By
From the * I. Department of Medicine, Section of Pathophysiology; Institute of Pathology; § III. Department of Medicine, University of Mainz, D-55101 Mainz, Germany;
Instituto di
Ricerche di Biologia Moleculare P. Angeletti, Pomezia, Rome, Italy; and ¶ Institute of Pathology,
University of Cologne, 51123 Cologne, Germany
Soluble cytokine receptors modulate the activity of their cognate ligands. Interleukin (IL)-6 in association with the soluble IL-6 receptor (sIL-6R) can activate cells expressing the gp130 signal transducer lacking the specific IL-6R. To investigate the function of the IL-6-sIL-6R complex in vivo and to discriminate the function of the IL-6-sIL-6R complex from the function of IL-6 alone, we have established a transgenic mouse model. Double-transgenic mice coexpressing IL-6 and sIL-6R were generated and compared with IL-6 and sIL-6R single-transgenic mice. The main phenotype found in IL-6-sIL-6R mice was a dramatic increase of extramedullary hematopoietic progenitor cells in liver and spleen but not in the bone marrow. In IL-6 single-transgenic mice and sIL-6R single-transgenic mice no such effects were observed. The high numbers of hematopoietic progenitor cells were reflected by a strong increase of peripheral blood cell numbers. Therefore, activators of the gp130 signal transducer like the IL-6-IL-6R complex may represent most powerful stimulators for extramedullary hematopoietic progenitor cells. gp130 activators may become important for the expansion of hematopoietic progenitor cells in vivo and in vitro.
IL-6 is a mediator of hematopoietic cell growth and differentiation acting on B cells, T cells, keratinocytes, neuronal cells, osteoclasts, and endothelial cells (for reviews,
see references 1 and 2). In the liver, IL-6 modulates the
transcription of acute phase response genes during acute and
chronic inflammatory states (3).
The IL-6-type cytokine family comprises IL-6, IL-11,
ciliary neurotrophic factor (CNTF)1, leukemia inhibitory
factor (LIF), oncostatin M (OSM), and cardiotrophin-1 (CT1). The biological action of these cytokines is mediated by multisubunit cell surface receptors that share a common
signaling subunit, the gp130 signal-transducing molecule
(4, 5). The subunits of these receptors are members of the
cytokine/growth hormone receptor family that have conserved cysteine and tryptophane residues in the extracellular domain and that signal via the JAK-STAT pathway (6).
IL-6 and IL-11 use a gp130 homodimer and OSM, CNTF,
LIF, and CT-1 use a heterodimer of gp130 and the 190 kD LIF receptor for the activation of intracellular signaling cascades (4). The receptor signaling complexes for CNTF, IL-6,
IL-11, and probably CT-1 (9) contain an additional ligand
specific subunit that is not required for LIF and OSM binding. Recently, a separate OSM-specific receptor has been
identified that also forms a heterodimer with gp130 (10).
Many soluble receptors (sR), like TNF-R, IL-1R, and
IL-4R, act antagonistically, competing with membrane receptors for the binding of the cytokines (11). However, some
agonistic cytokine-sR complexes stimulate signal-transducing
receptors, rendering cells sensitive to the respective cytokines that lack specific cytokine receptors on their surface.
Such cells on their own would not be able to respond to
the respective cytokine alone. This pathway, recently termed
transignaling, is used by the cytokines of the IL-6 family
(14, 15).
On target cells, IL-6 first binds to a specific IL-6 receptor
(IL-6R To discriminate between biological functions of IL-6
alone and those of the IL-6-IL-6R complex, double-transgenic mice coexpressing human IL-6 and the human soluble IL-6R (IL-6-sIL-6R mice) were generated and compared with sIL-6R (25) and IL-6 (26) single-transgenic mice and nontransgenic littermates. There were striking
differences regarding the phenotypes of single and doubletransgenic mice. IL-6-sIL-6R mice were smaller and displayed a marked hepatosplenomegaly caused by an extreme
expansion of extramedullary hematopoietic progenitor cells.
No such phenotype was observed in single-transgenic and
nontransgenic littermates. This study identifies the gp130 signal-transducing protein to be a most powerful mediator
of hematopoietic cell growth and differentiation in vivo.
Generation of Transgenic Mice.
