Mechanisms involved in the weak alloimmunogenicity of Thy-1 on mouse brain

Immunization of mice with allogeneic brain homogenates fails to elicit an appreciable response to the Thy-1 antigen. The aim of the present study was to determine the mechanism of the low immunogenicity of allogeneic brain. Immunization of C3H mice with freshly prepared mixtures of AKR/J thymus cells with either AKR/J or C3H brain homogenates elicited a primary response to the Thy-1.1 antigen as effectively as immunization with AKR/J thymus cells. When the mixtures were incubated overnight prior to injection, only AKR/J brain homogenate but not C3H brain homogenate abrogated the capacity of AKR/J thymus cells to elicit an anti-Thy-1.1 response. Suppressor T cells did not seem to be responsible for the inability of brain to elicit an anti-Thy-1 response, as mice that received cyclophosphamide 2 days prior to injection of brain did not produce Thy-1 antibodies. Antigenic competition also did not seem to be the cause for the weak immunogenicity of Thy-1 on brain, as mixtures of thymus and brain were capable of eliciting a primary response. When spleen cells from mice hyperimmunized against Thy-1 were transferred to normal syngeneic mice, subsequent immunization with brain homogenate was capable of eliciting Thy-1 antibodies. These results indicate that Thy-1 on brain may resemble a hapten, in being incapable of eliciting a primary immune response, but behaves like a complete antigen in boosting a secondary response.