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(Journal of Leukocyte Biology. 2001;70:881-886.)
© 2001 by Society for Leukocyte Biology

Senescence in innate immune responses: reduced neutrophil phagocytic capacity and CD16 expression in elderly humans

S. K. Butcher*, H. Chahal*, L. Nayak{dagger}, A. Sinclair{dagger}, N. V. Henriquez*, E. Sapey{ddagger}, D. O’Mahony{dagger} and J. M. Lord*

* MRC Centre for Immune Regulation, Birmingham University Medical School, Birmingham, United Kingdom; and Departments of
{dagger} Geriatric Medicine and
{ddagger} Medicine, University Hospital, Birmingham, United Kingdom

Correspondence: Dr. J. M. Lord, MRC Centre for Immune Regulation, Birmingham University Medical School, Birmingham B15 2TT, UK. E-mail: J.M.Lord{at}bham.ac.uk


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Elderly humans are more susceptible to bacterial infections because of declining immune status. We have investigated the effect of aging on neutrophil bactericidal responses, comparing neutrophil function in healthy, young (23–35 years) and elderly (>65 years) volunteers. Superoxide generation in response to fMLP was slightly increased in neutrophils from elderly donors, and serum from the elderly was able to opsonize E. coli efficiently. In contrast, phagocytic index was significantly lower in neutrophils from the elderly, compared with young donors (P<0.005). CD11a and CD11b expression was not affected by age, but CD16 was significantly reduced in neutrophils from elderly donors (P<0.0001). CD16 expression and phagocytic index were measured in the same neutrophils using FITC-labeled E. coli, PE-conjugated anti-CD16 antibody, and CD16 expression correlated with phagocytic index (r=0.83; P<0.05). In elderly patients with bacterial infection, CD16 expression remained low. We propose that reduced neutrophil CD16 expression and phagocytosis contribute to human immunesenescence.

Key Words: immunesenescence • aging • neutrophil-function


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
If current population trends continue, by the year 2025, one in five of the population in the Western world will be over 65 years of age [1 , 2 ]. However, although life expectancy for people living in developed countries is steadily increasing, the age at which good health can be expected to continue remains at approximately 10 years less. The elderly have a higher morbidity and mortality from infectious diseases, and it is now widely accepted that compromised immune function is a consequence of normal human aging and a primary cause of increased disease risk in the elderly [3 ]. There is significant literature concerning age-related changes in the acquired immune system [3 4 5 ]. For example, elderly humans have a diminished ability to generate high-affinity antibodies after immunization [6 ], and their CD4+ T cells show a decreased ratio of naïve-to-memory phenotype [7 ], resulting in decreased responsiveness to new antigen challenge. In contrast, much less is known about the effect of age on the innate immune system.

Neutrophils mediate the immediate host response to bacterial and fungal infections, which are largely responsible for the higher rates of mortality and morbidity in the elderly population [8 ]. Neutrophils are short-lived (half-life, 12–18 h), post-mitotic granulocytic cells that are produced in vast numbers (1–2x1011 per day) in the bone marrow. Several studies have shown that neutrophil numbers in the blood [9 , 10 ] and neutrophil precursors in the marrow [9 ] are not lowered in the healthy elderly; thus, neutrophil supply does not appear to be a major source of immune compromise in the innate response to bacterial infection. Similarly, the chemotactic responses of neutrophils do not appear to be significantly affected by age. In vitro studies of chemotaxis have shown migratory responses of neutrophils from healthy, elderly subjects to be unaltered [2 , 11 ], and adhesion of neutrophils from elderly subjects to endothelium was also unchanged [2 , 12 ]. The two remaining potential sources of compromised neutrophil bactericidal function are phagocytosis and bactericidal mechanisms such as superoxide generation and degranulation. These aspects of neutrophil function have been the focus of this study.

The few studies of neutrophil phagocytosis in the elderly are consistent in that they have shown an adverse effect of age on neutrophil phagocytic ability [13 14 15 ]. The reduced response of neutrophils to Staphylococcus aureus [15 ] is clinically important because of the increased susceptibility to this pathogen in elderly subjects [8 ]. However, only one study has looked at the number of neutrophils with phagocytic capacity and the number of bacteria ingested per neutrophil [15 ], the phagocytic index. This is important, because a simple reduction in cells able to phagocytose microbes could be counteracted by increased production and recruitment of neutrophils to sites of infection. Adequate recruitment of neutrophils to sites of infection would be significantly offset and the ability of the elderly to resolve an infection compromised if the neutrophils recruited had a reduced phagocytic capacity.

