Experimental Gerontology 34 (1999) 959 –970

Phenotypic and functional characteristics of
circulating monocytes of elderly persons૾
Hoss M. Sadeghia,*, Jack F. Schnelleb, John K. Thomasa,
Parunag Nishaniana, John L. Faheya
a

Center for Interdisciplinary Research in Immunology and Disease (CIRID) and the Jonsson Comprehensive
Cancer Center, UCLA School of Medicine, Los Angeles, CA 90024-1747, USA
b
Borun Center for Gerontological Research, Reseda, CA 91335, USA
Manuscript received June 7, 1999; manuscript accepted 7 September 1999

Abstract
Aging is associated with impairment of immune functions. Age-dependent alterations in T-cells
are well known. Although the pivotal role of monocytes in immune regulation by their production
of proinﬂammatory and inhibitory cytokines is acknowledged, limited information is available on
monocyte changes in aging. The present study focused on phenotypic changes in circulating
monocytes in elderly subjects and in the level of cytokines they produce. The results demonstrated
a signiﬁcant expansion of CD14dim/CD16bright circulating monocytes in elderly. In contrast, the
majority of circulating monocytes of healthy young individuals were CD14bright/CD16dim. The
CD14dim/CD16bright monocytes are considered to have phenotypic evidence for activation.
Furthermore, signiﬁcant increases of constitutive production of monocytic cytokines including
interleukin (IL)-1␤, IL-1receptor antagonist, and IL-6 by nonstimulated monocytes from elderly
was also indicative of activation. This was also observed when monocytes from elderly were
cultured with autologous lymphocytes. However, after stimulation, signiﬁcantly lowered IL-1␤
production was observed and IL-6 and IL-10 tended to be higher in the elderly. Collectively,
these results indicate that monocytes of aged individuals, in contrast to a younger population
exhibit in vivo activation as well as imbalanced production of cytokines. Such age-related
alterations in monocytes may contribute to impaired immune competence of aging. © 1999
Elsevier Science Inc. All rights reserved.
Keywords: Senescence, Monocyte, CD14/CD16, Cytokines

૾ Supported by NIH Grants AI07126-20 and AG 13013
* Corresponding author. Tel: ϩ1-310-206-8252; fax: ϩ1-310-206-1318.
E-mail address: hsadeghi@ucla.edu (H.M. Sadeghi).
0531-5565/99/$ – see front matter © 1999 Elsevier Science Inc. All rights reserved.
PII: S 0 5 3 1 - 5 5 6 5 ( 9 9 ) 0 0 0 6 5 - 0

