Ageing is associated with a decline in peripheral blood CD56^bright NK cells

Natural killer (NK) cells are a heterogenous group of large granular lymphocytes derived from bone marrow precursors which are phenotypically distinct from T and B lymphocytes. NK cells have been recognised to constitute up to 10% of peripheral blood lymphocytes (PBMC) and are also found in peripheral tissues including the peritoneal cavity, placenta and liver [1, 2]. NK cell activation is governed by a balance of signals through activatory and inhibitory receptors [3] although activation in response to downregulation of MHC class I molecules on the target cell is a potent influence. NK cells represent the first line of defence against virus-infected and neoplastic cells and initiate cell lysis and cytokine production in the absence of prior antigenic stimulation [1, 2, 4]. The potential role of NK cells in the control of cancer has been suggested by the observation of an inverse correlation between the NK cell count and incidence of malignant disease in cohort studies of humans with further support derived from murine studies [5‐7]. NK cell participation has been reported in the immune response to several viruses including HIV, hepatitis B and C, but it may have particular relevance in host defence against herpes viridae [7‐9]. The relevance of NK cells in the host defence against herpes viruses is best illustrated in the case of isolated NK cell deficiency resulting in a range of systemic herpes viral infections [10].

The NK cell phenotype is characterized by lack of expression of the CD3 complex together with variable expression of CD16, CD56 and CD57. CD56 is an integrin with homotypic adhesion properties and in recent years studies have demonstrated the existence of two NK cell subsets based on the intensity of CD56 expression. These CD56^bright and CD56^dim subsets are thought to differ in their tissue homing properties due to differential patterns of expression of chemokine receptors and adhesion molecules. CD56^bright NK cells lack or express low levels of CD16 and represent only 10% of circulating NK cells but are the dominant NK cell subset within lymph nodes [11]. In contrast, the major CD56^dim NK cell population has a granular phenotype and exhibits more cytotoxic activity than CD56^bright NK cells. In addition this subset expresses higher levels of CD16 and is thus more potent in antibody-directed cell cytotoxicity (ADCC). In contrast, CD56^bright NK cells are the primary source of immunoregulatory cytokines such as IL-10, IL-13 and GM-CSF following antigen recognition with minimal production of these molecules being observed from CD56^dim NK cells [3]. This cytokine response is likely to be critical in the activation of the adaptive immune response by NK cells.

Ageing is associated with an increased mortality and morbidity from infectious disease and cancer and this is at least partly related to the development of immune senescence. Previous reports have demonstrated alterations in NK cell number and phenotype in association with age. In this study we have extended these observations to enumerate NK cells with respect to the CD56^bright and CD56^dim subsets, and have correlated these findings with age.

The decline in immune function with increasing age is termed immune senescence and leads to impaired responses to vaccination, an increased incidence of autoimmune disorders and increased morbidity and mortality to infectious disease [12‐15]. Immune senescence has been attributed to a number of factors including thymic involution and memory T cell accumulation resulting in contraction of the T cell repertoire. More recently, debate has focused on the role of innate cellular immunity with regard to impaired immune function. Remarque et al. have reported that elderly individuals with low NK numbers have a three-fold increased risk of mortality in the first two years of follow up compared to those with high NK cells [16]. Functional studies have measured NK cell activity against the K562 tumour cell line and demonstrate impaired NK cell cytotoxicity in elderly donors [17]. Further evidence is derived from studies on centenarians, regarded as a model example of healthy ageing, who have been reported to have well preserved NK cell cytotoxicity [18]. Our data shows that NK cell numbers are relatively stable with advancing age but as the total peripheral blood lymphocyte count decreases there is a small increase in the proportion of the lymphoid compartment occupied by NK cells.

Our findings demonstrate that the number of CD56^bright NK cells declines with advancing age which may have considerable implications for NK cell function in the elderly cohort. This decline was apparent across all three age groups indicating a gradual decline with healthy ageing. The CD56^dim NK cell subset remains relatively constant but occupies a greater portion of the peripheral blood lymphoid pool with advancing age. Our findings are in keeping with Krishnaraj (1997) who also reports a significant reduction in the proportion of CD56^bright cells with relative sparing of the CD56^dim subset [19]. This contrasts with Borrego et al. (1999) who reports an expansion in the proportion in CD56^dim cells but little change in CD56^bright NK cells [20]. Differences in the methodology employed in these studies are likely to account for the variation in results.

NK cell associated cytotoxic function is thought to be mediated primarily through the CD56^dim NK cell subset whereas the CD56^bright NK cells can be thought of primarily as a cytokine producing subset. The CD56^dim cell is more cytotoxic than the CD56^bright NK cell subset [21] and the morphological appearance of CD56^dim cells shows greater granularity [1]. A substantial body of data indicates that the CD56^bright subpopulation plays a critical role in the early innate immune response. CD56^bright NK cells have a higher proliferative capacity than CD56^dim NK cells [22] and are also the primary source of NK cell-derived cytokines including TNF-α, IFN-γ, IL-10, IL-13, and GM-CSF [11]. NK cell culture with lipopolysaccharide-activated macrophages, providing an endogenous monokine source, resulted in a greater than 6-fold IFN-y production from CD56^bright NK cells in comparison with CD56^dim NK cells [11].

The importance of NK cells in the priming of adaptive immunity is becoming clearer in recent years and CD56^bright NK cells play an important role in the activation of dendritic cells [23]. A recent study found that lipopolysaccharide cultured with immature dendritic cells induced proliferation of peripheral blood NK cells restricted to CD56^bright NK cell subset which was also the main source of IFN-γ associated with dendritic cell interaction [24]. CD56^bright NK cells also interact with monocytes in a reciprocal fashion thereby promoting inflammation [25].

CD56 expression also determines the anatomical location of the major NK cell subsets. CD56^bright NK cells are the predominant NK cell subset within lymphoid tissue and inflammatory lesions and exhibit differential expression of a range of chemokine receptors and adhesion molecules including CD62L, CCR7 and CXCR3 [24]. The CD56^bright NK cell subset constitutively expresses high affinity IL-2R and produces IFN-γ in response to picomolar doses of IL-2 from T cells through interactions within secondary lymphoid organs [11].

These observations suggest that a reduction in CD56^bright NK cells may contribute to the impairment in immunity towards newly encountered foreign pathogens that is associated with ageing. The role of NK cells in the initiation of adaptive T and B cell responses [26] may be particularly impaired in elderly individuals due to lower circulating numbers of the CD56^bright NK cells.

We have shown that the number of CD56^bright NK cells within peripheral blood decreases with age. As this population plays a critical role in the cytokine response of the innate immune system these findings suggest that a decline in CD56^bright NK cell number may contribute to the development of immune senescence.