In this study, we have measured phagocytic index in neutrophils from healthy, young and elderly donors as well as measuring superoxide generation in response to formyl-Met-Leu-Phe (fMLP). Our data show reduced phagocytic index in neutrophils from elderly donors, with no reduction in superoxide generation. Phagocytosis is initiated by the interaction of specific receptors on the surface of the neutrophil with particulate ligands on the microbe. The key receptors inducing phagocytosis of bacteria by neutrophils are those for the Fc region of immunoglobulin G (IgG; Fc{gamma}RIII/CD16 and Fc{gamma}RII/CD32) and for the complement molecules C3b (CD35/CR1) and C3bi (CD11b/CR3), which are bound to the surface of the microbe. Analysis of expression of receptors for complement (CD11b) and Ig (CD16) on neutrophils revealed that CD16 expression was significantly reduced in the elderly. This loss of CD16 could be a major factor contributing to reduced neutrophil function and increased susceptibility to bacterial infections in the elderly. Indeed, when we examined CD16 expression in elderly patients with bacterial infection and neutrophilia, we found that CD16 expression remained low. This would suggest that neutrophils released recently from the bone marrow during infection also have reduced CD16 expression and that the altered expression of CD16 on neutrophils may arise in the bone marrow of the elderly.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Human volunteers
Healthy, elderly subjects (mean, 68.2 years; range, 65–81 years) meeting the immunogerontological criteria of the SENIEUR protocol [16 ] were used in the study. Healthy, young volunteers (mean, 25.9 years; range, 23–35 years) were recruited from staff and students in the medical school. Elderly patients (mean, 83.6 years; range, 78–96 years) with bacterial chest infection and neutrophilia were also studied; those with chronic inflammatory disease, kidney disease, diabetes, or receiving steroid treatment were excluded. In each experiment, approximately equal numbers of males and females were used, and informed consent was given.

Neutrophil isolation
Human neutrophils were isolated from venous blood, taken by venepuncture from consenting volunteers, on Percoll (Sigma Chemical Co., St. Louis, MO) density gradients as previously described [17 ]. Briefly, blood was drawn and dispensed immediately into 50 ml sterile polypropylene centrifuge tubes containing 30 mg ethylenediaminetetraacetate (EDTA) and mixed gently. The blood was then mixed with Hespan (Fresenius Medical Care, Lexington, MA) at a ratio of 1:7 ml blood to sediment erythrocytes. The leukocyte fraction was removed, mixed 1:1 with sterile phosphate-buffered saline (PBS), then layered onto Percoll gradients consisting of 5 ml 54% Percoll and 2.5 ml 78% Percoll. Gradients were spun for 25 min at 1100 rpm (MSE Centaur II, Fisher Scientific, Loughborough, UK) at room temperature. The lower granulocyte band was then removed, washed in sterile PBS, and resuspended in RPMI 1640 medium (Invitrogen Life Technologies, Carlsbad, CA) containing 10% fetal calf serum (Sera Laboratories International, Crawley, U. K.), 2 mM glutamine, 100 U/ml penicillin, and 100 µg/ml streptomycin. Neutrophil preparations were assessed for purity by differential staining using a commercial Giemsa staining kit (Diff-kwik, Baxter Healthcare, Deerfield, IL) and were routinely >95% neutrophils.

Measurement of superoxide generation in response to fMLP
Neutrophil respiratory burst in response to fMLP was determined in isolated neutrophils by measuring the generation of superoxide in a lucigenin-based assay [18 ]. The cells were washed once in RPMI 1640 medium, once in PBS, and resuspended in Hanks’ balanced saline solution (HBSS) 1% bovine serum albumin (BSA) buffer at 2 x 106 cells/ml. The cell suspension (100 µl) was added to plastic chemiluminometer tubes followed by 20 µl lucigenin (2.5 mM in HBSS; Sigma Chemical Co.) and incubated at 37°C in a water bath for 5 min. Two control tubes were used in each assay, one included no stimulus (fMLP) and the other, no cells. The assay was initiated by the addition of 100 nM fMLP (Sigma Chemical Co.) and superoxide generation measured over 10 min, as the release of light from lucigenin excited by reactive oxygen species. Results were expressed as integral counts per minute, determined from the area under the curve of the reaction for the first 10 min.

