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Regular Articles |
§
From the Faculty of Tropical Medicine,*
Mahidol
University, Bangkok, Thailand; the Center for Tropical
Diseases,
Cho Quan Hospital, Ho Chi Minh City,
Viet Nam; The Oxford-Wellcome Centre for Tropical
Diseases,
Nuffield Department of Clinical
Medicine, University of Oxford, and the Department of Clinical
Biochemistry and Cellular Science,§
University
of Oxford, United Kingdom
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Abstract |
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Introduction |
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Methods |
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Procedures
Post-mortem sampling was performed only after fully informed permission was granted from attendant relatives. Where a full autopsy was performed (in Vietnam only), these samples were taken after removal of the brain. In six cases samples were examined from three sites: medulla, cerebellum and cortex. When an autopsy was not performed samples were taken using a Vim-Silverman needle inserted through the superior orbital foramen or the foramen magnum. For this study the specimens were not sectioned, but smeared by placing the brain specimen between two glass microscope slides, pressing these together, and making thin smears.19 This method preserves long fragments of capillaries and venules. Brain biopsies were performed within 2 hours of death, and autopsies within 6 hours. Slides were fixed in absolute methanol and stained by the reverse Field's method.20 Malaria parasites and pigment were easily identified. Intravascular parasites were counted and staged. Peripheral blood slides were prepared either from blood taken post-mortem (by ventricular puncture) or, if an ante-mortem blood smear had been taken within 4 hours before death, this was used.
Morphological Assessment of Parasite Stage of Development
In Vitro Study
In order to validate the subsequent histopathological assessment
of parasite stage of development, in vitro cultures two of
different isolates of P. falciparum were synchronized
carefully by repeated exposure to sorbitol until a 1-hour time window
was obtained. Smears were then taken throughout one asexual life cycle
(every 2 hours until 42 hours, then every 1 hour). The parasite
morphology at precise times before the midpoint of schizogony in
vitro (see Figure 1
) formed the basis of the assessment of
parasite development stages in the cerebral vessels. The effects of
IC99 concentrations of quinine and artemether on parasite
morphology were assessed from in vitro cultures of parasite
isolates with predetermined antimalarial susceptibility.
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Within the cerebral vessels the red cells and the malaria parasites were compressed within the confines of the packed sequestered capillaries or venules. The diameter of a capillary is smaller than that of an erythrocyte, and red cells have to bend and deform in their passage through the microvasculature. The separation between red cells in brain smears was often difficult to determine, but the individual parasite nuclei and cytoplasm and malaria pigment were readily identified. In order to assess the effects of red cell compression within narrow vessels on parasite morphology, simulation experiments were conducted in the rat. The midpoint capillary diameters in the rat are smaller than those in humans, and so distortion should have been greater in the experiments than in the clinical samples. Large volume in vitro P. falciparum cultures (5 ml of blood at 15% hematocrit and 10–30% parasitemia) were prepared and infused via a 26-gauge catheter inserted into the aortic arch of anesthetized (6% chloral hydrate given by intraperitoneal injection) male Wistar rats. The inferior vena cava and descending aorta were clamped, and an equal volume of blood removed as that infused. The objective was to fill the cerebral vessels with human blood from the parasite culture. As the infusion was finished, cardiac arrest was induced. The brain was removed and smears from gray matter prepared in exactly the same way as the post-mortem slides were prepared in the clinical study. P. falciparum infected red cells at different stages of development in the cerebral microvasculature of the rat in brain smears were compared with those in heart blood smears.
Parasite Staging and Counting
In the brain smear from each case of fatal falciparum malaria a
minimum of 5 capillaries were selected randomly, the parasites counted,
and at least 10 examined for stage of development. A minimum of 20
sequential red cells in each capillary were examined. Discrete sections
of capillaries or venules containing less than 20 red cells were not
assessed. Eight stages of parasite development (approximate parasite
age) were recorded: tiny (0–6 hours) small (6–16 hours), and large
rings (16–26 hours); early (26–30 hours), middle (30–34 hours), and
late trophozoites (34–38 hours) and schizonts with up to (38–44
hours) or more than five nuclei (44–48 hours) (Figure 1)
. A total of
100 parasites were staged. The distribution of malaria pigment in each
vessel was also noted, and any white cells or gametocytes were
recorded. Thin film peripheral blood smears on admission and before
death were examined in the same way for parasite count and stage of
development assessment. Each brain and blood smear was examined
independently by two people, and the smears re-read if there was
significant disagreement.
