The major glycoprotein of the sheath of Litomosoides carinii microfilariae (gp22) was analysed for its amino acid and amino sugar composition. It is rich in proline, glutamine/glutamic acid and glycine and contains (N-acetyl)galactosamine. The N-terminal amino acid sequence was determined up to position 37. It consists of a group of 6 repeats of the pentapeptide sequence methionine-glycine-proline-glutamine-proline with two minor modifications in repeats 3–6, while the first two repeats follow the general pattern more loosely. Identical N-terminal amino acid sequences were found in at least two other sheath polypeptides (33 kDa, 39 kDa). Antisera prepared against 3 overlapping synthetic peptides corresponding to the amino terminus of gp22 recognized different epitopes. They all reacted with identical patterns of sheath polypeptides. The antisera failed to recognize antigens of 4th-stage larvae of L. carinii. In contrast, cross-reacting epitopes were detected in other parasite stages. Antisera reacted with material surrounding embryos and microfilariae in the uterus of females, and caused patchy fluorescence on the sheath of blood-derived and in vitro-released microfilariae.

This study was conducted to identify surface antigens of the microfilarial sheath of Litomosoides carinii which are accessible to antibodies. Rabbit antisera were raised against the soluble and insoluble fractions of purified sheaths by extracting them with a buffer containing 2-mercaptoethanol and sodium dodecylsulphate. These sera and rabbit hyperimmune sera directed against homogenates of total microfilariae, mature (i.e. microfilariae liberating) female parasites and excretory–secretory products of adult females were able to agglutinate live and formaldehyde-fixed microfilariae. When the antisera directed against sheath constituents were administered to patently infected Mastomys coucha, the microfilaraemia of these animals was rapidly reduced and remained low for a period of 2–3 weeks. Antibodies specifically binding to the microfilarial surface were immunoaffinity-purified on formaldehyde-fixed microfilariae. The antibodies react with sheath antigens of 40 and 120 kDa molecular mass which are produced by the epithelium of the distal uterus of the mature female, secreted and attached to the surface of the sheaths. A 120 kDa antigen recognized by anti-sheath surface antibodies was also detected in the excretory–secretory products of in Vitro-cultured immature female L. carinii from day 30 post-infection onwards. In the excretory–secretory products of mature adult female parasites recovered on day 130 post-infection, this 120 kDa molecule was absent. However, material reacting with the antibody was detected in the stacking gel of SDS-polyacrylamide gels. This finding may indicate that the basic units forming the 120 kDa antigen of immature adults or microfilarial sheath surface antigens occur in a highly polymerized form in the excretory–secretory products of mature female parasites.