Assessment of phagocytosis of fluorescein isothiocyanate (FITC)-labeled Escherichia coli
Phagocytosis of E. coli by neutrophils was measured using a commercial fluorescence-based kit (Phagotest, Orpegen, Germany), according to the manufacturer’s instructions. Briefly, 100 µl venous blood was incubated at 4°C for 10 min, then mixed with 20 µl of a suspension of FITC-labeled E. coli (1x109 bacteria/ml), and opsonized with whole human serum. Control tubes were kept at 4°C to monitor adherence of the labeled bacteria to neutrophils in the absence of uptake. The phagocytosis test tubes were incubated at 37°C for 10 min. Samples were then placed immediately at 4°C and mixed with FITC quench-buffer to eliminate fluorescence from bacteria sticking to the neutrophil cell surface. The blood was then washed twice in PBS, the erythrocytes were lysed, and the leukocytes were fixed for 20 min with the lysing/fixation solution provided at room temperature. The cells were then washed twice and finally incubated with a nuclear counter-stain for 10 min at 4°C. The assay was performed in duplicate and analyzed by flow cytometry, gating against the level of green fluorescence (FL1) seen in the 4°C control sample. Neutrophils were distinguished from other leukocytes by gating on forward- and side-scatter, and any remaining E. coli were excluded using the red fluorescent (FL2) stain to identify cells having diploid DNA. Ten-thousand events were measured to assess the percentage of neutrophils that had phagocytosed E. coli and the mean fluorescence intensity (MFI)—a measure of the number of bacteria taken up per cell. Data were then expressed as the neutrophil phagocytic index: Phagocytic index = % phagocytic neutrophils x MFI.

Analysis of cell-surface molecules by flow cytometry
Neutrophils were isolated as described above and resuspended at 2 x 106 cells/ml. Cells (2x105) were then washed and incubated with blocking buffer (PBS containing 10% human serum), followed by incubation with FITC-conjugated monoclonal antibodies (mAbs) to CD11a, CD11b, and CD16 (Dako, Bucks, UK) at saturating concentrations at 4°C. Cells were washed and fixed in PBS containing 4% paraformaldehyde for 10 min at 4°C. Fluorescence was analyzed by flow cytometry (Coulter Epics XL, Luton, UK), and 5000 events were collected per sample.

Phagocytosis versus CD16 expression
Whole blood (100 µl) was kept at 4°C for 10 min prior to assessment of phagocytosis of E. coli and CD16 expression in a dual-labeling protocol. A cooled FITC-labeled E. coli suspension (Orpegen Pharma, Heidelberg, Germany) was added to the blood and incubated for 10 min at 37°C, and a control was kept at 4°C. The tubes were then placed immediately at 4°C to stop further phagocytosis. For CD16 analysis, the blood was then incubated with saturating levels of unconjugated anti-CD16 mAb (Dako) for 15 min at 4°C. The blood was then washed and incubated at 4°C for 15 min with a phycoerythrin (PE)-conjugated rabbit anti-mouse IgG antibody. Controls were treated with mouse IgG followed by anti-mouse IgG antibody. All samples were then treated with FITC quench, washed twice in PBS, and resuspended in lysis/fixation buffer (Orpegen Pharma). The samples were analyzed by flow cytometry to give a MFI of CD16 labeling and phagocytic index.

Opsonization efficiency of serum and effect of IgG depletion on E. coli phagocytosis assay
To determine whether serum from elderly donors was able to opsonize E. coli as efficiently as serum from young donors, unopsonized FITC-labeled E. coli were combined with serum from young and old donors, and the ability of neutrophils from a single donor to phagocytose the E. coli was determined. Unopsonized (40 µl) FITC E. coli (Orpegen Pharma) was washed twice in PBS, incubated at 37°C for 40 min, and resuspended in 800 µl complement fixation buffer (ICN Pharmaceuticals, Costa Mesa, CA) and 100 µl donor serum. The E. coli were then washed twice in PBS and resuspended at 2 x 107 per 20 µl in PBS. Neutrophils (100 µl) at 2 x 106 cells/ml were then incubated for 2 h in RPMI 1640 containing 2% BSA and were then combined with 50 µl each donor serum and 20 µl E. coli opsonized with the same serum. The phagocytosis assay was then performed as described above.

