Constitutive Expression of MHC Class II Genes in Melanoma Cell Lines Results from the Transcription of Class II Transactivator Abnormally Initiated from Its B Cell-Specific Promoter 1 Virginie Deffrennes,* Jocelyn Vedrenne,* Marie-Claude Stolzenberg, ² Janet Piskurich, ‡ Giovanna Barbieri, 2 * Jenny P. Ting, § Dominique Charron,* and Catherine Alcaı ¨de-Loridan 3 * In melanoma cell lines, two different patterns of MHC class II expression have been described, either an IFN -inducible ex- pression of HLA-DR and HLA-DP, with a faint or null expression of HLA-DQ, resembling that described for melanocytes, or a constitutive expression, i.e., IFN- independent, of all three HLA-D isotypes. As this latter phenotype has been associated with a more rapid progression of melanoma tumors, we have analyzed in different melanoma cell lines the molecular mechanisms leading to this abnormal pattern of MHC class II expression. In agreement with the evidence of a coordinate transcription of the HLA-D genes in these cell lines, we have shown the constitutive expression of CIITA (class II transactivator) transcripts, CIITA being known as the master switch of MHC class II expression. Unexpectedly, these transcripts initiate from promoter III of the CIITA gene, a promoter that is mainly used constitutively in B lymphocytes. This expression was further shown to occur through factor(s) acting on the enhancer located upstream of CIITA promoter III, which was previously described in epithelioid cells as an IFN- -response sequence. The hypothesis of a general abnormality of the IFN- transduction pathway was dismissed. Constitutive transcription of CIITA from promoter III having been observed in unrelated melanoma cell lines, we propose the hypothesis that this phenomenon might not be a random event, but could be linked to the neoplasic state of the melanoma cells. The Journal of Immunology, 2001, 167: 98 –106. M elanoma arise from melanocytes that are pigment- producing cells derived from the neural crest, and represent the most lethal type of cutaneous cancers. As chemotherapy is poorly efficient on this type of cancers (1), mainly on distant metastasis, immunotherapy has been consid- ered. With the evidence of specific tumor Ags presented by MHC class I molecules (reviewed in Ref. 2), vaccination of patients with tumoral peptides (3, 4) was undertaken. This was demonstrated to be efficient, even though moderated by results showing that these treatments might induce immune escape of the tumors through MHC class I loss of expression (5) or mutations in the vaccinated tumor Ag (4). More recently, attention has focused on the role of CD4 + lym- phocytes in tumor elimination (see Ref. 6 for a review) with the evidence of tumor Ag-specific CD4 + lymphocytes (7, 8). How- ever, even though both CD4 + and CD8 + cells infiltrate the mel- anoma tumors (9), these cancer cells often acquire the capacity to escape immune surveillance through lymphocyte anergy via mech- anisms that are not yet unraveled (10). In addition, melanoma cells often display a constitutive expression of MHC class II molecules (11, 12), which is usually restricted to professional APC (13). Al- though MHC class II presentation of tumor Ags by melanoma cells has been described (14), MHC class II expression was shown to be associated with a better progression of primary melanoma and a higher metastatic dissemination (15). MHC class II constitutive expression in melanomas is considered nowadays as a progression marker (16, 17). Therefore, it was proposed that MHC class II- expressing tumors might mimic an APC and induce lymphocyte anergy by the lack of accessory signals (18). Within the hypothesis of a role in the immune escape and tolerance induction, mecha- nisms leading to the constitutive expression of MHC class II Ags in melanoma cells must be investigated. MHC class II molecules are represented by three isotypes, HLA-DR, HLA-DQ, and HLA-DP, consisting of a heterodimer of transmembrane glycoprotein  and  encoded by distinct genes. As mentioned above, constitutive expression of MHC class II molecules is restricted to professional APC such as dendritic cells, macrophages, and B lymphocytes. However, expression of MHC class II molecules is inducible by various cytokines, of which the most potent is IFN- in many of cells such as fibroblasts, epithelial cells (see Ref. 13 for a review), or melanocytes (19). In addition to a tissue-specific expres- sion, MHC class II molecules are subject to a differential expression depending on cell activation with T lymphocytes or differentiation in *Unite ´ Institut National de la Sante ´ et de la Recherche Me ´dicale, Unite ´ 396, Centre de Recherches Biome ´dicales des Cordeliers, Paris, France; ² Unite ´ Institut National de la Sante ´ et de la Recherche Me ´dicale, Unite ´ 429, Ho ˆpital Necker, Paris, France; ‡ Department of Basic Sciences, Mercer University Medical School, Macon, GA 31207; and § Lineberger Comprehensive Cancer Center, Department of Microbiology and Immunology, University of North Carolina, Chapel Hill, NC 27599 Received for publication October 31, 2000. Accepted for publication April 18, 2001. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 This work was supported in part by the Fondation pour la Recherche Me ´dicale, Institut National de la Sante ´ et de la Recherche Me ´dicale (Progre `s), and the Associ- ation pour la Recherche sur le Cancer. V.D. was the recipient of a grant from the Association pour la Recherche sur le Cancer, and J.V. was supported by the Ligue Nationale contre le Cancer et la Fondation pour la Recherche Me ´dicale. 2 Current address: Istituto di Biologia dello Svillupo, Consiglio Nazionale delle Ricerche (CNR), 152 via Ugo La Malfa, Palermo, Italy. 3 Address correspondence and reprint requests to Dr. Catherine Alcaı ¨de-Loridan. Unite ´ d’Immunoge ´ne ´tique Humaine, Institut National de la Sante ´ et de la Recherche Me ´dicale, Unite ´ 396, Centre de Recherches Biome ´dicales des Cordeliers, 15 rue de l’Ecole de Me ´decine, 75270 Paris cedex 06, France. E-mail address: Catherine. Alcaide@bhdc.jussieu.fr Copyright © 2001 by The American Association of Immunologists 0022-1767/01/$02.00