Immunology Letters, 25 (1990), 213-216 Elsevier IMLET 01455
Tumour necrosis factor, fever and fatality in falciparum malaria D o m i n i c Kwiatkowski M R C Laboratories, Fajara, The Gambia
I. Summary In Gambian children with Plasmodium falciparum infection, uncomplicated malaria fever is associated with moderately elevated circulating tumour necrosis factor (TNF) levels. We review clinical and in vitro data suggesting that schizont rupture stimulates bursts of TNF production that mediate paroxysms of malaria fever, and that the fever could possibly be beneficial to the host. Mean plasma TNF levels are ten times higher in fatal cerebral malaria than in uncomplicated malaria, providing support for the hypothesis that excessive TNF production may contribute to the pathology of severe infection. 2. Introduction
Plasmodium falciparum infection can persist for weeks or months at relatively stable parasitaemia, accompanied by intermittent fever [1]. Young African children have repeated infections, associated with a fatality rate in the order of 1°70 [2]. There is presently considerable interest in the idea that malaria fatality may relate to levels of tumour necrosis factor (TNF) production by the host. As anticipated by Ian Clark [3], various forms of experimental data now indicate that TNF acts (probably indirectly) to suppress the parasite, while there is increasing evidence that TNF may also contribute to the pathogenesis of fatal malaria (reviewed in [4, 5] and in this volume (by Clark, Taverne and Grau)). This has led to speculation that a moderate amount of TNF may be good for the host but an excess is Key words." Plasmodium falciparum; Fever, Cerebral malaria; Tumour necrosis factor Correspondence to: Dominic Kwiatkowski, Molecular Infectious Diseases Group, Institute of Molecular Medicine, John Radcliffe Hospital, Oxford OX3 9DU, U.K. Fax: +44-865-68941.
bad. This paper reviews evidence in favour of this hypothesis from our studies of Gambian children with acute falciparum malaria. 3. TNF production in relation to malaria fever The primary manifestation of P. falciparum infection in young Gambian children is fever. Lifethreatening complications occur in only a small minority of infections, and will be considered later (Fig. 1). Gambian children with uncomplicated falciparum malaria have significantly higher plasma TNF levels (measured by ELISA) than children with other infections of comparable severity [6]. Fig. 2 shows the frequency distribution of TNF levels found in a more recent sample of children with uncomplicated malaria fever. In this sample, median TNF levels were in the region of 3 0 - 4 0 pg/ml but there was a wide scatter, and 14% of samples showed levels of over 100 pg/ml. This scatter might be caused by many factors, but one important consideration is that TNF is probably released intermittently so that levels fluctuate with time. Our initial evidence for this came from an in vitro experiment in which P. falciparum was cultured in erythrocytes, in the presence of human monocytes [6]. The rate of TNF production was estimated by sampling the supernatant every 6 h and compared to the rate of production in paired control cultures containing unparasitised erythrocytes : the ratio of the two provided a TNF stimulation index (Fig. 3). Schizont rupture was followed by a new generation of rings, and by a significant increase in the rate of TNF production. It is interesting that malaria fever occurs at just this point in the parasite life cycle [1]. TNF is a potent endogenous pyrogen, and the experiment suggested that paroxysms of malaria fever may be mediated through bursts of TNF release,
0165-2478 / 90 / $ 3.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
213
TNF s t i m u l a t i o n i n d e x
'
FEVER
CEREBRAL MALARIA death
~~
i
g
~
............... SEVERE
ANAEMIA
2 -,
Fig. 1. Major clinical manifestations of acute falciparum malaria in young G a m b i a n children.
