Restore senile immunity

Interventions aimed at restoring immune function in the elderly

Interventions to restore appropriate immune function in the elderly Richard Aspinall, Pierre Olivier Lang, Immunity & Ageing, 2018.

Translated by Evgenia Ryabtseva
For references and references, see the original article.

Advanced age is characterized by an increased likelihood of developing immune dysfunction. Since the number of elderly people in the global population is growing progressively worldwide, there is a possibility of an increase in the prevalence and frequency of infectious diseases caused by both common and suddenly emerging pathogens. The result of this will be an increase in mortality and morbidity, with a corresponding increase in the risk of reduced capacity and the development of disability. Maintaining immune function at a stable level throughout life can be associated with a significant economic effect.

Improvement of immune functions in the elderly can be achieved with the help of a therapeutic approach aimed at rejuvenation, stimulation or support of the innate immune system in order to optimize its work. From the point of view of economic efficiency, such a therapeutic approach can be a difficult task because of the aspects associated with it: identifying people who will receive the maximum benefit; identifying the most appropriate approach for them and evaluating its effectiveness. An alternative approach is to keep active or provide stronger incentives through vaccination, in addition to greater cost-effectiveness, is the best option in the near term. This review examines both approaches.

Background

The slow and inexorable growth in the number of elderly people occurring throughout the world over the past few decades will have a significant impact on medical services, as well as on the epidemiology of vector-borne (contagious) and non-transmissive diseases. It is expected that the frequency of development of the latter will reach unprecedented values. Currently, people aged 65 and older travel more often and longer distances than their parents or grandparents. They are also physically more active than their peers a few decades ago. These factors play a role in changing the epidemiology of diseases. Another factor contributing to the problem is that today the globe is closely connected into a single system, and it may take several hours for any person or pathogen to cross the entire planet, which has recently been demonstrated by H5N1, H1N1, MERS, SARS, chikungunya fever and other outbreaks caused by newly emerging pathogens. We also need to include in this algorithm increased susceptibility, which is partly due to the extinction of immune function in the elderly. All together, these factors will contribute to a shift in the profile of common and newly emerging infectious diseases.

Proactive measures must be taken to preserve this growing sector of the general population and maintain its functional independence in everyday life. Vaccination is one of the most effective medical interventions ever undertaken by humans and prevents millions of infections worldwide every year. However, it is believed that vaccines are less effective in protecting the elderly. One of the main reasons that gave rise to this statement is the extinction of the effectiveness of immunity in this population. The results of studies conducted in different countries indicate the global nature of the problem of age-related extinction of the immune system, but very few works have been devoted to finding a solution to it. One of the tested approaches was an attempt to restore the immunity of representatives of this population to levels comparable to those characteristic of young people. Another, more practical approach is based on the assumption that a weakened immune system can be provoked to develop an immune response by exposure to a much stronger stimulus. This review discusses both of these approaches.

Recipients

Finding people who will receive specific therapy to restore immune function is a very difficult task. The main problem is the need to identify a person whose immune competence is insufficient to cope with new and/or common pathogens, but who looks healthy at the same time. Some responsible persons approached this very pragmatically. To an outsider, the principles on which this choice is based look like this:

– physiological aging of the immune system or its insufficiency is associated with old age;

– people are considered old when their age reaches a certain threshold, which in developed and developing countries is often taken to be 60-65 years old, which corresponds to the retirement age;

– since people over the age of 65 are old, they have a weakened immune system by default;

– therefore, anyone over the age of 65 should receive vaccines containing adjuvants or an increased amount of antigens to compensate for the extinction of the immune system.

The problem with such a categorical approach is the use of a precisely defined criterion, in this case age, together with the level of dysfunction of the immune system, which not only cannot be accurately measured, but is also poorly related to the aging process. According to the authors, in this case there is a need for methods of quantitative and qualitative assessment of violations of the functionality of the immune system.