Immunohistochemical Staining
Brain tissue from autopsy cases was snap-frozen in liquid nitrogen. Tissues were either taken immediately post-mortem by blind needle biopsy or at open autopsy. Control tissues were taken at autopsy in the United Kingdom from cases of sudden death excluding patients who died from systemic sepsis, central nervous system disease, or ischemia. Small tissue fragments were either cut directly from the biopsy cores with a scalpel blade or a 20-µm section cut on a cryostat. These were then smeared onto Vectabond-coated slides, air dried, and fixed in 100% acetone for 10 minutes. Slides were immunostained immediately using a standard indirect immunoperoxidase technique. The monoclonal antibodies used for recognition of the host receptor antigens and constitutive control antigens were as follows: class I HLA = W6/32; class II HLA = C3/43; CD31 = JC70A; VCAM-1 = 1.4C3; E-selectin = 1.2B6 (DAKO, UK); chondroitin sulfate = CS-56 (reacts to both CS-A and CS-C) (Sigma, UK); ICAM-1 = 15.2 (gift of Dr. N. Hogg); CD36 = ES IVC-7 (gift of Dr. E. Van der Schoot, Amsterdam); thrombospondin = 189/53.2 (gift of Dr. J. Dawes, Edinburgh); P-selectin = RUU-SP 2.15.1 (gift of Dr. R. Niewenhuis, Utrecht). All antibodies were either used as supernatants or reconstituted from lyophilized ascites in PBS/0.1% BSA and stored at 4°C. The monoclonal primary antibody was added to the sections for 30 minutes at room temperature in a moist chamber. Slides were then washed three times in PBS before adding the second layer of peroxidase conjugated goat anti-mouse immunogloblins (DAKO P0447) at 1:50 dilution in PBS/0.1% BSA. The solution was removed after 30 minutes of incubation at room temperature, washed in PBS three times, and then stained with the chromogen fast DAB tablets in PBS (Sigma, UK). After incubating for 8 minutes the sections were then washed in tap water, counterstained with hematoxylin and mounted with Aquamount.
Slides were examined and graded blindly by two independent observers (K.L. and G.T.). The degree of staining and the number of vessels showing staining were assessed using a semi-quantitative score: -, negative (no endothelial cell staining); +/-, scattered endothelial cells positive on <25% vessels; +, positive endothelial cell staining on >25% and <80% vessels; ++, strong positive endothelial staining on >80% vessels. The percentage of cases and intensity of staining was then compared between control and malaria cases.
Statistical Analysis
The data from these analyses were not normally distributed and nonparametric methods were used for all statistical analyses. The Wilcoxon signed rank test was used to compare brain and peripheral blood results. The Mann-Whitney U test was performed when comparing the effects of the different antimalarial treatments and the Spearman rank correlation coefficient measured associations with time from treatment to death. To determine if the age of parasites differed across brain capillaries the mean and standard deviation of all parasites pooled within a subject were calculated. Using these distributions five datasets of 10 values were randomly simulated for each subject. The standard deviation of the simulated means was compared to the standard deviation of the observed means (for each brain capillary) using the Wilcoxon signed rank test. The level of significance was 5% and all analyses were performed using the statistical package SPSS for Windows (SPSS Inc.).
