Natural Killer Cells (NK)
Involved in the direct recognition and elimination of virus-infected, stressed and malignant cells, NK cells are large granular lymphocytes that comprise approximately 10-15 % of the circulating lymphocyte pool. In humans, NK cells are defined by a CD356+ surface phenotype. However, they are not a homogeneous population as based on the differential surface expression of CD56, NK cells are assigned to one of two major subsets, namely CD56DIM and CD56BRIGHT. Despite the fact that ageing results in a reduction in NK cell production and proliferation , numerous studies have reported a significant increase in the percentage and/or absolute number of CD356+ cells with age [33, 34], suggesting the presence of long-lived NK cells in the circulation of older adults . At the subset level, ageing is associated with an increased proportion of CD56DIM NK cells and a reduction in the proportion of CD56BRIGHT NK cells [33, 34], alterations that culminate in a significantly increased circulating CD56DIM:CD56BRIGHT ratio with age [33, 35].
Results of two recent murine models of infection suggest ageing results in aberrant NK cell migration. Following influenza  or mousepox  infection, migration of NK cells to the spleen, lungs or lymph nodes was shown to be significantly reduced in aged mice during the early post infection phase, a defect that was assigned to an age-related impairment in NK cell maturation . Whilst no study to date has investigated the effect of age on the migratory capacity of human NK cells, groups have examined its impact on the expression of adhesion and chemo- kine receptors. With the exception of the IL-8 receptor CXCR1, whose surface density is reduced with age , comparable expression of the adhesion molecule CD2 and the chemokine receptors CCR3 and CCR5 has been reported for NK cells from young and older adults [35, 38].
Granule exocytosis and death receptor ligation are two contact- dependent mechanisms utilised by NK cells to eliminate transformed cells . Of these, granule exocytosis, which is characterised by the secretion of the pore-forming protein perforin and the serine protease granzyme B, is the primary cytotoxic mechanism of NK cells. Numerous studies have examined the impact of age on this form of NK cell defence , with the general consensus that at the single cell level, NK cell cytotoxicity (NKCC) is reduced with age [33, 41, 42]. Given that NK cells from older adults recognise and bind to transformed cells as efficiently as those from younger subjects [42, 43] , a post-binding defect is thought to underlie the age- related impairment in NKCC . At the molecular level, the induction of NKCC is regulated by signals transmitted through surface expressed activatory and inhibitory receptors. Currently, it is unclear as to what effect ageing has on NK cell receptor expression, with a series of conflicting observations reported by a number of independent groups (reviewed in ). However, data has been presented that suggests defective intracellular signalling may underlie the age-related impairment in NKCC. Following target cell stimulation, Mariani et al. found that due to an age-related delay in phosphatidylinositol 4,5-bisphosphate hydrolysis, NK cells from older adults generated significantly lower amounts of the second messenger inositol 1,4,5-triphosphate . If responsible for the age-associated reduction in NKCC . one would predict a functional consequence of this impaired intracellular signalling. Recently, we demonstrated that when bound to transformed cells, NK cells from older adults exhibit impaired polarisation of perforin-containing lytic granules to the immunological synapse, a defect that was associated with reduced perforin secretion . Importantly, we observed no age-related difference in perforin release when NK cells were stimulated with PMA and ionomycin (J. Hazeldine, unpublished observations). As agents that bypass cell surface receptors, this comparable functional response of NK cells from young and older adults to PMA and ionomycin treatment suggests that it is defective signalling proximal to the NK cell membrane that underlies the age-related impairment in NKCC.
Although renowned for their cytotoxic nature, it is becoming increasingly recognised that NK cells are a rich early source of immunoregulatory cytokines and che- mokines. For example, activated NK cells secrete TNF-a, IL-8 and interferon gamma (IFN-y), bestowing upon them the ability to amplify on-going innate immune responses and influence the early phases of adaptive immune responses
[44-46]. When challenged with inflammatory cytokines, NK cells from older adults respond with significantly increased secretion of IFN-y, macrophage inflammatory protein-1-alpha (MIP-1a) and IL-8. However, the levels generated are significantly lower than those produced by NK cells from younger subjects [45, 46]. Akin to humans, NK cells from aged mice exhibit aberrant cytokine production, with significant impairments reported in IFN-y production in models of influenza infection  and lipopolysaccharide challenge .
The recurrent and severe episodes of viral infection experienced by subjects with selective NK cell deficiency demonstrate the importance of NK cells in host defence . However, do the more subtle abovementioned age-related changes in NK cell function impact upon the health and/or well-being of older adults? To date, very few studies have attempted to answer this question. However, from the data available, reduced NKCC does appear to have clinical consequences. For example, retrospective and prospective studies have reported relationships between low NK cell activity in older adults and (1) a past history of severe infection, (2) an increased risk of future infection, (3) a reduced probability of surviving infectious episodes and (4) infectious morbidity [49-51]. Related to this increased risk of infection, reduced NKCC prior to and following influenza vaccination in older adults has been shown to be associated with reduced protective anti-hemagglutinin titres, worsened health status and an increased incidence of respiratory tract infection .