Genetic algorithms identify individuals with high risk of severe liver disease caused by schistosomes

Schistosomes induce severe hepatic disease, which is fatal in 2-10% of cases, mortality being higher in cases of co-infection with HBV or HCV. Hepatic disease occurs as a consequence of the chronic inflammation caused by schistosome eggs trapped in liver sinusoids. In certain individuals, the repair process leads to a massive accumulation of fibrosis in the periportal spaces. We and others have shown that genetic variants play a crucial role in disease progression from mild to severe fibrosis and explain why hepatic fibrosis progresses rapidly in certain subjects only. We will review here published findings concerning the strategies that have been used in the analysis of hepatic fibrosis in schistosome-infected individuals, the genetic variants that have associated with fibrosis, and variants in new pathways crucial for fibrosis progression. Together, these studies show that the development of fibrosis is under the tight genetic control of various common variants with moderate effects. This polygenic control has made it possible to develop models that identify schistosome-infected individual at risk of severe hepatic disease. We discuss the performances and limitations of these models.

Low plasma haptoglobin is a risk factor for life-threatening childhood severe malarial anemia and not an exclusive consequence of hemolysis

Severe Malarial Anemia (SMA), a life-threatening childhood Plasmodium falciparum malaria syndrome requiring urgent blood transfusion, exhibits inflammatory and hemolytic pathology. Differentiating between hypo-haptoglobinemia due to hemolysis or that of genetic origin is key to understand SMA pathogenesis. We hypothesized that while malaria-induced hypo-haptoglobinemia should reverse at recovery, that of genetic etiology should not. We carried-out a case-control study of children living under hyper-endemic holoendemic malaria burden in the sub-Saharan metropolis of Ibadan, Nigeria. We show that hypo-haptoglobinemia is a risk factor for childhood SMA and not solely due to intravascular hemolysis from underlying schizogony. In children presenting with SMA, hypo-haptoglobinemia remains through convalescence to recovery suggesting a genetic cause. We identified a haptoglobin gene variant, rs12162087 (g.-1203G > A, frequency = 0.67), to be associated with plasma haptoglobin levels (p = 8.5 × 10^-6). The Homo-Var:(AA) is associated with high plasma haptoglobin while the reference Homo-Ref:(GG) is associated with hypo-haptoglobinemia (p = 2.3 × 10^-6). The variant is associated with SMA, with the most support for a risk effect for Homo-Ref genotype. Our insights on regulatory haptoglobin genotypes and hypo-haptoglobinemia suggest that haptoglobin screening could be part of risk-assessment algorithms to prevent rapid disease progression towards SMA in regions with no-access to urgent blood transfusion where SMA accounts for high childhood mortality rates.

Liver ultrasound elastography for the evaluation of periportal fibrosis in schistosomiasis mansoni: A cross-sectional study

BACKGROUND

ARFI elastrography has been used as a noninvasive method to assess the severity of liver fibrosis in viral hepatitis, although with few studies in schistosomiasis mansoni. We aimed to evaluate the performance of point shear wave elastography (pSWE) for predicting significant periportal fibrosis (PPF) in schistosomotic patients and to determine its best cutoff point.

METHODOLOGY/PRINCIPAL FINDINGS

This cross-sectional study included 358 adult schistosomotic patients subjected to US and pSWE on the right lobe. Two hundred two patients (62.0%) were women, with a median age of 54 (ranging 18-92) years. The pSWE measurements were compared to the US patterns of PPF, as gold standard, according to the Niamey classification. The performance of pSWE was calculated as the area under the ROC curve (AUC). Patients were further classified into two groups: 86 patients with mild PPF and 272 patients with significant PPF. The median pSWE of the significant fibrosis group was higher (1.40 m/s) than that of mild fibrosis group (1.14 m/s, p<0.001). AUC was 0.719 with ≤1.11 m/s as the best cutoff value for excluding significant PPF. Sensitivity and negative predictive values were 80.5% and 40.5%, respectively. Whereas, for confirming significant PPF, the best cutoff value was >1.39 m/s, with specificity of 86.1% and positive predictive value of 92.0%.