The generation of human sIL-6R
and human IL-6 transgenic mice has been described in detail elsewhere (25, 26). By crossing homozygous sIL-6R and IL-6 mice,
hemizygous double transgenic mice (IL-6-sIL-6R mice) were generated that coexpressed both transgenes. Expression of the sIL-6R
transgene was driven by the neonatal phosphoenolpyruvate carboxykinase (PEPCK) promoter, which is transcriptionally activated at day 3 after birth and does not produce intrauterine developmental effects (25). The IL-6 transgene was driven by the
murine metallothionein-1 promoter (26). Serum concentration of
IL-6 ranged between 10 and 20 ng/ml in IL-6 and IL-6-sIL-6R mice; concentrations of sIL-6R ranged between 4 and 8 µg/ml
in sIL-6R and IL-6-sIL-6R mice (data not shown).
Antibodies.
The following antibodies were used: anti-Gr-1
(RB6-8C5), Mac-1 (M1/70), B220 (RA3-6B2), CD4 (H129.19),
CD8a (53-6.7) (all PharMingen, San Diego, CA). mAbs were used
as lineage markers. Anti-c-kit (2B8) and Sca-1 (E13-161.7) (both
PharMingen) were used as hematopoietic stem cell markers. FITC-
or PE-conjugated antibodies were used.
Cell Sorting and Flow Cytometric Analysis.
Freshly isolated spleen,
liver, and bone marrow cells were obtained according to methods
previously described (27). Cells were washed three times with
RPMI 1640 medium and suspended at a concentration of 2 × 105 cells/ml of PBS containing 2.5% FCS and 0.05% sodium
azide before use. Double staining using anti-CD45 (30F11.1)
(PharMingen) mAbs and various lineage and hematopoietic stem
cell-specific mAbs was performed. The cell suspensions were incubated for 30 min on ice with mAbs against cell surface molecules in PBS containing 2.5% FCS and washed three times with
the same buffer. The fluorescence of the cells were examined using a FACScan® (Becton Dickinson, Mountain View, CA).
Organ Weight Recording and Histomorphological Analysis.
At ages
indicated in the figures, three mice each of single-transgenic and
nontransgenic littermates and IL-6-sIL-6R mice were sacrificed,
and liver, spleen, and total body weights were recorded. Mean
values of the ratio of organ weight:body weight are shown. Organs were fixed in 4% formaldehyde/PBS and embedded in paraffin. 5-µm sections were stained with haematoxylin-eosin.
Immunohistochemistry.
Immunohistochemistry was performed
using the indirect peroxidase staining method. Rat anti-mouse
Ly-6, rat anti-mouse CD3, rat anti-mouse B220, (PharMingen),
and rat anti-mouse Ia (Boehringer Mannheim, Germany) were
used as primary antibodies. As a secondary antibody, peroxidaseconjugated goat anti-rat IgG (H+L) (Dianova, Hamburg, Germany) was used.
In Vitro Colony-forming Assays.
The number of hematopoietic
progenitor cells was determined in clonogenic assays as described
previously (28). Cell samples were obtained from femurs and suspensions of spleen and liver derived from one individual animal at
the age of 8-12 wk. The culture mixture for the assays consisted
of 0.8% methyl cellulose, 1% BSA, 10% FCS, 5% human plasma
and growth factors. Cells were seeded in agar cultures at a concentration of 104/well, and CFU-GM and BFU-E were stimulated with G-CSF (50 ng/ml) plus murine IL-3 (300 U/ml) or
stem cell factor (150 ng/ml) plus IL-3 plus erythropoietin (2 U/ml),
respectively. The total number of colonies per organ was calculated using the cellularity obtained for each organ and for the
bone marrow, assuming that each femur represents 6% of the total marrow.
Peripheral Blood Counts.
In general anesthesia, blood was obtained by cardiac puncture and analyzed in a CellDyn 3500 hemocounter (Abbott, Delkenheim, Germany).
The IL-6 transgene expression was regulated by
the mouse metallothionein-1 promoter. The sIL-6R transgene expression was driven by the rat PEPCK promoter,
which acts neonatally and permits evaluation of the influence
of gene products after birth without intrauterine developmental effects. A distinct phenotype was observed in IL-6- sIL-6R mice compared with single-transgenic mice: IL-6 and
sIL-6R single-transgenic and nontransgenic littermates (Fig.
