Human CTLA-4 is expressed in situ on T lymphocytes in germinal centers, in cutaneous graft-versus-host disease, and in Hodgkin's disease.

Cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4, CD152) is a molecule expressed on in vitro activated T cells. CTLA-4 shares important sequence homology with CD28 and binds to the same ligands, CD80 (B7-1) and CD86 (B7-2). CTLA-4 probably functions as a negative regulator of T lymphocyte activation in the mouse, although this remains to be proven for human T lymphocytes. We have developed new monoclonal antibodies against human CTLA-4 and have investigated the in situ expression of CTLA-4 in a wide variety of normal and pathological human tissues expressing CD80 and CD86. As revealed in this study, CTLA-4 is expressed on thymocytes in thymic medulla, on a subset of CD4+ T lymphocytes in germinal centers of follicular hyperplasia, on T cells, mainly CD8+, infiltrating skin affected by graft-versus-host disease, and on T cells, mainly CD4+, infiltrating Hodgkin's disease lesions. In immunoelectron microscopy, CTLA-4 was found on the plasma membrane as well as in the hyaloplasm and cytoplasmic vesicles, in agreement with its pattern of expression on in vitro activated T cells. Interestingly, no or at most scarce expression of CTLA-4 was found in granulomatous lymph nodes, T-cell-mediated inflammatory diseases, or non-Hodgkin's lymphomas, regardless of their expression of CD80 or CD86. Thus, expression of CTLA-4 appears to be induced in selective pathological conditions in vivo. The pathways leading to selective induction of CTLA-4 and its role in the pathophysiology of these conditions need to be further investigated.