Reduced cellular immune reactivity in healthy individuals during the malaria transmission season.

Antigen-induced cellular immune responses are suppressed during acute malaria. The present study engages the possibility that malaria-induced alterations in cellular immune reactivity extend beyond the clinical disease. Thus, lymphoproliferative responses of healthy individuals were diminished during the malaria transmission period in individuals living in an area of highly seasonal, unstable malaria transmission. This finding may have important implications for the design of studies of stimulatory properties of antigens using lymphocytes of endemic origin.

Cell-mediated immune responses to Plasmodium falciparum purified soluble antigens in sickle-cell trait subjects.

To determine the possible differences in the immune response to Plasmodium falciparum between sickle-cell trait (Hb AS) and normal haemoglobin (Hb AA) individuals, we examined 35 Hb AS and 24 Hb AA subjects matched for age and microenvironment. Their age was 2-55 years and all lived in a malaria endemic area 300 km south of Khartoum. Antibodies to ring-infected erythrocyte surface antigen (Pf155/RESA) and to circumsporozoite (CS) protein (anti-NANP40) indicated equal exposure to falciparum malaria. Peripheral blood mononuclear cells (BMNCs) from 20/35 (57%) Hb AS subjects compared with 10/24 (42%) Hb AA subjects, responded to affinity-purified P. falciparum soluble antigens (SPAg). Of those responding to SPAg, 9 (26%) Hb AS subjects and only two (8%) Hb AA subjects had high responses. The mean proliferative response to SPAg of BMNCs from Hb AS individuals was significantly higher than in Hb AA individuals (P less than 0.025). Responses of BMNCs to PPD and PHA were also higher among Hb AS individuals and correlated positively with responses to SPAg. These findings support the hypotheses that the sickle-cell trait protects individuals from P. falciparum infections, at least in part, by modulating the immune response.

T-cell responses in malaria.

Malaria is caused by infection with protozoan parasites of the genus Plasmodium. It remains one of the most severe health problems in tropical regions of the world, and the rapid spread of resistance to drugs and insecticides has stimulated intensive research aimed at the development of a malaria vaccine. Despite this, no efficient operative vaccine is currently available. A large amount of information on T-cell responses to malaria antigens has been accumulated, concerning antigens derived from all stages of the parasite life cycle. The present review summarizes some of that information, and discusses factors affecting the responses of T cells to malaria antigens.

Cell-mediated immune responses to soluble Plasmodium falciparum antigens in residents from an area of unstable malaria transmission in the Sudan.

This paper describes immune responses to P. falciparum infection in individuals living in an area of highly seasonal, unstable malaria transmission. The in vitro cellular immune responses of peripheral blood mononuclear cells (PBMC) from 36 Sudanese donors to a complex of affinity purified soluble P. falciparum antigens (SPag) and two components thereof (Ag1 and Ag7) were examined and compared to humoral immune parameters. In 29/36 Sudanese donors, SPag induced a significant lymphoproliferative response in vitro. In contrast only 3/27 Danish donors never exposed to malaria responded to SPag. Ag1 and Ag7 induced significant lymphoproliferation in 9/34 and 13/36 Sudanese donors respectively, whereas no Danish donors responded. Significant interferon-gamma production was observed in 16/27, 1/5 and 3/12 Sudanese donors when stimulated by SPag, Ag1 and Ag7 respectively. Lymphoproliferative responses to SPag correlated with proliferative responses to Ag1 and Ag7, and with Spag-induced interferon-gamma production. These results indicate that T-cell clones recognizing epitopes on Ag1 and Ag7 have been expanded in the studied population as a result of exposure to P. falciparum infection. The T-cell parameters did not correlate with the presence of antibodies to SPag, Pf155/RESA or a crude parasite sonicate or with the schizont IFA titers of the plasma. This indicates that parameters outside the degree of exposure to P. falciparum influence the cellular immune responses to malaria.

Immunoglobulin G reactivities to rhoptry-associated protein-1 associated with decreased levels of Plasmodium falciparum parasitemia in Tanzanian children.

In the Muheza region of Tanzania, an area with holoendemic malaria, the proportion of responders with IgG enzyme-linked immunosorbent assay reactivities to recombinant rhoptry-associated protein-1 (rRAP-1) as well as IgG reactivities to a repeat region of the acidic-basic repeat antigen (ABRA) increased with age. The proportion of responders with IgM reactivities to rRAP-1 increased with age in the first three decades. However, levels of IgG reactivities to rRAP-1 did not increase with age, indicating high levels of reactivities among young children. High P. falciparum densities were only detectable in children less than five years of age; in this group the proportion of IgG responders to rRAP-1 and to the ABRA repeat region was low but levels of IgG reactivities to rRAP-1 were inversely correlated with parasite density, suggesting that immune recognition of the antigen may be associated with resistance to infection. On the other hand, levels of IgG reactivities to the repeat region of ABRA increased with parasite densities in children 1-4 years of age. Two different profiles of IgG reactivities to rRAP-1 and to ABRA are detectable in young Tanzanian children and the Ig reactivities against rRAP-1 may be a component of the immune reactions restricting parasite multiplication.