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Brain smears were obtained from 50 Vietnamese and Thai patients who died from complications related to severe malaria (49 P. falciparum, 1 P. vivax) between 1986 and 1994. There were 13 women (age range 16–51) and 37 men (age range 13–67). The mean age was 32 years. Cerebral malaria was the main complication in 20 patients, while 12 had both cerebral malaria and renal failure, and 8 had renal failure without cerebral symptoms. Pulmonary edema was the principle cause of death in 6 cases. One patient died from algid malaria, one from lactic acidosis without cerebral involvement, and one from supervening staphylococcal septicemia. The patient who died with vivax malaria was a 40-year-old woman who was admitted fully conscious but with severe anemia (hematocrit 7%), jaundice, and hypoglycemia (plasma glucose 2.0 mmol/L). The P. vivax parasite count was 0.6%. She was treated with intravenous artesunate as coincident P. falciparum could not be excluded with confidence and was transfused with blood. Parasite clearance time was 48 hours. On the third day of admission she died suddenly and could not be resuscitated. Of the other 49 patients who died from falciparum malaria, 16 had hyperparasitemia on admission (parasitemia >10%).
The mean duration of preceding disease was 6.5 days (range 3–11). Elective treatment with intravenous quinine alone was given in 18 cases and artemether in 1 case; the other patients were randomized to receive either intramuscular quinine (18 cases) or artemether (12 cases) in the double blind trial.18 Before admission to hospital 23 patients had received quinine, 2 artesunate, 2 pyrimethamine-sulphadoxine, 1 mefloquine, and 1 chloroquine treatment. The median duration of antimalarial treatment before death was 2 days (range 1.5 hours to 7 days).
Pathological Findings
Complete data were available on the parasite count per 100 red blood cells for the brain and per 1000 red cells for blood for 46 of the 50 cases studied. Four were excluded from the quantitative analysis (3 P. falciparum, 1 P. vivax). The three patients with falciparum malaria died from renal failure. Each had cleared the peripheral parasitemia and no parasites were found in the brain, although significant amounts of malaria pigment were still evident in the small vessels, indicating that sequestration had occurred at some time. No parasites and no pigment were seen in the brain vessels of the patient who died from vivax malaria.
Sequestration
For the 46 patients with residual malaria parasites, the parasite
count in the brain median (range) = 66.5% (1–99) was
significantly higher than in the peripheral blood median (range) =
1.4% (<0.1–36.6) (p < 0.001). This confirms
significant sequestration in the cerebral microvasculature. To
investigate whether sequestration in the brain was associated with the
duration of or type of antimalarial drug treatment an index reflecting
the proportion of sequestered erythrocytes at the time of death was
calculated.
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Stage of Parasite Development
Assessment in Vitro
The different stages of P. falciparum development from
the tightly synchronized in vitro culture are shown in
Figure 1
, together with identifying
characteristics. The majority of parasites in the peripheral blood
smears were immature ring forms, with very few mature trophozoites or
schizonts. In vitro artemether at IC99
concentrations arrested parasite growth and led to altered parasite
shape and a significant change in red cell to cytoplasm and nuclear
cytoplasm ratios of medium and large rings (Figure 2)
. Artemether did not affect assessment
of the tiny ring stage or the trophozoite or schizont stage assessments
based on malaria pigment production and number of nuclei. Quinine at
IC99 concentrations had much less of an effect on ring form
morphology. Although both drugs affected the morphology and staining of
the parasite cytoplasm, neither drug affected the overall distribution
assessment or caused disappearance of pigment.
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In the simulation experiments in 10 rats, the malaria parasites, particularly the younger forms, appeared smaller in the cerebral vessels than in the peripheral (heart) blood. We investigated whether the following relationships were preserved: (a) the relative size of the parasite to the size of the parasitized red cell; (b) the amount of parasite cytoplasm in relation to the nucleus; (c) the amount and distribution of malaria pigment (hemozoin); (d) the number of parasite nuclei.
