I stumbled upon an interesting open access paper a few days ago, linked below, in which the authors present their view of immunosenescence, the age-related failure of the immune system, as being in part a process wherein some cells of the adaptive immune system change their characteristics and function to become more like innate immune system cells. It makes for interesting reading, though it is worth bearing in mind that the immune system as a whole is fantastically complex, and in many ways still a dark and unmapped forest. It is easy to theorize unopposed when there is such a lot of empty space remaining on the map, making it hard to argue concretely about the relative importance of various mechanisms and observations. This poor understanding of the intricacies of the immune system is why autoimmune diseases and immune aging are largely lacking in effective treatments, and why the best of the prospective cures are those that sidestep the entire question of specific causes and mechanisms in face of the Gordian strategy of destroying the entire immune system in order to start over with new stem cells and immune cells.
As you might know, the immune system of most higher animals is two-layered. The layer that evolved first, and which remains the entirety of the immune system in lower animals such as insects, is known as the innate immune system. It reacts quickly, generates inflammation, and reacts in the same, predictable way to every threat. It has no memory and does not reconfigure its operations in response to circumstances and history. Later in evolutionary history, a second layer known as the adaptive immune system came into being, a more sophisticated set of functions resting on top of the existing innate mechanisms. The innate immune system reacts to intruders, and then the adaptive immune system records the nature of the threat and responds in its own manner, augmenting the attack. As the name suggests, the adaptive immune system maintains a memory and adjusts its operations in order to more aggressively destroy pathogens that it has encountered in the past. As anyone in the field will tell you, however, this high level picture of cleanly divided dualism is overly simplistic, however. There are numerous grey areas and incompletely understood complexities at the border between the two sides of the immune system.
Given that the adaptive immune system can adapt, its failure with aging is in large part a matter of acquired misconfiguration. There is only a small influx of new immune cells in adults, and this puts an effective limit on the number of immune cells that is supported at any one time. The inevitable problem in a space-limited system that keeps a continual record of history is that it runs out of space: evolutionary pressures produced the trade-off of a system that works very well out of the gate in young people, but fails sometime in later life. An old adaptive immune system is burdened with too many cells devoted to memory and too few cells devoted to attacking new threats. That is on top of the progressive failures that occur due to the the growing burden of the molecular damage that accompanies aging: persistent metabolic waste products such as cross-links and lipofuscin, mitochondrial damage, diminished stem cell activity, and so forth. The innate immune system has its own problems that arise from this damage, but is less prone of the issue of misconfiguration.
Understanding exactly how aging progressively harms the intricate choreography of the immune response is a massive project, and nowhere near completion. It is possible to judge how far along researchers are in this work by the side effect of the quality of therapies for autoimmune disease, which are malfunctions in immune configuration, and largely incurable at the present time. From a practical point of view, and as mentioned above, the best prospects for effective treatments in the near future involve destroying and recreating the immune system. That works around our comparative ignorance by removing all of the problems that researchers don't understand in addition to ones that they do. Destroying the immune system can only be done with chemotherapy at the moment, which no-one would undergo unless there was no choice in the matter given that it has significant negative effects on long-term health, but once new methods of selective immune cell destruction are developed, lacking side-effects, then we can start to talk about treating immune aging by rebooting the immune system.
