Kyoko Hayakawa, MD, PhD
Office Phone: 215-728-5362
1. B Cell Subset Development and Maintenance
ATA (anti-thymocyte/Thy-1 autoantibody) is an autoreactive specificity encoded by an un-mutated B cell receptor (BCR) that is associated with the CD5+ B1 cell subset (J Exp Med. 172:869, 1990). By establishing ATA BCR transgenic mouse lines on Thy-1 positive or negative backgrounds using Thy-1 gene knockout mice, we discovered that the presence of self-antigen, in turn, has a positive effect on B cells with this specificity. The presence of self-antigen also promotes the accumulation of ATA B1 cells, and induces ATA autoantibody in serum, a process we termed "positive selection," (Science 285:113, 1999). The question now is whether this "positive selection" occurred in all developing B cells with this specificity, regardless of fetal or adult origin. While our answer was yes, the interpretation of our results is complex.
First, we found that ATA B cell positive selection is not a general feature of adult B cell development exposed to normal (wild type) levels of Thy-1 antigen; rather, such exposure leads to maturation arrest, a form of "negative selection," (J Exp Med. 197:87, 2003). However, if the amount of self-antigen is decreased, we observed a type of "positive selection," producing a unique population of B cells in the spleen, residing in the marginal zone (MZ B cells), (Immunity 23:297, 2005). In contrast, the major population of mature B cells normally found in this region, and referred to as follicular B cells (FO B or B2), are not generated if Thy-1 self-antigen is present. Thus, the level of BCR signaling experienced by newly formed B cells critically influences development of the three mature B cell compartments: B1 > MZ B >> FO B.
Although such research with μTg or μκTg mouse lines has significantly advanced our understanding of B cell subsets, a concern remains about potentially higher tonic BCR signaling due to expression of the transgene at a level higher than physiologic, which might have promoted survival of naïve FO B cells. In addition, Ig isotype switching, hypermutation and memory B generation could not be addressed with this μTg system. Thus, we generated a VH3609/D/JH2 targeted insertion into the JH locus, a "knock-in" mouse line VH3609t. By crossing this VH3609 knock-in line with different light chain transgenic mouse lines, we can now address new questions, previously unaddressed with the heavy chain transgene system.Top
2. B-1 Cell and B Cell Chronic Leukemia/Lymphoma
B cell chronic lymphocyte leukemia/lymphoma (B-CLL) exhibits distinctively restricted BCR usage and it has long been speculated that BCR signaling plays a role in the development of this disease. The characteristic, CD5+ surface phenotype, of B-CLL cells accounts for the involvement of BCR signaling, as supported by our research that the level of CD5 induction on B cells is correlated with the strength of BCR cross linking (Immunity. 23:297, 2005). However, direct evidence for (self) antigen involvement for B-CLL is currently lacking, and the exact cellular origin(s) and antigen exposure required to develop CD5+ B-CLL cells remain in dispute. Furthermore, B lymphomagenesis, along with aging, is likely to involve multiple genetic and/or epigenetic alterations. To assess the role of the B1 subset in leukemia and discover potential mechanism(s) of transformation, we are currently investigating a CD5+ B CLL mouse model, in which VH-Eμ TCL1 Tg animals expressing an AKT co-activator TCL1A oncoprotein is in the B-lineage. These studies involve crossing this TCL1 Tg line with various BCR transgenic mouse lines developed in our laboratory, that exhibit predominant B1, MZ B or FO B cell subset development respectively. Experiments combining such TCL1Tg mice with our autoantigen-controllable BCR Tg mice provides an opportunity to identify the cellular origin(s) of CD5+ B lymphoma in mice, dissecting the stepwise progression of CD5+ B lymphomagenesis.Top