J. Anat. (1998) 192, pp. 13–23, with 7 figures Printed in the United Kingdom 13

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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