Influence of molecular structure on the tolerogenicity of bacterial dextrans. II. The alpha1--3-linked epitope of dextran B1355.

Native dextran B512 is a near-linear glucose polymer with 96 per cent alpha1--6 and 4 per cent alpha1--3 linkages and a molecular weight (mol. wt) of 8 X 10(7). Sheep RBC sensitized with its O-stearoyl derivative (prepared by a modified method) have been used satisfactorily in direct PFC assays. B512 immunizes BALB/c mice optimally with doses of 1--10 mug and produces B-cell tolerance with 1 mg upwards. The specificity of the response determined by PFC inhibition analysis, is directed towards an alpha1--6-linked epitope. High dose tolerance is not preceded by immunity and is stable on cell transfer to irradiated recipients in which responsiveness becomes perceptible after 4--6 weeks. Progressive depolymerization of this polysaccharide reduces immunogenicity and tolerogenicity, both of which are extinguished when the mol. wt falls to 2 X 10(4). Optimal immunization with B512 is succeeded by partial tolerance (previously characterized by analogous levan experiments as a B-cell exhaustion process). The tolerance threshold dose of B512 is reduced 1000-fold during immunosuppression with cyclophosphamide. PFC inhibition studies supported the contention that tolerogenicity of polysaccharides is influenced by their overall binding capacities. A direct relationship between inhibitory and tolerogenic activities was found both with B512 fractions of varying mol. wt and with heterologous dextrans. The similarities between B512 and levan argue against the association of a highly branched structure with greater tolerogenicity. The effect of reducing the percentage of alpha1--6 linkages in dextrans suggests, however, that epitope density probably plays a contributory role in determining the outcome of interaction between polysaccharides and B cells.