The size of the parasite in relation to the size of the red cell (a) was distorted and could not be assessed reliably, largely because of the difficulty in identifying reliably the red cell borders. The parasite cytoplasmic dimensions (b) were significantly smaller in the brain vessels, and there was compression of the food vacuole. For example, for parasites at the young ring stage in smears from culture the mean cytoplasmic diameter was 0.67 (0.14) compared with 0.54 (0.08) in the rat brain vessels (p = 0.002). However, the ratio of the thickness of the cytoplasm on the opposite side of the food vacuole in relation to the diameter of the nucleus still allowed a stage assessment, and this was not significantly different to that in the heart blood smears; eg small rings 1.86 (0.32) in culture versus 1.94 (0.33) in the brain (p = 0.48). The other relationships were preserved; in particular intra- and extraparasitic malaria pigment (c) was easily identified and quantitated. There was no significant difference between the overall stage distributions assessed in the rat heart blood and rat cerebral vessel smears (p = 0.26). These data suggested that stage assessments derived from the in vitro experiments could be made in the cerebral vessels in the clinical samples.
Morphological Criteria
The following criteria were used in the assessment of parasites in the brains of the fatal cases. Red cells were identified by their pink (hemoglobin) coloration and, if possible, discrete borders. It was not always possible to distinguish the red cell borders. Malaria parasites were identified from their characteristic staining and intravascular location. Ring stages were identified by their characteristic shape, mature trophozoites by the presence of associated malaria pigment and schizonts by the presence of more than two nuclei. Criteria for distinguishing between merozoites in an intact schizont, a ruptured schizont, and ring forms in newly invaded erythrocytes were established in brain smears from five cases where all were evident. The mean (SD) distance between adjacent merozoites in formed schizonts, choosing the five most separated merozoites, was 0.33 (0.23) (N = 139), compared to 0.77 (0.65) µm (N = 127) between merozoites liberated from a ruptured schizont, and 3.29 (2.06) µm (N = 191) between the parasite nuclei in adjacent ring infected erythrocytes. Thus circular collections of merozoites with an average intermerozoite separation of <0.5 µm were considered intact schizonts, irregular collections of merozoites with an average separation between 0.5 and 1.25 µm were considered ruptured schizonts, and parasite nuclei separated by >1.25 µm, apparently surrounded by a red cell, were considered as newly invaded erythrocytes.
Clinical Study
Although all stages of malaria parasite development were seen in
these fatal cases, the most frequent parasite form found in the brain
was the late trophozoite stage (Figure 3
, Table 1
). Using
the midpoint of the modal stage to represent the parasite age (in
hours) for each patient, the brain vessels were found to have parasites
that had a modal age approximately 21 hours older than the peripheral
blood: median (range) = 32 (3–46) compared with 11 (3–36) hours
(P < 0.001). The age of parasites in both the
brain and the blood was not significantly different between those
treated with quinine (N = 25) and artemether;
(N = 5) median (range) for the brain was 32
(3–46) versus 36 (3–46) hours respectively and for the
blood was 11 (3–36) versus 21 (3–32) hours, respectively.
Peripheral parasitemias in the other patients were too low for accurate
assessment of stage distributions. The age of parasites in the brain
was correlated negatively with the time from starting treatment to
death (Spearman correlation coefficient = -0.42,
p = 0.004). There was no relationship between the
peripheral blood stage and duration of treatment (Spearman correlation
coefficient = 0.11, p = 0.56).
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If it is assumed that there is no reason why death should be
associated with a particular stage of parasite development, then the
distribution of mature stages in the cerebral vessels should be random
between patients. To assess this the trophozoite and schizont counts
were normalized, by converting each to the number per hour, to adjust
for the difference in their respective time spans. Schizonts were often
seen (Figure 4)
but were significantly
under-represented relative to trophozoites in the cerebral
microvasculature. The median ratio of schizonts to trophozoites was
0.15 (range, 0.0 to 11.7) and this differed significantly from the
expected value of 1 (p < 0.001). Examining
multiple sites from six brains demonstrated no evidence that this was
due to a sampling error. Those brains which had large numbers of
schizonts had them in every site sampled, and those that had none in
one site had none in others. Thus there were no obvious differences
between sites from the same brain.