Adequate functioning of the immune system

One of the problems of the immune system is that its work is impossible to see and therefore difficult to quantify. The immune system provides protection against many potential pathogens that we may encounter on a daily basis. Their inability to cause the disease is not recognized and is not realized. There is no method at our disposal for calculating the frequency of exposure to pathogenic organisms, and we are aware of the role of the immune system in our survival only in its absence or with the development of certain diseases. Of course, people with insufficient activity of the immune system demonstrate increased susceptibility to opportunistic organisms, a weakened response to vaccination, as well as an increased likelihood of morbidity and mortality associated with infections. However, infections are dangerous not only for these people: healthy people with a normally functioning immune system also suffer from viral, bacterial, fungal and parasitic diseases.

Unlike organs such as the skin, which differ in the clear visibility of age-related changes and the availability of methods for their assessment, there are no devices or methods that would allow assessing the overall functionality of the human immune system and the rate of its change. It is possible to identify people with an inadequately low response to influenza virus antigens, which makes them susceptible to the corresponding strain of the virus. However, this is only one of a small number of vaccination options in which the immune system provides a pre-known amount of specific antibodies. Clinicians wishing to assess the competence of the human immune system can prescribe a blood test with a count of the total number of cells, T- and B-lymphocytes and their subpopulations, as well as measuring the concentration of immunoglobulins in serum and the presence of specific antibodies. These data form a general idea of some elements of the immune system, but do not provide the possibility of a functional assessment of a person's ability to respond to a specific pathogen, except in cases where the obtained indicators are much lower than normal values. Therefore, identifying elderly people with normal parameters and impaired immune system functions can be a difficult task.

Thus, any attempt to restore immunity in the elderly, first of all, requires the use of simple methods to assess the effectiveness of the process. These methods should: (1) be related to the function being evaluated, (2) provide relatively fast results, (3) be relatively non-invasive, and (4) be implemented using simple equipment.

Factors contributing to the extinction of the immune system functionality

Changes in the fatty composition of tissues vary from person to person and throughout life. In general, with age, there is a decrease in fat-free body weight with simultaneous accumulation of adipose tissue in different locations. This is most noticeable in the primary lymphoid organs and thymus, the color of the tissue of which visually changes due to an increase in the number of adipocytes, leading to the extinction of the main function of these organs, which consists in the production of lymphocytes. Similarly, the age-related accumulation of adipose tissue changes the color of the bone marrow from red to yellow, which is accompanied by the loss of the microenvironment necessary for the formation of B-lymphocytes. In addition to changes in the rate of production of new lymphocytes, the extinction of the immune system may also be associated with the longevity of certain clones of these cells. This process is very complicated, since the duration of the period during which lymphocytes are capable of replication is limited, and the earlier many of them reach their limit, the faster functional failures in their repertoire form, as well as the ability to respond to certain influences decreases.

Age-related adipose tissue overgrowth is also an important source of inflammation, which has a serious impact on systemic metabolism and contributes to the development of life-threatening conditions, such as tissue resistance to insulin, type 2 diabetes mellitus and cardiovascular diseases. However, during the development of diet-induced obesity or age-related obesity, immunocompetent cells infiltrate adipose tissue and proliferate in it. Thus, cells of acquired immunity, including T- and B-lymphocytes, make a significant contribution to the development of chronic mild inflammation. Lymphocytes regulate the attraction of nonspecific immune cells into adipose tissue, while producing a wide range of pro-inflammatory cytokines. This shifts the balance between beneficial and harmful inflammation, which, in turn, contributes to the age-related functional extinction of the immune system and the development of conditions that affect health and well-being.

Approaches to restoring or stimulating immune function

In the case of the elderly, we may try to improve their immune function in a specific or non-specific way, and we may also consider rejuvenating their immune system by reversing the extinction of their major lymphoid organs. Each of these approaches is discussed below.