CONCLUSIONS/SIGNIFICANCE

pSWE was able to differentiate significant from mild PPF, with better performance to predict significant PPF.

A Functional IL22 Polymorphism (rs2227473) Is Associated with Predisposition to Childhood Cerebral Malaria

Cerebral malaria (CM) is a severe complication of Plasmodium falciparum infection. This encephalopathy is characterized by coma and is thought to result from mechanical microvessel obstruction and an excessive activation of immune cells leading to pathological inflammation and blood-brain barrier alterations. IL-22 contributes to both chronic inflammatory and infectious diseases, and may have protective or pathogenic effects, depending on the tissue and disease state. We evaluated whether polymorphisms (n = 46) of IL22 and IL22RA2 were associated with CM in children from Nigeria and Mali. Two SNPs of IL22, rs1012356 (P = 0.016, OR = 2.12) and rs2227476 (P = 0.007, OR = 2.08) were independently associated with CM in a sample of 115 Nigerian children with CM and 160 controls. The association with rs2227476 (P = 0.01) was replicated in 240 nuclear families with one affected child from Mali. SNP rs2227473, in linkage disequilibrium with rs2227476, was also associated with CM in the combined cohort for these two populations, (P = 0.004, OR = 1.55). SNP rs2227473 is located within a putative binding site for the aryl hydrocarbon receptor, a master regulator of IL-22 production. Individuals carrying the aggravating T allele of rs2227473 produced significantly more IL-22 than those without this allele. Overall, these findings suggest that IL-22 is involved in the pathogenesis of CM.

FOXP3+ Regulatory T Cells in Hepatic Fibrosis and Splenomegaly Caused by Schistosoma japonicum: The Spleen May Be a Major Source of Tregs in Subjects with Splenomegaly

Schistosoma eggs cause chronic liver inflammation and a complex disease characterized by hepatic fibrosis (HF) and splenomegaly (SplM). FOXP3+ Tregs could regulate inflammation, but it is unclear where these cells are produced and what roles they play in human schistosomiasis. We investigated blood and spleen FOXP3+ Tregs in Chinese fishermen with lifelong exposure to Schistosoma japonicum and various degrees of liver and spleen disease. FOXP3+ Tregs accounted for 4.3% of CD4+ T cells and 41.2% of FOXP3+CD4+ T cells; they could be divided into CD45RA-FOXP3hi effector (eTregs) and CD45RA+FOXP3low naive Tregs. Blood Treg levels were high in severe HF (+1.3; p = 0.004) and in SplM (+1.03, p = 0.03). Multivariate regression showed that severe HF (+0.85, p = 0.01) and SplM (+0.97; p = 0.05) were independently associated with the higher proportion of Tregs in the blood. This effect was mostly due to an increase in the proportion of eTregs in the blood of HF+++ (+0.9%; p = 0.04) and SplM (+0.9%; p = 0.04) patients. The proportion of eTregs expressing CXCR3 in the blood was lower in the HF+++ patients (37.4 +/- 5.9%) than in those with milder fibrosis (51.7 ± 2%; p = 0.009), whereas proportion were similar for cells expressing CD25hi, CCR7, and CTLA-4. Splenectomy improves symptoms and was associated with decreases in blood FOXP3+ Treg (-2.5; p<0.001) and eTreg (-1.3; p = 0.03) levels. SplM spleens contained a high proportion of eTregs with CXCR3, CCR5 and CTLA4 upregulation and CCR7 downregulation. This, and the strong expression of ligands of CXCR3 and CCR5 in the liver (n = 8) but not in the spleen suggested that spleen eTregs migrated to Th1-infiltrated liver tissues. Such migration may be attenuated in hepatosplenic patients due to lower levels of CXCR3 expression on Tregs (p = 0.009). Thus, higher blood Treg levels are associated with severe liver disease and splenomegaly. Our data are consistent with the hypothesis that the spleen is a major source of Tregs in subjects with splenomegaly. In most cases, Tregs migrate to the Th1-infiltrated liver and the lower levels of CXCR3+ Tregs in the blood of patients with severe schistosomiasis suggest that decreases in Treg migration sites of inflammation may aggravate the disease.