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1. Introduction
Aging is associated with a decline in host responses to infectious and inﬂammatory
stimuli that may contribute to increased susceptibility to infections and neoplasia in
elderly people. In studies of both humans and animals, the age-dependent decline in
immune competence has been mainly related to functional and phenotypic changes in the
T cell population (Thomon et al., 1989; Xu et al., 1993; Hulstaert et al., 1994), whereas
changes in monocytes have not yet been well characterized.
Monocytes are recognized as critically important for effective immune function. They
are the ﬁrst lines of defense against invading pathogens. Monocytes play a pivotal role in
immune regulation by production of cytokines such as IL-1␣, IL-1␤, IL-6, IL-8, IL-12,
and Tumor Necrosis Factor (TNF)-␣. These cytokines are mediators of the acute inﬂammatory responses. They also play a key role in the process of growth and differentiation
of immunocompetent lymphocytes. The production of these proinﬂammatory cytokines is
regulated by inhibitory cytokines, such as IL-10 (Chernoff et al., 1995) and IL-1receptor
antagonist (RA) (Dinarello et al., 1991), which are also monocyte products.
Monocytes are identiﬁed by CD14 antigen which is a 53-KD glycosyl-phosphatidylinositol-anchored cell surface glycoprotein. CD14 is also the receptor for lipopolysaccharides
(LPS) and LPS/LPS binding protein (Wright et al., 1990). The majority (Ͼ90%) of circulating
monocytes in healthy young individuals express substantial level of the cell surface molecule
CD14 and are CD14ϩ/CD16Ϫ. In addition, a cell population has been deﬁned that shows a low
level of CD14 and high level of CD16 (Fc␥ receptor type III) expression (Passlick et al., 1989;
Ziegler–Heitbrock et al., 1996). When circulating monocytes are activated there is an upregulation of CD16 (Clarkson et al., 1988; Andreesen et al., 1990) and a down modulation of CD14
expression (Bazil et al., 1991). This circulating CD14dim/CD16bright subpopulation of activated monocytes has been shown to expand in inﬂammatory syndromes such as AIDS
(Thieblemont et al., 1995), bacterial sepsis (Fingerle et al., 1993), solid tumors (Saleh et
al., 1995; Schmid et al., 1995), and tuberculosis (Vanham et al., 1996). Furthermore, we
have demonstrated previously that monocytes from HIV-infected individuals constitutively produce proinﬂammatory cytokines both in vivo and in vitro (Thieblemont et al.,
1995; Sadeghi et al., 1995). These ﬁndings suggest a proinﬂammatory role of the
circulating CD14dim/CD16bright monocyte subpopulation.
Monocyte changes in aging have been identiﬁed in murine and human studies. Murine
monocytes from aged animals showed reduced antigen presentation and decreased TNF-␣
and IL-6 production after stimulation (Effros et al., 1991). Studies in humans have mainly
been done with peripheral blood mononuclear cell (PBMC), which contain approximately
10 lymphocytes to each monocyte (Fagiolo et al., 1992; Daynes et al., 1993; Catania et al.,
1997). However, in these studies changes in CD14dim/CD16bright monocytes with age have
not been determined. A separate study of isolated monocytes from elderly reported
reduction in IL-1 production after LPS-stimulation as well as decline in other monocyte
functions such as tumoricidal activity and superoxide production (Mclachlan et al., 1995).
However, the effect of age on other monocytic cytokines, including inhibitory cytokines,
as well as phenotype changes remain uncertain.
The present study has been undertaken to determine the extent of monocyte activation
in elderly individuals and to investigate if the capacity of these cells to produce cytokines
in response to stimuli is altered. Phenotypic analyses were used to identify monocyte
activation. We have also evaluated the spontaneous and LPS-stimulated production of

H. Sadeghi et al. / Experimental Gerontology 34 (1999) 959 –970

961

pro-inﬂammatory cytokines in cultures of isolated monocyte as well as in cocultures of
monocytes with autologous lymphocytes.
2. Materials and methods
2.1. Study population
The study subjects were 63 generally inactive and frequently urinary incontinent
nursing home elderly residents with an average age of 87.6 (range 72–100). The average
length of residency in nursing home was 25 months (SD ϭ 28.7), 80% were female and
90% were Caucasian. Within 14 days of blood draw for this study, two out of the 63
subjects had clinical evidence of illness. None of the subjects was taking any medication
that is known to affect immune parameters. We have taken care in selecting our young
controls to be of the same ethnic background and sex, as the predominant elderly
population. Thus, the control group included 19 young healthy blood donors with average
age of 30.5 (range 20 – 47). The specimens from both age groups were tested under the
same conditions, at the same time and by the same researcher.
2.2. Reagents
Fluorescent conjugated mouse monoclonal antibodies (mAb) directed against CD14
(LeuM3, IgG2b) and CD16 (Leu-11a, IgG1) and isotype-control mAb (mouse IgG1, IgG2)
from Beckton Dickinson (Mountain View, CA, USA) were used. The medium for cell culture,
RPMI 1640 containing L-glutamine and antibiotics (penicillin 100 U/mL and streptomycin 100
␮g/mL) were purchased from Bio–Whittacker (Walkersville, MD, USA). Histopaque, PHA,
and LPS (E-Coli serotype 026-B6) were purchased from Sigma (St. Louis, MO, USA).
Reagents for Non-Speciﬁc Estrase (NSE) were a kind gift of Drs. N. Haeffner–Cavaillon and
J.-M. Cavaillon (INSERM Unite 430, Broussais Hospital and Pasteur Institute, Paris, France).
2.3. Flow cytometry
Immunophenotypic analyses were performed by using two colors (CD14/CD16) monoclonal antibodies (mAb) staining of anticoagulated (EDTA) whole blood samples. Monoclonal antibodies were diluted at 1Ϻ5 in phosphate buffer saline supplemented with 2%
normal calf serum (NCS) and 0.1% sodium azide (Az). Sixty microliter (␮L) of diluted
ﬂourescent conjugated mAb was added to aliquots of 50 ␮L of whole blood. The mixture
was incubated for 15 min at room temperature before lysing red blood cells by 2 mL of
FACS lysing solution (Beckton Dickinson). Cell pellets were washed twice with 2 mL of
buffer solution. All FACS analysis was performed immediately. Monocytes were gated using
forward scatter, accounting for the cell dimension and side scatter, measuring cells granularity
and the proportion of cells that were stained was quantiﬁed using a FACScan ﬂow cytometer
(Fig. 1). At least 5000 events were collected in list mode for monocytes and analysis was
performed using MacIntosh computer equipped with FACScan Cell Quest software.
2.4. Puriﬁcation of monocytes
Peripheral blood mononuclear cells (PBMC) were prepared by density centrifugation
of anticoagulated whole blood over Ficoll–Hypaque for 10 min at 300 ϫ g at room

