Gamma/delta T lymphocytes are affected in the elderly

Abstract

γδ T lymphocytes are considered to represent a link between the inflammatory response and adaptive immunity. In the present paper we investigated whether these cells play any role in the remodeling of the immune system described in the elderly. We show that the absolute number of γδ T cells in peripheral blood of old and very old subjects is reduced. Moreover, γδ T cells from old people and centenarians show enhanced levels of the early activation marker CD69 both after culture in medium alone and in LPS-stimulated cells. Furthermore, they show a basal increased production of tumor necrosis factor (TNF)-α as evaluated at the single cell level. Additionally, the response of these cells to IPP in ‘in vitro’ cultures is in part impaired. These results suggest that the high level of basal activation of γδ T cells is due to the ‘inflamed’ environment. However, the changes in number and function of γδ T lymphocytes might influence the resolution of inflammatory immune responses in the elderly.

Introduction

Many studies have described a large variety of modifications in cells and functions of the immune system in the elderly. The number and the proliferative ability of CD4⁺ T lymphocytes is reduced, whereas the number of NK and T cells expressing NK-associated antigens is increased, although the function of NK cells evaluated on a ‘per cell’ basis is impaired (Candore et al., 1992, Di Lorenzo et al., 1999, Miller, 1996, Pawelec et al., 1999, Potestio et al., 1999, Solana and Mariani, 2000). The pattern of cytokine production is also altered, and a shift towards a type 2 response has been hypothesized (Caruso et al., 1996, Caruso ae al., 2000, Shearer, 1997). On the whole, the ‘immunosenescent’ status has been described as characterized by an impairment of adaptive immunity. Conversely, many studies demonstrate that successful aging is related to well conserved immune functions (Pawelec et al., 1999, Caruso ae al., 2000, Franceschi et al., 1995, Wikby et al., 1998). However, immune responses are differently affected by the aging process and some of them decline while others, such as the less-specialized responses, remain unchanged or increase, i.e. there is remodeling of immune system (Franceschi et al., 1995, Franceschi et al., 2000).

Age-related modifications of immune parameters have been also studied in centenarians who are considered exceptional individuals who escaped the main diseases that cause death (Franceschi et al., 1995). Additionally, studies on centenarians have demonstrated that the ability of the immune system to combat the main diseases is not due to the complete integrity of the system itself but to a functional remodeling. In fact, although in these individuals adaptive immunity is in some way modified, good defensive capability is due to a state of inflammation that maintains ‘on’ the linkages with the adaptive response (Caruso ae al., 2000, Franceschi et al., 1995, Franceschi et al., 2000).

T cells bearing γδ TCR may be considered a good linkage between the inflammatory response and adaptive immunity. Many of them are double-negative cells (CD4⁻CD8⁻) and, as a consequence, and, as expected, they recognize antigens without major histocompatibility complex (MHC) restriction. They are found in normal numbers in MHC I and II class negative mice and show a peculiar anatomical distribution that depends on the receptor chains used. In humans, most of the cells using the δ1 chain are located in the intestinal epithelium, whereas cells using the δ2 chain predominate in the circulation. A similar preferential localization is known in mice and in cattle. The ligands recognized by γδ T cells have been extensively studied and now we know that these cells may exert cytotoxic activity responding to non-polymorphic molecules expressed on target cells. Moreover, they directly interact with components of the inflammatory branch of immune responses and may lead towards type 1 or type 2 adaptive immunity. They also act by virtue of chemokine and cytokine production after stimulation by non-peptidic microbial antigens, such as phoshorylated non-peptidic metabolites (Biswas et al., 2000, Cipriani et al., 2000, De Libero, 1997, Egan and Carding, 2000, Fearon and Locksley, 1996, Hayday, 2000, Janeway et al., 1988, Spada et al., 2000).

In a previous paper, we have demonstrated that the percentage of γδ T lymphocytes in peripheral blood of old subjects is decreased when compared to that of young subjects. However, these cells show a higher level of the early activation marker CD69 after 3 h of culture with both lipopolysaccharide (LPS) and complete medium alone. These data suggested to us that γδ T lymphocytes in the elderly are in an activated condition and that they may act as ‘first aid’ cells to replace the impaired adaptive immunity (Colonna Romano et al., 2000).

Due to the role that γδ T lymphocytes play between the natural and the adaptive immune response, we focused our attention on γδ T lymphocytes in the elderly and centenarians, to evaluate whether γδ T lymphocytes might play a part in the immune remodeling (Franceschi et al., 1995, Franceschi et al., 2000; as given earlier). In the present paper we confirm and extend the datum of a low level of γδ T cells in elderly (and centenarians) evaluating the absolute number of cells according to age. Furthermore, we have evaluated the expression of CD69 in unstimulated cultures and in cultures stimulated by LPS, which activates γδ T cells via monocytes, or by the non-peptidic phosphoantigen isopentenylpyrophosphate (IPP) that selectively activates γδ T lymphocytes (De Libero, 1997, Hayday, 2000, Lahan et al., 1998). Moreover, we have assayed the ability of γδ T cells from old and very old subjects to produce tumor necrosis factor (TNF)-α evaluated at the single cell level.

We demonstrate that γδ T cells from the old and centenarians, although numerically reduced, show basal and LPS-stimulated enhanced levels of CD69 and a basal increased production of the cytokine TNF-α evaluated at the single cell level. Additionally, the response of these cells to IPP in ‘in vitro’ cultures is in part impaired.