Antibody‐dependent cell‐mediated immune reactions to Loa loa microfilariae in amicrofilaraemic subjects

Summary Antibody‐mediated mechanisms that could be important in controlling microfilaraemia in Loa loa infected amicrofilaraemic adults (mf‐ve) were studied. These subjects were selected as having a verified ocular passage of an adult L. loa but being amicrofilaraemic and without recent diethylcarbamazine treatment. Sera from 37 mf‐ve subjects were compared to 14 sera from heavily (> 4000 mf/ml) infected subjects (mf+ve) and 9 sera from Caucasian control subjects for their reactions with L. loa mf (mf). Many mf‐ve sera (22/37) were strongly positive in immunofluorescence (IFAT) on living mf. Mf+ve sera were negative, or only weakly positive, and Caucasian sera were negative. Clinical signs were not significantly different between IFAT reactive and non‐reactive mf‐ve subjects. Approximately half of the IFAT positive, mf‐ve sera were also able to agglutinate mf: no other sera were active in this test. Titres ranged from log2 3–6 and in most cases, 9/11, the agglutination reaction was mercaptoethanol‐sensitive. Antibody‐dependent cellular adherence was studied using mf and leukocytes from uninfected donors. Using cryopreserved mf many heat‐inactivated mf‐ve sera gave strong reactions with obvious adherence by 4 h and few motile mf remained by 16 h but when fresh mf were employed these reactions were weak. However, addition of complement to many (10/11) mf‐ve sera considerably enhanced adherence to fresh mf. The effect of various treatments on the complement source indicated a role for both the classical and alternative pathways. The cells attached to mf were mainly neutrophils (83%) with some eosinophils (15%) and few mononuclear cells (2%). The common occurrence of antibodies able to mediate complement‐dependent adherence of polymorphonuc‐lear leukocytes to L. loa mf in the sera of mf‐ve subjects may indicate that such a mechanism is important in controlling microfilaraemia in vivo. Copyright © 1992, Wiley Blackwell. All rights reserved