Formation of Plasmodium falciparum gametocytes in vivo and in vitro relates to transmission intensity

In vitro sensitivity to antimalarial drugs and polymorphisms in Pfg377 gene in Plasmodium falciparum field isolates from Mewat, India

The malaria infection is the interplay of several complex factors, among which drug resistance and gametocyte biology are the crucial ones. Measurements of the drug sensitivity assays and prevalence of gametocyte genes play a major role in malaria eradication programmes. The purpose of this study was to assess the current drug sensitivity profile in the field isolates and type the Pfg377 gene. In vitro drug sensitivity assay was used to assess the susceptibility of Plasmodium falciparum field isolates collected from Mewat (Haryana), located in Northern region of India to four anti-malarial drugs. The inhibitory concentrations (IC₅₀) for the four drugs viz Chloroquine (CQ), Artesunate (AS), Sulphadoxine (SD) and Pyrimethamine (PYR) were in the range of 10.11-113.2, 2.26-4.08, 13.31-43.91 and 0.76-4.91 nM respectively in the evaluated 25 field isolates. The Pfg377 allele typing in the 26 field isolates revealed different types of haplotypes (A, B, C and D) varying by the size and number of repeats and deletions. The Pfg377 allelic types in the cryopreserved and adapted isolates of P. falciparum were compared to determine the parasite lines in them. The allele types or presence of different clones in the same sample could not be correlated significantly with resistance to any of the four drugs tested in the study. Our study revealed different drug sensitivity profiles of field isolates from Mewat region and allele typing of Pfg377 gene revealed different haplotypes of field isolates were circulating in parasite population.

Epidemiology and infectivity of Plasmodium falciparum and Plasmodium vivax gametocytes in relation to malaria control and elimination

Malaria remains a major cause of morbidity and mortality in the tropics, with Plasmodium falciparum responsible for the majority of the disease burden and P. vivax being the geographically most widely distributed cause of malaria. Gametocytes are the sexual-stage parasites that infect Anopheles mosquitoes and mediate the onward transmission of the disease. Gametocytes are poorly studied despite this crucial role, but with a recent resurgence of interest in malaria elimination, the study of gametocytes is in vogue. This review highlights the current state of knowledge with regard to the development and longevity of P. falciparum and P. vivax gametocytes in the human host and the factors influencing their distribution within endemic populations. The evidence for immune responses, antimalarial drugs, and drug resistance influencing infectiousness to mosquitoes is reviewed. We discuss how the application of molecular techniques has led to the identification of submicroscopic gametocyte carriage and to a reassessment of the human infectious reservoir. These components are drawn together to show how control measures that aim to reduce malaria transmission, such as mass drug administration and a transmission-blocking vaccine, might better be deployed.

Acquired immune response to defined Plasmodium vivax antigens in individuals residing in northern India

In this investigation, we evaluated the naturally acquired immune response to Plasmodium vivax stage-specific antigens in individuals of different age groups belonging to malaria endemic areas of northern India. Four synthetic peptides containing both B- and T-cell epitopes from P. vivax circumsporozoite protein, merozoite surface protein-1, apical membrane antigen-1 and gametocyte surface antigen-1 were used to determine both humoral and cellular immune responses. Immunity, in terms of antibody response and T-cell proliferation against these stage-specific peptides, has been observed in the study subjects. The results demonstrated age-dependent antibody response in this population. Forty two patients were diagnosed with P. vivax. There was a significant association (P=0.013) between number of antibody responders and recognition of stage-specific epitopes by antibodies. The antibody response to B-epitopes of P. vivax CSP, MSP1, AMA1 and GAM1 was associated with age; adults responded more frequently to these antigens than did younger children. In this population, 66% (201/304) cases showed seropositivity to all peptides and 13% (41/304) showed negative response. Peripheral blood mononuclear cells of more than 75% of individuals proliferated in response to stimulation by all four epitopes. In conclusion, the results demonstrated immunogenicity of the epitopes to P. vivax in population of this endemic zone.

Plasmodium falciparum circumsporozoite protein: epidemiological variations among field isolates prevalent in India

OBJECTIVE

To investigate the extent of genetic variations in T-helper-cell epitopic regions of circumsporozoite (CS) protein in Plasmodium falciparum field isolates collected from different regions of India at different phases of malaria transmission.

METHODS

Genomic DNA was isolated from 507 P. falciparum wild-parasite isolates obtained from six geographical locations of India at three time points coinciding with malaria transmissions. The T-helper-cell epitopic regions were polymerase chain reaction (PCR)-amplified and the products were purified and then sequenced.

RESULTS

Based on sequences, nine variants were found among isolates and they were categorized into nine groups (V-1 to V-9), where V-1 and V-2 were observed in all three time points (TP). The variants V-1 to V-4 in TP-1; V-1, V-2, V-5 to V-8 in TP-2; and V-1, V-2, V-5 and V-9 in TP-3 were present and they showed restricted heterogeneity. During peak transmission (TP-2), parasite populations were more diverse and heterogeneous and the variants regionally unbiased and restricted. However, the alleles of V-6 and V-9 in both Th2R and Th3R showed identical sequence variation with those observed in other geographical regions of the world. The remaining seven groups did not show such similarity.

CONCLUSION

The Th2R and Th3R epitopes are implicated in host immune response to P. falciparum. The polymorphism in these epitopic regions indicates antigenic diversity, which may cause adverse outcome of a subunit vaccine including the CS prototype variant. Therefore, the formulation of a vaccine considering the restricted local repertoire parasite populations may be helpful.

A longitudinal investigation of Plasmodium falciparum malaria in children in northern India

A group comprising 27 young children (1-4 y of age) suffering from uncomplicated falciparum malaria were studied to characterize the isolates and to measure humoral immune responses during acute infection and after recovery. Finger prick blood from each individual was collected on d 1. After treatment with chloroquine, a further blood sample was collected from each child on d 7, 30, 90 and 180 for assay of antibody responses to P. falciparum antigens. Isolates from individual patients were incubated in vitro for demonstration of rosette formation, assay of plasmodial growth rate and analysis of Pfcrt gene polymorphism. Out of 27 isolates of P. falciparum, 20 showed formation of rosettes in vitro. The growth rate at 96 h varied widely among the isolates. In Pfcrt gene analysis at 76-codon site, 14 showed wild-type Lys 76, 7 showed mutant type Thr 76 and 6 had mixed type. 14 children, all with anaemia on d 7, showed a positive direct antiglobulin test (DAT). Sera positive by ELISA IgG on d 90 also showed parasite growth inhibitory activity in vitro. Significant levels of IgG, IgG1 and IgG3 subclass antibodies against MSP1 were detected in 14 sera collected on d 90. On d 180, there was a decline in IgG and its subtypes. These findings suggest that a variability in isolates may occur in one and the same seasonal area, making children prone to infection. As a consequence, they develop antibodies during recovery phase from an acute attack, which remain in circulation for a period of 4-5 months. After that, a decline in antibody level may again make them susceptible to the disease. Prevalence of different serotypes in a small area may suggest the complexity of malaria transmission.