Ultrastructural and cytochemical evaluation of sepsis ......J. Anat. (1998) 192, pp. 13–23, with 7...
Transcript of Ultrastructural and cytochemical evaluation of sepsis ......J. Anat. (1998) 192, pp. 13–23, with 7...
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Ultrastructural and cytochemical evaluation of sepsis-induced
changes in the rat pulmonary intravascular mononuclear
phagocytes
BALJIT SINGH1*, KATHLEEN J. DOANE2 AND GARY D. NIEHAUS3
"Department of Anatomy and Physiology, Atlantic Veterinary College, Prince Edward Island, Canada and #Departments of
Microscopic Anatomy and $Physiology, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio, USA
(Accepted 12 August 1997)
Sepsis stimulates an increase in the number and activity of mononuclear phagocytes in systemic host-defence
organs. The present study was conducted to define the ultrastructural and cytochemical characteristics of the
mononuclear phagocytes that sequester in the lung microvasculature of septic rats. Fourteen rats were
challenged with a single intraperitoneal injection of saline (0±5 ml}100 g), E. coli (2¬10(}100 g) or glucan
(4 mg}100 g), and euthanased 2, 4, or 7 d later. The lungs were inflation fixed and processed for
transmission electron microscopy. Cellular morphology was used to identify the intravascular mononuclear
phagocytes and acid phosphatase (AcPase) expression was monitored as an index of cellular differentiation
and activation. Control rats contained a limited number of monocytes in the pulmonary vasculature. In
contrast, large numbers of activated mononuclear phagocytes were seen in the microvasculature within 48 h
of treatment with either microbial product. The recruited pulmonary intravascular mononuclear phagocytes
(PIMP) exhibited AcPase-reactive Golgi complexes, accumulation of secretory vesicles and other features of
cell activation consistent with enhanced biosynthetic activity. Subsequent electron microscopy, conducted 4
and 7 d posttreatment, suggested that a progressive decline in the number and activity of PIMPs then
occurred. In order to quantify the sepsis-induced accumulation of AcPase-positive PIMP, the experimental
challenges were repeated in 11 rats and, 48 h later, tissue samples were evaluated by light microscopy for
tartrate-insensitive acid phosphatase. Control rats exhibited 0±148³0±107 AcPase-positive PIMP}alveoli. E.
coli and glucan challenged animals exhibited significant (P! 0±01) increases in AcPase-positive mononuclear
phagocytes, with 0±782³0±073 and 0±636³0±170 PIMP}alveoli respectively. The results demonstrate that
focal sepsis stimulates a significant, but transient, recruitment of activated mononuclear phagocytes into the
rat pulmonary microvasculature.
Key words : Mononuclear phagocyte system; host defence.
Protection against blood-borne pathogens is provided
by mononuclear phagocytes lining the microvascu-
lature of the liver, spleen, lungs and bone marrow
(Benacerraf et al. 1957). Each mammalian species
exhibits this mononuclear phagocyte system (MPS)
but there is significant interspecies variability in the
relative contribution of the normal liver and lungs to
total systemic host defence (Niehaus et al. 1980). The
Correspondence to Dr Gary D. Niehaus, Department of Physiology, NEOUCOM, 4209 State Route 44, Box 95, Rootstown, OH 44272-
0095, USA. Tel : 1 330 325-2511, ext. 258; fax: 1 330 325-2524; e-mail : gdn!riker.neoucom. edu
* Current address : Texas Agricultural Experimental Station, Texas A & M University System, 7887 North Highway 87, San Angelo, TX
76901, USA.
human possesses a liver-dominant host-defence sys-
tem in which hepatic Kupffer cells avidly engulf most
circulating microbes while relatively few of the
prokaryotic cells accumulate in the pulmonary micro-
vasculature. However, radiographic studies (Hammes
et al. 1983; Marigold et al. 1983; Shih et al. 1986) have
shown that septic patients often exhibit a significant
enhancement of nonembolic localisation of blood-
borne particulates in the lung microvasculature. It has
been suggested that the pulmonary sequestration of
test particulates in the septic patient is mediated by
active mononuclear phagocytes lining the pulmonary
microvasculature (Klingensmith & Ryerson, 1973).