To determine the contribution of immunoglobulin binding to phagocytosis of E. coli in the assay used here, serum depletion was performed using protein A/G sepharose (Santa Cruz Biotechnology, Santa Cruz, CA). The depleted serum was passed through a 0.2 µm filter (Gelman Sciences, Ann Arbor, MI) and used in the phagocytosis assay as described above. Depletion of IgG was confirmed by Western blotting using a horseradish peroxidase (HRP)-conjugated anti-IgG antibody (Sigma Chemical Co.) and enhanced chemiluminescence (ECL) detection system (Amersham, Arlington Heights, IL).

Statistics
Differences among groups were determined using a t-test, and a P value of <0.05 was taken as a significant difference between means. Correlations were determined using Pearson’s correlation analysis.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Superoxide response in neutrophils from elderly donors
Figure 1 shows the response of neutrophils isolated from healthy, young (23–35 years) and elderly (>65 years) donors to fMLP. The generation of superoxide was not reduced in the elderly group and in fact, was slightly higher than the young age group (P<0.05). Increased responsiveness to fMLP can be an indicator of the priming of neutrophils. However, the magnitude of the increase and the fact that the elderly donors were in good health and unlikely to have been exposed to priming agents such as pro-inflammatory cytokines suggest this is unlikely.



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  		Figure 1. Neutrophil superoxide response to fMLP. Neutrophils were isolated from 23 healthy, young (23–35 years) and elderly (>65 years) volunteers and incubated for 10 min with fMLP as described in Materials and Methods. Superoxide generation for each volunteer is shown, and the median for each group is marked.

 
Phagocytosis in neutrophils from elderly donors
Phagocytosis of FITC-labeled E. coli was determined by fluorescein-activated cell sorter (FACS) analysis. The percentage of neutrophils that was able to phagocytose the E. coli was reduced in neutrophils from elderly donors—77.6 ± 6.2% phagocytes compared with 86.6 ± 8.1% in the young age group (n=11; P<0.01). When the level of bacteria ingested, indicated by the MFI, was taken into account and used to calculate the phagocytic index, the effect of age was even more marked (Fig. 2 ). Phagocytic index was reduced by approximately 50% in the elderly group compared with the young age-group (P<0.005). Therefore, the elderly have fewer neutrophils that are able to phagocytose, and those cells with phagocytic ability ingest fewer bacteria per cell than neutrophils from young donors. Decreased phagocytic capacity of neutrophils from elderly donors was confirmed using opsonized yeast stained with procion rubine and enumeration of ingested yeast by light microscopy (unpublished results).



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  		Figure 2. Phagocytosis of E. coli by neutrophils of young and elderly donors. Phagocytosis of E. coli by neutrophils was measured in whole blood using a commercial kit (Phagotest, Orpegen Pharma). Blood was incubated with FITC-labeled E. coli for 30 min, and uptake of bacteria was determined by FACS analysis according to the manufacturer’s instructions. Data for the phagocytic index (% of neutrophils that had phagocytosed E. coli x MFI) for neutrophils from 11 healthy, young (23–35 years) and elderly (>65 years) donors are shown. The median for each population is marked.

 
In the phagocytosis assay used, the E. coli are supplied opsonized with whole-serum immunoglobulin and complement. To determine whether the ability of serum from the elderly to opsonize infecting microbes could be compromised, serum from young and elderly volunteers was used to opsonize FITC-labeled E. coli. Figure 3 shows that E. coli opsonized by serum from five elderly and young donors were phagocytosed equally well by neutrophils from a different, young donor. Thus, the serum from elderly subjects appears to contain adequate immunoglobulin and complement to opsonize E. coli for ingestion by neutrophils.



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  		Figure 3. Opsonization of E. coli with serum from young and elderly donors. Serum from five healthy, young and elderly donors was incubated with unopsonized FITC-labeled E. coli prior to addition of neutrophils from a single, young donor. Phagocytic index was measured as described in Materials and Methods, and data shown are mean ± SD.

