The tetrasaccharide repeating unit of the capsular polysaccharide of Kkebsiella serotype 11, K11PS, comprises the following sequence: [-*3)-I8-D-GlcpA-(1--.3)-a-D-Galp-(1--3)-4-D-Glcp-(l--I with a 4,6-0-(1carboxyethylidene)-a-D-galactopyranosyl residue linked to 0-4 of the glucuronic acid residue. Octasaccharide (OS) derived from K11PS by bacteriophage +11-associated glycanase, was coupled to bovine serum albumin and to keyhole limpet hemocyanin. The immunogenicity of various antigens after intraperitoneal immunization was studied by measuring the levels of circulating antibodies. Injection of BALB/c mice with K11PS resulted in induction of 2-mercaptoethanol-sensitive immunoglobulin M antibodies. The responses observed in BALB/c nulnu mice and in male (CBA/N x C3H/HeN)Fl mice indicate that K11PS is a thymus-independent type 2 antigen. Immunization of BALB/c mice with either OS-bovine serum albumin or OS-keyhole limpet hemocyanin resulted in the induction of circulating 2-mercaptoethanol-resistant immunoglobulin G antibodies.Results in BALB/c nulnu mice indicate that the OS-protein conjugates are thymus-dependent antigens. Since the OS-keyhole limpet hemocyanin conjugate induced antibodies in both (CBA/N x C3H/HeN)Fl females and males, we propose to refer to this kind of antigen as a thymus-dependent type 1 antigen, whereas OS-bovine serum albumin, which evoked immunoglobulins in (CBA/N x C3H/HeN)Fl females only, can be referred to as a thymus-dependent type 2 antigen.Bacterial polysaccharides are generally considered to be thymus-independent (TI) antigens (4, 6) and are characterized by the induction of mainly immunoglobulin M (IgM) and IgG3 antibodies after primary immunization (17, 29), without the induction of immunological memory (35). Moreover, the immune response toward TI antigens is developed only at later stages of the ontogeny, which has important consequences for the vaccination of neonates (27,35).For vaccinations aiming at long-lasting protection against diseases, it would be desirable to immunize with thymus-dependent (TD) antigens (35). TD antigens are supposed to induce immunological memory and thus to provoke a state of prolonged resistance to infections (36). Furthermore, the development of TD forms of a particular TI antigen raises the possibility of immunizing young animals with more success than with the TI antigen itself.In earlier experiments with Streptococcus pneumoniae type 3 capsular polysaccharide (S3PS) and hexasaccharideprotein conjugates, we showed that the S3PS behaved as a TI antigen, whereas the hexasaccharide-protein conjugates behaved as TD antigens (35). To extend the results obtained with S3PS and hexasaccharide-protein conjugates of S. pneumoniae, we used the capsular polysaccharide of Klebsiella serotype 11, K11PS, as a novel model antigen. This polysaccharide was chosen because of the availability of bacteriophage +11-associated glycanase, which could be used to depolymerize K11PS. Enzymatic digestion of K11PS results in the cleavage of the ,(1-3-3) glycosidic linkage betw...