! 0
triggered by the simultaneous rupture of a large number of schizonts. Confirming this association in vivo proved more difficult, since fever paroxysms in falciparum malaria are often poorly defined, and since it would not be ethical to do sequential studies on untreated patients. Instead, we have recently analysed single blood samples from almost 400 children who presented to clinic with acute falciparum malaria. The blood film allowed us to identify those in whom schizont rupture had recently occurred, and we found that this group had, on average, a significantly higher degree of fever and higher TNF levels than those in whom schizont rupture had occurred less recently (D. Kwiatkowski, L. Kuyateh, A. Hill, J. Castracane, D. Brewster, B. M. Greenwood; submitted for publication). Furthermore, levels of both TNF and interleukin-1 alpha (IL-lc~, another pyrogenic cytokine) were significantly associated with %
100 8060-
40 20 0 m a l a r i a (n-196) "~
'10 pg/ml
~]
10-20 pg/ml
n o n - m a l a r i a (n-121) ~
20-40 pg/ml
~
~40 pg/ml
Fig. 2. Frequency distribution of plasma T N F levels in young G a m b i a n children with uncomplicated acute falciparum malaria, and in children with a variety of other infections. Both groups were well enough to be treated as outpatients.
214
6
12
18
24
30
36
42
48
54
60
66
72
36
42
48
54
60
66
72
l° I RINQ8
8~
~
SCHIZONTS
% ~arasitaemia
,! 2~
oi
O
6
12
18
24
30
Hours
Fig. 3. Synchronised erythrocytic cultures of P falciparum plus h u m a n mononuclear cells in vitro. Upper panel shows rate of T N F production, expressed as a stimulation index (see text). Lower panel shows the time course of parasite development: schizont rupture occurs at about 42 h. (Reproduced with permission of Clin. Exp. Immunol.)
the degree of fever whereas interferon gamma (a non-pyrogenic cytokine) was not. These findings strengthen the hypothesis that TNF and IL-1 are mediators of malaria fever, but at this stage we obviously cannot rule out the possibility that other endogenous pyrogens [7] may also be involved. Do these findings tell us anything about the biological role of TNF in falciparum malaria? Moderate elevation of the circulating TNF level appears to be usual, and is compatible with good outcome. TNF production is linked to the parasite life-cycle, and we infer that fluctuations in TNF levels are at least partly responsible for the intermittent nature of malaria fever. As well as the general evidence that TNF may be protective in malaria [4, 5] two recent observations suggest that the host fever response may specifically act to suppress parasite growth in
human infection. We have observed that febrile temperatures are inhibitory to the growth of P falciparum in vitro (Fig. 4) [8], and elsewhere in this volume Kamini Mendes describes parasite-suppressive serum factors in Plasrnodium vivax infection, that are maximal at the time of the fever paroxysms. This poses an apparent paradox for, if fever is indeed inhibitory to the parasite, why does infection persist despite recurrent fever? We propose that fever does not act to eliminate infection but rather to regulate parasite density. Classical studies of human malaria showed that erythrocytic parasites multiply exponentially in the early phase of infection, but that the parasite population density tends to equilibrate after the onset of fever. In other words, parasitaemia tends to stabilise at the level which causes fever: for P. falciparum this is around 101°/I, and for P vivax 108/1 [1]. This phenomenon is suggestive of a negative feedback mechanism, whereby fever occurs only when parasite population density exceeds a certain threshold value, suppressing parasite growth and ultimately tending to stabilise population density at (or just above) the threshold value. This hypothesis, which also relates to the question of why malaria fever is periodic, is discussed in greater detail else-
128-
4-
2E
Illa
0.5 0.25
0.125 I
I
I
I
I
I
0
1
2
3
4
5
Days Fig. 4. Effect of temperature on the growth of P. falciparum in erythrocytic culture in vitro. Closed squares denote 37 °C, open squares 40 °C. (Reproduced with permission of J. Exp. Med.)