1 A, left; data not shown) developed normally with respect to
weight gain, food and water intake, appearance, and behavior.
However, IL-6-sIL-6R mice (Fig. 1 A, right) were remarkably smaller compared with single-transgenic and nontransgenic littermates. When body weights were measured over
a period of 20 wk, it became evident that the IL-6-sIL-6R mice developed reduced body weights as soon as 6 wk after
birth (Fig. 1 B). At the age of 20 wk, mean values obtained
in IL-6-sIL-6R mice were around 20-25 g compared with
40 g in single-transgenic and nontransgenic littermates (Fig.
1 B). Moreover, at autopsy and at histopathological analysis, IL-6-sIL-6R mice were found to have markedly reduced body fat in all regions of the organism compared
with single-transgenic littermates (data not shown).
Most remarkably, at the age of 4-6 wk, IL-6-sIL6R mice displayed distended abdominal regions (Fig. 1 A).
Single-transgenic and nontransgenic littermates were normal in this respect. The abdominal distension was caused by a
marked increase of liver and spleen weights relative to the
total body weight. Time course experiments (Fig. 2, A and
B) revealed steadily increasing relative liver and spleen
weights in IL-6-sIL-6R mice, while the respective relative
organ weights in single-transgenic and nontransgenic littermates were unchanged. At wk 20, the relative liver weight (Fig. 2 A) had duplicated and the relative spleen weight
(Fig. 2 B) had increased by a factor of 5 when compared
with single-transgenic and nontransgenic littermates.
The gross morphology of livers and spleens were examined in mice at 8 wk of age (Fig. 2 C). Livers and spleens of
8-wk-old IL-6R mice (Fig. 2 C, left) and IL-6 mice and nontransgenic littermates (data not shown) were completely normal. Livers of 8-wk-old IL-6-sIL-6R mice (Fig. 2 C, right),
however, appeared dark in color and had a rugged and
humpy surface. This appearance was caused by round, whitish, and with respect to the liver surface elevated foci. The
spleens of IL-6-sIL-6R mice were clearly enlarged in size (Fig. 2 C, right). The enlargement of the spleen is even more remarkable in light of the fact that IL-6-IL-6R mice had half the body weight of single-transgenic and nontransgenic littermates (see Fig. 1 B).
The increase of liver
and spleen weight in IL-6-sIL-6R mice was caused by a
marked extramedullary proliferation of hematopoietic cells in
both organs (Fig. 3, A-D), which was not detected in any
other parenchymal organ. No extramedullary hematopoiesis was found in single-transgenic and nontransgenic littermates (Fig. 3, A
Also, a large number of megakaryocytes was detected in
IL-6-sIL-6R mice (Fig. 3, B and D) but not in single-transgenic and nontransgenic littermates (Fig. 3, A Plasmacellular infiltrates in the liver and spleen with consecutive involvement of all parenchymal organs were observed in 50% of both IL-6 (Fig. 3, C These results clearly demonstrate that the presence of the
IL-6-sIL-6R complex leads to extramedullary accumulation
of hematopoietic progenitor cells, which are unresponsive
to IL-6 alone.
To characterize further the nature of the hematopoietic cells in the liver, immunohistochemistry was performed with antibodies to surface antigens
of neutrophils (Ly-6), monocytes/macrophages (Ia), B cells
(B220), and T cells (CD3). As early as 4 wk after birth, IL-6-
sIL-6R mice showed single positive neutrophils (Fig. 4 A)
and monocytes/macrophages (Fig. 4 A
Cells harvested from liver, spleen, and bone marrow
from single-transgenic and nontransgenic littermates and
from IL-6-sIL-6R mice were subjected to FACS® analysis
to determine the percentage of macrophages, granulocytes, progenitor cells, and B and T lymphocytes present in these
organs. In the bone marrow, there were no significant differences detected between IL-6-sIL-6R mice and singletransgenic and nontransgenic littermates with respect to the
various surface antigens tested (data not shown). In the
spleen of IL-6-sIL-6R mice (Table 1, upper panel), the number of granulocytes, macrophages, and Sca-1+ progenitor
cells increased by a factor of 2.3, 1.5, and 2, respectively, compared with single-transgenic or nontransgenic littermates. B cells were increased 2.3-fold in both IL-6 singletransgenic and in IL-6-sIL-6R mice compared with nontransgenic littermates. CD4+ and CD8+ lymphocytes were
not different among the groups of mice tested.
