Role of T cells and cytokines in fatal and resolving experimental babesiosis: protection in TNFRp55-/- mice infected with the human Babesia WA1 parasite.
Hemmer RM,
Ferrick DA, Conrad PA
J Parasitol
2000 Aug;86(4):736-42
20412654
Bab microti & WA1 mice T cell.tif
(Whole
discussion from this reference is cited below, because the authors give a
thorough explanations for the role of TNF-a and other cytokines (including a
lot of references) - and this is possibly a common pathogenetic 'pathway' for
several (all?) intracellular parasitic infections ?)
DISCUSSION:
TNF-a is a pleiotropic cytokine that appears to be an integral component of the immunologic defense network. TNF-a seems to be required for resistance to several intracellular facultative bacteria and parasites, including Listeria monocytogenes and Leishmania major (Havell, 1989; Green et al., 191990; Kaufmann, 1993). In contrast, other evidence demonstrates the detrimental effects of TNF-a on the host.
TNF-a is involved in triggering the lethal effects of
cachexia, septic shock syndrome, inflammation, and other systemic
manifestations of disease (Tracey and Cerami, 1994). The fact that this cytokine is crucial in
organizing the events involved in both immunity and disease demonstrates the
complexity of TNF-a function.
Previous studies
from this laboratory demonstrated that WA1 babesial infection results in
pulmonary edema, infiltration and adhesion of mononuclear cells to the
pulmonary veins, and endothelial cell activation, as demonstrated by
hypertrophy and upregulation of intracellular adhesion molecule-1 (Hemmer et
al., 1999). There is much evidence that these inflammatory responses can be
mediated by TNF-a (Mulligan et al., 1993; Urquhart, 1994; Luca et al., 1997).
The effector functions
of TNF-a include increased vascular permeability, upregulation of adhesion
molecule expression on endothelial cells, and stimulation of other changes in
vascular endothelium that contribute to tissue injury. Thus, the pathology
observed in WA1-infected mice correlates with known biological effects of TNF-a
and suggests a role for its activity in the WA1-associated disease.
In the present study we examined the difference in TNF-a production by CD4+, CD8+, and gd+ splenic T-cell subpopulations in fatal and resolving Babesia infections.
Upregulation
of TNF-a production by gd+ T cells in fatal WA1 infections, but not in
resolving infections, suggested that TNF-a production was detrimental to the
host.
Additional
support for the role of TNF-a in the pathogenesis of WA1 came from infections
of TNFRp55-/- mice. These mice are resistant to either lipopolysaccharide or
bacterial superantigen-induced septic shock, a syndrome mediated by TNF-a
(Pfeffer et al., 1993).
Furthermore,
in the characterization of TNFRp55-/- mice, it was shown that inoculation of
TNF-a triggered mononuclear cell infiltration of lung, liver, and kidney in
C57BL/6 mice, but not in TNFRp55-/- mice (Neumann et al., 1996). This
provides evidence that the TNFRp55 receptor is necessary for TNF-a mediated
leukocyte adhesion to activated endothelium.
In our study, the
TNFRp55-/- mice survived a lethal WA1 infection, thus demonstrating that
abolishing TNFRp55 function eliminated an important pathogenic mechanism of the
WA1 parasite. This result suggests that during a WA1 infection, signaling
through TNFRp55 promotes the recruitment of leukocytes to the lungs, and the
outcome is activation of the vascular endothelium, pulmonary edema, respiratory
distress, and death. Taken together, these results implicate TNF-a as an
essential component of the WA1-associated pathology.
In addition
to TNF-a, other cytokines were involved in Babesia infections. IFN-g, another
proinflammatory cytokine, was shown to have significanfly higher peak
expression by CD8+ T cells in WA1-infected mice compared to mice infected with
B. microti. IFN-g is the principal macrophage-activating cytokine and central
in the regulation of TNF-a (Rudin et al., 1997). There
is evidence that IFN-g synergizes with TNF-a to enhance an inflammatory
response (Tracey and Cerami, 1994). Upregulation
of both of these cytokines during the WA1 infection could account for the
severity of the WA1-associated disease.