where [9]. The argument concerns only human malaria: in experimental rodent malaria, parasite population dynamics are different and fever is not generally observed. 4. TNF production in fatal malaria The majority of hospital deaths due to P. falciparum infection in Gambian children are associated with the syndrome of cerebral malaria: i.e., unrousable coma, often accompanied by fits and hypoglycaemia. It is very rare for a Gambian child with cerebral malaria to have multiorgan failure of the sort seen in South-East Asia [10], but nevertheless the case fatality is around 20% despite hospital treatment. At the main government hospital in The Gambia in 1989, cerebral malaria accounted for over a third of all paediatric deaths (D. Brewster, personal communication). Our understanding of the relationship between TNF production and disease severity is incomplete. Studies in Malawi have found that, amongst children with cerebral malaria, circulating TNF levels are significantly higher in fatal cases than survivors, but data on normal TNF levels in Malawian children with falciparum malaria are not yet available [11]. Our initial studies of plasma TNF levels in Gambian children [6] revealed no significant difference between cerebral malaria and uncomplicated malaria, but the numbers were relatively small (19 in the cerebral malaria group) and the data may have been confounded by the temporal fluctuation of plasma TNF levels that we have discussed above. We have recently studied 113 Gambian children shortly after admission to hospital with strictly defined cerebral malaria (28 of whom died), comparing them with 179 cases of uncomplicated childhood malaria (D. Kwiatkowski, A. Hill, I. Sambou, J. Castracane, P. Twumasi, D. Brewster, B. M. Greenwood; submitted for publication). Plasma TNF levels were widely scattered, with considerable overlap between the different groups, but there was a highly significant shift in the distribution of TNF levels with increasing disease severity. Compared to levels in the mild malaria group, geometric mean TNF levels were approximately twice as high in those children who had cerebral malaria but survived, and ten times as high in children who went on to die of cerebral malaria. TNF levels were significantly higher in chil215
dren with h y p e r p a r a s i t a e m i a ( > 5 × 10n/l) a n d with h y p o g l y c a e m i a ( < 2 m m o l / l ) . Since h y p e r p a r a s i t a e m i a a n d h y p o g l y c a e m i a are also a s s o c i a t e d with fatal o u t c o m e we p e r f o r m e d m u l t i v a r i a t e analysis, which c o n f i r m e d a strong a s s o c i a t i o n between T N F levels a n d fatality t h a t was i n d e p e n d e n t o f glucose a n d p a r a s i t a e m i a . T h e IL-lo~ d a t a showed a generally similar picture, whereas IFN-,y levels did n o t show a simple r e l a t i o n to disease severity. T N F levels o f over 500 p g / m l in cerebral m a l a r i a were a s s o c i a t e d with over 60°70 mortality. F u r t h e r m o r e , if we a s s u m e an overall m o r t a l i t y in the region on 1% for acute falc i p a r u m m a l a r i a in general, o u r d a t a i m p l y t h a t a T N F level above 500 p g / m l in a child with m a l a r i a fever carries a relative risk o f d e a t h in the o r d e r o f 50, a l t h o u g h we w o u l d require further i n f o r m a t i o n to d e t e r m i n e a precise figure. Taken t o g e t h e r with the growing evidence t h a t T N F is a critical p a t h o l o g i c a l m e d i a t o r in lethal end o t o x i c s h o c k [12] a n d in certain types o f lethal rodent m a l a r i a [4, 5, 13] these findings strengthen the hypothesis t h a t excessive T N F p r o d u c t i o n in h u m a n m a l a r i a m a y c o n t r i b u t e to a fatal o u t c o m e . However the causal link has n o t yet been f o r m a l l y e s t a b l i s h e d a n d the logical next step m a y be s o m e f o r m o f antiT N F i n t e r v e n t i o n s t u d y in those m a l a r i a p a t i e n t s who are at high risk o f death. T h e striking elevation o f p l a s m a T N F levels in fatal m a l a r i a also raises the q u e s t i o n o f w h a t determines the level o f T N F p r o d u c t i o n . Elsewhere in this v o l u m e Taverne a n d colleagues [14] r e p o r t progress in d e f i n i n g T N F - s t i m u l a t i n g c o m p o n e n t s o f plasm o d i a l culture s u p e r n a t a n t s . Possibly parasites vary in their T N F - s t i m u l a t o r y activity, a n d host genetic a n d i m m u n o l o g i c factors p r o b a b l y influence the level o f T N F response. In a d d i t i o n , o u r d a t a suggest t h a t p a t t e r n o f schizont r u p t u r e w o u l d affect T N F p r o d u c t i o n , a n d it is p o s s i b l e t h a t the high m u l t i p l i c a t i o n rates a n d a s y n c h r o n o u s g r o w t h o f P. falcipar u m m a y o c c a s i o n a l l y generate sustained high levels o f T N F p r o d u c t i o n t h a t are deleterious to the host.