) (16). This IL-6-IL-6R complex induces the homodimerization of two gp130 molecules (17, 18), leading
to intracellular signaling events. Soluble forms of the IL-6R
complex are generated by limited proteolysis from the cell
surface (19, 20). Elevated concentrations can be found in
inflammatory diseases (21). In vivo, the sIL-6R acts as a
serum-binding protein for IL-6 and prolongs the plasma
half life of IL-6 (25). However, the biologic function of the
IL-6-IL-6R complex in vivo is unknown.
IL-6-sIL-6R Mice Are Smaller and Have a Reduced Body
Weight.
Fig. 1.
Weight abnormalities in IL-6-sIL-6R mice. (A) An IL-6-
sIL-6R double-transgenic mouse on the right side and a sIL-6R singletransgenic mouse on the left side at the age of 8 wk. (B) Total body
weight recorded of IL-6-sIL-6R mice, of single-transgenic mice, and of nontransgenic littermates.
[View Larger Versions of these Images (97 + 21K GIF file)]
Fig. 2.
Organ weight and gross morphology of liver and spleen. Liver (A) and spleen (B) weight of IL-6-sIL-6R, IL-6, and sIL-6R mice and nontransgenic littermates. Organ weight is presented as liver weight/total body weight and spleen weight/total body weight, respectively. Gross morphology
of livers and spleens of sIL-6R mice (C, left) and IL-6-sIL-6R mice (C, right) at 12 wk of age.
[View Larger Versions of these Images (18 + 21 + 95K GIF file)]
-D
; data not shown). In the livers and
spleens of IL-6-sIL-6R mice, multiple hematopoietic foci
were present, which increased in size with time. At 8 wk,
there were occasional foci (Figs. 3, A and B) present in both
organs. However, at 16 wk, complete involvement of liver
and spleen with multiple and large confluent hematopoietic
foci accompanied by loss of the parenchymal organ architecture was demonstrated (Fig. 3, C and D). Determination of enzymatic values in the serum (LDH, alkaline phosphatase, and transaminases) did not show any alteration either
in IL-6-sIL-6R mice or in single-transgenic and nontransgenic littermates (data not shown), indicating that the liver
function of the mice was not compromised.
Fig. 3.
Histomorphological analysis of livers and spleens in IL-6-sIL-6R
(A-D), and IL-6 (A-D
) mice. (A)
Liver tissue with several hematopoietic
foci containing predominantly granulopoietic precursor cells and terminally
differentiated segmental granulocytes
(IL-6-sIL-6R, 8 wk). (A
) Normal liver
tissue with absence of hematopoietic foci in an 8-wk-old IL-6 transgenic
mouse. (B) Spleen tissue with activated
extramedullary hematopoiesis showing
enlarged granulopoietic and erythropoietic areas as well as increased number of
megakaryocytes (IL-6-sIL-6R, 8 wk).
(B
) Spleen tissue from an IL-6 transgenic mouse at 8 wk showing regular
distribution of red and white pulp and
little hematopoiesis. (C) Liver tissue
with large hematopoietic focus displaying blastic precursor cells and terminally
differentiated granulocytes (IL-6-sIL6R, 16 wk). (C
) Liver tissue from an
IL-6 transgenic mouse at 16 wk with
periportal plasmacytoma infiltrate. (D)
Spleen tissue with destroyed architecture
and complete involvement by hematopoiesis (IL-6-sIL-6R, 16 wk). (D
)
Spleen tissue completely infiltrated by
plasmacytoma (IL-6, 16 wk). The scale
bars indicate 100 µm.
[View Larger Version of this Image (106K GIF file)]
-D
). Remarkably, megakaryocytes were present only in the spleen (Fig.
3, B and D) and not in the liver (Fig. 3, A and C). Megakaryocytes were found as soon as the age of 8 wk (Fig. 3 B) and
further increased in number at the age of 16 wk (Fig. 3 D).
and D
) and IL-6-
sIL-6R mice (data not shown) around the age of 16 wk. In
IL-6 single-transgenic animals, plasmacellular infiltrates were
the only histopathological abnormalities detected. However,
in IL-6-sIL-6R mice, extramedullary hematopoiesis (Fig. 3,
A-D) was the most dominant histopathological finding observed in all mice examined.