Recovery by enhanced stimulation or metabolic manipulation

Food

One of the often overlooked factors is the significant amount of energy required to trigger an immune response. Naive T-lymphocytes, which are usually in a state close to rest, should be activated by changing their metabolism when they encounter an antigen. After successful activation, there is an increased absorption of nutrients, an increase in the activity of oxidative phosphorylation in mitochondria, and cell metabolism becomes predominantly glycolytic. This precedes an increase in energy requirements and precursors of cellular components necessary for proliferation. The cells of many elderly people do not receive enough energy to form a successful immune response due to inadequate nutrition or changes in intestinal permeability to certain substances. Studies have shown that the addition of energy sources and trace elements to the diet of elderly people helps to strengthen the immune response to vaccination against influenza virus and pneumococcal infection. Changes in intestinal permeability disrupt the intake of trace elements such as zinc, as well as vitamin D. However, despite convincing scientific evidence confirming the importance of zinc and vitamin D for the functioning of the immune system, studies involving older people have not brought unambiguous results.

Rapamycin

The Streptomyces hygroscopicus fungus found in the soil of Easter Island produces a compound of the macrolide family called rapamycin, which has the ability to inhibit the proliferation of mammalian cells and has an immunosuppressive effect. Due to these properties, it has been approved by the U.S. Food and Drug Administration (FDA) as a drug for the treatment of certain types of cancer, including kidney cancer. The usual dose of the drug for cancer patients is about 10 mg per day. Subsequent experiments on mouse models have shown that at lower doses rapamycin can improve the body's response to influenza virus antigens by increasing the number of virus-specific antibodies. These results prompted specialists to conduct clinical trials in which participants received placebo or rapamycin at a dose of 0.5 mg daily, or 5 mg or 20 mg weekly. After a 6-week course and a 2-week break, all participants were vaccinated with a trivalent flu vaccine. At the same time, the participants of the experimental group had a significant improvement in the immune response compared to the participants of the control group.

Adjuvants

One of the approaches to restoring an effective immune response to the antigen contained in the vaccine in the elderly and ensuring adequate levels of protection is the introduction of an adjuvant into the vaccine. Classical adjuvants based on aluminum salts have recently begun to be replaced with water-oil emulsions. However, the use of adjuvants is a double-edged sword, as they increase the risk of developing both local and systemic adverse reactions. The mechanisms of action of adjuvants are not completely clear. It is believed that they can act as an antigen depot, prolonging its presence on the periphery of the body, or interact with antigen-recognizing elements on antigen-presenting cells, or stimulate a local inflammatory reaction by attracting more cells to the injection zone. The use of adjuvants may also be limited to rarely used vaccines. For example, it is impractical to vaccinate an elderly person annually for 30 years (from 65 to 95) with an anti-influenza vaccine containing an adjuvant. This can lead to the fact that all specific clones of reacting cells will quickly exhaust their potential, which will lead to the opposite result.

Increasing the amount of antigen

It has been demonstrated that increasing the amount of antigen in the vaccine dose also improves the immune response that develops in an elderly body, but this relationship is not directly proportional. That is, a tenfold increase in the dose of the antigen will not strengthen the corresponding immune response by 10 times. Vaccines with an increased antigen content include an influenza vaccine with a 4–fold increase in the amount of antigen compared to the standard dosage (60 micrograms instead of 15 micrograms of hemagglutinin for each of the three influenza strains: A/H1N1, A/H3N2 and B) and Zostavax - a live attenuated vaccine against chickenpox virus, intended exclusively for the elderly containing more than 14 times the number of active viral units compared to the standard children's vaccine. The latest vaccine increases resistance to chickenpox virus in the elderly and significantly reduces the risk of shingles and postherpetic neuralgia.

Specific rejuvenation

Adoptive transfer of specific clones

Latent viral infections are one of the aspects often attributed to serious problems of the elderly. The mechanisms of suppression of these viruses work throughout life, but as they age, they weaken. A good example is carriers of the chickenpox virus (human herpes virus type 3, or HHV-3), who in old age may develop episodes of shingles and manifest postherpetic neuralgia. The mechanisms of suppression of cytomegalovirus activity (CMV or HHV-5) also weaken with age, which causes the detection of active virus in urine. Studies have shown that the adoptive transfer (introduction of donor immune cells "trained" for the antigen of a certain pathogen) of cytomegalovirus-specific T-lymphocytes allows to stop the active form of the disease. While this therapeutic approach is in the early stages of clinical trials and will not be offered to elderly people in the foreseeable future, more recent studies demonstrate the possibility of obtaining virus-specific T-lymphocytes from human own stem cells.

