Leukemic cell expressed CTLA-4 suppresses T cells via down-modulation of CD80 by trans-endocytosis

Abstract

Abstract Despite clinical utility in targeting the immune checkpoint, Cytotoxic T Lymphocyte Antigen 4 (CTLA-4), on T cells, its function on non-T cells remains unaddressed, especially in the context of therapy. We define an immunosuppressive role for tumor expressed CTLA-4 in Chronic Lymphocytic Leukemia (CLL). Most primary CLL samples were intracellularly CTLA-4+ (12/14) but surface CTLA-4− (14/14). Co-culture with activated T cells induced surface expression of CTLA-4 on CLL B cells. To mechanistically study CLL B cell expressed CTLA-4, we generated CLL-derived Mec1 and OSU-CLL cell lines to inducibly express CTLA-4. These cell lines express the ligands for CTLA-4, CD80 and CD86. Expression of CTLA-4 on Mec1 and OSU-CLL resulted in decreased CD80 expression on CD80+ cells with rescue upon CTLA-4 blockade (N=3). Co-culture of CTLA-4+ Mec1 and CTLA-4+ primary CLL cells with CD80-GFP+ Hek293 cells revealed transfer of CD80-GFP into primary CLL cells (N=2) and the Mec1 cell line. This finding was consistent with the ability of CTLA-4+ T cells to trans-endocytose CD80. Co-culture of T cells with Mec1 CTLA-4+ cells resulted in decreased IL2 production (N=3, p=0.0172). The loss of IL2 signified decreased co-stimulation as a result of tumor expressed CTLA-4. We generated an in vivo model to study tumor CTLA-4 using the TCL1 murine model of CLL. Murine CTLA-4+ (mCTLA-4) TCL1 tumor cells were engrafted into human CTLA-4+/+mFcγR−/− immune competent mice. MCTLA-4 on the tumor cells but not human CTLA-4 on the host cells was blocked with an anti-mCTLA-4 antibody. Effect on disease progression and overall survival are currently being assessed. We present a paradigm shift whereby CTLA-4 is an immunosuppressive protein irrespective of expression on T cells.