IL-22 and IL-22 binding protein (IL-22BP) regulate fibrosis and cirrhosis in hepatitis C virus and schistosome infections (CCR4P.202)

IL-22 acts on epithelia, hepatocytes and pancreatic cells and stimulates innate immunity, tissue protection and repair. IL-22 may also cause inflammation and abnormal cell proliferation. The binding of IL-22 to its receptor is competed by IL-22BP. We addressed the question of the role of IL-22 and IL-22BP in hepatic fibrosis caused by schistosomes or by HCV. We found that schistosome eggs stimulate the production of IL-22 transcripts and inhibit the accumulation of IL22-BP transcripts in schistosome-infected mice and that schistosome eggs selectively stimulate the production of IL-22 by blood leukocytes from individuals chronically infected with S.japonicum. High IL-22 levels in cultures correlated with protection against hepatic fibrosis and portal hypertension. To test further the implication of IL-22/IL-22BP in hepatic disease, we analyzed common genetic variants of IL22RA2, which encodes IL-22BP, and found that the genotypes AA, GG of rs6570136 (p=0.003; OR=2), and CC, TT of rs2064501 (p=0.01; OR=2), were associated with severe fibrosis in Chinese infected with S.japonicum. We confirmed this result in Sudanese and Brazilians infected with S.mansoni. The aggravating genotypes were associated with high IL22RA2 transcripts levels. These same variants were also associated with HCV-induced fibrosis and cirrhosis. These results provide strong evidence that IL-22 protects against and IL-22BP aggravates liver fibrosis and cirrhosis in humans with chronic liver infections.

IL-22 and IL-22 binding protein (IL-22BP) regulate fibrosis and cirrhosis in hepatitis C virus and schistosome infections

Interleukin (IL)-22 acts on epithelia, hepatocytes, and pancreatic cells and stimulates innate immunity, tissue protection, and repair. IL-22 may also cause inflammation and abnormal cell proliferation. The binding of IL-22 to its receptor is competed by IL-22 binding protein (IL-22BP), which may limit the deleterious effects of IL-22. The role of IL-22 and IL-22BP in chronic liver diseases is unknown. We addressed this question in individuals chronically infected with schistosomes or hepatitis C virus (HCV). We first demonstrate that schistosome eggs stimulate production of IL-22 transcripts and inhibit accumulation of IL22-BP transcripts in schistosome-infected mice, and that schistosome eggs selectively stimulate production of IL-22 in cultures of blood leukocytes from individuals chronically infected with Schistosoma japonicum. High IL-22 levels in cultures correlated with protection against hepatic fibrosis and portal hypertension. To test further the implication of IL-22/IL-22BP in hepatic disease, we analyzed common genetic variants of IL22RA2, which encodes IL-22BP, and found that the genotypes, AA, GG of rs6570136 (P = 0.003; odds ratio [OR] = 2), and CC, TT of rs2064501 (P = 0.01; OR = 2), were associated with severe fibrosis in Chinese infected with S. japonicum. We confirmed this result in Sudanese (rs6570136 GG [P = 0.0007; OR = 8.2], rs2064501 TT [P = 0.02; OR = 3.1]), and Brazilians (rs6570136 GG [P = 0.003; OR = 26], rs2064501 TC, TT (P = 0.03; OR = 11]) infected with S. mansoni. The aggravating genotypes were associated with high IL22RA2 transcripts levels. Furthermore, these same variants were also associated with HCV-induced fibrosis and cirrhosis (rs6570136 GG, GA [P = 0.007; OR = 1.7], rs2064501 TT, TC (P = 0.004; OR = 2.4]).

