and McCune, JM (2012). Immunosenescence and HIV. Current Opinion in Immunology, 24: 1-6.

Originally published as an article (in the Cooler Minds Prevail series) in Cryonics magazine, June, 2013