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It is believed that parasites <26 hours old circulate freely in
the bloodstream, whereas parasites >26 hours old are largely stuck in
the capillaries and venules. In order to see whether the proportion of
young parasites present in the brain was the same as that which would
be expected from the numbers present in circulating blood, the
proportion of red cells containing parasites <26 hours old was
calculated (for the brain red cells containing parasites >26 hours old
were excluded from the denominator). The brain was found to have a
considerably higher proportion of red cells containing young parasites
compared to peripheral blood: median (range) = 19.0% (0–90%)
versus 1.8% (0–36.2%) (p = 0.001).
All stages of parasites in the first half of the asexual life cycle
were over represented (ie this preponderance was not confined to the
more mature ring form parasites) (Figure 5 A–C)
. The proportion of all parasites
<26 hours old that were very young rings (aged 0–6 hours) was similar
in the brain and peripheral blood; median (range); 21.3% (0–100%)
compared with 28.6% (0–94.4%), respectively,
(p = 0.22). In four of the 50 cases, more than
70% of the parasites in the brain were ring forms.
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Individual capillaries within each brain showed very wide
variations in the numbers of parasitized red blood cells (PRBC). In
several brains some capillaries contained only unparasitized red cells,
whilst others in the same section had 100% (all 20/20) of red cells
parasitized. Stage of development also varied considerably (Figure 6)
. For example, one capillary might have
60% of parasites in the later ring forms, with no trophozoites, while
another capillary from the same brain had 80% late trophozoites with
no rings. In some smears where a particularly long vessel was present
clustering of different stages could be seen within a single vessel
(Figure 7)
. To see if the age of
parasites differed between vessels within a subject, the mean age of
parasite development and its standard deviation were calculated for
each capillary. These values were then compared with the mean and
standard deviation for all the staged parasites (ie the sum of all 5
vessels counted). In order to determine if the individual capillary
means and their variances differed from that which would be expected
from a single distribution of parasite stages, five datasets of 10
values were simulated from the overall distribution of that subject.
The simulated means and their standard deviations were compared to
those observed. There was significantly more variation of parasite age
between capillaries than would be expected from a single distribution
of parasites; median (range) = 4.12 (0.52 to 15.47) compared to
1.89 (0.58 to 5.71) respectively (p < 0.001).
Thus each vessel tended to have a single relatively narrow age
distribution of sequestered parasites.
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Samples were available from 13 of the fatal cases of falciparum malaria and 10 controls.
Control Brains
The control brains showed a reproducible cerebrovascular
endothelial cell immunophenotype similar to that seen in previous
reports.21
Brain microvascular endothelium expressed class
I HLA and CD31 constitutively and strongly. Focal scattered staining of
most vessels for ICAM-1 and thrombospondin was also seen, the latter in
a granular intracytoplasmic perinuclear pattern. P-selectin staining
was also focal, cytoplasmic and granular, suggesting a resting storage
pattern in Weibel Palade bodies. Very focal and weak CD36 and VCAM-1
staining was seen in some control cases, but many vessels did not
express these antigens. E-selectin was uniformly negative in blood
vessels, suggesting that this antigen is not expressed on normal brain
endothelial cells, but variable expression was seen on some neuronal
dendrites. Focal sparse chondroitin sulfate staining was seen on
endothelial cells in two control cases, but stronger staining was seen
on some perivascular macrophages. In general we did not observe CS
positivity on control brain endothelial cells. These results are
summarized in Table 2
. Control brain
endothelial immunophenotype was thus class I HLA++,
CD31++, class II HLA+/-,
ICAM-1+/-, TSP+/-, CD36-,
E-selectin-, VCAM-1-, P-sel+/-,
CS-.
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The staining patterns of putative host sequestration receptors on
vessels in brains from fatal cases of malaria differed to that in the
controls. The proportion of cases showing staining, and the intensity
of staining was compared between control and malaria cases (Table 2
,
Figures 8 and 9
). The
intensity and distribution of ICAM-1 staining was increased, with
nearly all vessels showing moderate to strong positive staining.
Endothelial staining for class II HLA was increased and there was
increased staining for VCAM-1 and E-selectin in a scattered pattern.