216
We need better u n d e r s t a n d i n g o f parasite p o p u l a tion d y n a m i c s a n d T N F kinetics to be able to address questions o f this sort.
Acknowledgements These ideas are the result o f discussions a n d j o i n t ventures with m y colleagues at the M e d i c a l Research C o u n c i l L a b o r a t o r i e s , F a j a r a a n d the Royal Victoria H c s p i t a l , Banjul, a n d I a m especially grateful to Dr. Brian G r e e n w o o d a n d Dr. H i l t o n W h i t t l e for their support and encouragement.
References [I] Kitchen, S. E (1949) in: Malariology (M. F. Boyd, Ed.) Vol. 2, pp. 966-1045. W. B. Saunders, Philadelphia. [2] Greenwood, B.M., Bradley, A.K., Greenwood, A.M., Byass, P., Jammeh, K., Marsh, K., Tulloch, S., Oldfield, F. S. J. and Hayes, R. (1987) Trans. R. Soc. Trop. Med. Hyg. 81, 478. [3] Clark, I. A. (1978) Lancet ii, 75. [4] Clark, I. A. (1987) Parasitol. Today 3, 300. [5] Clark, I. A. and Cowden, W. B. (1990) in: Tumor Necrosis Factors: Structures, Regulation and Functions (B. B. Aggarwal and J. T. Vilcek, Eds.) Marcel Dekker, New York, in press. [6] Kwiatkowski, D., Cannon, J. G., Manogue, K. R., Cerami, A., Dinarello, C. A. and Greenwood, B. M. (1989) Clin. Exp. Immunol. 77, 361. [7] Dinarello, C. A. (1987) Lymphokines 14, 1. [8] Kwiatkowski, D. (1989) J. Exp. Meal. 169, 357. [9] Kwiatkowski, D. and Greenwood, B. M. (1989) Parasitol. Today 5, 264. [10] Phillips, R. E. and Warrell, D. A. (1986) Parasitol. Today 10, 271. [11] Grau, G. E., Taylor, T. E., Molyneux, M. E., Wirima, J. J., Vassali, P., Hommel, M. and Lambert, R H. (1989) New Engl. J. Med. 320, 1586. [12] Tracey, K. J., Vlassara, H. and Cerami, A. (1989) Lancet i, 1122. [13] Grau, G. E., Fajardo, L. E, Piguet, P.-E, Allet, B., Lambert, R-H. and Vassali, P. (1987) Science 237, 1210. [14] Taverne, J., Bate, C. A. W. and Playfair, J. H. L. (1990) lmmunol. Lett. 25, 207.
Tumour necrosis factor, fever and fatality in falciparum malaria D o m i n i c Kwiatkowski M R C Laboratories, Fajara, The Gambia
I. Summary In Gambian children with Plasmodium falciparum infection, uncomplicated malaria fever is associated with moderately elevated circulating tumour necrosis factor (TNF) levels. We review clinical and in vitro data suggesting that schizont rupture stimulates bursts of TNF production that mediate paroxysms of malaria fever, and that the fever could possibly be beneficial to the host. Mean plasma TNF levels are ten times higher in fatal cerebral malaria than in uncomplicated malaria, providing support for the hypothesis that excessive TNF production may contribute to the pathology of severe infection. 2. Introduction
Plasmodium falciparum infection can persist for weeks or months at relatively stable parasitaemia, accompanied by intermittent fever [1]. Young African children have repeated infections, associated with a fatality rate in the order of 1°70 [2]. There is presently considerable interest in the idea that malaria fatality may relate to levels of tumour necrosis factor (TNF) production by the host. As anticipated by Ian Clark [3], various forms of experimental data now indicate that TNF acts (probably indirectly) to suppress the parasite, while there is increasing evidence that TNF may also contribute to the pathogenesis of fatal malaria (reviewed in [4, 5] and in this volume (by Clark, Taverne and Grau)). This has led to speculation that a moderate amount of TNF may be good for the host but an excess is Key words." Plasmodium falciparum; Fever, Cerebral malaria; Tumour necrosis factor Correspondence to: Dominic Kwiatkowski, Molecular Infectious Diseases Group, Institute of Molecular Medicine, John Radcliffe Hospital, Oxford OX3 9DU, U.K. Fax: +44-865-68941.