Seasonal changes in cell mediated immune responses to soluble Plasmodium falciparum antigens in children with haemoglobin AA and haemoglobin AS.

In this longitudinal study peripheral blood mononuclear cells (PBMC) were obtained before and during the malaria season from healthy HbAA and HbAS children. Cells were compared for proliferation in response to stimulation by soluble Plasmodium falciparum antigens (SPAg) or purified derivative of tuberculin (PPD). The lymphoproliferative responses to SPAg of the paired PBMC samples showed 2 distinct seasonal changes in relation to the haemoglobin phenotype. In HbAA children, the lymphoproliferative responses to SPAg were suppressed during the malaria season. In contrast, they were enhanced in HbAS children during the malaria season. No distinct seasonal change in the response to PPD was found in relation to the haemoglobin phenotype. The study points to the role of the sickle cell trait in modulating the cellular immune responses to falciparum malaria.

Sickle cell disease in Sudan.

The clinical, haematological and biochemical features of 50 Sudanese patients with sickle cell disease (SCD) were determined. Of 23 patients with complete family data, 21 had sickle cell anaemia (homozygous HbSS), 2 had sickle-cell/beta+thalassaemia but none had sickle cell/beta Othalassaemia. The remaining 27 patients had HbSS phenotype. 84% of patients were from the Baggara tribe in western Sudan, where HbS is a natural extension of the west African HbS belt. 21 patients were children under 2 years old; 19 were 3-10 years old; and the remaining 10 were over 10 years old. Young patients presented mainly with painful vaso-occlusive crisis, severe anaemia, hand and foot syndrome, fever, underweight, malnutrition and various infectious diseases. All patients had mild to moderate cardiac enlargment; 42% had a moderately enlarged spleen but only 10% had an enlarged liver; 20% had infarctive lesions of long bones and another 8% had Salmonella osteomyelitis. Leg ulcers, priapism, enuresis and cholelithiasis were not observed. Patients had a mean haemoglobin concentration of 7.3 g/dl; reticulocyte count of 15.1%; serum bilirubin of 2.1 mg/dl; HbA2 level of 2.8% and HbF of 7%. Thus, the observed pattern of SCD in Sudan is comparable to the severe type described for Africans and not comparable to the benign form found in Shiite Moslem Arabs of Saudi Arabia. 6 adults with mild SCD had HbF levels below 5%. Amelioration of the disease, therefore, does not seem to be related to HbF levels; nor was it possible to relate it to high levels of erythrocyte 2,3-diphosphoglycerate.

Serological evidence for remarkably variable prevalence rates of Toxoplasma gondii in children of major residential areas in United Arab Emirates.

The major objective of this study was to determine the seroprevalence rate of Toxoplasma gondii in children citizens of United Arab Emirates (UAE) main residential areas. Questionnaire information, clinical data and blood samples were obtained from 1006 primary school children residence of seven out of nine districts of UAE. ELISA was used for detection of antibodies against the immunodominant surface antigen (SAG1) of T. gondii. The sensitivity and specificity of the employed ELISA were 98.4 and 99.1%, respectively using 'Eiken' latex agglutination test as a reference test. The seroprevalence rates were remarkably variable in different residential areas and ranged between 3.5% for Dubai and 34.6% for Sharjah, with an overall prevalence of 12.5% for the seven districts. Rear of ruminants at home and consumption of raw milk associated significantly (P<0.05) with exposure to T. gondii. UAE children exposed to T. gondii infection had a significantly higher hepatomegaly rate (P<0.05) and complained more of various symptoms at the time of sampling (P<0.01) compared to unexposed children. This study urges for more population studies to further elucidate the prevalence rates of toxoplasmosis in UAE in relation to age, gender, place of residence and risk factors.

No effect of human serum and erythrocytes enriched in n-3 fatty acids by oral intake on Plasmodium falciparum blood stage parasites in vitro.

To examine the effect of n-3 polyunsaturated fatty acids (n-3 PUFA) on the erythrocytic growth of Plasmodium falciparum, serum and erythrocytes were separated from blood of a healthy donor before and after he had taken fish oil capsules for 8 days. Such intake supplied an amount of eicosapentaenoic acid (EPA, 20:5n-3) of 3.5 g/d and docosahexaenoic acid (DHA, 22:6n-3) of 2.5 g/d and 24 mg/d of total tocopherol. Post-intake fish oil serum (post-s) and erythrocytes (post-e) were tested in vitro for inhibitory activity against blood stages of P. falciparum compared with pre-intake serum (pre-s) and pre-intake erythrocyte (pre-e). Also the effect of EPA and arachidonic acid (AA, 20:4n-6) on the erythrocytic growth of P. falciparum was tested using in vitro assays. The results show that both post-s and post-e had no antimalarial activity on P. falciparum. No differential antimalarial effect was observed for 20:5n-3 compared with 20:4n-6, which at high concentrations (> 40 microM) had an anti-schizont-growth effect.

