Our findings demonstrate that following a shortage in supply of newly developed T cells due to thymic involution, peripheral T cell compartment in aged mice displays a sequence of phenotypic and functional changes important for the functioning of the entire immune system. These changes follow a characteristic pattern. The first phase was predictable by the known effects of T cell homeostasis in lymphopenic conditions and the effect of the functional avidity of the TCR on peripheral T cell homeostasis. In this phase, there was an increase of both CD4+ and CD8+ T cells with a phenotype (CD3lowCD5hi) indicative of relatively higher functional avidity for self-peptide/MHC complexes . The second phase is characterized by development of functional erosion of T cells, caused by a defect in TCR-mediated activation.
Self-peptide/MHC complexes are involved in many aspects of T cell physiology, promoting thymocyte differentiation [38, 39], enabling peripheral survival and homeostasis of naïve T cells and modulating their activation by antigens [40–42]. Under lymphopenic conditions residual T cells proliferate to reconstitute their nearly normal numbers . T cells with higher avidity for self-peptide/MHC complexes enjoy advantage and expand more relative to the low avidity T cells [16–18]. During and or consequent to the expansion process T cells acquire phenotype of activated/memory T cells and acquire effector functions [43–47]. In addition to the faster disappearance of T cells with naive phenotype, the accumulation of high avidity T cells and their partial activation is potentially dangerous due to increased risk of autoimmune disorders. In fact, lymphopenic conditions are known to be associated with autoimmune phenomena . However, the incidence of autoimmune diseases in general does not increase in elderly, despite development of lymphopenia.
In humans, each autoimmune disease has a characteristic pattern of incidence. Although average peak of incidence differs for each individual autoimmune disease, a general trend suggests that most autoimmune diseases develop either during puberty (juvenile type diseases) or during mature reproductive life of individuals. For example, lupus erythematosus affects primarily women of childbearing age, and most frequently begins between ages of 15 and 40 years . The average age of onset of multiple sclerosis is 28-30 years . The number of new cases in both diseases, as well as other autoimmune diseases, reduces with further age. What could be the reason for this decline? The function of the immune system declines with aging in both mice and humans, limiting its ability to respond to infections and vaccines [2, 15]. The changes are mainly due to dysfunctions in the T cell compartment while the activity of B cells and innate immunity are less affected [2, 51]. However, these changes occur at age of 70 or higher, and are unlikely responsible for the decline in incidence of autoimmunity after the ages of 30-40. Our results showing attenuated clinical EAE in aged mice are in agreement with the incidence of human autoimmune disorders. Concomitant changes in T cell phenotype and function of old mice suggest that reasons for reduced autoimmunity may be intrinsic to T cells. This notion is further supported by an earlier occurrence of EAE attenuation in MTB TCRβ transgenic mice, since the transgene is expressed by T cells and hence affects primarily the function of T cells. Interestingly, we have previously shown an age-dependent arrest in the progression of lupus in F1 offsprings of MTB and lupus-prone BXSB strain . This arrest was coupled with reduced activation of T cells in vivo and reduced numbers of natural regulatory T cells, suggesting again a mechanism intrinsic to T cells. The numbers and function of natural regulatory T cells in MTB mice on B6 background (used in the present study) are indistinguishable from the WT mice , arguing against the role of these cells in reduced susceptibility of MTB mice to EAE. It remains to be determined whether intrinsic mechanisms additional to the two identified here (increased IL-10 production and functional arrest) may be involved, such as possibly changes in IL-17 production- a cytokine important for development of EAE .
It is tempting to speculate that the strategy of the immune system to counteract age associated increased risk of autoimmunity is promotion of differentiation of T cells with a potential to secrete IL-10 (so called Tr1 cells). In support of this notion, development of multiple sclerosis in humans is associated with defective development of Tr1 cells that secrete IL-10 [54, 55]. IL-10 secretion as a result of chronic high-avidity TCR engagement has been described in other experimental models [34–37], and increased IL-10 production associated with aging has been reported in both aged mice  and humans . Our results showing an earlier onset of IL-10 mRNA levels in mice with artificially higher TCR avidity for self-peptide/MHC complexes clearly supports this possibility, although this may not be the only mechanism affecting the function of the immune system in type A mice. Subsequent reduction, however, suggests that the control of enhanced T cell reactivity for self by IL-10 is temporary, and persisting chronic stimulation leads to a functional shut down.
Because IL-10-deficient mice develop enterocolitis , IL-10 is thought to be involved in maintenance of tolerance to self. However, IL-10 can also exert immunostimulatory properties, such as stimulation of B cell proliferation and differentiation into the antibody-secreting cells, and differentiation of CD8+ T cells into effector cells . Despite these stimulatory functions of IL-10, the effect of IL-10 in most studies of autoimmune diseases is one of regulation. Thus in EAE, systemic administration of IL-10 prior to EAE induction prevents the development of the disease [59, 60]. In contrast to the actively induced EAE, injection of IL-10 exacerbated adoptively transferred form of the disease . Removal of IL-10 by gene inactivation increases the severity of the disease [62–64], suggesting that IL-10-production has a physiological role in dampening the course of the EAE. In lupus, the levels of IL-10 found in the serum of affected patients correlate with the disease activity . This could suggest involvement of IL-10 in the pathogenesis of the disease, but also (apparently unsuccessful) attempts of the immune system to regulate the ongoing autoimmune response. The former possibility is supported by ameliorating effects of anti-IL-10 antibody treatment in lupus patients , as well as in NZB hybrid mice . However, these early results were countered with the findings of new studies. Thus, genetic deficiency of IL-10 resulted in significantly enhanced disease, while the treatment with recombinant IL-10 ameliorated the disease in the MRL model . Furthermore, continuous low levels of IL-10 achieved by gene therapy approach also diminished the disease activity in NZB hybrid congenic mouse model . Therefore, the exact role of IL-10 in lupus remains to be established.
T cell dysfunction resulting in progressive difficulties to raise immune responses have been described in elderly humans and mice [15, 70]. Therefore, these findings suggest that T cell dysfunctions associated with aging can at least partly be explained by adaptive alterations in high-avidity T cells caused by their autoreactivity. Our findings parallel those of tumor infiltrating T cells that become non-functional if their TCR is of high, but not low affinity for antigen . Thus, while their immediate impact may be effective, high avidity T cells may not be most desirable for long-term protection and/or preservation of immunological memory, as they are likely to functionally erode earlier than the low avidity T cells.