The current study was conducted to characterise the
sepsis-induced increase in active mononuclear phago-
cytes that occurs in the pulmonary microvasculature
of a model species which shares the human liver-
dominant mononuclear phagocyte system.
Local interactions between microbes and mono-
nuclear phagocytes stimulate a system-wide change in
the capacity of the MPS to respond to subsequent
bacteraemia. There is an increase in the number and
size of phagocytes located in the primary organs of the
rat mononuclear phagocyte system following intra-
venous infusion of glucan, a microbial product
(Filkens et al. 1964). Intravenous infusion of en-
dotoxin also stimulates sequestration of mononuclear
phagocytes in the lung microvasculature (Warner et
al. 1994). Additionally, the phagocytic capacity of
pulmonary intravascular mononuclear phagocytes
(PIMP) is significantly enhanced following endotoxin
challenge (Warner et al. 1994). Thus sepsis appears to
mediate an enhanced recruitment and activation of
pulmonary intravascular mononuclear phagocytes.
Based on the preceding observations, we have
developed the global hypothesis that induction of
local sepsis results in upregulation of the mononuclear
phagocyte system so that each component of that
host-defence system increases its capacity to engulf
and neutralise foreign and effete particulates. The
current study was conducted to demonstrate that
intraperitoneal sepsis induces an increase in the
number of mature mononuclear phagocytes in the
pulmonary microvasculature. Light and electron
microscopy was used to define the time-dependent
changes in ultrastructural and cytochemical charac-
teristics of pulmonary intravascular mononuclear
phagocytes following intraperitoneal infection by
either live gram negative bacteria, Escherichia coli (E.
coli) or glucan (a β polysaccharide purified from the
wall of baker’s yeast). Focal injection of either
microbial agent stimulated a significant (P! 0±01), 4-
fold increase in the number of PIMP at 48 h
postchallenge. Those acutely sequestered mono-
nuclear phagocytes exhibited the morphological and
cytochemical characteristics of differentiated, matur-
ing phagocytes. However, the number, maturity and
activation status of the phagocytes decreased at 96 h
and were near normal at 168 h after the single
intraperitoneal injection of microbial products. These
data demonstrate that focal nonvascular sepsis stimu-
lates a rapid but transient upregulation of the primary
systemic host-defence cells of the lung.
The studies were reviewed by the Institutional Animal
Care and Use Committee and were conducted in
accordance with the principles defined in the Guide
for the Care and Use of Laboratory Animals (NIH
85-23).
Electron microscopy
Fourteen male Sprague–Dawley rats (369±8³73±5 g)
were divided into 3 groups. Two control animals were
evaluated 0 or 7 d after intraperitoneal injection of
saline (0±5 ml}100 g). Animals in the experimental
groups were challenged either with E. coli (2¬10(}100 g i.p.) or with glucan (4 mg}100 g i.p.) and evalu-
ated 2, 4 or 7 d (2 rats}d) later. Pilot studies (data not
shown) demonstrated that the nonlethal bacterial
challenge (2¬10( E. coli}100 g) resulted in an asymp-
tomatic animal which exhibited increased numbers of
activated mononuclear phagocytes in the pulmonary
microvasculature. A similar evaluation of glucan
confirmed previous reports that 4 mg}100 g of body
weight (Filkens et al. 1964) activated the mononuclear
phagocyte system. On the day of the experiment, the
animals were anaesthetised (1±0 mg xylazine and
10 mg ketamine HC1}100 g; i.p.) and intubated
through a midline incision. The animals were euthan-
ased (0±5 ml Ketaset i.v.), a midline thoracotomy
performed, and the lungs inflation fixed with 2%
glutaraldehyde and 2±5% paraformaldehyde in 0±2
sodium cacodylate buffer (1±0 ml}100 g). The lungs
were excised and incubated for 30 min in a beaker of
the same fixative (4 °C). The lungs were then cut into
parasagittal slices (2–4 mm thick) and 10–15 random
samples collected. At least 1 large sample (3–5
mm}side) was collected from each lobe of the lung.