) in the liver. At the age of 6 wk, there were increased numbers of scattered
neutrophilic granulocytes (Fig. 4 B) and small clusters of
monocytic cells (Fig. 4 B
) present in the liver. Large confluent foci of both cell types were detected at the age of 16 wk
(Figs. 4, C and C
). Immunohistochemistry using antibodies directed against B and T cell epitopes revealed that B cells
were also present, but to a lesser extent, and that T cells were
absent (data not shown). In single-transgenic and nontransgenic littermates, immunohistochemistry using the above
mentioned antibodies gave negative results (data not shown).
Fig. 4.
Immunohistochemical
analysis of the Ly-6 (A-C), and
Ia (A-C
) surface protein expression on hematopoietic liver
cells in IL-6-sIL-6R double-transgenic mice. Granulocytic (A) and
monocytic/macrophage (A
) cells
are almost completely absent in
the liver at the age of 4 wk. At 6 wk, scattered granulocytic (B)
and small clusters of monocytic/
macrophage cells (B
) appear in
the liver, representing early hepatic extramedullary hematopoiesis. At the age of 12 wk, there are
large confluent hepatic foci showing predominantly granulopoietic differentiation (C), but also
significant monopoietic cells (C
). The scale bars indicate 100 µm.
[View Larger Version of this Image (146K GIF file)]
The most prominent findings were seen in the liver of IL-6-sIL-6R mice (Table 1, lower panel). Granulocytes, macrophages, Sca-1+ progenitor cells, and B cells were absent or present only scarcely in single-transgenic sIL-6R mice and nontransgenic littermates. Macrophages and B cells were increased 15-fold and 45-fold in IL-6 single-transgenic mice, respectively, compared with sIL-6R single-transgenic or nontransgenic littermates. All cell lines (granulocytes, macrophages, progenitor cells, and B cells) were increased by a factor of 200-300 in IL-6-sIL-6R mice compared with IL-6R single-transgenic or nontransgenic littermates.
These results show that hematopoietic cells in the liver and spleen of IL-6-sIL-6R mice were mainly granulocytes and monocytes, to a lesser extent B cells, and not T cells.
Massive Hematopoietic Progenitor Cell Proliferation in the Liver and Spleen.In view of the extramedullary hematopoiesis
observed in liver and spleen, we investigated whether the
continuous stimulation of the gp130 signal transducer affected committed hematopoietic progenitor cells of the
granulocytic-monocytic and erythocytic lineages in different hematopoietic organs. The frequency of hematopoietic progenitor cells in the bone marrow, spleen, and liver of
transgenic and nontransgenic mice was determined using in
vitro clonogenic assays. The IL-6-sIL-6R-mediated gp130
stimulation had distinct effects on different hematopoietic
lineages in extramedullary organs: the total number of
CFU-GM and BFU-E in spleen and liver derived from
IL-6-sIL-6R mice was increased ten times when compared with IL-6-sIL-6R single-transgenic mice and nontransgenic littermates (Fig. 5, A and B). Most interestingly, in
the bone marrow, the relative and absolute number of clonogenic hematopoietic cells was not elevated in IL-6-sIL6R mice compared with IL-6 and sIL-6R single-transgenic
and nontransgenic mice (Fig. 5 C).
These data show that continuous gp130 activation leads to a massive increase of extramedullary committed hematopoietic progenitor cells and not to an increase of bone marrow-derived progenitor cells.
