Antibody isotypes in sera of equine fetuses aborted due to Leptospira interrogans serovar pomona-type kennewicki infection
Introduction
The equine epitheliochoreal placenta does not allow transfer of maternal immunoglobulins to the fetus and so maternal immunoglobulins are received postnatally via colostrum. However, a number of studies have demonstrated partial immunocompetence in the later stage equine fetus. Mixed lymphocyte culture-responsive cells are detected in the thymus and spleen at 100 and 200 days of gestation, respectively (Perryman et al., 1980). Detectable quantities of IgM but little or no IgG are found at 200 days (Perryman et al., 1980). Neutralizing antibodies to T2 coliphage have been detected following immunization at 200 days (Martin and Larson, 1973), and immunization with keyhole limpet hemocyanin induces both antibody and T-cell activity (Hanant et al., 1992). Antibody to equine herpes virus-1 has also been detected in aborted fetuses (Whitwell et al., 1992), and fetuses at 232 days of gestational age have made putative IgGa, IgGb and a trace of IgG(T) in response to in utero immunization with attenuated Venezuelan equine encephalomyelitis virus (Morgan et al., 1975). However, these isotype responses were identified by immunoelectrophoretic analysis, and were not quantified.
Large amounts of IgG1 and traces of IgG2 are found in fetal lamb sera in response to antigenic stimuli (Fahey and Brandon, 1978), whereas in the adult sheep these isotypes are present in nearly equal concentrations. In the horse, IgGb synthesis is inapparent in neonatal serum yet IgGb becomes the predominant immunoglobulin in sera of adults, replacing IgGa which is found in most neonatal sera and which subsequently increases after the first month of life (Sheoran et al., 2000). Characterization of equine immunoglobulin isotype responses to known antigens at various stages of life provides information useful in predicting vaccine efficacy at these different stages.
Leptospiral infection predominantly involving serovar pomona-type kennewicki has caused placentitis, abortion, stillbirth and premature live birth in horses (Jackson et al., 1957, Ellis et al., 1976, Ellis et al., 1983, Hodgin et al., 1989, Poonacha et al., 1993, Poonacha et al., 1994, Williams et al., 1994). The gestational age of the aborted fetus ranges from 3.6 months to full term with a mean of 9 months. The outcome varies with the age of the fetus and with the virulence of the Leptospira (Hanson, 1977). Some late term abortions result in birth of live foals that develop normally. Leptospira are found in the placenta and in the liver, kidney and other tissues of the foal. It is unknown what proportion of abortions result from placentitis alone or from a combination of placentitis and the effects of direct fetal infection. Sera of fetuses that aborted due to type kennewicki infection had both IgM and IgG, and specific agglutinating activity (Poonacha et al., 1994). The aim of the present study was to further characterize antibody isotype responses to Leptospira in sera of late term equine fetuses aborted due to L. interrogans serovar pomona-type kennewicki infection. In addition, specific isotype responses of their dams, and of other horses positive for agglutinating antibody, were investigated.
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Sera
Fetal sera were obtained at necropsy in the Livestock Disease Diagnostic Center, Lexington, KY from seven fetuses aborted due to natural Leptospira infection as confirmed by serology, fluorescent antibody microscopy and culture. Sera were available from mares of three of the fetuses. In addition, sera positive for agglutinating antibody to L. interrogans serovar pomona from 10 adult horses including some males were studied.
Purification of IgG subisotypes and IgA
Affinity chromatography utilizing mouse-ascites MAbs specific for equine
Agglutinating antibodies
Sera from both fetuses and dams agglutinated L. interrogans serovar pomona (Table 1).
Antibody isotype responses to L. interrogans serovar pomona-type kennewicki
Leptospira-specific IgM responses dominated serum antibody isotype responses of fetuses, mares and horses except Sundance, Buddy and Amy (Table 2, Table 3); IgGa and IgG(T) dominated fetal serum and adult serum IgG antibody responses, respectively, except for 14,244 (Table 2, Table 3). IgGa antibody responses of dams and fetuses were similar. Fetuses did not make specific IgGb antibody responses whereas strong
Discussion
This is the first study that showed that IgGa dominated the fetal serum IgG response to Leptospira infection with little or no IgGb and IgG(T) synthesis. In contrast, Leptospira-specific IgG(T) together with lesser yet strong IgGa and IgGb responses were characteristic of both aborting mares, and infected horses. As expected, isotype responses were dominated by IgM in both fetuses and adults, since bacterial polysaccharides such as leptospiral lipopolysaccharides (LPS) have been shown to elicit
Acknowledgements
This study was supported by the Keeneland Association.
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Present address: Department of Biomedical Sciences, Tufts University School of Veterinary Medicine, 200 Westboro Road, North Grafton, MA 01545, USA.