 
Expression of surface CD11a, CD11b, and CD16
We have measured expression of CD16 and CD11b, together with CD11a, which has been shown previously to be unaffected by age [12 ]. FACS analysis confirmed that expression of CD11a was unaffected by age (MFI, 3.59±0.3 for young and 3.51±0.5 for old donors) and revealed that CD11b expression (Fig. 4 A ) was also not significantly lower in the neutrophils from elderly donors. In contrast, levels of surface CD16 (Fig. 4B) were significantly reduced (P<0.0001) in the elderly group. To determine whether the reduced CD16 expression of neutrophils might arise during neutrophil production in the bone marrow, we also examined CD16 expression on neutrophils of elderly patients with bacterial infections and confirmed neutrophilia. We found that CD16 expression remained low (P<0.0002) during neutrophilia (Fig.4B) , suggesting that neutrophils are released from the marrow with reduced CD16 expression.



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  		Figure 4. Effect of age on expression of CD11b and CD16. Neutrophils from healthy, young (23–35 years) and healthy, elderly (>65 years) volunteers were immunostained with FITC-conjugated mAbs to (A) CD11b (n=17) or (B) CD16 (n=23). Neutrophils from elderly (>65 years) patients with bacterial infections were also assessed for CD16 expression (n=19). Fluorescence was analyzed by flow cytometry, and data are expressed as MFI for each sample. The median value is shown for each age group.

 
Relevance of CD16 expression to neutrophil phagocytic index
As the E. coli used in the phagocytosis assays was opsonized with immunoglobilin and complement, it was important to establish the relevant contribution of CD16 to phagocytosis in this system. Figure 5 A shows that depletion of IgG from serum, by pretreatment with protein A/G agarose prior to coating E. coli, reduced the phagocytic index by approximately 30%. Western blotting of serum using an antihuman IgG antibody showed that the majority of IgG had been removed by the depletion protocol (Fig. 5B) .



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  		Figure 5. Effect of removal of IgG on neutrophil phagocytosis. Serum from young and old, healthy donors was treated with protein A/G agarose to remove IgG. (A) The depleted serum was used to opsonize FITC-labeled E. coli prior to incubation with isolated neutrophils and measurement of phagocytic index. Data shown are the mean of triplicate values from a single experiment representative of two performed. (B) Depletion of IgG was checked by Western blotting of serum samples (5–20 µl) using an antihuman IgG antibody.

 
In addition, using a dual-labeling technique in which neutrophils were incubated with FITC-labeled E. coli prior to fixation and labeling with a PE-conjugated anti-CD16 antibody, it was possible to determine the relationship of CD16 expression to phagocytic index. Analyzing neutrophils from six different donors using this method confirmed a positive correlation (r=0.83; P<0.05) between CD16 expression in the neutrophil population and phagocytic index (Fig. 6 ).



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  		Figure 6. Effect of CD16 expression on neutrophil phagocytosis. Neutrophils from six healthy volunteers were incubated with FITC-labeled E. coli prior to fixation and staining for CD16 with a PE-conjugated anti-CD16 antibody. (A) Neutrophils were analyzed for CD16 expression and uptake of E. coli by FACS analysis, and two representative FACS dot plots and single-channel histograms are shown. (B) Data for CD16 expression (MFI) and phagocytic index were calculated and are shown for each sample. The linear regression value for the plot was r = 0.83, with P < 0.05.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The aging process has significant and deleterious effects on immune responses in humans, resulting in increased incidence of inflammatory disease and susceptibility to infections. In this study, we have examined neutrophil function in healthy, elderly humans, because compromised functioning of innate immune responses might contribute to increased bacterial infections in the elderly [8 ], at least as much as reduced adaptive responses. Superoxide responses were not reduced in neutrophils from elderly donors and were, in fact, slightly higher than the response of neutrophils from young donors. The degree of enhancement was not sufficient to indicate priming, which would be expected to increase responsiveness to fMLP by three- to fivefold [19 ], but does show that a significant killing mechanism in neutrophils was not decreased by age. In contrast, phagocytic capacity was reduced significantly in the elderly, and this was accompanied by reduced CD16 expression. This is the first study describing the effect of age on the expression of Fc-gamma receptors on human neutrophils. Furthermore, CD16 expression was shown to correlate positively with neutrophil phagocytic index, suggesting that Fc-mediated phagocytosis was adversely affected by age.

