Functional properties of CD4⁺CD28⁻ T cells in the aging immune system

Abstract

The aging immune system is characterized by a progressive decline in the responsiveness to exogenous antigens and tumors in combination with a paradoxical increase in autoimmunity. From a clinical viewpoint, deficiencies in antibody responses to exogenous antigens, such as vaccines, have a major impact and may reflect intrinsic B cell defects or altered performance of helper T cells. Here we describe that aging is associated with the emergence of an unusual CD4 T cell subset characterized by the loss of CD28 expression. CD28 is the major costimulatory molecule required to complement signaling through the antigen receptor for complete T cell activation. CD4⁺CD28⁻ T cells are long-lived, typically undergo clonal expansion in vivo, and react to autoantigens in vitro. Despite the deficiency of CD28, these unusual T cells remain functionally active and produce high concentrations of interferon-γ (IFN-γ) and interleukin-2 (IL-2). The loss of CD28 expression is correlated with a lack of CD40 ligand expression rendering these CD4 T cells incapable of promoting B cell differentiation and immunoglobulin secretion. Aging-related accumulation of CD4⁺CD28⁻ T cells should result in an immune compartment skewed towards autoreactive responses and away from the generation of high-affinity B cell responses against exogenous antigens. We propose that the emergence of CD28-deficient CD4 T cells in the elderly can partially explain age-specific aberrations in immune responsiveness.

Introduction

The immune system undergoes characteristic changes with aging. The increased risk for infections and higher incidence of malignancies in the elderly have been attributed to declining immunocompetence. As T lymphocytes have a central role in the generation of immune responses, their function in the aging immune system is of particular interest (Hirokawa, 1992, Miller, 1996).

There is evidence that the ability to generate novel T cells is severely impaired with progressing age (Robey and Fowlkes, 1994, Weissman, 1994). T cell lymphopoiesis is primarily achieved in the thymus, an organ which begins to involute with the onset of reproductive maturity (George and Rittner, 1996). How changes in thymic function affect T cell homeostasis is incompletely understood. Several studies have investigated age-related changes in the functional and phenotypic profiles of T cells. With aging, the proportion of CD4⁺ T cells expressing a memory phenotype (CD45RO⁺, CD29^high, CD44^high) increases at the expense of T cells with a naive phenotype (CD45RA⁺) (Lerner et al., 1989, Nagelkerken et al., 1991, Ernst et al., 1993, Gabriel et al., 1993, Dobber et al., 1994). Although it could be expected that memory immune responses are boosted in older individuals, clinical observation has suggested the opposite. Memory responses appear to be particularly impaired in the elderly. Underlying mechanisms are not understood, but it is possible that changes in signal transduction and lymphokine production have considerable influence (Shi and Miller, 1993, Song et al., 1993, Utsuyam et al., 1993, Engwerda et al., 1994). Along these lines, it has been described that, with progressing age, T cells shift from a preferential TH1 functional pathway to a TH2 commitment (Hobbs et al., 1993). Also, early signal transduction events critically involved in T cell activation have been found to undergo age-associated shifts. As a common denominator, these studies have indicated decreased responsiveness to TCR signaling and to costimulatory molecule-mediated signals.

From a clinical point of view, dysfunction of humoral immune response is particularly important in the aged population. The efficacy of vaccines, such as influenza and pneumococcus, are markedly lower in the elderly as compared to younger individuals (Degelau et al., 1994). Influenza vaccination can induce protection in 60–70% of younger individuals, but in only 30–40% of the elderly. Prior exposure to infectious agents with the subsequent generation of antibodies appears not to compensate for this disadvantage. Studies in aged rodents have indicated that in vivo antibody responses might be characterized by reduction in antibody titer and affinity (Riley et al., 1989, Nicoletti and Cerny, 1991, Miller and Kelsoe, 1995).

High-affinity memory B cell responses, the ultimate goal of vaccination, are generated in spatio-functional specialized sites designated germinal centers (MacLennan, 1994, Kelsoe, 1995). Germinal centers provide a unique environment bringing together B cells, T cells, and follicular dendritic cells. Germinal center formation is a T cell-dependent process (Jacobson et al., 1974, Stedra and Cerny, 1994), and it has been reported that the number of germinal centers characteristically declines with age. T cells equipped for a functional role in germinal centers have been found to express the CD4 marker, acquire expression of CD57, and downregulate expression of Thy-1 (Bouzahzah et al., 1995, Zheng et al., 1996). Their functional profile is incompletely understood but they seem to produce IL-2 and not IL-4.

Recent studies have began to shed light on the molecules critically involved in facilitating the T–B cell interaction required for the successful induction of high-affinity antibodies. Upon activation, CD4 T cells upregulate cell surface expression of CD40 ligand which interacts with CD40 on the surface of B cells. B cell activation, proliferation, and differentiation have all been related to CD40-mediated signals. The critical role of the CD40–CD40 ligand interaction in humoral immune responses is emphasized by the identification of genetic defects in the CD40 ligand gene (Callard et al., 1993, Facchetti et al., 1995). Patients with mutations in the CD40 ligand gene develop X-linked hyper-IgM syndrome, a congenital immunodeficiency associated with the failure to generate IgG antibodies in germinal centers. In mice the formation of germinal centers can be completely inhibited by antibodies binding the CD40 ligand molecule (Foy et al., 1994).

A second well-characterized receptor–ligand pair has a crucial role in T–B interaction. CD28 molecules expressed on the surface of CD4 T cells interact with CD80 or CD86 molecules found on activated B cells. In CD28 null mice, B cells do not undergo somatic hypermutation and germinal centers are not formed (Ferguson et al., 1996). CD28 is constitutively expressed on CD4 T cells, while CD80 and CD86 are induced after stimulation, mainly on antigen-presenting cells. It has been proposed that a lack of the CD28–CD80/86 interaction in germinal centers is responsible for the decline in somatic mutation in germinal centers of aged mice. Support for this hypothesis has come from studies describing lack of expression of CD86 in the germinal center of aged mice (Miller et al., 1994).

We have recently made the observation that CD28-deficient CD4 T cells can emerge in humans (Schmidt et al., 1996a). These unusual CD4 T cells are consistently detected in patients with rheumatoid arthritis (RA) (Martens et al., 1997). While investigating conditions able to induce CD28 deficiency in CD4 T cells, we found that the emergence of CD4⁺CD28⁻ T cells is age related. As a substantial proportion of CD4 T cells can lack CD28 expression, we have been interested in the functional consequences of the absence of this costimulatory molecule on CD4 T cells. Here we describe that CD4⁺CD28⁻ T cells are deficient in providing help to B cells, suggesting that the accumulation of CD4⁺CD28⁻ T cells could be one mechanism of impaired humoral responses in the elderly. Despite the lack of CD28, these T cells can, however, produce cytokines such as IFN-γ, and are thus equipped with the ability to promote pro-inflammatory responses. We propose that CD4⁺CD28⁻ T cells have a role in the paradoxical development of increased autoimmune responses and impaired response to exogenous antigens in the aging immune system.