A, Microchimerism (donor MHC class II+ cells) detected by immunohistochemistry 14 days posttransplant, in spleens of C3H recipients of B10 cardiac allograft pretreated (day –7) with either immature donor-derived DC±anti-CD40L mAb, or mature donor DC. Whereas infusion of donor mature DC 7 days before heart transplant resulted in reduced levels of microchimerism, pretreatment with immature donor DC, especially in combination with anti-CD40L mAb, resulted in the highest observed levels of donor class II+ cells. B, This effect became more pronounced with time posttransplant, and the highest incidences of microchimerism were detected in animals with long-surviving grafts, 100 days posttransplant. Anti-CD40 mAb administration alone did not affect the incidence of donor MHC class II+ cells compared with PBS controls. Magnification, A×200, B×100 (left); ×1000 (right).