All samples were collected from the parenchyma in
order to exclude the large vessels and airways located
in the centre of each lobe. The samples were immersed
for an additional 3 h in the same fixative, 60 min in
1±5% osmium tetroxide and then stored (4 °C) in
fresh buffer.
Acid phosphatase cytochemistry
Acid phosphatase was used as a marker of mono-
histiocytic differentiation (Lord et al. 1990; Sokol et
al. 1993) and activation (Bevilacqua et al. 1991). The
cytochemistry was performed on lung tissue using
Gomori’s lead capture method (Borgers et al. 1991;
Singh et al. 1995). The tissues were incubated at
room temperature in a solution of 10 m sodium
14 B. Singh, K. J. Doane and G. D. Niehaus
Fig. 1. Mononuclear phagocytes in the pulmonary microvasculature of control rats. The cytoplasm of the immature cells (Mo) was devoid
of vacuoles (a) or AcPase staining (b). AS, alveolar space; Ep, type II alveolar epithelial cells ; arrowhead, type I alveolar epithelial cell ; LB,
lamellar bodies ; En, endothelium; bar, 1 µm.
β-glycerophosphate, 30 m lead nitrate and 50 m
acetate buffer (pH 5±0) for 3 h. The samples were then
rinsed with 1% ammonium sulphide and acetic acid
(Borgers et al. 1991). A medium that was deficient in
the sodium β-glycerophosphate substrate was used to
process the control tissues.
Processing for transmission electron microscopy
The tissues were dehydrated in ascending concentra-
tions of ethyl alcohol and propylene oxide, and
embedded in epoxy resin. Semithin sections were cut
on a Reichert Jung Ultracut microtome, stained with
toluidine blue and examined by light microscopy to
select areas for ultrathin sectioning. The ultrathin
sections were contrasted with uranyl acetate and lead
citrate before being examined with a Hitachi-100S
transmission electron microscope.
Light microscopy
A light microscopy study was conducted to quantify
the accumulation of maturing AcPase-positive mono-
nuclear phagocytes that occurred in the lung micro-
vasculature at 48 h after induction of intraperitoneal
sepsis. On d 0, 11 male Sprague–Dawley rats
(190³7 g) were divided into 3 groups and challenged
by i.p. injection of (1) saline (0±5 ml}100 g; n¯ 3) ;
(2) E. coli (2¬10(}100 g; n¯ 4) ; or (3) glucan
(4 mg}100 g; n¯ 4). After 48 h the animals were
anaesthetised (1±0 mg Rompum, 10 mg Ketaset}100 g i.p.) and intubated through a midline incision.
The animals were euthanased (0±5 ml Ketaset i.v.),
a midline thoracotomy performed, and the lungs
inflation fixed with 4% paraformaldehyde in 0±1
phosphate buffer with 4±5% sucrose and 3% NaCl,
pH 7±5 (1±0 ml}100 g). The lungs were excised and
incubated for 180 min in a beaker of the same fixative
(4 °C). The lungs were then cut into parasagittal slices
(2–4 mm thick) and 10–15 random samples collected.
One or more sample(s) (3–5 mm}side) were collected
from each lobe of the lung. All samples were collected
from the parenchyma in order to exclude large vessels
and airways. The samples were transferred to fresh
buffer and stored (4 °C). The samples were cryopro-
tected in 7% sucrose in PBS, embedded in Tissue
Freezing Medium (Fisher Scientific, Pittsburgh, PA)
and frozen. The embedded samples were cut into 6 µm
thick sections (Minotome Cryostat, IEC, Needham,
MA) and mounted on Vectabond slides (Vector
Laboratories, Inc., Burlingame, CA). The slides were
stained for tartrate-insensitive acid phosphatase (kit
387-A; Sigma Chemical Co., St Louis, MO), which
occurs specifically in maturing mononuclear phago-
Sepsis-induced changes in rat PIMP 15
Fig. 2. Accumulation of mononuclear phagocytes (M) in the pulmonary microvasculature at 48 h after intraperitoneal injection of E. coli (a)
or glucan (b). Higher magnification micrographs of the accumulating mononuclear phagocytes from rats treated with E. coli (c) or glucan
(d ) show features including prominent centrioles (thin arrow), Golgi complexes (G), the formation of vacuoles, and plasma membrane
projections in contact (pointed arrows) with the capillary endothelium. The morphology suggests that the accumulating PIMP are activated.
