The immune response of mice to the α-l-6 epitope of dextran (Dx) B512 was found to be under genetic control. The congenic mouse strains A, A.CA, A.SW, A.TH, and A.TL exhibited a specific defect in their response to α-l-6. Also strain CBA/N was unresponsive to α-1-6, but the mechanism of unresponsiveness was found to be different.

Unresponsiveness to α-l-6 in congenic A strains was not due to suppressor cells. Although these strains failed to respond to the α-l-6 epitope, they responded strongly to the hapten Fluorescein isothiocyanate (FITC) conjugated to Dx, indicating that the Dx can function as an efficient carrier in these strains. Dx was a potent polyclonal B-cell activator in congenic A strains as well as in high responder strains. Polyclonally-activating concentrations of lipopolysaccharide (LPS) failed to induce the synthesis of anti-α- l-6 antibodies in congenic A strains, although antibodies of all other specificities studied were produced. However, in high responder strains, LPS induced the synthesis of anti-α-l-6 antibodies. It was concluded that congenic A strains do not express V genes coding for antibodies against α-l-6.

In contrast, strain CBA/N failed to respond to both the α-l-6 and FITC epitope on Dx, whereas they could respond to FITC conjugated to horse erythrocytes. Dx induced a very small, if any, polyclonal antibody response in B cells from CBA/N mice or male CBA/N x DBA hybrids, whereas Dx was a very potent polyclonal B-cell activator in female hybrids. It is concluded that CBA/N mice are nonresponders to Dx or haptenated Dx, because the cell population that can respond to the polyclonal B-cell activating properties of Dx is severely depleted.

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