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H. Sadeghi et al. / Experimental Gerontology 34 (1999) 959 –970

Fig. 1. Two-color ﬂow-cytometric analysis of circulating monocyte subpopulation in whole blood. Whole blood
was drawn in EDTA vacucontainer tubes and cells were stained using CD14-FITC and CD16-PE monoclonal
antibodies (Becton Dickinson). Monocytes were gated according to their forward and side scatter characteristics.
The percentage of CD14Ϯ/CD162ϩ circulating monocytes for one representative aged individual (left plot) and
one representative young donor (right plot) are shown.

temperature. Monocytes were enriched by adherence to plastic culture dish (Costar,
Cambridge, MA, USA) for 45 min at 37°C, 95% humidity, and 5% CO2. Nonadherent cells
were removed by three washes of the cells by the medium. Under these conditions, each batch
of isolated adherent cells contained more than 85% monocytes as assessed by histochemical
staining for Non-Speciﬁc-Estrase positivity (NSEϩ) (Haeffner–Cavaillon et al., 1989).
2.5. Cytokine production
Monocyte (1 ϫ 106 NSEϩ cells/ml) was cultured in the absence or presence of
autologous lymphocytes and in the absence of any exogenous stimuli for 20 h. Cells were
also cultured in the presence of LPS (0.1 ␮g/ml) for 20 h. In a preliminary studies we have
observed that cytokine production by monocytes (such as IL-1) reaches a plateau in the
presence of 0.1 ␮g of LPS/ml/1 ϫ 106 monocytes and after 18 h of culture. Thus, all
further experiments were done at these conditions. After 20 h of culture, plates were
centrifuged and supernatant was collected for the assessment of extracellularly released
cytokines. Cell pellets were adjusted with the equivalent volume of the medium and
underwent three freeze-thaw cycles for the assessment of the IL-1␣, which is an intracellular cytokine. Production of cytokines in the supernatant collected from monocytes
and from autologous cultures of monocytes with lymphocytes was assessed using speciﬁc
ELISA [IL-1␣, IL-1␤, and IL-1RA by R&D systems (Minneapolis, MN, USA); IL-10 by
Immunotech (Westbrook, ME, USA); IL-6 by Immunogenetics (Belgium), and TNF-␣ by
Genzyme (Cambridge, MA, USA)].
2.6. Statistical analyses
We have used Wilcoxon sign rank test to analyze the results presented in Fig. 1 because
the values were not normally distributed and there was one outlier in each age group
which we have not eliminated. For all other results t-test were used for statistical analyses
because the results were normally distributed. Results were expressed as mean Ϯ SEM
and considered signiﬁcant if p Ͻ 0.05.

H. Sadeghi et al. / Experimental Gerontology 34 (1999) 959 –970

963

Fig. 2. Increase of circulating CD14dim/CD16bright monocyte subpopulation in elderly individuals. Results from
ﬂow cytometric analysis of whole blood from aged (n ϭ 63) and young donors (n ϭ 19) are shown. Horizontal
lines indicate mean Ϯ SEM values for each population. p ϭ 0.006 by Wilcoxon sign rank test between age
groups.