Asterisk, interstitium; hollow arrows, mitochondria ; N, nucleus ; AS, alveolar space; p, platelet ; Ep, epithelial cell ; En, endothelium;
bar, 1 µm.
16 B. Singh, K. J. Doane and G. D. Niehaus
Fig. 3. High magnification views of E. coli treated PIMP (a) show that the cells exhibit prominent interchromatin granules (arrows) in the
nucleus (N) as well as cytoplasmic vacuoles (V). (b) Both secretory vesicles (small thick arrows) associated with the Golgi complex (G) and
a plasma membrane projection (large arrow) are seen in contact with the endothelium (En). Hollow arrows, mitochondria ; bars, 0±2 µm.
cytes and macrophages. Using a light microscope
(model BHT; Olympus Optical Co., Tokyo, Japan),
10 high power fields (¬1000 magnification under oil)
were evaluated for each of 5 tissue samples (i.e. a total
of 50 fields}lung; approximately 500 alveoli). Septal
AcPase-positive, mononuclear phagocytes (indicated
by 2 or more red cytoplasmic granules of acid
phosphatase) were counted, while the heavily stained
alveolar macrophages were ignored. The number of
mature mononuclear phagocytes per alveolus was
determined.
Statistics
The light microscopy data are reported as mean³..
One-way analysis of variance and Student’s unpaired
t tests were applied to the data. Significance was
assumed when P! 0±05.
Control rats exhibited a limited number of intra-
vascular monocytes. The immature, unactivated
status of the monocytes was indicated by their lack of
cytoplasmic vacuoles (Fig. 1a), lysosomes or AcPase
staining (Fig. 1b). Additionally, most cells demon-
strated limited physical interactions with the capillary
endothelium, suggesting that they were transiting the
lung vasculature. Thus unprimed rats appear to
possess few resident pulmonary intravascular mono-
nuclear phagocytes (PIMP).
The electron micrographs indicated that PIMP
accumulated in the pulmonary microvasculature at
48 h after experimental induction of intraperitoneal
sepsis either with E. coli (Fig. 2a) or glucan (Fig. 2b).
The sequestered phagocytes exhibited multiple cyto-
plasmic projections that were in contact with the
capillary walls, suggesting the development of focal
areas of adhesion with the endothelium (Fig. 2c, d ).
These adherent mononuclear phagocytes were more
mature than their nonprimed sisters. Cells from both
E. coli-primed and glucan-primed rats showed signs of
activation, such as prominent centrioles and Golgi
complexes (Fig. 2c, d ). Many PIMP exhibited both
distinct interchromatin granules in the nuclear matrix
(Fig. 3a) and Golgi complexes in association with
secretory vesicles (Fig. 3b). Additionally, E. coli-
primed (Fig. 4a) and glucan-primed (Fig. 4b) PIMP
Sepsis-induced changes in rat PIMP 17
Fig. 4. Acid phosphatase expression in Golgi complexes (G) and lysosomes (L) demonstrating the presence of mature mononuclear
phagocytes in the pulmonary microvasculature at 48 h after E. coli (a) or glucan (b) challenge. A higher magnification view (c) of the
mononuclear phagocytes in panel a shows the localisation of AcPase in Golgi complexes (G). Similar Golgi-associated AcPase reactivity is
shown in a high magnification view of a pulmonary mononuclear phagocyte (d ) from a glucan-treated rat. N, nucleus ; E, endosome; hollow
arrows, mitochondria ; IM, interestitial macrophage; AS, alveolar space. Bars : a, b, 1 µm; c, d, 0±25 µm.