Aging is associated with a general decline in immune function, contributing to a higher risk of infection, cancer, and autoimmune diseases in the elderly. Such faulty immune responses are the result of a profound remodeling of the immune system that occurs with age, generally termed as immunosenescence. While the number of naïve T cells emerging from the thymus progressively decreases with age as a result of thymic involution, the memory T cell pool expands and exhibits significant changes in the phenotype and function of antigen-experienced T cells, particularly evident in the CD8+ T cell compartment. Chronic immune activation due to persistent viral infections, such as cytomegalovirus (CMV) and Epstein-Barr virus (EBV), is one of the main drivers contributing to the accumulation of highly differentiated antigen-specific CD8+ T lymphocytes that have characteristics of replicative senescence. In combination with the depletion of the peripheral pool of naïve T cells, the accumulation of these terminally differentiated T cells with age skews the immune repertoire and has been implicated in the impaired immune responses to new antigens and vaccination in the elderly
Natural killer cells and αβCD8+ T lymphocytes are the two major cell lineages with constitutive cytotoxic activity and have a crucial role in the recognition and killing of abnormal cells. However, the paradigm for the recognition of target cells is fundamentally different between these two cell types: conventional αβCD8+ T cells rely on the T cell receptor (TCR) to recognize specific peptides presented by major histocompatibility complex class-I (MHC-I) molecules, whereas NK cells use a repertoire of germ line-encoded receptors to detect "missing self" or "altered-self" antigens and directly kill abnormal cells, without prior sensitization. Besides antigen specificity, the development of immunological memory is conventionally another distinctive feature between NK and T cells, categorizing them into distinct arms of the immune system and the innate and adaptive immune system, respectively.
Nevertheless, accumulating evidence supports the existence of NK cell memory, as well as evidence for TCR-independent responses mediated by αβCD8+ T lymphocytes, suggesting that the conventional limits between the innate and adaptive arms of the immune system may be not as distinct as first thought. NK and T lymphocytes have a common origin from a lymphoid progenitor cell in the bone marrow, and recent comparative proteomic and transcriptomic studies have demonstrated a remarkably close proximity between effector αβCD8+ T lymphocytes and NK cells, reiterating an evolutionary ancestry and shared biology between the two cell lineages.
An increasing body of literature reveals the existence of subsets of T cells with features that bridge innate and adaptive immunity. These cells typically co-express a TCR and NK cell lineage markers, distinguishing them from NK cells and other innate lymphoid cells, which lack the expression of a TCR or somatically rearranged receptors. Functionally, innate-like T cells respond to TCR ligation but are also able to respond rapidly to danger signals and pro-inflammatory cytokines, independently of TCR stimulation, resembling innate cells. Recently, subsets of conventional αβCD8+ T cells expressing NK cell markers and intraepithelial T cells have been included in this vaguely defined group of innate-like T cells. Despite the similarities in phenotype and function, there are clear differences in ontogeny and tissue distribution between them.
In this review, we will discuss recent evidence that aging is associated with the expansion of a subset of conventional αβCD8+ T cells with phenotypic, functional, and transcriptomic features that resemble NK cells. Such innate-like αβCD8+ T cells have the characteristics of terminally differentiated T cells, and the acquisition of functional NK receptors is most likely part of a general reprograming of the CD8+ T cell compartment during human aging, to ensure broad and rapid effector functions. We propose that innate-like αβCD8+ T cells share important features with other innate-like T cells; however, fundamental differences in origin and development separate them from truly innate cells. Interestingly, these cells are also differentially affected by aging, suggesting distinct roles in immune responses at different times of life. Evidence indicates that chronological aging is associated with accumulation of cells combining features of both the innate and adaptive arms of the immune system, most likely to compensate for functional defects of conventional NK and CD8+ T cells with age. We propose that senescent CD8+ T cells should not be seen as a dysfunctional population but instead a functionally distinct subset, which uses recently acquired NK cell machinery to maintain rapid effector functions throughout life. Contrary to the classic paradigm that peripheral TCR ligation is essential for T cell activation, this subset of highly differentiated T cells has impaired TCR responsiveness and may be non-specifically activated by inflammatory cytokines or after ligation of innate receptors. The switch to an innate mode of function may shed light on the mechanisms that allow highly differentiated CD8+ T cells to maintain functionality, despite the loss of TCR signal functions.
Our understanding of the physiological significance of the expression of NKRs on T cells is still incomplete, and the identification of the molecular mechanisms and the transcriptional regulators underpinning the development of innate features in T cells is essential. Most importantly, it will be important to understand how the intersection between innate and adaptive immune features may be manipulated to enhance immune function and to use this information to develop new approaches to improve immunity in the elderly.