Nonspecific rejuvenation

Blood transfusion

Heterochronous parabiosis – surgical connection of blood flows of two animals of different ages – has been successfully used in the past to identify factors that weaken the severity of age-related changes in the cardiovascular system and cognitive function. Experiments on modifying the immune system of an older parabiont with the help of an external factor coming from the bloodstream of a young parabiont showed that while the precursors of T-lymphocytes of both parabionts can populate the partner's thymus, this does not ensure the restoration of age-related atrophy of the thymus, indicating the presence of a defect in the thymic environment itself. This dismisses the option of rejuvenation of the immune system with the help of a factor contained in the blood, but the idea of rejuvenation of the peripheral immune system by transfusion of young blood has a long history. In the 1920s, Professor A.A. Bogdanov tested the hypothesis that blood transfusion of young people to old people is able to rejuvenate the latter. He conducted an experiment on himself by transfusing 11 doses of blood, but the 12th transfusion led to his death in 1928.

Usually, a donor donates 500 ml of blood for transfusion. This volume contains approximately 7.5×10 ⁸ T-lymphocytes. In people aged 20-30 years, at least 50% of these cells will be represented by naive lymphocytes and resting memory cells. It would be advisable to take the donor volume of blood from people on a regular basis, isolate leukocytes from it and store them in ampoules in liquid nitrogen before a person reaches old age. Memory cells will have a repertoire that depends on pathogens and antigens that a person encountered before blood collection, so their ability will be somewhat limited compared to the capabilities of the population of naive T-lymphocytes. This population should include cells that have recently emerged from the thymus and have a wide repertoire of receptors. In aging people, age-related changes occur in the populations of peripheral T and B lymphocytes, which include the appearance of gaps in the repertoire of antigen-specific receptors, which subsequently narrows, as well as the presence of leukocytes that have exhausted the possibilities for division or are approaching their limit. Returning your own white blood cells to the bloodstream later in life can help solve these problems. Of course, there are questions about the number of cells returned to the recipient, the ability of these cells to integrate into the general population, as well as the presence of receptors for specific pathogens on the surface of transfused lymphocytes. Preliminary answers to these questions have been obtained in animal experiments, but for a more complete understanding of the issue, clinical studies are necessary. Despite this, several companies have already appeared offering services for the isolation of leukocytes in the early stages of life and their storage in liquid nitrogen to return to the bloodstream at a more advanced age.

Interleukin-7

Interleukin-7 (IL-7) is a cytokine produced by stromal cells of primary or secondary lymphoid organs. It binds to a cell surface receptor consisting of a specific alpha chain (CD127) and a non-specific gamma chain (CD132), also part of receptors for other cytokines, including interleukins 2, 4, 9, 15 and 21. Simultaneous expression of CD127 and CD132 is recorded at various stages the development of T cells, starting from early progenitor cells and ending with memory cells. In the early intrathymic stages of T-lymphocyte development, the interaction between interleukin-7 and its receptor is responsible for cell survival. In addition, it is presumably responsible for the diversity of T-cell receptors and is involved in the clonal expansion of mature thymocytes before their transition to the pool of naive T-lymphocytes. In peripheral T-lymphocytes, the interaction of interleukin-7 and its receptor is mainly associated with maintaining the number of cell clones by ensuring cell survival and proliferation of both naive T-lymphocytes and memory cells. The idea of the possibility of using interleukin-7 as a rejuvenating agent originates in a study by Bhatia et al., in which young mice were injected with antibodies to this cytokine for a long period of time. As a result, the animals developed severe atrophy of the thymus and a decrease in the cellularity of this organ, similar to that observed in old mice. Interleukin-7 therapy allowed to reverse the age-related atrophy of the thymus in old animals, which ensured the restoration of thymus functions and improved the functions of peripheral T cells. However, it should be noted the importance of the concentration of interleukin-7 in this situation. Experiments on transgenic mice synthesizing different amounts of this cytokine showed that mice with the highest production of it had fewer thymocytes than ordinary animals of the control group. At the same time, their differentiation mechanism was disrupted at an early stage of T-lymphocyte development.