The data concerning superoxide responses are in agreement with previous publications showing a normal response to fMLP in the elderly [12 , 20 ]. However, in contrast to the data regarding fMLP, studies concerning superoxide generation in response to particulate stimuli have indicated a reduced response to these agents in the elderly. Wenisch and coworkers [15 ] have shown that superoxide generation was decreased in response to S. aureus but not to E. coli, an observation with particular clinical relevance because of the reduced ability of the elderly to resolve infection by gram-positive bacteria [8 ]. We also found that superoxide generation in response to E. coli was not affected by age (unpublished results). It is possible that superoxide responses to E. coli, which involve binding of lipopolysaccharides by CD14 [21 ], may be unaffected by age, although responses to gram-positive bacteria, which are more dependent on complement and Fc-receptor ligation, are attenuated. Indeed, the Fc receptor-mediated superoxide response has been shown already to be reduced significantly in the elderly [22 ], again identifying attenuation of Fc-mediated responses as a significant factor in age-related neutrophil functional decline.

The notion that altered Fc responses contribute significantly to the decline in neutrophil function with age is supported by our data concerning phagocytosis of E. coli. The Fc-gamma receptor clearly plays a key role in dictating the level of the phagocytic response in neutrophils, because removal of IgG markedly reduced phagocytic index. In addition, the level of CD11b was essentially unaltered in the elderly, indicating that loss of CD16 was a specific age-related effect rather than a reflection of a general decline in surface-protein expression. The decline in CD16 levels with age, shown here, provides the first data on the molecular events underlying attenuated Fc-mediated phagocytosis in the elderly.

Expression of CD32, the other low-affinity Fc-gamma receptor present on resting neutrophils, has not been studied here. CD16 is expressed at 4–5 times the level of CD32 and is thought to play the dominant role in binding IgG on neutrophils [23 ]. CD64, the high-affinity Fc-gamma receptor, is only expressed on primed neutrophils [24 ]. Therefore, loss of CD16 would be expected to reduce phagocytic efficiency, and this is borne out by our data.

Although reduced phagocytosis in neutrophils from elderly subjects has been shown previously [13 14 15 ], the molecular basis of this decline has not been established. Our data indicate that reduced phagocytosis in the elderly is intrinsic to the neutrophil itself, because serum from elderly donors was able to opsonize bacteria efficiently. Previous studies demonstrating that immunoglobulin and complement levels were within the normal range in the elderly [2 ] are in agreement with this conclusion. Emmanuelli et al. [14 ] showed that phagocytosis of unopsonized bacteria was not reduced in the elderly, suggesting that receptors for innate recognition of bacterial components (e.g., CD14) were not affected by age and that the reduced response of neutrophils to opsonized E. coli was likely to be mediated by a reduced expression of the relevant receptors or as a result of compromised signalling function. The data shown here support a role for reduced Fc-gamma receptor expression in neutrophil functional decline in the elderly. Currently, we are investigating whether there is an effect of aging on Fc-receptor signals involved in phagocytosis.

To determine whether reduced expression of CD16 had arisen in the circulation of the elderly or was already present on neutrophils as they were released from the bone marrow, we measured CD16 expression in the elderly during neutrophilia. Bacterial infections increase the production and release of neutrophils from the bone marrow into the circulation, an important factor in combating infection. It is interesting that despite a profound neutrophilia in the elderly patients, CD16 expression remained low, suggesting that the alteration in CD16 expression arises in bone marrow. Moreover, although the elderly can mount an adequate neutrophilia, the neutrophils produced to combat infection also have reduced CD16, limiting their ability to combat the infection. In conclusion, we have shown that phagocytic capacity is reduced as humans age, and reduced expression of the Fc-gamma receptor CD16 may contribute to this decline. Preliminary data suggest that CD16 expression is already low on neutrophils released from the bone marrow. Because phagocytosis of bacteria by neutrophils is crucial in the early phase of infection, if this function is compromised in the elderly, it will lead to a reduced ability to combat bacterial infections.


   ACKNOWLEDGEMENTS
 
S. K. B. and H. C. are supported by a grant from the Biotechnology and Biological Sciences Research Council (BBSRC) as part of the Science of Ageing (SAGE) initiative into normal human aging.

Received March 12, 2001; revised July 23, 2001; accepted August 2, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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