18 B. Singh, K. J. Doane and G. D. Niehaus
Fig. 5. Light micrographs of tartrate-insensitive AcPase-positive
mononuclear phagocytes in the rat lung microvasculature at 48 h
after i.p. injection of saline (a), E. coli (b) or glucan (c). Small
arrowheads, AcPase-positive PIMP; large arrowheads, AcPase-
positive alveolar macrophages. ¬400.
demonstrated an enhanced expression of AcPase.
With either priming agent, the expressed AcPase was
associated with Golgi complexes (Fig. 4c, d ). Thus,
the 48 h mononuclear phagocytes exhibited the char-
acteristics of activated and rapidly differentiating
macrophages.
Light microscopy (Fig. 5) confirmed the accumu-
lation of maturing mononuclear phagocytes in the
lung microvasculature at 48 h postsepsis. The pul-
monary microvasculature of control rats possessed
0±148³0±107 AcPase-positive cells in each alveolus
(Fig. 6), with the PIMP exhibiting light-to-moderate
Fig. 6. Tartrate-insensitive, AcPase-positive PIMP at 48 h after i.p.
injection of saline (n¯ 3), E. coli (n¯ 4) or glucan (n¯ 4). Data
presented as means³.. * P! 0±01.
AcPase staining. At 48 h after i.p. injection either of
live E. coli or glucan, there were 0±782³0±073 and
0±636³0±170 PIMP}alveolus, respectively. Addition-
ally, the primed PIMP appeared to stain more
intensely for AcPase. Thus induction of sepsis stimu-
lated a significant (P! 0±01) accumulation of matur-
ing PIMP in the lung microvasculature.
The AcPase-positive cells remaining in the micro-
vasculature at 96 h posttreatment continued to possess
lysosomes and Golgi complexes that were reactive for
AcPase (Fig. 7a, b). However, the mature PIMPs
tenure in the microvasculature appeared to be tran-
sient. Electron microscopy suggested that the number
of PIMP had began to decrease at 96 h and was near
normal at 168 h postinflammation. Thus within 7 d
following induction of focal sepsis, an acute ac-
cumulation of mature activated PIMP was followed
by a progressive disappearance of the cells from the
pulmonary microvasculature. Since differentiated cells
cannot regress to a precursor stage, the mature PIMP
either diapedesed into the alveoli or were transported
to another location by the blood.
Histological changes in the pulmonary interstitium
of primed animals may suggest that some transseptal
movement of vascular leucocytes occurred. Electron
micrographs of primed lungs at 96 h postsepsis
showed evidence of widened interstitial spaces and
limited disruption of the alveolar epithelium (Fig.
7a, c, d ). Since the structural derangements did not
result in any overt signs of respiratory distress, it is
possible that the altered pulmonary structure is
evidence of normal efflux pathways for cells moving
from the vasculature, across the interstitium and into
the alveoli.
Sepsis-induced changes in rat PIMP 19
Fig. 7. Characteristics of the pulmonary intravascular mononuclear phagocytes and the lung interstitium at 96 h post-challenge. In a, a
pulmonary intravascular mononuclear phagocyte from a rat treated with E. coli exhibits AcPase both in Golgi complexes (G) and lysosomes
(L). In b a morphologically mature pulmonary intravascular mononuclear phagocyte is seen in the lung of a glucan-treated rat. The widened
pulmonary interestitial space (asterisks) observed after intraperitoneal injection of either E. coli (a, c) or glucan (d ) suggests disorganisation
20 B. Singh, K. J. Doane and G. D. Niehaus
In the normal rat, mononuclear phagocytes contribute
to the pulmonary sequestration of blood-borne parti-
culates (Collet & Policard, 1962; Frankel et al. 1962;
Aullenbacher et al. 1983; Niehaus & Karve, 1996).