Rejuvenation of the immune system in old rhesus macaques who received an injection of the recombinant drug monkey interleukin-7 before vaccination against influenza was manifested by an increase in the output of T-lymphocytes from the thymus and higher titers of hemagglutinins compared to control group animals who received an injection of saline solution. The first clinical study of the therapeutic effects of interleukin-7 was conducted with patients with metastatic cancer. The results of a later double-blind phase IIa study conducted under placebo control with patients with lymphocytopenic metastatic breast cancer, some of whom received injections of recombinant interleukin-7 before chemotherapy, demonstrated its ability to induce a significant increase in the number of T-lymphocytes. The results of another study involving HIV-infected patients who received antiviral therapy in parallel showed that repeated cycles of administration of recombinant human interleukin-7 were well tolerated by patients, most of whom recorded a stable recovery of the T-cell population. To date, there have been no published data on the treatment of healthy elderly people with interleukin-7.

Surgical or chemical castration

Early studies on rodents have demonstrated that surgical castration can lead to the restoration of age-related atrophy of the thymus, but this effect is temporary. It has also been shown that chemical castration using luteinizing hormone releasing hormone agonists (luteinizing hormone releasing hormone agonists) effectively induces thymus repair in old rodents and primates. Experiments involving humans suggest that this may form the basis of a promising approach to improving immunity in older men. However, the side effects associated with such therapy will alienate many from choosing it.

Keratinocyte growth factor

Keratinocyte growth factor belongs to the family of fibroblast growth factors, which includes secreted proteins that transmit signals to tyrosine kinase receptors, and intracellular non-signaling proteins that act as cofactors for calcium channels reacting to electrical impulses. Keratinocyte growth factor is the seventh member of this family and a secreted signaling molecule that interacts with its receptor expressed on thymus epithelial cells and induces proliferation of thymus epithelial cell precursors and mature epithelial cells in fetal thymus cultures. The results of early experiments on a mouse model indicate that subcutaneous administration of keratinocyte growth factor to mice for three consecutive days at a dose of 5 mg/kg per day provides enhanced thymopoiesis for two months. Moreover, such therapy restored thymopoiesis in old animals. These results prompted the researchers to conduct experiments on primates, whose bone marrow was restored after total irradiation by introducing CD34⁺ hematopoietic stem cells. The introduction of keratinocyte growth factor provided an increase in the number of naive T-lifocytes in the lymph nodes compared to animals of the control group, while a larger number of cells were identified as having recently emerged from the thymus. In addition, after the introduction of the factor, the architecture of the thymus in animals was preserved for at least 12 months after bone marrow restoration.

Palifermin is a recombinant form of keratinocyte growth factor of bacterial origin, used in the therapy of patients undergoing chemotherapy or radiotherapy to stimulate the growth of mucosal cells. The positive results of experiments on great apes have prompted scientists to conduct research involving HIV-infected patients receiving antiretroviral therapy. The hypothesis was taken as a basis, according to which low CD4⁺ cell levels in these patients are due to a decrease in thymic function and palifermin therapy should increase the content of these cells in their blood. However, a randomized double-blind clinical trial under placebo control revealed no significant changes either in the number of recently released thymus cells, nor in the number of naive cells, nor in the size of the thymus. The dosage used in such studies varied from 20 to 60 micrograms/kg of body weight, which is significantly less than the dose of 5 mg/kg of body weight administered to mice in the original model. This may explain the absence of changes in the number of T-lymphocytes in the human body, whereas the features of the structure of the thymus may prevent an increase in its size during a limited period of the experiment.