3. Results
3.2. CD14dim/CD16bright subpopulation of circulating monocytes in aged individuals
We investigated the presence of circulating CD14dim/CD16bright monocytes in healthy
elderly population in comparison to young healthy controls using two-color immunoﬂuorescence. A representative example of the ﬂow cytometry data is shown in Fig. 1.
Circulating monocytes expressing CD14dim/CD16bright were signiﬁcantly increased in
elderly subjects as compared to young controls (mean Ϯ SEM: 14.2 Ϯ 0.9 vs. 10.0 Ϯ 1.0,
respectively, p ϭ 0.006) (Fig. 2). A wide variation in expression of CD14dim/CD16bright
between individuals was evident (range 3 to 34%). The percent circulating CD14dim/
CD16bright subpopulation of monocytes in healthy individuals varies from 3 to 13%,
according to our study, and 3 to 8% according to literatures (Kreutz et al., 1992;
Ziegler-Heitbrock et al., 1993; Schmid et al., 1995; Frankenberger et al., 1996). Therefore,
we have chosen mean percentage Ϯ SD (13%) as the cut-off point. Thirty-four of the 63
(54%) elderly subjects, but only 1 of 19 (5%) young controls, showed increased circulating CD14dim/CD16bright monocytes (Fig. 2). The increase in circulating CD14dim/
CD16bright monocyte subpopulations between aged and young subjects was statistically
signiﬁcant ( p ϭ 0.006). Thus, the expansion of this monocyte subpopulation is a
characteristic feature of aging and suggests a state of in vivo monocyte activation in the
majority of this elderly population.

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H. Sadeghi et al. / Experimental Gerontology 34 (1999) 959 –970

Fig. 3. Spontaneous production of cytokines by monocytes from aged subjects. Isolated monocytes (1 ϫ 106
NSEϩ cells/ml) from aged individuals (ﬁlled bars) and from young donors (open bars) were cultured alone or
with autologous lymphocytes for 20 hours. The concentration of IL-1␣ (A) in cell lysates and of IL-1␤ (B),
IL-1ra (C), IL-6 (D), and TNF␣ (E) in culture supernatants were measured by speciﬁc ELISA. The number of
subjects in each study group is shown within the bars. The data represent mean (pg/mL) Ϯ SEM. Using t-test,
p-values for the differences between age groups are indicated.

3.2. Spontaneous in vitro production of cytokines by separated monocytes from elderly
subjects
To assess the function of monocytes of elderly subjects in the context of the existing
chronic monocyte activation, the constitutive production of cytokines (IL-␣, IL-1␤,
IL-1ra, IL-6, and TNF-␣) by isolated monocytes cultured in the absence of any exogenous
stimuli for 20 h were studied (Fig. 3). The levels of IL-1␤, IL-1ra, and IL-6 spontaneously
produced by monocytes of aged subjects was signiﬁcantly higher than production by
monocytes from young control (Fig. 3). The difference in the amount of intracellular
IL-1␣ between the two groups was of borderline signiﬁcance ( p ϭ 0.051), whereas there
have been no signiﬁcant differences observed in the levels of TNF-␣ (Fig. 3) and IL-8
(data not shown).
3.3. Effect of lymphocytes on spontaneous production of cytokines by monocytes from
aged subjects
It has been reported that production of IL-1RA, IL-8, and IL-10 by monocytes, isolated
from healthy young donors, is enhanced by the presence of lymphocytes whereas production of IL-1␤ and TNF-␣ remained unaffected (Sadeghi et al., 1999). Human CD4ϩ

H. Sadeghi et al. / Experimental Gerontology 34 (1999) 959 –970

965

Table 1
Production of cytokine by LPS-stimulated monocytes isolated from elderly

Mononuclear
cells

IL-1␣

Monocytes
Aged
4.6 Ϯ 0.3
Young 4.2 Ϯ 0.6
P
0.237
Monocytes ϩ
Lymphocytes
Aged
7.5 Ϯ 0.5
Young 6.6 Ϯ 0.8
P
0.178