CONCLUSIONS

These results provide strong evidence that IL-22 protects against and IL-22BP aggravates liver fibrosis and cirrhosis in humans with chronic liver infections. Thus, pharmacological modulation of IL-22 BP may be an effective strategy to limit cirrhosis.

IL2RA genetic variants reduce IL-2-dependent responses and aggravate human cutaneous leishmaniasis

The outcome of Leishmania infections varies substantially, depending on the host and the parasite strain; infection may be asymptomatic or cause mild or severe skin ulcers (cutaneous leishmaniasis [CL]), limited or disseminated lesions, or lethal visceral disease. We previously reported an association between IL-2R mutations and susceptibility to visceral leishmaniasis in children infected with Leishmania donovani. In the present study, we evaluated the possible role of IL-2 signaling in human CL. We first showed that the transcripts of several genes of the IL-2 pathway were abundant in skin lesions caused by Leishmania braziliensis. We then carried out a genetic analysis, focusing on major genes of the IL-2 pathway. We used a family-based approach and found that polymorphisms of several genes appeared to be associated with CL in a Brazilian population. Moreover, two polymorphisms of the IL2RA gene were significantly and independently associated with CL. We confirmed this result in a second Brazilian sample (also exposed to L. braziliensis) and in Iranians infected with Leishmania tropica: IL2RA rs10905669 T (Pcombined = 6 × 10(-7)) and IL2RA rs706778 T (Pcombined = 2 × 10(-9)) were associated with greater susceptibility to lesion development. These alleles were also correlated with a poor IFN-γ response and poor FOXP3(+) regulatory T cell activation. Thus, IL-2 plays a crucial role in protection against the cutaneous ulcers caused by Leishmania, and the IL-2 pathway is a potential target for strategies aiming to control Leishmania-related diseases.

Genetic analysis of human predisposition to hepatosplenic disease caused by schistosomes reveals the crucial role of connective tissue growth factor in rapid progression to severe hepatic fibrosis

Schistosome worms inhabit mammalian mesenteric veins. Their eggs cause chronic inflammation, which progresses to periportal fibrosis in 5 to 30% of cases, increasing portal blood pressure and leading to esophageal varices. Episodes of bleeding cause hepatic necrosis and may ultimately lead to hepatic failure and the death of the patient. Schistosome infections can also cause pulmonary hypertension and heart failure. The mechanisms of fibrogenesis and fibrolysis are beginning to be unraveled, but it remains unclear why disease occurs only in certain subjects, as also observed for other types of chronic liver inflammation, as in hepatitis C or B. We summarize here the results that showed that fibrosis progression is determined by a genetic locus on chromosome 6. The CCN2 gene at this locus, encodes CTGF that is a crucial regulator of fibrosis. Two groups of CCN2 polymorphisms independently modulate the progression of hepatic fibrosis. These results were obtained in an Asian population, but were extended to humans living in Africa and South America and are presently tested in liver fibrosis of other etiological origins.

Cytokine regulation of periportal fibrosis in humans infected with Schistosoma mansoni: IFN-gamma is associated with protection against fibrosis and TNF-alpha with aggravation of disease

Hepatic periportal fibrosis, which affects 5-10% of subjects infected by Schistosoma mansoni, is caused by the T cell-dependent granuloma that develop around schistosome eggs. Experimental models of infection have shown that granuloma and fibrosis are tightly regulated by cytokines. However, it is unknown why advanced periportal fibrosis occurs only in certain subjects. The goal of the present study was to evaluate the cytokine response of S. mansoni-infected subjects with advanced liver disease in an attempt to relate susceptibility to periportal fibrosis with an abnormal production of cytokines that regulate granuloma and fibrosis. Fibrosis was evaluated by ultrasound on 795 inhabitants of a Sudanese village in which S. mansoni is endemic: advanced periportal fibrosis was observed in 12% of the population; 35% of the affected subjects exhibited signs of portal hypertension. Age (odds ratio (OR), 11.5), gender (OR, 4.2), and infection levels (OR, 2.2) were significantly (p < or = 0.01) associated with hepatic fibrosis. Cytokines produced by egg-stimulated blood mononuclear cells from 99 subjects were measured (75 with no or mild fibrosis; 24 subjects with advanced fibrosis). Multivariate analysis of cytokine levels showed that high IFN-gamma levels were associated with a marked reduction of the risk of fibrosis (p = 0.01; OR, 0.1); in contrast, high TNF-alpha levels were associated with an increased risk (p = 0.05; OR, 4.6) of periportal fibrosis. Moreover, infection levels were negatively associated with IFN-gamma production. These results with observations in experimental models strongly suggest that IFN-gamma plays a key role in the protection of S. mansoni-infected patients against periportal fibrosis, whereas TNF-alpha may aggravate the disease.

