CD19 mAb treatment significantly reduced the production of autoantibodies reactive with histones, ssDNA, and dsDNA, whereas control mAb treatment was without effect (Fig. effectivein vivotreatment for non-Hodgkin’s lymphoma (47). Unconjugated anti-CD20 mAb therapy may also ameliorate the manifestations of rheumatoid arthritis, systemic lupus erythematosus, idiopathic thrombocytopenic purpura, and hemolytic anemia, as well as other immune-mediated diseases (810). Despite the effectiveness of this therapy, most pre-B and immature B lymphoblastic leukemias and many other B cell malignancies do not express CD20, express CD20 at low levels, or lose CD20 expression after CD20 mAb immunotherapy (7). Moreover, only half of non-Hodgkin’s lymphoma patients respond to CD20-directed immunotherapy, and CD20 mAb therapy does not reverse the production of pathogenic autoantibodies. However, CD19 is usually a structurally unique cell surface receptor that is expressed from the earliest stages of pre-B cell development until B cell terminal differentiation into plasma cells (11). Thereby, CD19, expressed by most pre-B-acute lymphoblastic leukemias (ALL), common-ALL, null-ALL, non-Hodgkin’s lymphomas, B cell chronic lymphocytic leukemias (CLL), prolymphocytic leukemias, and hairy cell leukemias, represents a potentially important new target for unconjugated mAb immunotherapy (1215). Developing immunotherapies and carrying out mechanistic studies in humans is usually challenging. Moreover, human studies primarily focus on changes in blood, which contains only 2% of the total lymphocyte pool in the normal adult human body (16). Thus, it is hard to accurately ascertain the effects of immunotherapies on the majority of B cells, which are found in peripheral lymphoid tissues. To overcome this difficulty, we developed a transgenic mouse model for assessing CD19-directed immunotherapies that is amenable to mechanistic studies and genetic manipulation and that may predict thein vivooutcome of human therapies. These mice express the humanCD19gene regulated by its endogenous promoter, which recapitulates the developmental pattern of human CD19 (hCD19) cell surface expression (1721). Because of CD19 overexpression, hCD19 transgenic (hCD19TG) mice also develop autoimmune disease (11,19,21). This preclinical model for immunotherapy thereby allowed the identification, characterization, and mechanistic examination of hCD19-directed therapies for early B lymphoblastic leukemias/lymphomas and autoimmunity. == Materials and Methods == Mice.Transgenic mice expressing hCD19 (TG-1 line) and their wild-type littermates were as described (17). TG-1 mice were generated from the original h191 founders (C57BL/6 B6/SJL), Cilostazol and were crossed onto a C57BL/6 background for at least seven generations. Fc receptor common chain (FcR)-/-mice (B6.129P2-Fcerg1tm1) from Taconic Farms were crossed and backcrossed with TG-1+/+mice to generate hCD19TG+/-FcR-/-and hCD19TG+/-FcR+/-littermates. Mice hemizygous for any c-Myctransgene [cMycTG, C57BL/6J-TgN(IghMyc); The Jackson Laboratory] were crossed with hCD19TG+/+mice to generate hCD19TG+/-cMycTG+/-offspring as determined by PCR screening (22,23). Rag1-/-(B6.129S7-Rag1tm1Mom/J) mice were from your Jackson Laboratory. Macrophage-deficient mice were generated by tail vein injections of liposome-encapsulated clodronate (0.1 ml per 10 g of body weight; Sigma) on days -2, 1, and 4 as explained (24). All mice were housed in a specific pathogen-free barrier facility and first used at 69 weeks of age. These studies were approved by the Duke University or college Animal Care and Use Committee. Abs and Cilostazol Immunofluorescence Analysis.The HB12a and HB12b (IgG1) mAbs were generated as described (25,26). Other mouse anti-hCD19 mAbs included FMC63 (IgG2a, Chemicon; and a nice gift from Heddy Zola, Child Health Research Institute, Adelaide, Australia) (2729), B4 (IgG1, Beckman Coulter) (12), and HD237 (IgG2b, Fourth International Workshop on Human Leukocyte Differentiation Antigens, Vienna, 1989), an isotype switch variant of the HD37 mAb HMGCS1 (30). The mouse anti-mouse CD19 (mCD19) mAb MB191 (IgA) was as explained (19). Mouse CD20-specific mAbs, MB20-11 and MB20-18, were as explained (31). Other mAbs included: B220 mAb RA36B2 (provided by Robert Coffman, DNAX); Thy1.2 mAb (Caltag, Burlingame, CA); and anti-mouse CD19 (1D3), CD5, CD43, and CD25 mAbs (BD Pharmingen). Isotype-specific and anti-mouse Ig or IgM Abs were Cilostazol from Southern Biotechnology Associates. For immunofluorescence analysis, single-cell leukocyte suspensions were stained on ice by using predetermined optimal concentrations of each Ab for 2030 min as explained (17). Next, 10,000 cells with the forward and side light-scatter properties of.