The BB1 Monoclonal Antibody Recognizes Both Cell Surface CD74 (MHC Class II-Associated Invariant Chain) as Well as B7-1 (CD80), Resolving the Question Regarding a Third CD28/CTLA-4 Counterreceptor

• Published in: The Journal of immunology (1950) 1998-09, Vol.161 (6), p.2708-2715
• Main Authors: Freeman, Gordon J, Cardoso, Angelo A, Boussiotis, Vassiliki A, Anumanthan, Anukanth, Groves, Richard W, Kupper, Thomas S, Clark, Edward A, Nadler, Lee M
• Format: Article
• Language: English
• Subjects: 3T3 Cells; Abatacept; Amino Acid Sequence; Animals; Antibodies, Blocking - pharmacology; Antibodies, Monoclonal - metabolism; Antibodies, Monoclonal - pharmacology; Antibody Specificity; Antigens, CD; Antigens, Differentiation - immunology; Antigens, Differentiation - metabolism; Antigens, Differentiation, B-Lymphocyte - genetics; Antigens, Differentiation, B-Lymphocyte - immunology; B7-1 Antigen - biosynthesis; B7-1 Antigen - immunology; Binding Sites, Antibody - immunology; CD28 Antigens - immunology; CD28 Antigens - metabolism; CHO Cells; Cloning, Molecular; COS Cells; Cricetinae; CTLA-4 Antigen; Histocompatibility Antigens Class II - genetics; Histocompatibility Antigens Class II - immunology; Humans; Immunoconjugates; Immunoglobulin Fc Fragments - metabolism; Interferon-gamma - pharmacology; Keratinocytes - metabolism; Mice; Molecular Sequence Data; Phenotype; Precursor B-Cell Lymphoblastic Leukemia-Lymphoma - immunology; Recombinant Fusion Proteins - metabolism; RNA, Messenger - biosynthesis; Tumor Cells, Cultured
• ISSN: 0022-1767; 1550-6606
• DOI: 10.4049/jimmunol.161.6.2708

The identification of all CD28/CTLA-4 counterreceptors is critical to our understanding of this pivotal pathway of T cell activation. Clouding our understanding has been the reported discrepancies in expression and function of the B7-1 (CD80) molecule based upon the use of the BB1 vs other anti-B7-1 mAbs. To resolve this issue, we have cloned a BB1-binding molecule from the BB1+B7-1(-) NALM-6 pre-B cell line. Here, we demonstrate that this BB1-binding molecule is identical to the cell surface form of CD74 (MHC class II-associated invariant chain). CD74-transfected cells bound the BB1 mAb but not other anti-CD80 mAbs, CD28-Ig, or CTLA4Ig. Absorption and blocking experiments confirmed the reactivity of BB1 mAb with CD74. A region of weak homology was identified between CD74 and the region of B7-1 encoding the BB1 epitope. Therefore, the BB1 mAb binds to a protein distinct from B7-1, and this epitope is also present on the B7-1 protein. Many of the puzzling observations in the literature concerning the expression of human B7-1 are resolved by an understanding that BB1 staining is the summation of CD74 plus B7-1 expression. This observation requires the field to reconsider studies using BB1 mAb in the analysis of CD80 expression and function.