Immunological aspects of cerebral lesions in murine malaria.

The majority of male C57Bl/Rij mice died infected with Plasmodium berghei early in the second week. Death was closely correlated to collapse of the thermoregulation of the body, with perivascular oedema and petechial haemorrhages in the brain. Mice that did not show a collapse of thermoregulation (temperature drop below 30 degrees C) and survived for more than 2 weeks after infection did not show haemorrhages. Development of this syndrome (temperature below 30 degrees C; early death; haemorrhages) during infection depended on the presence of the spleen and was prevented by irradiation of the spleen or a timely treatment with dexamethasone, anti-T-cell serum or immune serum.

Stage-specific proteins of Plasmodium berghei-infected red blood cells detected by antibodies of immune mouse serum.

The asynchronously developing malaria parasite Plasmodium berghei was synchronized using in vitro cultivation techniques (Mons et al. 1985). After the infection of naive mice, preparations of parasitized erythrocytes with a high level of synchronism could be obtained. Immunofluorescence and immunoblotting techniques using serum from immunized mice were applied to determine stage-specific immunogenic molecules in the parasitized erythrocyte preparations. These techniques allowed the detection of not only parasite-derived but also altered-self molecules. Membrane fluorescence of infected erythrocytes was detected only in preparations containing late trophozoites and schizonts. The appearance of this fluorescence pattern coincided with the presence of immunogenic polypeptides of mol. wt. greater than 200 kD, 86 kD, and 56 kD especially, among some other polypeptides. Preliminary experiments using lactoperoxidase-catalyzed radioiodination suggested that the greater than 200 kD and 56 kD molecules were present at the erythrocyte surface. One molecule with mol. wt. 153 kD was associated with the presence of ring-infected erythrocytes. However, membrane fluorescence of ring-stage-infected erythrocytes was not found. Noninfected erythrocytes sometimes showed membrane fluorescence.

Plasmodium berghei: relative immunogenicity of infected reticulocytes and infected oxyphilic red blood cells.

Hyperbleeding of mice 1 day before and 1 day after infection with Plasmodium berghei resulted in a more aggravated infection. Parasitemia rose significantly faster, but the mean survival time of these mice was not significantly different from control mice. At Day 5 of infection, parasites were almost exclusively in reticulocytes in contrast to control infections in which parasites were found in oxyphilic erythrocytes at Day 5 after infection. Purified parasitized reticulocytes taken from hyperbled mice at Day 5 after infection contained more young developmental parasite stages than purified parasitized oxyphilic erythrocytes taken from normal mice at Day 5 to 7 after infection. Parasitized reticulocytes were more readily opsonized by antibodies from immune serum when compared to parasitized oxyphilic red blood cells and when used to stimulate immune spleen cells the former were better stimulator cells than the latter. Results suggest either that parasitized reticulocytes are more immunogenic then parasitized oxyphilic red blood cells or that suspensions of parasitized reticulocytes contain more immunogenic parasite stages than suspensions of parasitized oxyphilic red blood cells.

Plasmodium berghei: reduction of the mouse's specific lymphoproliferative response in relation to corticosterone and pregnancy.

Spleen cells from mice immune to Plasmodium berghei exhibited a significantly increased in vitro proliferative response to parasitized reticulocytes compared to spleen cells from normal mice. The specific response to malaria antigen was decreased in spleen cells from pregnant immune mice in contrast to the nonspecific response to the mitogen phytohemagglutinin. Addition of mouse serum to spleen cell cultures of immune mice depressed both the phytohemagglutinin and the specific proliferative response, whereas serum of pregnant mice exerted an even stronger inhibition than serum of nonpregnant mice. Charcoal adsorption of mouse sera for the elimination of steroid hormones removed the serum dependent immunosuppression from normal as well as pregnant serum. Corticosterone added to the spleen cell cultures depressed also the proliferative response. These findings demonstrate that the response to malaria antigen is decreased in immune mice during pregnancy. The possible effect of serum corticosterone on the depression of the immune response is discussed.

Pregnancy-induced recrudescences strengthen malarial immunity in mice infected with Plasmodium berghei.

A considerable proportion of mice lose acquired immunity to Plasmodium berghei during the first pregnancy. Immune parous mice, however, have a better immune status than virgin mice, the risk of loss of immunity during a subsequent pregnancy is greatly reduced, the capacity to clear parasites is enhanced, and the maintenance of immunity is less dependent on certain splenic functions. The establishment of improved immunity is dependent on the presence of proliferating parasites during the second half of pregnancy when immunosuppression results in recrudescence. Immune reactivity is also improved after a (chemotherapeutically controlled) recrudescent infection provoked by immunosuppressive treatment of immune mice with corticoids or anti-T cell serum. This mimics the situation encountered during pregnancy. Hence, improved immunity after pregnancy is a consequence of a reconfrontation of a suppressed and/or convalescent immune system with proliferating parasites.