Extramedullary Hematopoietic Progenitor Cells Are Functionally Normal.Peripheral blood cell numbers were determined at the age of 8, 16, and 24 wk in IL-6-sIL-6R mice
and compared with numbers measured in IL-6 and sIL-6R
single-transgenic mice and nontransgenic littermates. Whereas
the peripheral blood cell numbers of single-transgenic and
nontransgenic littermates were unchanged over the period
of time tested, a most dramatic increase of white blood cells
consisting mainly of neutrophilic granulocytes was observed
in IL-6-sIL-6R mice (Fig. 6, A and B). At the age of 8 wk,
there was no difference observed between IL-6-sIL-6R
mice and single-transgenic mice and nontransgenic littermates. However, at the age of 16 wk, white blood cells and
neutrophils increased by a factor of 10 and 17, respectively,
and at the age of 24 wk by a factor of 27 and 48, respectively, in IL-6-sIL-6R mice. Platelets (Fig. 6 C), red blood cells (Fig. 6 D), and hemoglobin values (Fig. 6 E) of IL-6-
sIL-6R mice increased by a factor of 2.1, 1.5, and 1.5, respectively, when compared with age-matched nontransgenic littermates and did not change in 16- and 24-wk-old
mice. In single-transgenic animals, peripheral blood cell
counts and hemoglobin values were not altered compared
with nontransgenic mice (Fig. 6, C-E). Thus, the expanded pool of hematopoietic progenitor cells in IL-6-sIL-6R mice
appears to retain its capacity to undergo proliferation and
differentiation into mature blood cells, resulting in expansion of circulating blood cells.
Our study demonstrates that the activation of the gp130 signal transducer by a complex of IL-6 and the sIL-6R represents a major stimulation of growth and differentiation of hematopoietic progenitor cells in vivo. The presence of IL-6 alone is not sufficient to stimulate these cells. As demonstrated by histomorphological and immunohistochemical data and in vitro clonogenic assays, in IL-6-sIL-6R mice an expansion of hematopoietic progenitor cells of the granulocytic-monocytic, megakaryocytic, and erythroid lineages was observed.
We conclude that these hematopoietic progenitor cells
express gp130 signal-transducing molecules on their surface
and lack the specific IL-6R. This notion is underscored
by a recently published study using gp130-deficient mice,
which were shown to exhibit a greatly diminished number
of hematopoietic progenitor cells (29). Thus, gp130 activation is required to stimulate hematopoietic progenitor cells.
Because IL-6 and IL-11 act via a gp130 homodimer, loss of
IL-6 can be compensated by IL-11, possibly explaining
why IL-6-deficient mice are viable (30, 31). In mice overexpressing cytokines like LIF (32) or OSM (33), requiring the gp130/LIF-R heterodimer for signal transduction, hematopoietic effects are scarce, indicating that hematopoietic
progenitor cells express little or no LIF-R on their surface.
This hypothesis is supported by findings from mice in
which the LIF-R has been deleted by targeted disruption
(34). These mice show no significant abnormalities of the
hematopoietic system.
The fact that increased B cell numbers are detected in spleen and liver of IL-6 and IL-6-sIL-6R transgenic mice supports the notion that B cells express the specific IL-6R on their surface and thus may be activated by IL-6 alone (35). Accordingly, IL-6 induced plasmacytosis in IL-6 and IL-6-sIL-6R mice by stimulating proliferation and maturation of B cells (36, 37).
In IL-6-sIL-6R double transgenic mice a marked reduction of body weight and body size associated with a reduction of body fat was observed compared with single-transgenic and nontransgenic littermates. Currently, we can not explain the reduced body size observed in these animals. However, the reduced body fat that we observed in IL-6- sIL-6R mice could be explained by the fact that the recently identified leptin receptor (OB-R) (38) is a signaltransducing protein similar to gp130, involving the same intracellular signaling molecules (39, 40). The OB-R is located in many tissues, including the hypothalamus, and is involved in the regulation of the size of adipose tissue mass through effects on satiety and energy metabolism. The natural ligand of the OB-R, leptin, a 16-kD protein, is expressed primarily in the white adipose tissue (41). It might be possible that the IL-6-sIL-6R complex activates gp130 molecules on OB-R-expressing cells, thereby activating leptinspecific signaling pathways.
We have provided evidence that the IL-6-sIL-6R complex in vivo induces extramedullary hematopoiesis in liver and spleen but not in bone marrow. It is known that there are resident hematopoietic stem cells in the adult liver (42). These cells might be expanded after stimulation by IL-6- sIL-6R in a concentration-dependent manner. Rather high concentrations of IL-6 (2 nM) and sIL-6R (>20 nM) are required for the activation of gp130-expressing cells in vitro (43). This is somewhat surprising considering that the KD of the IL-6-sIL-6R interaction is in the range of 1 nM (16, 44). A possible explanation could be that the average half-life of the IL-6-IL-6R complex is shorter than the time required to assemble the functionally active IL-6-IL6R/gp130 complex. We have recently demonstrated that the sIL-6R acts predominantly at the site of its generation (M. Peters et al., submitted for publication). Because liverspecific promoters drive the expression of the transgenes used in the present study, one could reason that sufficient concentrations of IL-6 and sIL-6R required for an effective stimulation of hematopoietic progenitor cells are found predominantly in the liver. Recently, transgenic mice coexpressing IL-6 and the membrane-bound IL-6R have been generated (45) in which soluble IL-6R was generated by shedding at levels 100-fold lower compared with IL-6- sIL-6R mice of the present study. No hematopoietic abnormalities were found in these mice, indicating that high levels of the IL-6-sIL-6R complex are required to stimulate hematopoietic progenitor cells.
