Parasite Immunology , 2001: 23: 163±167 Research note Human Bancroftian filariasis: loss of patent microfilaraemia is not associated with production of antibodies to microfilarial sheath A.K.SATAPATHY, P.K.SAHOO, J.J.BABU GEDDAM, M.C.MOHANTY & B.RAVINDRAN Division of Immunology, Regional Medical Research Centre, ICMR, Bhubaneswar, India SUMMARY Antisheath antibodies have been incriminated in elimina- tion of circulating microfilariae in human filariasis since a very significant inverse association has been consistently demonstrated between the two parameters. An attempt was made in the present study to seek empirical proof for the above proposal. Two cohorts of 43 and 73 microfilariae (mf) carriers were examined after 13 and 10 years, respectively, for mf as well as antisheath antibodies. The first cohort was also examined for the presence of circulating filarial antigen (CFA). Of the 43 mf carriers examined after 13 years, 62´8% were free of circulating mf although only 3´7% of them had demonstrable antisheath antibodies. Approximately 93% of this cohort (with or without current microfilaraemia) tested positive for CFA after 13 years indicating continued presence of adult filarial worms in the host after loss of mf in circulation. When the second cohort of 73 mf carriers were examined after 10 years, 30 were found to be amicrofilaraemic and only 6´66% of them had demonstrable antisheath anti- bodies. We conclude that, in human Bancroftian filariasis, elimination of circulating microfilariae may not be mediated by antibodies to microfilarial sheath. Keywords Wuchereria bancrofti, protective immunity, antisheath antibodies, circulating filarial antigen, Bancroftian filariasis INTRODUCTION Lymphatic dwelling filarial helminths survive in human hosts for several years without functionally being recog- nized by the host immune system. Whether or not humans develop protective immunity against various developmental stages of the parasite has been a subject of intense debate. Animal models such as cats and dogs have provided evidence for existence of protective immunity against animal filarial parasites (1,2). While there has been no broad consensus on development of acquired immunity against larval and adult stage parasites, antimicrofilarial immunity in human lymphatic filariasis is believed to be mediated by antibodies to microfilarial sheath (3,4). In some animal models, such as cats and dogs, follow-up investigations have revealed that antibodies are absent during microfilaraemic phase and when antibodies to the surface of microfilariae (mf) appear the event is associated with loss of circulating mf (5±7). Several in vitro investigations on antibody mediated cell adherence (ADCC) performed with several filarial parasites and host cells also indicated that ADCC could be mediated only by those sera containing antibodies reactive to the surface of mf. These observations have been interpreted to conclude that microfilarial phase is sustained in the absence of antisheath antibodies and that appearance of such anti- bodies would lead to elimination of mf presumably by a process of ADCC (8±10). In the absence of follow-up investigations of mf carriers vis-a Á-vis antisheath antibodies in human filariasis, the inverse association observed between the two parameters in cross- sectional studies were interpreted to imply loss of patent microfilaraemia that circulating mf are eliminated as a consequence of appearance of antisheath antibodies in Brugian as well as Bancroftian filariasis (3,4,10). Some of the recent findings have however, raised serious doubts on the above interpretation in human filariasis. High prevalence q 2001 Blackwell Science Ltd 163 Correspondence: B.Ravindran, Division of Immunology, Regional Medical Research Centre (ICMR), Chandrasekharpur, Bhubaneswar- 751016, India (e-mail: immunol@dte.vsnl.net.in). Received: 29 August 2000 Accepted for publication: 30 October 2000