J. Med. Microbio1.-Vol. 22 (1986), 269-275 0 1986 The Pathological Society of Great Britain and Ireland A monoclonal antibody reacting with a determinant on leptospiral lipopolysaccharide protects guinea pigs against leptospirosis B. H. JOST, B. ADLER, T. VINH, and S. FAINE Department of Microbiology, Monash University, Clayton, 3 168, Victoria, Australia Summary. An IgA monoclonal antibody (MUM/F 1 - 1 /copenhageni) was produced from a mouse immunised with Leptospira interrogans serovar copenhageni. The antibody showed partial serogroup specificity by agglutination and by reaction in enzyme immunoassay, and opsonised homologous leptospires for phagocytosis by cultured mouse macrophages. Immunodiffusion and Western-blotting experiments indicated that MUM/Fl- 1 lcopenhageni reacted with a carbohydrate determinant in the leptospiral lipopolysaccharide. Daily administration of purified MUM/F 1 - 1 / copenhageni IgA before and after challenge with 2 x lo8 virulent homologous leptospires passively protected newborn guinea pigs against lethal leptospirosis. Introduction It is well established that development of a humoral immune response during leptospirosis is important in resistance to infection (Adler and Faine, 1976, 1977, 1978a and b; Adler et al., 1980a). Immunity appears to depend on the production of agglutinating, opsonising antibody; cell-mediated immune mechanisms have no detectable role. Although the immune response to infection with leptospires is well documented, the nature of the antigens important in protection remains unclear. Many preparations of leptospiral antigens have been reported (Faine, 1974) but only a few have been purified for definitive study. One of these anti gens, the lept o spiral lipopol y saccharide (LPS) structurally resembles endotoxins of other gram- negative bacteria, although it has different chemical and biological properties (Vinh et al., 1986). As leptospiral LPS elicits the production of protective antibody during infection (Adler and Faine, 1978a and b), it is possible the LPS is an important protective antigen. Antibodies against LPS from other gram-negative organisms can be used to passively protect against homologous infection and LPS can also be used as an immunogen against experimental infection (Pennington and Kuchmy, 1980; Colwell et al., 1984; Kirkland and Ziegler, 1984; Sawada et al., 1984). To determine the nature of the leptospiral anti- Received 4 Feb. 1986; accepted 23 Apr. 1986. gens that stimulate the production of protective antibody, monoclonal antibodies (MCAs) have been produced. This strategy has been used success- fully for other organisms, notably the malaria parasite Plasmodium (Potocnjak et al., 1980; Holder and Freeman, 1981; Boyle et al., 1982). Anti- leptospiral MCAs have been described (On0 et al., 1982, 1984; Adler and Faine, 1983a and b; Kobayashi et al., 1984), but have been mainly of taxonomic significance. If these MCAs can be used to passively protect against infection in a suitable animal model, they should be useful to identify and characterise potential protective antigens whose determinants could be the basis for immunogens suitable for human and veterinary use. This paper reports the production and characteri- sation of a protective MCA directed against a determinant on the leptospiral LPS. Materials and methods Lep t ospires Leptospira interrogans serovar copenhageni strain designated H45 in this laboratory was isolated from a rat (Faine and van der Hoeden, 1964) and serovar copen- hageni strain L136 was provided by Professor R. Yana- gawa, Hokkaido University, Sapporo, Japan. L. illini serovar illini was obtained from A.D. Alexander, Walter Reed Medical Center, Washington D.C., USA. The other leptospiral serovars used in this study were provided by N. Stallman, W.H.O. Leptospira Reference Laboratory, 269