Today's open access paper discusses possible approaches to the treatment of immunosenescence, the age-related decline in effectiveness of the immune system. Unfortunately it is largely a tour of compensatory treatments, ways to force the cells of the immune system into greater or more useful activity without addressing any of the underlying causes of immunosenescence. Many of these methodologies have serious side-effects, are disruptive of normal immune function and overall health, and cannot be applied for the long term. Checkpoint inhibition, or the delivery of recombinant IL-7, for example, both of which are used as short term interventions to treat cancer.
The path to actually fixing the aged immune system by addressing causes is quite different. It would involve restoring the thymus from atrophy in order to restore a more youthful pace of production of T cells. Replacing the hematopoietic stem cell population to ensure that the right balance of immune cells are produced in the bone marrow. Reversing the degeneration of lymph nodes, where immune cells coordinate. Clearing out the populations of worn, malfunctioning, and misconfigured immune cells in tissues and bloodstream. This is a lot of work, but it is an oversight to omit these active lines of research and development from any review of ways to treat immunosenescence.
The one approach outlined at length in this open access paper that does address a plausible cause of immunosenescence is vaccination against cytomegalovirus (CMV). Near everyone is silently infected by late life, and the adaptive immune system becomes ever more devoted to trying and failing to clear this persistent viral infection. Ever more T cells are specialized to CMV, leaving ever fewer available for other tasks. As the supply of new T cells diminishes with age, this overspecialization becomes a serious issue, contributing greatly to the decline in immune function.
The authors here make the point that all of the necessary knowledge and technology already exists to put together a viable, widely used vaccine for CMV, but the will to do so is absent. We live in a world in which HPV vaccination became a reality, however, and CMV is arguably far worse when it comes to costs and suffering. Perhaps, at some point in the years ahead, the slow machineries of regulation will come to the point at which people are regularly vaccinated against CMV in order to reduce the impact of aging on immune function. I think it likely that selective destruction of CMV-specialized immune cells is more likely to emerge as a branch of therapy before that happens, however.
Until a few decades ago, a very small fraction of the population would reach 80 years of age. Now this is a frequent event, with the average life expectancy for a newborn to have risen to 80 years in most Western European countries. However, the increase in lifespan does not coincide with increase in healthspan. The link between aging and disease is in part a reflection of the functional changes in the immune system of older people. Different factors contribute to the development of age-related immune dysfunction, but the epilog of an aged immune system is an increased propensity toward a reduced resistance to infection, poorer responses to vaccination, and the development of age-related diseases.
The analysis of the contributing factors to this profound immune remodeling has revealed a complex network of alterations that influence both innate and adaptive arms of the immune system. The diversity of cells, molecules and pathways involved in this remodeling, and their ability to influence each other, including the intra- and inter-individual variability of the immune response, make it hard to identify interventions that can be predicted to improve or, at least, to maintain the immune function in older adults. Within the past few years, numerous studies of the underlying mechanisms of age-related immune decline have laid the groundwork for the identification of targeted approaches, focusing on interventions able to target the hallmarks of immunosenescence.
Taking into account the role of HCMV in the decrease of naïve T cells and increase of memory T cells, the reduction of the latent/lytic viral load, by vaccination and/or antiviral drugs, should be beneficial to diminish HCMV-associated immunosenescence. As a result of 40 years of work, there are many candidate HCMV vaccines. Therefore, we know the antigens needed in a HCMV vaccine, and that vaccination can be protective. To reach the goal of an effective HCMV vaccine, now we need a concentrated effort to combine the important antigens and to generate durable responses that will protect for a significant period.
Further, Letermovir is an antiviral agent that inhibits HCMV replication by binding to components of the terminase complex. In patients undergoing hematopoietic stem cell transplantation, Letermovir daily prophylaxis is effective in preventing clinically significant HCMV infection when used through day 100 after transplantation, with only mild toxic effects and with lower all-cause mortality than placebo. However, there is no suggestion yet for the use of antiviral therapy as a strategy for prophylactic mitigation of immunosenescence.