bad. This paper reviews evidence in favour of this hypothesis from our studies of Gambian children with acute falciparum malaria. 3. TNF production in relation to malaria fever The primary manifestation of P. falciparum infection in young Gambian children is fever. Lifethreatening complications occur in only a small minority of infections, and will be considered later (Fig. 1). Gambian children with uncomplicated falciparum malaria have significantly higher plasma TNF levels (measured by ELISA) than children with other infections of comparable severity [6]. Fig. 2 shows the frequency distribution of TNF levels found in a more recent sample of children with uncomplicated malaria fever. In this sample, median TNF levels were in the region of 3 0 - 4 0 pg/ml but there was a wide scatter, and 14% of samples showed levels of over 100 pg/ml. This scatter might be caused by many factors, but one important consideration is that TNF is probably released intermittently so that levels fluctuate with time. Our initial evidence for this came from an in vitro experiment in which P. falciparum was cultured in erythrocytes, in the presence of human monocytes [6]. The rate of TNF production was estimated by sampling the supernatant every 6 h and compared to the rate of production in paired control cultures containing unparasitised erythrocytes : the ratio of the two provided a TNF stimulation index (Fig. 3). Schizont rupture was followed by a new generation of rings, and by a significant increase in the rate of TNF production. It is interesting that malaria fever occurs at just this point in the parasite life cycle [1]. TNF is a potent endogenous pyrogen, and the experiment suggested that paroxysms of malaria fever may be mediated through bursts of TNF release,
0165-2478 / 90 / $ 3.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
213
TNF s t i m u l a t i o n i n d e x
'
FEVER
CEREBRAL MALARIA death
~~
i
g
~
............... SEVERE
ANAEMIA
2 -,
Fig. 1. Major clinical manifestations of acute falciparum malaria in young G a m b i a n children.
! 0
triggered by the simultaneous rupture of a large number of schizonts. Confirming this association in vivo proved more difficult, since fever paroxysms in falciparum malaria are often poorly defined, and since it would not be ethical to do sequential studies on untreated patients. Instead, we have recently analysed single blood samples from almost 400 children who presented to clinic with acute falciparum malaria. The blood film allowed us to identify those in whom schizont rupture had recently occurred, and we found that this group had, on average, a significantly higher degree of fever and higher TNF levels than those in whom schizont rupture had occurred less recently (D. Kwiatkowski, L. Kuyateh, A. Hill, J. Castracane, D. Brewster, B. M. Greenwood; submitted for publication). Furthermore, levels of both TNF and interleukin-1 alpha (IL-lc~, another pyrogenic cytokine) were significantly associated with %
100 8060-
40 20 0 m a l a r i a (n-196) "~
'10 pg/ml
~]
10-20 pg/ml
n o n - m a l a r i a (n-121) ~
20-40 pg/ml
~
~40 pg/ml
Fig. 2. Frequency distribution of plasma T N F levels in young G a m b i a n children with uncomplicated acute falciparum malaria, and in children with a variety of other infections. Both groups were well enough to be treated as outpatients.