n IL-1␤

n IL-1RA

n IL-6

n TNF␣

n IL-10

n

27 4.6 Ϯ 0.8* 27 35 Ϯ 3.5 24 15.2 Ϯ 3.2 17 5.1 Ϯ 0.6 22 1.3 Ϯ 0.4 8
12 7.8 Ϯ 1.6 12 33 Ϯ 7.7 11 11.9 Ϯ 2.6 11 6.5 Ϯ 0.3 10 0.6 Ϯ 0.2 4
0.040
0.284
0.221
0.169
0.060
27 5.7 Ϯ 1.3 15 109 Ϯ 15.5 15 38.3 Ϯ 0.8 17 8.1 Ϯ 1.5 24 10.5 Ϯ 2.5 8
11 9.4 Ϯ 2.8 10 117 Ϯ 17.7 10 29.8 Ϯ 5.5 11 15.2 Ϯ 3.9 11 8.6 Ϯ 0.8 4
0.104
0.359
0.104
0.055
0.243

Isolated monocytes (1 ϫ 106 NSEϩ/ml) were cultured in the absence or presence of non-adherent mononuclear cells and in the presence of LPS (0.1 ␮g/ml) for 20 h. The data present mean Ϯ SEM of the concentration
(ng/mL) of the cytokine.
* p Ͻ 0.050 by Student t-test between age groups.

and CD8ϩ T lymphocytes selectively modulate the production of IL-ra and IL-8 by
stimulated monocytes. Because there is a deﬁcit of lymphocyte function in a majority of
our study subjects (manuscript in preparation) as well as an expansion of activated
monocytes we have studied the effect of the presence of autologous lymphocytes on in
vitro monocyte function and on spontaneous release of monocytic cytokines. We have
assessed the constitutive production of IL-1␣, IL-1␤, IL-1RA, IL-6, and TNF-␣ by
puriﬁed monocytes, from elderly and young controls, cultured (at equal number) alone or
with autologous lymphocyte and in the absence of any exogenous stimulus for 20 h.
Spontaneous production of IL-1␣, IL-1␤, and IL-6 by monocytes from elderly subjects
were signiﬁcantly increased by the presence of autologous lymphocytes ( p ϭ 0.010, p ϭ
0.020, and p ϭ 0.009, respectively) whereas the differences in the levels of IL-1RA and
TNF-␣ were not signiﬁcant ( p ϭ 0.080 and p ϭ 0.053, respectively). The constitutive
production of cytokines by monocyte from young controls, whether cultured alone or in
the presence of autologous lymphocytes, have been barely detectable (IL-1␣, IL-1␤, and
TNF-␣) or non-signiﬁcant (IL-6, IL-1RA). As nonactivated lymphocytes from healthy
young controls do not produce any detectable cytokines, these results further indicate that
monocytes from elderly individuals are activated and this activation is further enhanced by
the presence of lymphocytes.
3.4. Cytokine production by LPS-stimulated monocytes from the aged subjects
To assess the changes in the capacity of monocytes to produce cytokines, we studied
the in vitro cytokine production by LPS-stimulated monocytes isolated from elderly and
the young controls (Table 1). Among the cytokines studied, there were no signiﬁcant
differences observed in the production of cytokines by monocytes from elderly persons
compared to young controls except for the production of IL-1␤, which was signiﬁcantly
reduced. Furthermore, the presence of lymphocytes did not make a difference between
aged and young subjects in any of the in vitro produced cytokines by monocytes (Table
1). Moreover, the production of IL-8 by LPS-stimulated monocytes from aged individuals

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H. Sadeghi et al. / Experimental Gerontology 34 (1999) 959 –970

cultured in the presence of autologous lymphocytes was also signiﬁcantly reduced (data
not shown). The levels of IL-10 and IL-1␣ in culture of LPS-stimulated monocytes from
elderly people were higher as compared to young controls, but the difference was not
signiﬁcant. LPS-stimulated lymphocytes do not produce the monocytic cytokines IL-1␣,
IL-1␤, or IL-1ra, and produce almost nondetectable amount of IL-8, IL-6, and TNF-␣
(data not shown). These observations indicate that monocytes from aged individuals
respond to stimulation but the balance of cytokines may be altered.