Chondroitin sulfate staining was also increased, with a larger number
of cases showing endothelial staining (Figure 8A)
compared to the
strong perivascular monocyte staining in controls (Figure 8B)
. Staining
with thrombospondin appeared to decrease, perhaps reflecting release of
this antigen from intracellular stores during endothelial activation.
There was no change in the strong and constitutive expression of CD31
and class I HLA (Figure 8C)
, and no increase in the patchy, low levels
of CD36 and P-selectin staining (Figure 8D)
. Thus endothelial cells in
the brain of malaria cases show increased expression of ICAM-1, VCAM-1,
E-selectin, CS, and class II HLA, and decreased TSP.
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. Accurate staging of parasite development was not
possible in the immunohistochemical sections as parasite morphology was
less well preserved, but with rough assessments (based on size,
intraparasitic malaria pigment, and number of nuclei) the variation in
endothelial staining along a vessel did not appear to be related to the
stage of development of the locally sequestered parasites. There were
no evident differences between the eleven cases of cerebral malaria and
the two fatal malaria cases who did not have cerebral malaria. Both
these cases also had variable increases in ICAM-1, VCAM-1, and
E-selectin staining. Some vessels had patchy class II staining on some
endothelial cells (Figure 9E)
, but in general if a vessel showed
positive staining for a marker of endothelial activation, this was
present throughout its length, rather than being related to the
presence of PRBC in one segment or over one endothelial cell. There was
little evidence for lymphocyte or platelet accumulation in vessels in
any case. Scattered monocytes were seen in several malaria cases
(CD36+, class I, and II HLA+ staining), and
numerous class II HLA positive microglial cells were seen (Figure 9F)
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Discussion |
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It is possible that the preparation of brain smears leads to
overestimation of sequestration, because capillaries without
cytoadherent red cells may be missed on microscopic examination, or may
be more likely to empty during the preparation of the brain smear, than
those with sequestered parasites. However, this is unlikely to be a
major confounder as capillaries and venules showing no evidence of
cytoadherent parasitized erythrocytes were often seen, whereas others
in the same section, several microns away, were intensely parasitized
(Figures 5 and 6)
. Assessment of the stage of parasite development
in vivo is also confounded by the effects of antimalarial
drug treatment and compression within the confines of narrow vessels.
However, in vitro and animal experiments indicated that
these would not lead to large errors in stage assessment. The
heterogeneity in the microvascular distribution of parasitized
erythrocytes in fatal malaria emphasises the importance of the vascular
endothelial receptors for parasitized red cell cytoadherence in
determining the extent and distribution of sequestration. Cytoadherence
is mediated by the binding of parasite derived adhesins, expressed on
the surface of the parasitized erythrocyte and also possibly
modifications in the host cell membrane, to vascular endothelial cell
receptors.1,24
The principal parasite adhesin is PfEMP1, a
high molecular weight, antigenically variant product of the
var gene superfamily.25-27
Multiple vascular
receptors for cytoadherence have been identified in vitro.
Quantitatively CD36 appears to be the most important receptor in the
body, although this molecule is not generally expressed on the surface
of cerebral vascular endothelium. ICAM-1 is probably the most important
receptor for cytoadherence in the brain.21
Thrombospondin,
E-selectin, VCAM-1, CD31/PECAM, and chondroitin sulfate
A28,29
have also all been shown capable of mediating
cytoadherence in vitro. Expression of some of these
cytoadherence receptors is up-regulated by proinflammatory cytokines.
This immunocytochemical study confirms our previous report that
cerebral microvascular expression of ICAM-1, E-selectin, and VCAM-1 is
increased in fatal malaria (Figure 9)
, and also shows for the first
time that chondroitin sulphate and HLA class II expression is
increased. CD-36 and P-selectin staining was very scanty. These
receptors do not appear to mediate intracerebral sequestration
(although CD36 is probably the major sequestration receptor outside the
brain).