214
6
12
18
24
30
36
42
48
54
60
66
72
36
42
48
54
60
66
72
l° I RINQ8
8~
~
SCHIZONTS
% ~arasitaemia
,! 2~
oi
O
6
12
18
24
30
Hours
Fig. 3. Synchronised erythrocytic cultures of P falciparum plus h u m a n mononuclear cells in vitro. Upper panel shows rate of T N F production, expressed as a stimulation index (see text). Lower panel shows the time course of parasite development: schizont rupture occurs at about 42 h. (Reproduced with permission of Clin. Exp. Immunol.)
the degree of fever whereas interferon gamma (a non-pyrogenic cytokine) was not. These findings strengthen the hypothesis that TNF and IL-1 are mediators of malaria fever, but at this stage we obviously cannot rule out the possibility that other endogenous pyrogens [7] may also be involved. Do these findings tell us anything about the biological role of TNF in falciparum malaria? Moderate elevation of the circulating TNF level appears to be usual, and is compatible with good outcome. TNF production is linked to the parasite life-cycle, and we infer that fluctuations in TNF levels are at least partly responsible for the intermittent nature of malaria fever. As well as the general evidence that TNF may be protective in malaria [4, 5] two recent observations suggest that the host fever response may specifically act to suppress parasite growth in
human infection. We have observed that febrile temperatures are inhibitory to the growth of P falciparum in vitro (Fig. 4) [8], and elsewhere in this volume Kamini Mendes describes parasite-suppressive serum factors in Plasrnodium vivax infection, that are maximal at the time of the fever paroxysms. This poses an apparent paradox for, if fever is indeed inhibitory to the parasite, why does infection persist despite recurrent fever? We propose that fever does not act to eliminate infection but rather to regulate parasite density. Classical studies of human malaria showed that erythrocytic parasites multiply exponentially in the early phase of infection, but that the parasite population density tends to equilibrate after the onset of fever. In other words, parasitaemia tends to stabilise at the level which causes fever: for P. falciparum this is around 101°/I, and for P vivax 108/1 [1]. This phenomenon is suggestive of a negative feedback mechanism, whereby fever occurs only when parasite population density exceeds a certain threshold value, suppressing parasite growth and ultimately tending to stabilise population density at (or just above) the threshold value. This hypothesis, which also relates to the question of why malaria fever is periodic, is discussed in greater detail else-
128-
4-
2E
Illa
0.5 0.25
0.125 I
I
I
I
I
I
0
1
2
3
4
5
Days Fig. 4. Effect of temperature on the growth of P. falciparum in erythrocytic culture in vitro. Closed squares denote 37 °C, open squares 40 °C. (Reproduced with permission of J. Exp. Med.)
where [9]. The argument concerns only human malaria: in experimental rodent malaria, parasite population dynamics are different and fever is not generally observed. 4. TNF production in fatal malaria The majority of hospital deaths due to P. falciparum infection in Gambian children are associated with the syndrome of cerebral malaria: i.e., unrousable coma, often accompanied by fits and hypoglycaemia. It is very rare for a Gambian child with cerebral malaria to have multiorgan failure of the sort seen in South-East Asia [10], but nevertheless the case fatality is around 20% despite hospital treatment. At the main government hospital in The Gambia in 1989, cerebral malaria accounted for over a third of all paediatric deaths (D. Brewster, personal communication). Our understanding of the relationship between TNF production and disease severity is incomplete. Studies in Malawi have found that, amongst children with cerebral malaria, circulating TNF levels are significantly higher in fatal cases than survivors, but data on normal TNF levels in Malawian children with falciparum malaria are not yet available [11]. Our initial studies of plasma TNF levels in Gambian children [6] revealed no significant difference between cerebral malaria and uncomplicated malaria, but the numbers were relatively small (19 in the cerebral malaria group) and the data may