4. Discussion
Aging is associated with changes in immune function. Animal and human studies have
shown major impairments of T cells with aging (Pisciotta et al., 1967; Sidman et al., 1987;
Ustuyama et al., 1987). Although monocytes are recognized as critically important for the
homeostasis of immune function, only limited data on age-related changes in monocytes
compared to lymphocytes are available (Effros et al., 1991; Mclachlan et al., 1995;
Murphy et al., 1996). In this study we focused on the effect of age on expansion of a
subpopulation of circulating monocytes and in cytokines produced by monocytes.
The heterogeneity of monocytes with respect to the expression of surface antigen
CD14bright and CD14dim is clearly established in the literature (Passlick et al., 1989; Wang
et al., 1992; Fingerle et al., 1993; Ziegler–Heitbrock et al., 1993). Although CD14bright
monocytes represent the main population, CD14dim cells are generally considered to
represent an activated subpopulation. Monocytes undergo morphologic and phenotypic
changes during the course of maturation into macrophages. These changes include upregulation of CD16, MAX.1 and HLA-DR, which are typical of tissue macrophages. The
CD14dim/CD16bright subpopulation of monocytes has been shown to be consistently
expanded in inﬂammatory syndromes as noted above and is presumed to reﬂect monocyte
activation. We found a signiﬁcant expansion of this CD14dim/CD16bright subpopulation of
circulating monocytes in elderly subjects that may indicate a state of in vivo monocyte
activation. This ﬁnding is consistent with the reported upregulation of other activation
markers, CD45RO, CD54 (ICAM-1), CD29 (␤1-integrin), CD32 (Fc␥R-II) on circulating
monocytes in elderly (Stohlawetz et al., 1998). Other studies have also shown high
expression of Class II antigen on circulating CD14dim/CD16bright monocytes by using
three color immunoﬂourescence (Ziegler–Heitbrock et al., 1993). Thus, although still in
the circulation, this subpopulation of monocytes has features in common with mature
tissue macrophages. Therefore, it seems that these circulating cells in elderly are monocytes in the process of maturation but that may have not reached the ﬁnal stage or may
have defects of cellular maturation; i.e. they do not go into tissue like macrophages of
young people. Further investigations are required to fully elucidate the mechanisms and
function of this subpopulation.
Furthermore, the signiﬁcant increase of neopterin, a marker of in vivo monocyte
activation, in the serum of these same elderly population conﬁrms previous report (Catania
et al., 1997) and supports further the hypothesis of monocyte activation (data not shown).
This activation of circulating monocytes cannot be due to experimental manipulation
because the specimens have been promptly processed in parallel with the control samples.
The possibility of clinical pathology conditions affecting monocyte activation can be
excluded because our study population consisted of elderly but not clinically ill nursing
home residents. None of the elderly have had clinical evidence of infection or been taking

