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

Dextran B1355 is a branched glucose polymer containing 57 per cent alpha1--6, 35 per cent alpha1--3 and 8 per cent alpha1--2/1--4 linkages. Direct PFC responses to B1355 can be measured with sheep RBC sensitized with its O-stearoyl or palmitoyl derivative, and, as shown by inhibition analysis, are specific for an eptiope which is dependent on alpha1--3 linkages. B1355 is a potent immunogen in BALB/c mice producing peak PFC levels which approach 10(6) per spleen following an optimal dose of 1 mg. By contrast, the alpha-1--3-linked epitope of B1355 is feebly tolerogenic, for even 10 mg still induces a strong initial response. Mice given 1--10 mg sustain PFC levels 1--2 log10 above background for several months, but do not respond further to restimulation. Full recovery is attained by their spleen cells within 1 week of transfer into irradiated recipients. Deeper tolerance to this epitope was attained in vivo only when these larger doses of B1355 were injected during cyclophosphamide suppression. Two exceptions to this weak tolerogenicty were found. First, BALB/c spleen cells developed durable partial alpha1--3 tolerance following 2-hour incubation with B1355 in vitro. Second, CBA mice were fully tolerized by doses of 1 mg upwards. It is argued from these and other data in the accompanying papers that the relative resistance of BALB/c mice to induction of alpha1--3 tolerance is explicable neither as part of a more general phenomenon based on macrophage activity nor as due to an inadequate epitope density. A possible explanation based on features of the genetically determined high alpha1--3 responsiveness of BALB/c B cells is discussed.