Changing expression of these ligands, and thus changing vascular endothelial cell receptivity for parasitized cell adhesins, is the most likely explanation for the uneven microvascular distribution of cerebral sequestration observed in this study. However, testing this hypothesis by relating the distribution of microvascular sequestration to different receptor types in autopsy specimens is problematic. Up-regulation can occur before or after sequestration. Mature trophozoites or schizonts in a vessel have been sequestered for hours, or if arrested by antimalarial treatment, for days before death. The increased expression of inducible receptors such as ICAM-1, VCAM-1, and E-selectin could result from systemic processes, such as the increased levels of proinflammatory cytokines, or from local changes related to microvascular obstruction and ischaemia. The finding that increased receptor expression tended to be present throughout a "sequestered" vessel, and not just at the site of parasitized red cell adherence, argues against the up-regulation of these receptors being solely a direct result of binding of PRBC to a particular cell. However, this does not preclude the possibility that binding could induce phenotypic changes in an individual cell, which would increase its adhesive potential. Also, there was no definite difference between patterns of staining in cerebral and non-cerebral malaria cases. These findings confirm that endothelial activation in the brain is not specific to cerebral malaria,30 and would support the hypothesis that endothelial cell activation can occur independent of PRBC binding.
Absence of a putative endothelial receptor at a site of sequestration may not exclude its involvement in cytoadherence if that receptor is transiently up-regulated and turned over rapidly. Because each case reflects a single time point during the disease process, which varies with each patient, differences in the temporal expression of particular receptors could be missed. However the co-expression of markers of endothelial activation such as E-selectin and VCAM-1, which have very different time courses of expression on endothelial cells in vitro, suggests that in vivo they are being subjected continuously to waves of activation on a more chronic time course, which does allow us to see both being expressed at the same time.
The parasites within the cytoadherent erythrocytes in each vessel or section of a vessel tended to be of similar age, whereas the distribution of parasite stages in the vessels overall was much broader, ie there was spatial clustering in parasite age. Presumably small blood vessels increase their receptivity to cytoadherence by increasing expression of these vascular ligands and select for those circulating parasitized erythrocytes newly capable of adhesion. All the red cells infected by more mature parasites would already be adherent (ie not circulating). Thus as the parasites mature and begin to express red cell surface adhesins, they are filtered out by those vessels newly capable of accepting them. Rolling is followed by static cytoadherence.31 Blood flow is not stopped by this process. There must be significant flow even through vessels partially occluded by cytoadherent parasitized erythrocytes to allow this filtration to continue, although the unparasitized erythrocytes presumably undergo considerable deformation in squeezing past the adherent and relatively nondeformable sequestered red cells.1 This is the only explanation to account for the considerably higher proportion of parasitized red cells in the cerebral vessels compared to the peripheral blood. This study also suggests that once parasitized red cells adhere, they do not detach again and circulate freely. If they did recirculate this would lead to mixing of stages, and the parasites sequestered in each vessel should reflect the broad stage distribution of malaria parasites in the second 24 hours of their development.
Heterogeneity in microvascular sequestration in the brain may well explain the rarity of permanent neurological damage in cerebral malaria. It is likely that microvascular obstruction and consequent ischaemia provides a strong stimulus to vasodilation.32 Blood flow through those capillaries and venules which are not obstructed by parasitized cells would be expected to be maximal. Altered vessel wall shear stresses would also induce release of the potent vasodilator nitric oxide.33 It has been suggested that nitric oxide may cause coma in cerebral malaria, although the source hypothesized has been cytokine up-regulated, inducible isoform of nitric oxide synthase (iNOS), rather than vascular wall endothelial isoform (eNOS), or the neuronal isoform (nNOS).2,3,34 Although there may be a background increase in vascular endothelial nitric oxide generation because of the systemic increase in proinflammatory cytokine release,35 local factors are likely to predominate before terminal shock develops. Release of vasodilatory mediators would be expected to be greatest adjacent to cytoadherent red cells. Thus delivery of oxygen and metabolic substrates in areas with partially occluded vessels may be sufficient to prevent permanent neuronal cell death. Even apparently blocked capillaries may still allow passage of uninfected red cells.