In resolving B. microti infections, increased production of both IL-10 and IL-4 by gd+ T cells occurred as the parasite load was decreasing. Thus. in B. microti infections, increased expression of IL-10 and IL-4 was important for resolution of the parasitemia.
One
striking observation was that gd+ T cells were the major cytokine-producing
cells in both WA1 and B. microti infections. gd+ T cells act as a first line of
defense against a pathogen and response of gd+ T cells to antigen happens
faster than that ab+ [?, hard to read on my copy] T cells (Boismenu and Havran,
1997). These responses indicate that the cytokine profile of gd+ T cells may regulate the functional
activities of both innate and acquired immune responses. Production of IFN-g
by gd+ T cells has been shown to prime
macrophages for TNF-a expression (Nishimura et al., 1995).
Other
studies of intracellular organisms imply that control of pathogen dissemination
and host tissue damage requires the regulatory influence of gd+ T cells (Rosat et al., 1993; Fu et al..
1994). These studies cast gd+ T cells
as an important regulatory cell for both innate and acquired immune responses.
Despite the fact that gd+ T cells were
the predominant cytokine-producing cells detected in our experiments, gd-/-
mice behaved similar to controls when they were infected with either WA1 or B.
microti. This result is not entirely surprising in light of the fact that gd+ T cells make up a small percentage of
all T cells, approximately 2-5% in the mouse spleen. Certainly other cell types
are major producers of cytokines. Macrophages and natural killer cells that
produce TNF-a and IFN-g, respectively, could contribute to the WA1-associated
pathology. The flow cytometry results showed limited involvement of cytokine
production by CD4+ and CD8+ T cells. This was probably not due to technical
difficulties, because IFN-g production was observed in CD8+ T cells, and IL-2
expression was detected in CD4+ T cells (data not shown). It is possible that
the cytokine expression was below the level of detection for this assay, or
that cytokines other than the ones we tested are involved in Babesia infections.
It is important to note that in this study, the T cells were not restimulated
in vitro, and this may explain the paucity of cytokine expression by CD4+ T
cells.
Because it was
well established that CD4+ and CD8+ T cells have many important functions in
defense against intracellular pathogens, we used CD4-/- and CD8-/- mice to
study the effects of an absence of these T-cell subpopulations on the outcome
of WA1 and B. microti infections.
There
was a significant delay in parasite clearance in both WA1- and B.
microti-infected CD4-/- mice. A similar persistence in parasitemia was
observed in mice depleted of CD4+ T cells by monoclonal antibody treatment (Shimada et al., 1996). These
results demonstrate a role for CD4+ T cells in the elimination of parasites
from the blood.
Following
WA1 inoculation, CD8-/- mice had a moderately increased survival rate. One possibility is that because
CD8+ T cells are a major source of lFN-g, the absence of CD8+ T cells reduces
the overall availabilty of IFN-g (Boehm et al., 1997). Consequently, the
synergistic effects of IFN-g and TNF-a are inhibited, leading to a reduced
inflammatory response and increased survival.
The
protection in B. microti-infected CD4-/- and CD8-/- mice suggests that either
CD4+ or CD8+ T cells, without the other subset present, but in the presence of
B cells, gd+ T cells, and innate immune cells can be sufficient for immunity.
Our results emphasize the plasticity and redundancy of CD4+ and CD8+ T-cell
functions in response to intracellular pathogens.
These
studies were undertaken to gain a more complete understanding of the role of T
cells and cytokines in the pathogenesis of WA1 infections and the resolution of
B. microti infections. We have shown that cytokine
responses of splenic T cells were different during fatal and resolving
infections and that cytokines contribute to both host pathology and host
immunity. TNF-a activity through TNFRp55 was essential to the WA1-associated
disease. Additional studies of T cells, B cells, endothelial cells,
and innate immune responses are needed to elucidate the different mechanisms
involved in fatal and resolving Babesia infections. Future experiments will be
directed at studying the role of cytokine-stimulated endothelial cells and
macrophages in WA1 pathology.