Transient depletion of T cells with high LFA-1 expression from peripheral circulation during acute Plasmodium falciparum malaria.

Acute P. falciparum malaria is associated with loss of in vitro T cell responsiveness to antigenic stimulation, and with high plasma levels of soluble interleukin 2 receptor (IL 2R). In the present study peripheral T cells from acute P. falciparum malaria patients from a malaria-endemic area of Sudan were analyzed for expression of cell surface antigens associated with T lymphocyte adhesion, activation and maturation. The results were compared to results from T cells obtained from the same donors either before the attack, or during convalescence. Most donors showed a remarkable loss of T cells with high expression of the surface marker LFA-1 (CD11a/CD18) during the clinical episode, in addition to the functional changes described above. Two donors that did not show phenotypic changes were furthermore characterized by having an unabated proliferative response and normal plasma IL 2R levels. All peripheral CD3+ T lymphocytes expressed LFA-1, which had a clearly bimodal distribution on these cells. The T cell subpopulation having high LFA-1 expression (LFA-1++) was composed of both memory and unprimed T cells, according to their expression of CD45RA and CD45R0. Analysis of expression of membrane-bound IL 2R (CD25) and ICAM-1 (CD54) did not reveal in vivo activated T cells in the peripheral blood of the patients. Taken together, these data suggest that circulating T cells recognizing parasite antigens are temporarily withdrawn from peripheral circulation during P. falciparum malaria.

Modulation of the cellular immune response during Plasmodium falciparum infections in sickle cell trait individuals.

Plasma and peripheral blood mononuclear cells (PBMC) were obtained from P. falciparum-infected individuals with and without the sickle cell trait at diagnosis and 7 days after treatment. HbAA and HbAS patients were compared for levels of plasma soluble IL-2 receptors (IL-2R) and the in vitro cellular reactivity to affinity-purified soluble P. falciparum antigens (SPAg), PPD and phytohaemagglutinin (PHA). At diagnosis, HbAS patients with clinical disease had lower plasma-soluble IL-2R levels and parasite counts than the corresponding HbAA patients, whereas HbAS and HbAA patients with asymptomatic infections had comparable soluble IL-2R levels and parasite counts. PBMC from HbAS patients had higher proliferation and IFN-gamma production in response to SPAg than PBMC from HbAA patients. The difference in the lymphoproliferative responses to SPAg between the two groups was evident in patients with asymptomatic infections. In all patients, the clinical severity, the soluble IL-2R levels and the parasite counts were directly related. The former two were inversely related to the proliferative responses to SPAg. After treatment, all the studied parameters were comparable in the two groups. The study indicates that during P. falciparum infection, HbAS compared with HbAA patients had lower in vivo cellular activation and higher in vitro cellular reactivity in response to soluble malaria antigens.

Lymphoproliferative responses to Plasmodium falciparum antigens in children with and without the sickle cell trait.

Blood mononuclear cells (BMNC) were isolated from sickle cell trait (HbAS) healthy donors and normal haemoglobin (HbAA) healthy donors resident in a P. falciparum endemic area of eastern Sudan. Blood samples were collected during the malaria season. BMNC were tested for their proliferative responses to phytohaemagglutinin (PHA), purified protein derivative of tuberculin (PPD) and to soluble P. falciparum antigens (SPAg). Higher responses to SPAg and PPD were observed in the HbAS children compared with the HbAA children, whereas no differences were observed among adults of the two phenotypes. Proliferative responses to PHA were comparable in all individuals tested. The significance of these findings in relation to haemoglobin phenotype, age and the possible immunoregulatory mechanisms operating in HbAS and HbAA children during the malaria season is discussed.

Loss of cellular immune reactivity during acute Plasmodium falciparum malaria.

Sixteen patients suffering from acute Plasmodium falciparum malaria were studied. All were residents of an area of unstable malaria-transmission in Eastern Sudan. Blood-samples were drawn at diagnosis, and 7 and 30 days later. Blood-samples from thirteen donors, drawn outside the malaria transmission season 5 months prior to the attack, were included in the study. Lymphoproliferative responsiveness to purified soluble malarial antigens and to the unrelated antigen PPD was lost during the acute phase of the disease in most donors, but was regained during convalescence, except in four donors recrudescing or reinfected by day 30. In contrast to the suppression of antigenic responses, cellular responses to phytohaemagglutinin (PHA) remained virtually unaffected. All donors showed elevated plasma-levels of soluble IL-2 receptor during the acute phase of the disease which normalized during convalescence. Five donors examined by fluorescence-activated cell sorting (FACS) showed no increase in surface expression of IL-2 receptor on peripheral lymphocytes. The data indicate that acute P. falciparum malaria causes a depletion of antigen-reactive T-cells from the peripheral circulation, probably due to homing of this cell-population to lymphoid tissues. It was also found that acute-phase plasma was suppressive to PPD-induced proliferative responses, indicating an additional suppressive mechanism operating in vivo.