Protective immunity in human Bancroftian filariasis: inverse relationship between antibodies to microfilarial sheath and circulating filarial antigens

The existence and the nature of protective immunity in human filariasis con tinues to be a subject of intense debate. While there is no broad consensus on functional immunity against larval and adult stage parasites, anti-micr ofilarial immunity has been demonstrated to be mediated by antibodies to th e microfilarial sheath. In the present study, circulating filarial antigens (CFA), a marker of active filarial infection in human Bancroftian. filaria sis, was found to be inversely associated with antibodies to microfilarial sheath in a cohort of 411 subjects representing all categories of filariasi s across the clinical spectrum of the disease. Approximately 80% of humans of all age groups (5-65 years) were found to have either CE;A or anti-sheat h antibodies. The inverse relationship observed between these two parameter s was found to be independent of the clinical manifestation; both symptomat ic and asymptomatic cases were found to display similar inverse association between CFA and anti-sheath antibodies. The prevalence of anti-sheath anti bodies in the paediatric group was found to be very high as compared to adu lts; 78% of children below the age of 10 years tested positive for anti-she ath antibodies although the mf rate and CFA rate were only 4.5% and 22.7%, respectively, in this age group, indicating that developing larvae or juven ile adult stage parasites could have been the source of antigenic stimulus for induction of antibodies to the microfilarial sheath.