Interactions of Plasmodium berghei-infected erythrocytes with complement.

Incubation of radioactively labeled parasitized (Plasmodium berghei) erythrocytes (PE) with adherent peritoneal exudate cells in the presence of 10% (v/v) fresh mouse serum (NMS) resulted in the uptake of a proportion of radioactive material (PE). Inactivation of the added serum by heat or zymosan treatment resulted in diminished uptake of radioactivity. These results suggest that PE activated complement. Incubation of fresh NMS with PE reduced the hemolytic complement level of the serum as shown by its subsequent decreased ability to lyse antibody-coated rabbit red blood cells. No such effect was found when uninfected erythrocytes from either infected or uninfected blood were preincubated with fresh NMS. Thus, PE or PE-derived material activated complement. Addition of EGTA during incubation of fresh NMS with PE did not inhibit the decrease in complement level. This indicated that complement was activated by the alternative pathway. Complement levels decreased even when fresh NMS and PE were incubated in the presence of EDTA (which inhibits both classical and alternative pathway activation), suggesting that a complement activating factor (or a complement inhibitor) was released from the PE. However, lysis of PE after incubation with either fresh rabbit or guinea pig serum did not occur unless anti-mouse erythrocyte antibody was added. The production of a complement-activating factor by PE might explain part of the decreasing complement levels during infection and might enable the parasite to escape from a complement-mediated defense mechanism of the host.

ACTH-dependent modulation of malaria immunity in mice.

Tetracosactrin, a synthetic adrenocorticotrophic hormone (ACTH) analogue delivered by osmotic minipumps implanted s.c. in mice induced a dose-dependent increase of plasma corticosterone levels. In mice with an established immunity to Plasmodium berghei the increase of the plasma corticosterone level due to tetracosactrin treatment correlated with loss of immunity against this malaria parasite. The observed plasma corticosterone levels associated with loss of malaria immunity were of the same order as those in mice that lost their immunity during pregnancy. Adrenalectomy before administration of the ACTH analogue prevented both the increase of plasma corticosterone and loss of malaria immunity. Adrenalectomized mice still lost their malaria immunity when treated with the synthetic corticoid dexamethasone. The effector function of malaria immunity is sensitive to corticoids, and, at least during pregnancy, the naturally occurring serum corticosterone level appears to be an important regulator of malaria immunity.

Malarial immunity in pregnant mice, in relation to total and unbound plasma corticosterone.

A pregnancy dependent loss of malarial immunity is accompanied by an (excessive) increase of total as well as free plasma corticosterone. This loss of immunity was largely prevented by adrenalectomy. Moreover, malarial immunity was more sensitive to dexamethasone immunosuppression during pregnancy. Primary infections are more virulent during pregnancy and like in recrudescent mice, cause excessive total and free plasma corticosterone levels. Corticosterone may be considered an immuno-regulatory serum factor during pregnancy, the endocrine regulation of which is disturbed in pregnant, infected mice.

Corticosterone regulation of the effector function of malarial immunity during pregnancy.

In the experimental Plasmodium berghei mouse model, as in human malaria, reduced maternal responsiveness and even loss of immunity were observed during pregnancy. Loss of immunity in the second half of pregnancy occurred during a period of elevated plasma corticoid levels. Further analysis showed that plasma corticoid levels were significantly higher in immunodepressed mice than in mice that remained immune throughout pregnancy. Plasma corticosterone levels differed increasingly from those in mice with persistent immunity towards recrudescence. In nonimmune infected controls, however, only a slight increase in plasma corticosterone, already present during the subpatent period, was measured. Blocking the maternal corticoid production by adrenalectomy delayed the increase of plasma corticosterone (fetoplacental origin) and reduced the number of mice that lost immunity during pregnancy considerably. The role of various plasma corticoid levels in the regulation of effector function of immunity during pregnancy is discussed.

Pregnancy associated recrudescence in murine malaria (Plasmodium berghei).

Depending on the strain, a variable proportion of mice solidly immune to the rodent malaria parasite Plasmodium berghei developed a recrudescence during pregnancy that was either transient or lethal. Recrudescence was not observed in all mice, and the rate was lower in gravida II as compared to gravida I mice. On the other hand a proportion of the mice that did not develop recrudescence exhibited a pregnancy associated clearance of persisting parasites in immune mice (premunition-sterile immunity), being more pronounced in gravida II than gravida I mice. Development of the mechanism of enhanced clearance is apparently parasite dependent. Enhanced clearance was manifest until day 4 of pregnancy. Pregnancy associated immunodepression was observed most strongly between day 4 and 16 of pregnancy, when the highest rates of recrudescence and associated mortality were found. Immunodepression apparently disappears at the end of pregnancy or shortly after parturition.

Depressed malarial immunity in pregnant mice.

A proportion of mice solidly immune to the rodent malaria parasite Plasmodium berghei exhibited a pregnancy-associated depressed immunity with a transient or even lethal recrudescence.