Evaluation methods

To illustrate the problems associated with the assessment of the general state of immunity, an example with blood pressure can be given. The results of long-term collection and analysis of blood pressure measurement results allowed doctors to determine the normal ranges of blood pressure indicators for different ages and/or for certain conditions. In the results, based on the optimal ratio between risk and benefit, various thresholds were selected for prescribing blood pressure-lowering drugs to relatively healthy, weakened and terminally ill patients. Unfortunately, to date, there are no such schemes for the immune system. Moreover, the work with the immune system is complicated by its exceptional complexity. Therefore, the development of a universal method that would make it possible to distinguish between the full, weakened and deficient statuses of the immune system to predict its individual power and strength of reaction to a certain antigen is a very difficult task.

Using the vaccination response as a direct measurement method

The assessment of individual immune reactivity by confrontation of the immune system with a common or new vaccine antigen, followed by a quantitative assessment of the quality of immune functions as a whole, is a very attractive goal. For example, the elderly are characterized by a blurred response to the flu vaccine, so the only method to determine the effectiveness of anti-aging/ restorative therapy is still to conduct randomized clinical trials comparing a group of elderly people receiving therapy with elderly people of the control group receiving placebo by evaluating the difference in titers (amount) of antibodies inhibiting hemagglutination. This approach has been successfully applied in clinical studies of rapalogs (analogues of rapamycin). Its disadvantage lies in the need to attract a large number of people to each group in order to obtain statistically significant results.

Longitudinal (long-term) studies in which the same person is observed and examined before and after therapy may look more attractive, but the problem is that if vaccination is used as an evaluation system, it must be carried out both before and after therapy. This can be a problem, especially with repeated vaccination against influenza, since the history of interaction with different subtypes of the virus affects the emerging immune response.

Repeated vaccinations with other vaccines, such as pneumococcal polysaccharide vaccines, may not be able to demonstrate a positive effect due to the low reactivity associated with repeated vaccination. Another option is to issue morbidity diaries to participants to monitor the number of infections throughout the study, as well as their duration. Compliance with the protocol in this case is no less important factor than the truthfulness of the participants. Subjective assessment, which is necessary in some cases, may be suboptimal, so many choose a more objective approach and resort to indirect assessment methods.

Indirect assessment methods

Clinicians may have access to a large set of biological tests. These include: a general blood test, counting of lymphocyte subpopulations, determination of immunoglobulin levels in the blood, as well as the presence of specific antibodies, but none of them can be used to reliably assess the rejuvenation of the immune system. This is mainly due to the fact that the values of these indicators may coincide for immunocompetent people and people whose immune response is below adequate. Approaches to determining improvements in the general state of immunity include: assessment of changes in the number of recent thymic emigrants, identification of the presence of a large number of naive lymphocytes or changes in the ratio of subpopulations of cells at different stages of differentiation, assessment of skin reactions or changes in the parameters of the immune risk phenotype.

Conclusion

At the root of the question of the possibility of rejuvenation of the immune system lies the possibility of changing the effect. To date, we do not have a method for changing the overall immune capacity of the body. Often, as this measure, clinical monitoring of infections developing in patients, especially unusual, non-attenuating, as well as those taking a recurrent course or progressing into systemic ones, is used. Such assessment methods are often subjective, and there are no objective laboratory methods for assessing the overall immune capacity, despite the long history of methods for quantitative analysis of the reaction to the antigen by determining the titer of antibodies or evaluating the individual T-cell response of the body to viral glycoprotein. We only know that the level of reaction is associated with protection against disease for a small number of pathogens.

The normal range of immune activity is also not defined, and as long as there is no method for assessing this activity, firstly, we do not have ways to determine a person's need for therapy and, secondly, we cannot assess the possible effect of such anti-aging therapy. The complexity of the problem was compounded by the understanding that a successful immune response is due not only to the production of a sufficient number of specific antibodies and effector T cells. Susceptibility to diseases is also due to the number of barriers of the innate immune system, such as the integrity of the skin, the washing effect of tears, saliva or urine, the functions of the ciliated epithelium and mucous membrane, as well as the reaction of neutrophils, macrophages and natural killer cells.