These cells are augmented by an accumulation of
macrophage-like phagocytes at 24 h after induction of
sepsis (Freudenberg, 1989; Warner et al. 1994) and, as
a result, particulate uptake in the lungs is enhanced
(Klingensmith & Lovett, 1974; Warner et al. 1994).
The current study both confirmed the acute phagocyte
sequestration and demonstrated that the sequestered
cells exhibit the indented nuclei, abundant lysosomes,
and large cytoplasm-to-nucleus ratio of differentiated
macrophages (Atwall et al. 1989, 1994). In species that
possess pulmonary intravascular macrophages (PIM),
the mononuclear phagocytes are permanently bound
to the endothelium by adhesion plaques (Winkler,
1988). While the rat exhibited similar focal PIMP-to-
endothelium adhesions, the adhesion was not per-
manent because the differentiated phagocytes dis-
appeared from the microvasculature within 7 d of the
microbial stimulation. Thus the current study demon-
strates that the sepsis-induced increase in pulmonary
intravascular mononuclear phagocytes is transient.
The elevated concentrations of AcPase in the PIMP
cytoplasm demonstrate the intense biosynthetic ac-
tivity that is characteristic of differentiated macro-
phages. Lysosomal AcPase increases during matu-
ration of mononuclear phagocytes (Schnyder &
Baggioline, 1978; Bevilacqua et al. 1991; de Chastel-
lier et al. 1993; de Chastellier & Berche, 1994; Kam-
barage et al. 1995) and contributes to the elevated
bacteriocidal capacity of those differentiated phago-
cytes (de Chastellier et al. 1993; de Chastellier &
Berche, 1994). Bacteria are degraded when the
lysosomal hydrolase is secreted into microbe-engorged
phagosomes (Steinman et al. 1983), as transmembrane
H+-ATPases optimise the phagosome pH (Mellman
et al. 1986). Production of the enzyme, controlled by
the bcg gene (de Chastellier & Berche, 1994), involves
progressive transport of the enzyme from the en-
doplasmic reticulum through the cis-Golgi network,
the Golgi stack and the trans-Golgi network before it
is stored in lysosomes (Rothman & Orci, 1992;
Mellman & Simons, 1992; Thomopoulos et al. 1992).
The current mononuclear phagocytes exhibited the
increased lysosomal AcPase concentrations of mature
of the interstitial matrix. The patchy collagen (C) and disrupted type I alveolar epithelium in c and uplifted type II alveolar epithelial cell
(Ep) in d are also consistent with interestitial damage. M, PIMP; N, nucleus ; Nu, nucleolus ; p, platelet ; AS, alveolar space; en, endothelium;
* interstitium; bars, 1 µm.
cells. The increased amounts of nuclear euchromatin
and presence of AcPase in the production organelles
indicate that active transcription and production of
the hydrolase were occurring. The data demonstrate
that the transiently sequestered cells are mature active
mononuclear phagocytes.
The current single septic episode induced seques-
tration of a transient PIMP population, possibly
followed by movement of the cells into the interstitium
and alveoli (Hance et al. 1985; Warnock et al. 1987;
Weinstock & Brain, 1988; Freudenberg, 1989; Sebring
& Lehnert, 1992). However, chronic pathologies may
prolong the phagocyte duration in the microvascu-
lature. Experimental hepatic injury causes a sustained
accumulation of hyperactivated PIM-like phagocytes
in the rat pulmonary microvasculature (Chang &
Ohara, 1994). It is unclear whether the population of
active cells is maintained by a constant turnover of
differentiated phagocytes or a prolonged duration of
individual macrophages. Regardless of the source of
PIMPs, it appears that a chronic pulmonary aug-
mentation of systemic host-defence can occur. This
experimental observation is consistent with the en-
hanced pulmonary systemic host-defence capacity
that is observed in patients with hepatic disease
(Keyes et al. 1973; Steinbach, 1972).
The response of the rat pulmonary systemic host-
defence mechanism both to sepsis and hepatic injury
mimics the human reaction to these pathologies.