Although cerebral malaria was associated with intense sequestration
within the brain, this was also seen in patients who did not lose
consciousness. This may reflect the time between admission and death
and does not disprove the concept that cerebral malaria is associated
specifically with cerebral sequestration. Other patients in this
series, particularly those who died after several days of antimalarial
treatment, had few or no residual parasites in their cerebral vessels.
This has been noted previously36,37
and has led some
investigators to suggest that cerebral sequestration is not a necessary
prerequisite for cerebral malaria.3
Nearly all such cases
reported had received many days of treatment, and in this series
residual pigment was evident within the cerebral microvasculature, even
if no parasites were seen (Figure 10)
.
Thus this relatively large study provides no support for the contention
that cerebral malaria can occur in the absence of cerebral
sequestration. Sequestration occurs in all patients with falciparum
malaria and is a consistent pathological feature of severe malaria. If
it was not then schizonts and mature trophozoites would be seen
regularly in peripheral blood films. The absence of parasites from the
brains of fatal cases indicates effective antimalarial treatment and
clearance of infected erythrocytes. Such treatment cannot always
reverse malaria related or secondary pathological processes in severely
ill patients.
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Although schizonts were under represented in this study, tiny and small ring form parasites were over-represented in the cerebral vessels. These stages are not thought to cytoadhere. If it is assumed that there is free mixing within the systemic circulation of nonadherent red cells,41 the proportion of young ring forms in the cerebral vessels should be smaller than that in the peripheral blood, where the denominator number of unparasitized red cells is larger. The reverse was found. Indeed there were over 10 times more young rings in the cerebral vessels than would be expected from a free mixing model. Could this be an artifact of the stage assessment of drug exposed parasites in cerebral vessels? The staging of malaria parasites by light microscopy is not very precise because of the difficulties in obtaining very tightly synchronized cultures, different growth rates, smearing, and staining, and the simple differences from one parasite with another. Compression of the parasitized erythrocytes within the cerebral capillaries could further confound interpretation. Although the red cell and parasite dimensions and shape were different in blood and brain smears, the amount of parasite cytoplasm, the number of nuclei, and, in particular, the presence of intraparasitic pigment were similar in simulation experiments in the rat. Antimalarial drug treatment also affected parasite morphology, and whereas quinine has little effect artemether significantly alters ring stage morphology and arrests development. However none of these confounders is likely to explain the presence of large numbers of small parasites without visible malaria pigment in the cerebral vessels. Malaria pigment, once formed, is insoluble and undeformable and is not resorbed.42 Intraparasitic pigment visible under light microscopy usually indicates that the parasite is more than 18 to 24 hours old.41 The absence of visible pigment is strong evidence that the parasite is in the first half of the asexual life cycle.
These data suggest that there is either sequestration or retention of the younger parasite forms in the cerebral microvasculature. Massive synchronous merogony would be an unlikely alternative explanation for this finding, and could not explain the grouping of small and large ring forms seen in some vessels. If schizogony occurred in a completely occluded vessel then a higher proportion of multiple infections would be anticipated, and some evidence of adjacent mature parasites or residual pigment should have been seen, which was usually not the case. It is possible that changes in surface charge or membrane fluidity in ring stage infected red cells could alter their normal transit through the microcirculation and lead them to accumulate, preferentially in branch vessels which are normal closed to flow. Whichever the mechanism, red cells containing young rings, hitherto thought incapable of sequestration, are concentrated considerably in the cerebral vessels. Whether this is because of adhesion to vascular endothelium or to other parasitized cells or simply reflects their reduced deformability is not known, but it does suggest that peripheral parasite counts are an even greater underestimate of the parasite burden in severe falciparum malaria than previously thought.
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Acknowledgements |
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Footnotes |
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Supported by The Wellcome Trust of Great Britain through the Wellcome- Mahidol University- Oxford Tropical Medicine Research Programme in Thailand, and the Wellcome Trust Clinical Research Unit, Center for Tropical Diseases in Vietnam.
Accepted for publication April 11, 1999.
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