Levels of specific antibodies towards the major antigenic determinant of slime-producing Staphylococcus epidermidis determined by an enzyme immunoassay and their protective effect in experimental keratitis.

Staphylococcus epidermidis is an important cause of bacterial keratitis. Certain S. epidermidis strains produce an extracellular slime layer rich in an acidic polysaccharide with a molecular size of 20 kDa (20-kDa PS). We have demonstrated that the level of 20-kDa PS-specific antibodies significantly rises after establishment of slime-producing S. epidermidis bacteraemia and, furthermore, that rabbit polyclonal antibodies to 20-kDa PS opsonize cells of slime-producing S. epidermidis to a great degree and promote their clearance by polymorphonuclear cells (Arch. Biochem. Biophys. 342 (1997) 389; J. Pharm. Biomed. Anal. 22 (2000) 1029). The purpose of this study was to examine the protective and therapeutic effects both of active immunization, using 20-kDa PS as antigen, and of passive administration of specific antibodies towards the 20-kDa PS in a rabbit keratitis model. For active immunization, 20 rabbits were subcutaneously immunized with 20-kDa PS, whereas for passive immunization specific polyclonal IgG antibodies against 20-kDa PS were administered to 20 rabbits 1 day before induction of infection. Clinical observations were made weekly for 1 month and levels of 20-kDa PS antibodies in serum and aqueous humor in both immunization groups were determined by an enzyme immunoassay. The levels of specific anti-20-kDa PS IgG in serum and aqueous humor following either active or passive immunization were significantly higher as compared with control groups (P<0.001). Although, actively immunized rabbits showed significantly less corneal damage than control animals, passively immunized ones were significantly better protected as compared with both control and those actively immunized. Obtained results suggest that 20-kDa PS plays crucial role in the pathogenesis of S. epidermidis keratitis and that both types of immunization significantly protect against corneal S. epidermidis pathology and damage.