Genetics of parasitic infections

Parasites cause much suffering mainly in countries of the southern hemisphere. Hundreds of millions of individuals are infected by schistosomes, leishmanias, plasmodiums, trypanosomes, and various other parasites, and severe clinical disease occurs in a sizable fraction of the infected population causing death and severe sequelae. The outcome, asymptomatic, subclinical or clinical disease, of an infection depends mostly on the parasite and on its host. Several groups analyzing the genetics of human susceptibility to parasites have began to identify the critical steps of the pathogenic mechanisms in a few parasitic infections such as malaria and schistosomiasis. The present article, which is not meant to be an exhaustive review of the field, illustrates the progresses made in this field from pioneer studies in animals to works in endemic populations using modern strategies of human genetics.

Induction of a protection against S. mansoni with a MAP containing epitopes of Sm37-GAPDH and Sm10-DLC. Effect of coadsorption with GM-CSF on alum

Studies of anti-S. mansoni immunological responses in individuals living in endemic areas identified immunogens (Sm37-GAPDH and Sm10-DLC) with vaccine candidate properties. Analysis of the epitopes of these immunogens indicated that: (i) Sm37-5 is a major B-cell epitope of Sm37-GAPDH and the IgG antibody reactivity toward this determinant is associated with resistance to reinfection; (ii) Sm10-T is a T-cell epitope of the major T-cell immunogen Sm10-DLC. This led us to test a multiple antigen peptide (MAP) containing Sm37-5 and Sm10-T as an anti-schistosome vaccine. This MAP induced a significant protective immune response in mice when injected in Freund's adjuvant or coadsorbed with GM-CSF on aluminium hydroxide. In the latter case the physical link between the cytokine and the antigen via the coadsorption on alum was necessary to obtain a protective response. Results of the antibody response indicated that when the MAP and GM-CSF were coadsorbed on alum, the antibody response against the Sm10-T epitope located in the NH(2)-terminal position was significantly amplified up to 30% of the anti-Sm37-5 response.

Genetic control of schistosome infections by the SM1 locus of the 5q31-q33 region is linked to differentiation of type 2 helper T lymphocytes

Human susceptibility to Schistosoma mansoni infections is controlled by the SM1 locus on chromosome 5 in q31-q33. This genetic region encodes cytokines which regulate the development of helper T lymphocytes. In the present work, a clonal analysis of CD4(+) T lymphocytes of homozygous resistant and homozygous susceptible subjects was undertaken to evaluate whether SM1 controls helper T-cell differentiation. Of 121 CD4(+) T-cell clones (TCC) from three susceptible (S) and three resistant (R) subjects, 68 proliferated when stimulated by parasite antigens. Parasite-specific TCC derived from susceptible subjects (33 STCC) produced 10- to 1,000-fold less interleukin-4 and -5 than TCC from resistant subjects (25 RTCC). Clones from both patient groups produced, however, the same amount of gamma interferon. Parasite-specific STCC were type 1 helper (Th1) or Th0/1, whereas RTCC were either Th2 or Th0/2. These results, together with the localization of SM1 in 5q31-q33, indicate that the SM1 locus controls the differentiation of Th2 lymphocytes.

