Immunochemical properties of oligosaccharide-protein conjugates with Klebsiella-K2-specificity. II. Protective capacity of the conjugates and anti-conjugate antibodies against infection with Klebsiella pneumoniae 01:K2 in mice.

Oligosaccharide subunits from Klebsiella pneumoniae B5055 capsular polysaccharide, covalently linked to protein carriers, the native polysaccharide, and killed whole bacteria, were compared with respect to their immunogenicity in mice and their capacity to protect mice against bacterial infection. The protection was studied (a) by active immunization and (b) by passive administration of antisera raised in rabbits using the different immunogens. The results were as follows: in the active protection experiments the octa- and dodecasaccharide conjugates were as effective as the polysaccharide in increasing the LD50 by a factor of 10(5) as compared to nonimmunized animals. Thus, these antigens were only slightly less effective than killed bacteria. In contrast the tetrasaccharide conjugate was at least 1000-fold less effective than all other antigens used. In the passive protection experiments these results were paralleled in that the antiserum against the tetrasaccharide conjugate also showed the lowest degree of protection, though, for methodical reasons, the differences in the LD50s were less pronounced. The drastically lower efficiency of the tetrasaccharide in comparision to its higher oligomers is in agreement with serological findings previously published (see first communication), which showed that two repeating units are the minimum requirement for a substantial representation of polysaccharide serological specificity.