In situ expression of B7/BB1 on antigen-presenting cells and activated B cells: an immunohistochemical study.

B7/BB1 is a physiological ligand for CD28, a receptor expressed on a major subset of T lymphocytes. B7/BB1 has been shown to be expressed on human blood dendritic cells and on in vitro activated (but not resting) B cells and monocytes. Ligation of CD28 with B7/BB1 up-regulates cytokine production and prevents the induction of anergy in T cells activated through TCR/CD3. We examined the in situ expression of B7/BB1 by immunohistochemistry with a novel mAb B7-24. Dendritic cells in skin (Langerhans cells), lymph node sinuses (veiled cells), and T cell zones of spleen and lymph nodes (interdigitating dendritic cells) were strongly positive for B7/BB1. B7/BB1 was also present on fetal thymus dendritic cells located at the cortico-medullar junction and the medulla, but absent in normal adult thymuses. Resident macrophages and endothelial cells did not stain, but in granulomatous inflammations B7/BB1 was found on macrophages and epitheloid cells. A subset of B immunoblasts and of germinal center B cells in lymph node and spleen was also found to express B7/BB1. Our findings on the distribution of B7/BB1 expression in tissues, in particular its expression on professional antigen-presenting cells, further substantiate the putative co-stimulatory role of B7/BB1 in T cell activation in vivo. The presence of B7/BB1 in fetal but not adult thymic medulla suggests a role for B7/BB1 in thymocyte maturation.