Usha Ponnappan

Department of Geriatrics, Medicine, Microbiology and Immunology, University of Arkansas for Medical Sciences, and GRECC, John L. McClellan Memorial VA hospital, VA Medical Research, GC143, 151/LR, 4300 West 7^th street, Little Rock, Arkansas 72205

Received 1/15/97 Accepted 1/29/98

2. INTRODUCTION: AGING AND IMMUNE FUNCTION

One of the extensively studied and well documented physiologic declines in the aged is immune senescence (1-5). Aging results in a significant decline in immune function which has been directly or indirectly linked to increased susceptibility to infections, autoimmunity, and cancer (2,4,6-11). Previous studies have clearly established that neither changes in numbers of lymphoid cells nor changes in cellular environment contribute to age-related immune dysfunction, thus suggesting that the underlying cause for this dysfunction may be found in regulatory changes within the immune apparatus (5,12,13). Age-associated decreases in both humoral and cell mediated immune responses have been well documented (14-17). The two main cellular components of the immune system mediating specific immune responses, i.e. T lymphocytes and B lymphocytes, exhibit functional deficits with advancing age (17-25). Although phagocytic activity has been reported to decrease with age (26), preliminary studies on antigen presentation by both macrophage and dendritic cells appear not to be affected with advancing age (27-29). Total immunoglobulin production is unchanged with age, while serum immunoglobulin components and specificities, show age-related alterations. In older humans, serum concentrations of IgG and IgA increase, whereas the concentration of IgM is either unchanged or decreases with age (30-33). While antibody responses against foreign antigens such as tetanus toxoid decrease with age (4,14,17,18), the production of antibodies directed towards self antigens such as thyroglobulin, nuclear proteins, and DNA has been documented to increase in aging humans (18,30,33,34), and these observations have often been the bases for theories of declining self/non-self regulation in the elderly. Other tests of B cell function such as proliferation for the large part demonstrate lowered responses after treatment with mitogens and antigens (2,19,35), however, scattered reports show either age-related increase or no change in B cell proliferative responses (2, 36). Although B lymphocyte function appears to be clearly affected with advancing age, it still remains to be unequivocally established whether this is independent of the actions of the regulatory T cells and/or their secreted cytokines.

2.1 T lymphocyte function during immune senescence

In contrast to studies dealing with B cells, most of the reports on age-related immune dysfunction point to a significant decline in T cell function. In fact, decline in T cell function has been proposed to be the central defect in immune senescence (2,4,5). The organ responsible for T cell maturation and function, the thymus, undergoes considerable involution with age. It has been suggested that the manifestations of immune senescence are all related to the involution of the thymus, however the precise role of thymic involution in immune senescence is unclear since this occurs as early as 20 years of age in humans while T cell deficits are not observed until the fourth or fifth decade of one's life (5,11,37). It cannot, however, be ruled out that the involution of the thymus may be a forerunner to immune deficits observed in the elderly.

Aging results in a few changes in the phenotypic profiles of T lymphocytes. It has been observed that the changes that accompany aging in the T cell compartment is that of a shift toward a greater number of memory (CD44hi/CD45RO⁺) and fewer naive (CD44lo/CD45RA⁺) T cells (38-41). The increase in ratio of memory to naive T cells is thought to be a reflection of the decreased supply of naive cells to the periphery accompanied by the continual antigen-driven conversion of naive to memory cells (2,42). Memory cells are thought to differ from naive cells in terms of activation requirements and cytokine production (43-45). It has been proposed that many age-related alterations in T cells are attributable to the increase in memory cells based on the poor activation capacity of memory cells (38,39,46-48), however this argues against some studies which have indicated that memory cells have similar or even enhanced activation capabilities compared to naive cells (43, 49-54). Furthermore, recent studies have provided evidence that CD45RA⁺ and CD45RO⁺ positive cells can interconvert thus, suggesting that the previous findings may not be absolute and may be meaningful only when discussed in the context of activating antigen (55,56). Therefore, the consequence of age-related increase in memory (CD45RO⁺) T cell subsets remains to be fully delineated. Although changes in the numbers of CD4 and CD8 T cell subsets have been demonstrated in humans, these changes are not consistent from study to study and are too small to account for the significant changes seen in the functional abilities of T cells (2).

T cell function in elderly humans is characterized by decreases in classic tests of T cell mediated immunity such as Mixed Lymphocyte Reactions (MLR) and Delayed Type Hypersensitivity responses (DTH) (2,4,13,15,57). Cytotoxic T cell (CTL) generation and activity has also been reported to decline with age (2,58,59). A hallmark feature of age-related T cell dysfunction is the inability of T cells from the elderly to proliferate at a level comparable to that seen in young individuals after activation with antigen or mitogens such as Phytohemagglutinin (PHA), Concanavalin A (Con A), or anti-CD3 (20-23). Diminished proliferative capacity of lymphocytes from elderly humans is also evident in in vitro responses to recall antigens, i.e. antigens to which they have been previously sensitized, such as Varicella Zoster and Mycobacterium Tuberculosis (4,14,15,24). The decrease in mitogen-induced proliferative responses have been demonstrated to be not attributable to alterations in cell number, viability, number or binding affinity of cell surface receptors on T cells (12,13,25,60,61), but rather to a decline in their ability to generate second messengers (2,20,62-65). Numerous studies have demonstrated significant age-related decrease in both IL-2 production and IL-2R-alpha expression following T cell activation (20,24,61,66-68). These defects are also evident at the level of gene expression as mRNA for IL-2 and IL-2-R-alpha is also decreased with age following mitogenic stimulation (61,66). The effect of age on mitogen induced generation of cytokines other than IL-2 has been studied less extensively and information is not consistent, however some studies have reported increases in IFN-gamma production by T cells, while others have reported increases in IL-10 and IL-6 in T cells and mouse spleen cells (69, 70-72). Taken together these studies appear to suggest an underlying defect in the generation of second messengers following T cell receptor mediated activation. The cascade of early biochemical events that follows TCR ligation ultimately trigger the transcription of genes including c-fos, c-myc, IL-2 and IL-2R-alpha whose protein products are essential for T cell differentiation and effector function (73,74,75). Activation of transcription factors is an important mechanism for the transmission of extracellular signals from the cytoplasm to the nucleus. Few newly emerging studies including our own have implicated a decrease in the induction of transcription factors for the resulting decline in signaling and proliferation observed in the aged (76-79). These studies indicate that in addition to defects occurring immediately following receptor ligation, defects also occur in downstream signaling events of which the induction of transcription factors may be vital to the events resulting in aberrant nuclear signaling. T cell activation results in the activation of several important transcription factors including Activator Protein-1 (AP-1), Nuclear Factor of Activated T cells (NFAT), and Nuclear Factor Kappa B (NF kappa B) which play key roles in mediating transcription of genes involved in the generation of T cell immune responses (73-75). NFAT is a transcriptional activator that is a specific target for signals from the antigen receptor and binds to sites in the IL-2 promoter. This complex is largely restricted to T lymphocytes and is thought to be responsible for the T cell-specific inducibility of IL-2 (73,80). Binding sites for NFAT have also been observed in the regulatory regions of genes for other cytokines such as IL-3, IL-4, GM-CSF, and TNF-alpha indicating that NFAT may regulate the expression of cytokine genes other than IL-2 (81-84).