Functionally, the human mononuclear phagocyte
system is poorly defined but it appears to exhibit
characteristics of a liver-dominant host-defence sys-
tem. Blood-borne test particulates are rapidly cleared
from the circulation (Salky et al. 1964) at removal
rates similar to those of liver-dominant species
(Dobson, 1957; Noller, 1957; Niehaus et al. 1980).
Approximately 1±5% of the blood-borne test parti-
culates accumulate in the normal human lung (Stern
et al. 1966; Keyes et al. 1973), a rate similar to that
seen in the rat (Niehaus et al. 1980). Scintigraphic
studies demonstrate that 80–90% of intravenously
infused **mTc-sulphur colloid rapidly sequesters in the
liver, with most of the remaining colloid accumulating
in the spleen and bone marrow (Mettler & Guiberteau,
1991). However, significant pulmonary sequestration
of **mTc-sulphur colloid has been reported during
liver cancer (Steinbach, 1972), hepatic injury (Keyes et
al. 1973; Brown & Starzl, 1969; Shih et al. 1986) or
infection (Hammes et al. 1983; Marigold et al. 1983;
Sepsis-induced changes in rat PIMP 21
Shih et al. 1986). It is postulated that the **mTc-
sulphur colloid is taken up by pulmonary intra-
vascular mononuclear phagocytes (Klingensmith &
Ryerson, 1973). The phagocytic uptake hypothesis is
supported by the demonstration of a population of
pulmonary intravascular macrophages in patients
with noninfectious disease (Dehring & Wismar, 1989)
and by histological evidence of increased pulmonary
mononuclear phagocytes in a patient which had
exhibited significant pulmonary accumulation of
**mTc-sulphur colloid (Imarisio, 1974).
Enhanced localisation of mononuclear phagocytes
in the pulmonary microvasculature may represent a
nonspecific response of the mononuclear phagocyte
system to any foreign agent. In other words, MPS
upregulation may occur regardless of the charac-
teristics of the invading agent. Clinical evidence of
nonspecific upregulation of the pulmonary intravas-
cular phagocyte system is provided by the increased
pulmonary microvascular sequestration of blood-
borne **mTc-sulphur colloid that occurs during Lassa
fever (Marigold et al. 1983), bacterial sepsis (Keyes et
al. 1973; Imarisio, 1974), infectious mononucleosis
(Hammes et al. 1983), or disseminated intravascular
coagulation (Smith et al. 1980; Teertstra et al. 1985).
In the current study, similar PIMP sequestration
occurred in response either to live E. coli bacteria or
glucan (Fig. 6). Those microbial products were chosen
because pretreatment with the gram-negative bacteria
E. coli enhances the host’s tolerance to subsequent
endotoxaemia (Prigal, 1969; Parant et al. 1976)
whereas glucan pretreatment significantly decreases
host tolerance (Bowers et al. 1986). The current results
confirm that host invasion of foreign products
stimulates a nonspecific sequestration of differentiat-
ing PIMP. The results do not indicate the functional
characteristics (phagocytic capacity, cytotoxin and
monokine production, etc.) of those sequestering
PIMP.
In summary, both the rat and human appear to
possess liver-dominant systemic host-defence mech-
anisms (Niehaus, 1989). Functionally, both species
appear to exhibit similar capacities to clear particu-
lates from the circulation (Niehaus et al. 1980).
Normally, very limited quantities of blood-borne
particulates are cleared by the pulmonary intravas-
cular mononuclear phagocytes in these species (Wein-
stock & Brain, 1988). However, sepsis or hepatic
injury significantly upregulates these pulmonary sys-
temic host-defence cells. The recruitment of mono-
nuclear phagocytes into the pulmonary capillaries
appears to be a nonspecific component of the host
response to inflammatory stimuli and absence of a
continuous stimulus leads to their migration from the
vasculature.
Ms Dorota Wadowska and Dr David E. Sims are
thanked for their assistance with the electron mi-
croscopy. The study was funded by an Ohio Research
Challenge Grant and intramural funds from the
Department of Physiology, NEOUCOM, as well as
intramural funds from the Department of Anatomy
and Physiology, Atlantic Veterinary College.
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