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.
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.
Freeman
G J
GJ
Department of Adult Oncology, Dana-Farber Cancer Institute, Brigham and Women's Hospital, Boston, MA 02115, USA. gordon freeman@macmailgw.dfci.harvard.edu
Cardoso
A A
AA
Boussiotis
V A
VA
Anumanthan
A
A
Groves
R W
RW
Kupper
T S
TS
Clark
E A
EA
Nadler
L M
LM
D016475:3T3 Cells; D000069594:Abatacept; D000595:Amino Acid Sequence; D000818:Animals; D019138:Antibodies, Blocking / Q000494:pharmacology; D000911:Antibodies, Monoclonal / Q000378:metabolism* / Q000494:pharmacology; D000918:Antibody Specificity; D015703:Antigens, CD; D000943:Antigens, Differentiation / Q000276:immunology / Q000378:metabolism*; D000944:Antigens, Differentiation, B-Lymphocyte / Q000235:genetics / Q000276:immunology*; D018122:B7-1 Antigen / Q000096:biosynthesis / Q000276:immunology*; D001666:Binding Sites, Antibody / Q000276:immunology; D018106:CD28 Antigens / Q000276:immunology / Q000378:metabolism*; D016466:CHO Cells; D019556:COS Cells; D060908:CTLA-4 Antigen; D003001:Cloning, Molecular; D006224:Cricetinae; D000949:Histocompatibility Antigens Class II / Q000235:genetics / Q000276:immunology*; D006801:Humans; D018796:Immunoconjugates; D007141:Immunoglobulin Fc Fragments / Q000378:metabolism; D007371:Interferon-gamma / Q000494:pharmacology; D015603:Keratinocytes / Q000378:metabolism; D051379:Mice; D008969:Molecular Sequence Data; D010641:Phenotype; D015452:Precursor B-Cell Lymphoblastic Leukemia-Lymphoma / Q000276:immunology*; D012333:RNA, Messenger / Q000096:biosynthesis; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D014407:Tumor Cells, Cultured
D019138:Antibodies, Blocking; D000911:Antibodies, Monoclonal; D015703:Antigens, CD; D000943:Antigens, Differentiation; D000944:Antigens, Differentiation, B-Lymphocyte; D018122:B7-1 Antigen; D018106:CD28 Antigens; D060908:CTLA-4 Antigen; C556706:CTLA4 protein, human; C556707:Ctla4 protein, mouse; D000949:Histocompatibility Antigens Class II; D018796:Immunoconjugates; D007141:Immunoglobulin Fc Fragments; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; C045084:invariant chain; D000069594:Abatacept; D007371:Interferon-gamma