In vitro recall of proliferative and cytolytic responses to minor histocompatibility antigens by dendritic cell enriched canine peripheral blood mononuclear cells

It is difficult in vitro to demonstrate existent in vivo sensitization of dogs and humans to minor histocompatibility antigens. Using conventional one-way mixed leukocyte culture, when sensitized blood cells are stimulated with MHC antigen-matched sibling PBMC bearing the minor histocompatibility antigens, there is usually no proliferative or cytotoxic response detected. We reported previously that 0 of 17 dogs sensitized by transfusion of dog leukocyte antigen-identical littermate blood had proliferative responses in mixed leukocyte culture when unfractionated sibling PBMC were used as stimulator cells. We reasoned that this result might be due to the inability of unfractionated PBMC to efficiently present minor histocompatibility antigens to the in vivo-primed T cells, a function thought best performed by dendritic cells. When we used a low buoyant density Percoll fraction of canine PBMC, shown previously to be enriched in dendritic cells, as stimulator cells, we were able to generate cytotoxic and/or proliferative responses in mixed leukocyte culture in all 5 dogs that had been sensitized to minor histocompatibility antigens by transfusions of dog leukocyte antigen-identical sibling littermate blood. By contrast, using unfractionated PBMC as stimulator cells, we found evidence of sensitization in only 1 of the 5 dogs. These data support the concept that the presentation of minor histocompatibility antigens, in contrast to major histocompatibility antigens, to the immune system may be restricted to a subpopulation of professional APC.