THE TISSUE DISTRIBUTION OF THE B7-2 COSTIMULATOR IN MICE - ABUNDANT EXPRESSION ON DENDRITIC CELLS IN-SITU AND DURING MATURATION IN-VITRO

B7-2 isa recently discovered, second ligand for the CTLA-4/CD28, T cel l signaling system. Using the GL-1 rat monoclonal antibody (mAb), we m onitored expression of B7-2 on mouse leukocytes with an emphasis on de ndritic cells. By cytofluorography, little or no B7-2 was detected on most cell types isolated from spleen, thymus, peritoneal cavity, skin, marrow, and blood. However, expression of B7-2 could be upregulated i n culture. In the case of epidermal and spleen dendritic cells, which become highly immunostimulatory for T cells during a short period of c ulture, the upregulation of B7-2 was dramatic and did not require adde d stimuli. Lipopolysaccharide did not upregulate B7-2 levels on dendri tic cells, in contrast to macrophages and B cells. By indirect immunol abeling, the level of staining with GL-1 mAb exceeded that seen with r at mAbs to several other surface molecules including intercellular adh esion molecule 1, B7-1, CD44, and CD45, as well-as new hamster mAbs to CD40, CD48, and B7-1/CD80. Of these accessory molecules, B7-21 was a major species that increased in culture, implying a key role for B7-2 in the functional maturation of dendritic cells. B7-2 was the main (>9 0%) CTLA-4 ligand on mouse dendritic cells. When we applied GL-1 to ti ssue sections of a dozen different organs, clear-cut staining with B7- 2 antigen was found in many. B7-2 staining was noted on liver Kupffer cells, interstitial cells of heart and lung, and profiles in the submu cosa of the esophagus. B7-2 staining was minimal in the kidney and in the nonlymphoid regions of the gut, and was not observed at all in the brain. In the tongue, only rare dendritic cells in the oral epitheliu m were B7-2(+), but reactive cells were scattered about the interstiti al spaces of the muscle. In all lymphoid tissues, Gl-1 strongly staine d certain distinct regions that are occupied by dendritic cells and by macrophages. For dendritic cells, these include the thymic medulla, s plenic periarterial sheaths, and lymph node deep cortex; for macrophag es, the B7-2-rich regions included the splenic marginal zone and lymph node subcapsular cortex. Splenic B7-2(+) cells were accessible to lab eling with GL-1 mAb given intravenously. Dendritic cell stimulation of T cells (DNA synthesis) during the mixed leukocyte reaction was signi ficantly (35-65%) blocked by GL-1. The block could be enhanced by addi ng 1G10 anti-B7-1 or by using CTLA-4 Ig, a ligand for both B7-1 and B7 -2. We conclude that B7-2, like other accessory molecules, is expresse d by many types of antigen-presenting cells. However, the regulation a nd extent of B7-2 expression seems to differ among cell types. Dendrit ic cells express very high levels, in several sites in vivo and after maturation into strong accessory cells in culture.