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967

any medication that is known to affect immune parameters. Therefore, in our study
population, we have not observed evidence to support involvement of clinical pathology
in monocyte activation. This is in accord with Gon et al. (1996) who has also demonstrated lack of correlation between clinical outcome and monocyte function in elderly
people suffering from pneumonia. Thus, increase in activated circulating monocytes may
be based on ongoing processes of aging of the immune system.
Chronic activation of monocytes in elderly was also evident by observing spontaneous
cytokine overproduction. Most reports have investigated PBMC where the lymphocytes
outnumbered the monocytes by ten to one (Nachbaur et al., 1991; Fagiolo et al., 1993;
Roubenoff et al., 1998; Straub et al., 1998). There have not yet been any reports on
constitutive cytokine production by isolated monocytes from elderly. Here, we studied
puriﬁed monocytes isolated from elderly and young subjects. We observed signiﬁcantly
higher amount of spontaneous secretion of IL-1␤, IL-1RA, and IL-6 by monocytes
isolated from elderly in contrast to young controls. The level of IL-1␣ has also been
increased but with borderline signiﬁcance ( p ϭ 0.051). TNF-␣ level was unchanged with
age, whereas IL-10 from unstimulated cells was undetectable. Our data, showing varying
patterns of change in secretion among the cytokines, is consistent with the well-documented independent regulation of cytokine production (Gauchat et al., 1985). Moreover,
our observed increase in secretion of IL-1␤ and IL-1RA (monocytic cytokines) is in
accord with the reported elevated serum levels of IL-1RA (Catania et al., 1997), IL-1␤
(Gon et al., 1996), and those produced by peripheral mononuclear cells in elderly
(Roubenoff et al., 1998). Taken together, these results provide further evidence, beside
phenotypic changes, that the monocytes of elderly are in a state of in vivo activation.
IL-6 in conjunction with other cytokines is involved in B-cell maturation and differentiation leading to production of speciﬁc antibodies. Increased IL-6 release by isolated
monocytes from elderly in our study is also consistent with the increased serum level of
IL-6 and production by peripheral mononuclear cells from elderly reported elsewhere
(Daynes et al., 1993; Ershler et al., 1993; Roubenoff et al., 1998; Straub et al., 1998). The
increase in IL-6 could lead to autoimmunity. Also, the bone resorbing activity of IL-6 is
well-documented (Wong et al., 1988; Bataille et al., 1991; Roodman 1992). IL-6 increases
activity of osteoclasts and thus contributes to bone degradation. The increased production
of IL-6 by monocytes from aged people may contribute to the pathogenesis of diseases
such as osteoporosis and autoimmunity that are common late in life (Nachbaur et al.,
1991; Merz et al., 1991).
Likewise, the secretion of monocytic cytokines was signiﬁcantly enhanced when these
cells have been co-cultured with autologous lymphocytes demonstrating the effect of
lymphocytes on monocytes. Isolated lymphocytes from our study population did not
release detectable amounts of monocytic cytokines (IL-1 and TNF-␣) (data not shown).
Thus these cytokines are mainly products of monocytes. On the other hand, IFN-␥ is a
well known monocyte activator and we found that production of IFN-␥ by PHA-stimulated whole blood from these same elderly individuals was signiﬁcantly increased as
compared to young controls (data not shown). We have also observed other age-associated
changes in lymphocytes of our elderly population (manuscript in preparation). Based on
the paracrine nature of cytokine production, changes in lymphocyte function in elderly
might be expected to inﬂuence monocytic cytokine production. Taken together these
results may in part explain the effect of lymphocytes on enhancement of cytokine
production by monocytes from elderly.
The functional capacity of monocytes alone or in the presence of autologous lympho-

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H. Sadeghi et al. / Experimental Gerontology 34 (1999) 959 –970

cytes was tested by LPS stimulation and cytokine production. In general, the production
of cytokines by cells isolated from aged and young populations were similar except for
decreased production of IL-1␤ and increased production of IL-10 by cultured monocytes
in the elderly. Our results are in agreement with other studies (Mclachlan et al., 1995;
Castle et al., 1999). Monocytic cytokines such as IL-1␤ and TNF-␣ have been reported to
possess antitumoral properties (Mclachlan et al., 1995). IL-1␤ is mainly a product of
activated monocytes and is involved in the inﬂammatory response and induction of fever.
It is also an early signal leading to T cell IL-2 gene induction. On the other hand, the role
of IL-10 in reducing T cell proliferation by downregulating the expression of MHC II and
co-stimulatory molecules on APC (de Waal Maleyft et al., 1991) and inhibition of IL-2
production (de Waal Maleyft et al., 1993) has been well documented. The lower IL-1␤ and
the higher IL-10 response may contribute to the defect in T cell proliferation observed in
these same elderly individuals (manuscript in preparation).
In summary, the present study indicates that aging may affect functions of monocytes.
Changes of surface antigen expression, constitutive cytokine production, and in capacity
of monocytes to produce cytokines may contribute to the deﬁciency of effective immune
functions often found in elderly. This chronic activation of monocytes and the complex
imbalance of the cytokines they produce are characteristic features of aged individuals and
may contribute to impaired immune competence and other disorders of aging.

Acknowledgments
We are grateful to Dr. N. Aziz (CIRID, UCLA, Los Angeles, CA) for assistance in
ELISA assays, Drs. N. Haeffner–Cavaillon and J.-M. Cavaillon (INSERM-U430 Broussais Hospital and Pasteur Institute respectively; Paris, France) for the generous gift of
nonspeciﬁc-estrase product. We are also thankful to Dr. R. Effros (UCLA, Los Angeles,
CA, USA) for reviewing this manuscript.

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