Extramedullary hematopoiesis in the spleen has been recognized after administration of recombinant G-CSF to mice (46). Splenic stimulation of progenitors in IL-6-sIL-6R mice indicates that cytokines of the IL-6 family can contribute to hematopoiesis in the spleen. Recently, it has been demonstrated that LIF overexpression results in moderate splenomegaly and slightly activated extramedullary hematopoiesis (32, 47). LIF-deficient mice show a reduction of hematopoietic progenitors in the spleen and bone marrow. However, hematopoietic spleen and bone marrow progenitor cells from those mice transplanted into lethally irradiated normal mice retained their normal function, indicating that LIF is required in the microenvironment to maintain stem cell numbers, rather than affecting the potential of hematopoietic progenitors (48). In light of these results, one might speculate, that similar to LIF, the IL-6- sIL-6R complex contributes to the microenvironment required by hematopoietic cells in spleen and liver. However, it is of interest that stimulation by IL-6-sIL-6R leads to far stronger hematopoiesis than stimulation by LIF.
It is possible that liver and spleen cells express other important growth factors like stem cell factor (49, 50), chemokines as pre-B cell-stimulating factor (51), or flt-3 (52), either in membrane-bound or soluble form. These cytokines might be involved as cofactors in the growth of hematopoietic progenitor cells in these organs.
Hematopoietic progenitor cells stimulated by the IL-6- sIL-6R complex are functionally normal. This is reflected by the highly elevated numbers of granulocytes, megakaryocytes, and erythrocytes in the peripheral blood. Thus, the expanded pool of hematopoietic progenitor cells appears to retain its capacity to undergo proliferation and differentiation into mature blood cells, resulting in expansion of circulating blood cells.
In summary, there are three major findings of this study. First, we provided evidence that continuous activation of the gp130 signal transducer by molecules like the IL-6-sIL-6R leads to a most effective stimulation of hematopoietic progenitor cells. Progenitor cells of the granulocytic-monocytic, megakaryocytic, and erythroid lines are involved. Second, IL-6-sIL-6R causes a stimulation of predominantly extramedullary and not bone marrow hematopoietic progenitor cells. Third, progenitor cells are functionally normal, retaining their capacity to undergo maturation into mature blood cells. Apart from expanding CD34+ cells obtained from cord blood in vitro (43), continuous activation of the gp130 signal transducer by agents like the hIL-6-hsIL-6R complex could be the most effective mechanism to reconstitute hematopoiesis in vivo or ex vivo in patients with cytopenic conditions.
Address correspondence to Malte Peters, I. Department of Medicine, Section of Pathophysiology, University of Mainz, Obere Zahlbacher Strasse 63, D-55101 Mainz, Germany.
Received for publication 30 October 1996
1 Abbreviations used in this paper: CNTF, ciliary neurotrophic factor; CT-1, cardiotrophin-1; LIF, leukemia inhibitory factor; OSM, oncostatin M; PEPCK, phosphoenolpyruvate carboxykinase; sIL-6R, soluble interleukin-6 receptor; TNF-R, tumor necrosis factor receptor.We thank B. Gilberg and K. Petmecky for excellent technical assistance and Dr. M. Blessing, Boehringer Ingelheim Research Group, I. Department of Medicine, University of Mainz, for continuous advice and discussions throughout the study. Dr. B. Lotz is thanked for the determination of the peripheral blood values.
This work was supported by the Deutsche Forschungsgemeinschalt (Bonn, Germany) and the Naturwissenschaftlich-Medizinisches Forschungszentrum (Mainz, Germany) to M. Peters, P. Schirmacher, C. Peschel, and S. Rose-John.
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