have been confounded by the temporal fluctuation of plasma TNF levels that we have discussed above. We have recently studied 113 Gambian children shortly after admission to hospital with strictly defined cerebral malaria (28 of whom died), comparing them with 179 cases of uncomplicated childhood malaria (D. Kwiatkowski, A. Hill, I. Sambou, J. Castracane, P. Twumasi, D. Brewster, B. M. Greenwood; submitted for publication). Plasma TNF levels were widely scattered, with considerable overlap between the different groups, but there was a highly significant shift in the distribution of TNF levels with increasing disease severity. Compared to levels in the mild malaria group, geometric mean TNF levels were approximately twice as high in those children who had cerebral malaria but survived, and ten times as high in children who went on to die of cerebral malaria. TNF levels were significantly higher in chil215
dren with h y p e r p a r a s i t a e m i a ( > 5 × 10n/l) a n d with h y p o g l y c a e m i a ( < 2 m m o l / l ) . Since h y p e r p a r a s i t a e m i a a n d h y p o g l y c a e m i a are also a s s o c i a t e d with fatal o u t c o m e we p e r f o r m e d m u l t i v a r i a t e analysis, which c o n f i r m e d a strong a s s o c i a t i o n between T N F levels a n d fatality t h a t was i n d e p e n d e n t o f glucose a n d p a r a s i t a e m i a . T h e IL-lo~ d a t a showed a generally similar picture, whereas IFN-,y levels did n o t show a simple r e l a t i o n to disease severity. T N F levels o f over 500 p g / m l in cerebral m a l a r i a were a s s o c i a t e d with over 60°70 mortality. F u r t h e r m o r e , if we a s s u m e an overall m o r t a l i t y in the region on 1% for acute falc i p a r u m m a l a r i a in general, o u r d a t a i m p l y t h a t a T N F level above 500 p g / m l in a child with m a l a r i a fever carries a relative risk o f d e a t h in the o r d e r o f 50, a l t h o u g h we w o u l d require further i n f o r m a t i o n to d e t e r m i n e a precise figure. Taken t o g e t h e r with the growing evidence t h a t T N F is a critical p a t h o l o g i c a l m e d i a t o r in lethal end o t o x i c s h o c k [12] a n d in certain types o f lethal rodent m a l a r i a [4, 5, 13] these findings strengthen the hypothesis t h a t excessive T N F p r o d u c t i o n in h u m a n m a l a r i a m a y c o n t r i b u t e to a fatal o u t c o m e . However the causal link has n o t yet been f o r m a l l y e s t a b l i s h e d a n d the logical next step m a y be s o m e f o r m o f antiT N F i n t e r v e n t i o n s t u d y in those m a l a r i a p a t i e n t s who are at high risk o f death. T h e striking elevation o f p l a s m a T N F levels in fatal m a l a r i a also raises the q u e s t i o n o f w h a t determines the level o f T N F p r o d u c t i o n . Elsewhere in this v o l u m e Taverne a n d colleagues [14] r e p o r t progress in d e f i n i n g T N F - s t i m u l a t i n g c o m p o n e n t s o f plasm o d i a l culture s u p e r n a t a n t s . Possibly parasites vary in their T N F - s t i m u l a t o r y activity, a n d host genetic a n d i m m u n o l o g i c factors p r o b a b l y influence the level o f T N F response. In a d d i t i o n , o u r d a t a suggest t h a t p a t t e r n o f schizont r u p t u r e w o u l d affect T N F p r o d u c t i o n , a n d it is p o s s i b l e t h a t the high m u l t i p l i c a t i o n rates a n d a s y n c h r o n o u s g r o w t h o f P. falcipar u m m a y o c c a s i o n a l l y generate sustained high levels o f T N F p r o d u c t i o n t h a t are deleterious to the host.
216
We need better u n d e r s t a n d i n g o f parasite p o p u l a tion d y n a m i c s a n d T N F kinetics to be able to address questions o f this sort.
Acknowledgements These ideas are the result o f discussions a n d j o i n t ventures with m y colleagues at the M e d i c a l Research C o u n c i l L a b o r a t o r i e s , F a j a r a a n d the Royal Victoria H c s p i t a l , Banjul, a n d I a m especially grateful to Dr. Brian G r e e n w o o d a n d Dr. H i l t o n W h i t t l e for their support and encouragement.
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