Development of eosinophil peroxidase activity and concomitant alteration of the antioxidant defenses in the liver of mice infected with Schistosoma mansoni

BACKGROUND/AIMS

The tropical parasite Schistosoma mansoni causes granulomatous inflammation following lodging of eggs in portal capillaries. In vitro studies indicated that the host reaction should involve reactive oxygen intermediates; however, it is not known what occurs in vivo at the site of the disease. Moreover, the ultimate pathophysiological effects of oxidative processes depend upon antioxidant factors, which are investigated in this study.

METHODS

We explored the changes in the major enzyme activities involved in liver redox metabolism during the course of infection and, for some of them, the mRNA expression. We also measured the reduced glutathione and lipid peroxide levels in the liver.

RESULTS

We found that the deposition of parasite eggs triggers the release of endogenous eosinophil peroxidase; enzyme activity developed in the immediate vicinity of the eggs and it increased dramatically with time. However, Cu,Zn-superoxide dismutase, catalase and glutathione peroxidase activities decreased drastically. In contrast, glutathione transferase was unaffected. There was no proportional decrease in mRNA levels for the H2O2 scavenging enzymes. Reduced glutathione concentrations also dropped as a result of infection. Lastly, a two-fold increase in the levels of hepatic products generated by lipid peroxidation was observed.

CONCLUSIONS

These results show that on the one hand oxidative processes occurred at the site of granulomatous inflammation and on the other hand the antioxidant capacity of the liver decreased, leading to the generation of lipid peroxides. The resulting imbalance between pro- and anti-oxidant processes may play a central role in the pathology associated with schistosomiasis.

Induction of a protective immunity against Schistosoma mansoni with ovalbumin-coupled Sm37-5 coadsorbed with granulocyte-macrophage colony stimulating factor (GM-CSF) or IL-12 on alum

A previous study has shown that Sm37-5 is a major B cell epitope of Sm37-GAPDH. This epitope is highly antigenic in human infections and IgG antibody reactivity toward this determinant is associated with adolescent resistance to reinfection. This led us to test a synthetic peptide corresponding to Sm37-5, coupled to ovalbumin, as an anti-schistosome vaccine. Although mice injected with Sm37-5-OVA in Freund's adjuvant showed significant protection, immunization in aluminium hydroxide failed to induce protection. The adjuvant effect of cytokines (GM-CSF or IL-12) associated with the antigen on alum was investigated. With each of these two cytokines, significant reductions in the worm burden were obtained (32-38% with GM-CSF and 27% with IL-12, respectively). In addition, a reduction of the egg number trapped in the liver of immunized mice was also observed. Thus, protections were obtained with formulations that could potentially be used in humans.

High prevalence of GB-C/hepatitis G virus in a Brazilian population with helminth infection

A study of GB-C virus/Hepatitis G virus (GBV-C/ HGV) infection was carried out in a rural population of Northeastern Brazil, in which the prevalence of schistosomiasis is 80-90%. Despite the absence of parenteral risk exposure, the prevalence of GBV-C/HGV markers of infection was found to be unusually increased: viremia, 16.4%; specific antibody, 18.3%. It is therefore suspected that helminth infection influenced the immune response to GBV-C/HGV infection by shifting the balance of cytokine responses from Th1 to Th2, resulting in a delayed viral clearance. Phylogenetic analysis of viral isolates did not provide evidence for high rates of sexual or mother-to-infant viral transmission. The study revealed that viral strains belonged to types 1 and 2 only (predominant in Africa and Europe, respectively), suggesting that GBV-C/HGV was introduced into the New World by white conquerors and black slaves since the 16th century.

