IgG SUBCLASS ANTIBODIES TO PLASMODIUM FALCIPARUM

The immunoglobulin G (IgG) subclass antibodies to Plasmodium falciparum blood stage antigens in the sera of 181 individuals living in malaria endemic area in Kanchanaburi Province, western Thailand, were determined by enzyme-linked immunosorbent assay (ELISA). In this study, IgG3 and IgG1 were shown to be the predominant subclasses. Generally, IgG2 were coexpressed with IgG1 and IgG3 while IgG4 was found to coexpress with other three IgG subclasses. The levels of specific IgG1, IgG2, and IgG3 increased significantly with age (r = 0.295, p = 0.000; r = 0.416, p = 0.000; r = 0.320, p = 0.000, respectively). The data indicate that the higher antibody production required continuous stimulation under natural condition. Furthermore, the levels of specific IgG1, IgG2 and IgG3 increased in immune individuals without clinical malaria, reported in adolescents and adults, were associated with malaria resistance. Similar results were found in children with different patterns of IgG subclasses in which the specific IgG2 and IgG3, but not IgG1 was related to resistance. of IgG1 and IgG3 in individuals to crude schizont extract were associated with protection against P. falciparum, whereas predominance of antibodies in other IgG subclasses and of IgM antibodies or overall low antibody levels were associated with disease susceptibility (Bouharoun-Tayoun et al, 1992). A recent study in Solomon Islanders also showed that the antibodies to crude schizont extract were distributed among four subclasses with IgG1 and IgG3 predominating (Rzepczyk et al, 1997). However, antibodies to MSA2 in Gambian sera were predominantly of IgG3 subclass (Taylor et al, 1995) which might contribute to the development of clinical protective immunity to malaria (Taylor et al, 1998). Similarly in a study in Senegal, IgG3 antibody to P. falciparum blood-stage extract was reported in association with lower risk of malaria attack, and recovery from severe malaria (Aribot et al, 1996; Sarthou et al, 1997). In contrast to the study in Madagascar, Correspondence: Dr Srisin Khusmith, Department of Microbiology and Immunology , Faculty of Tropical Medicine, 420/6 Rajvithi Road, Bangkok 10400, Thailand. Tel: (662) 2460056 ext 1594 E-mail: tmskm@mahidol ac.th SOUTHEAST ASIAN J TROP MED PUBLIC HEALTH Vol 32 No. 2 June 2001 248 the levels of total IgG and IgG subclass to P. falciparum extract were higher in nonprotected subjects than in protected ones (Dubois et al, 1993). However, in infants, the level of IgG1 to MSP1 was shown to be negatively correlated with protection (Branch et al, 2000), while IgG2 was related to a decrease in the risk of P. falciparum infection (Deloron et al, 1997). So far, the mechanisms that are intimately involved in the secretion of an IgG subclass which confer protection are not yet understood. It may depend upon many factors including the parasite strain, the parasite antigens used in the analysis, and the host genetic background. The aim of the present study was to investigate in peoples living in a malaria endemic area in western Thailand whether they develop IgG antibodies directed against blood stage P. falciparum extract. The distribution of IgG subclass patterns was determined and the influence of age as well as malaria attack on the level of each IgG subclass were also evaluated. MATERIALS AND METHODS Study area and population The Bongty subdistrict, Saiyok district, Kanchanaburi Province is situated in a rural area of Thailand 150 km west of Bangkok along the Thai-Myanmar border where malaria is endemic. The area of study covered four villages with a total population of 689: 406 children (58.9%) and 283 adults (41.1%). The climate in this region is warm and humid with the temperature ranging from 28oC to 35oC. The rainy season goes from May to October. The villages are located on the bank of a permanent fresh water river. In 1999, approximately 9% and 3% of the population were infected with P. falciparum and P. vivax, respectively.