Genetic localization of a locus controlling the intensity of infection by Schistosoma mansoni on chromosome 5q31-q33

Three hundred million individuals are at risk of infection by schistosomes and around 200,000 die each year of this disease. Severe clinical disease in schistosomiasis is often the consequence of heavy infection which, in several endemic areas, are determined largely by the susceptibility/resistance of individuals. Previously, we reported evidence, based on a segregation analysis in Brazilian pedigrees, that intensity of infection by Schistosoma mansoni was influenced by a major gene, indicating that host genetic factors are probably critical in controlling schistosome infection and disease development. To localize this gene, referred to as SM1, we performed a genome-wide study on 142 Brazilian subjects belonging to 11 informative families Our results show a linkage to only one region, on chromosome 5q31-q33, with maximum two-point lod scores of +4.74 and +4.52 for D5S636 and the colony stimulating factor-1 receptor marker (CSF1R), respectively. This was corroborated by multipoint analysis, indicating a close proximity to CSF1R as the most likely location of SM1. This region contains several candidate genes encoding immunological molecules that were shown to play important roles in human protection against schistosomes.

Identification of a major T cell immunogen in the anti-schistosome response of adult residents in an area endemic for Schistosoma mansoni

Vaccine-induced immunity to Schistosoma mansoni infection depends on the specific priming of certain T cell subsets and on the recall of this response by natural infections months or years after vaccine administration. Thus, those schistosome proteins that activate T cells in individuals stimulated by natural infections are potential candidate vaccine antigens. In the present study, we identified and purified one such T cell-stimulating antigen and evaluated its immunological properties in subjects living in an area endemic for Schistosoma mansoni. Chromatography fractions (gel filtration, followed by ion exchange chromatography) of soluble extracts of schistosomula were screened for their ability to stimulate schistosome-specific T cell clones derived from a subject sensitized by natural infection. A fraction stimulating most clones was identified and characterized. A few nanograms of this fraction, containing a major 9-10-kDa component, stimulated the T helper cells of most adults living in an endemic area of Brazil, and was able to trigger a strong cutaneous immediate hypersensitivity reaction. In contrast, children reacted weakly to this antigen preparation both in blastogenesis and in skin tests, although they mounted a significant reaction to crude larval antigen preparations. In conclusion, this work identifies a schistosomula antigen that induces a strong T cell response in adults sensitized by natural infections. This T cell response develops gradually in children and adolescents, is apparently not restricted by the HLA haplotypes common in the study area, and allows the production of parasite-specific IgE antibodies. Thus, this T cell response has some features of the immune response that is believed to protect chronically exposed humans from reinfection.

Environmental, genetic and immunological factors in human resistance to Schistosoma mansoni

The design of programs for the control of endemies requires the knowledge of the principal factors that determine parasite transmission and infection levels in exposed populations. In the studies summarized in this article, the role of environmental and host specific factors in the infection by S. mansoni have been evaluated. It is shown that a limited number of factors actually influences infection intensity: water contacts, age, and sex were shown to account for 20 to 25% of infection variance, while 35 to 40% of it was accounted for by the effect of a major codominant gene. A remarkable fact is the important weighting (around 55% of the variance) of factors (the major gene and age) that influence human capacities of resistance. This observation strongly supports control measures aimed at increasing human resistance, such as vaccination. The effect of age on the development of resistance has now been observed in several studies on S. mansoni or S. haematobium. It is, therefore, a constant finding in schistosomiasis infections that resistance develops extremely slowly requiring a long period of exposure to the parasite and repeated infections. These studies provide strong incentives to increase efforts in the evaluation of the immune response of subjects living in endemic areas. Such evaluations are necessary to define vaccine and vaccination programs, and they are also urgently needed to evaluate the effects of chemotherapy on the development of immunity in children and adolescents, as well as on the persistence of protective immunity in adults. Immunological studies begin to provide a clearer picture of the role of acquired immunity in human protection against S. mansoni. It is increasingly clear that the slow development of resistance in children, as well as its alteration in certain age groups, are related to the maturation of parasite specific immunity and its alteration by specific immune factors. Thus, the development of resistance is associated with the maturation of IgE-dependent immunity, whereas blocking Ab may interfere in children and adolescents with the expression of full resistance. This finding raises the question as to whether a vaccine could include major allergens without triggering the well-known deleterious side effects associated with hypersensitivity reactions. The absence of such reactions in subjects with high parasite-specific IgE levels who are exposed to daily infections suggests that this may be feasible.