Comprehensive Analysis of BAFF Binding Receptor Profiles and Receptor Occupancy in B Cell Chronic Lymphocytic Leukemia: Identification of Discrete Phenotypic Subgroups.
Authors: Tschumper Renee C. , Darce Jaime R. , Wu Xiaosheng , Mihalcik Stephen A. , Jelinek Diane F. 
Published: October 2019

Abstract
B cell-activating factor (BAFF) is known to regulate normal B cell development and homeostasis primarily by signaling through the high affinity receptor, BAFF-R, one of three BAFF binding receptors (BBRs). BAFF also binds two other receptors, BCMA and TACI with lesser affinity. We have recently shown that normal peripheral blood (PB) B cells express high levels of prebound soluble BAFF, which is lost upon B cell activation. Because of BAFF’s activity on normal B cells, we have been interested in the roles of BAFF and BBRs in B cell chronic lymphocytic leukemia (B-CLL). We and others have demonstrated that BAFF promotes primary CLL B cell survival and that serum BAFF levels are elevated in some patients. Although CLL B cells are known to express BBRs, a comprehensive and quantitative analysis of BBR levels and CLL B cell capacity to bind BAFF has not yet been done. We began this study by characterizing the level of soluble BAFF bound to freshly isolated CLL B cells, measured by both western blot analysis and flow cytometry. To assess receptor occupancy, cells were incubated with or without exogenous BAFF before assessing anti-BAFF reactivity and changes in median fluorescence intensity (ΔMFI; defined by dividing the MFI of the anti-BAFF antibody by the MFI of the isotype matched control antibody) were calculated. Normal B cells have higher detectable levels of bound BAFF with a ΔMFI ranging from 16 to 35 (mean=22.2). Upon addition of exogenous BAFF, the ΔMFI range increased to 27–96.6 (mean=49.1; n=8). Thus, despite evidence of prebound BAFF, clearly not all BBRs were occupied on normal PB B cells. By contrast, the levels of prebound BAFF on CLL B cells were significantly lower with a ΔMFI ranging from 1 to 13.1 (mean=2.7; n=36). Of note, 10/36 patients did not exhibit increased anti-BAFF reactivity upon incubation with exogenous BAFF (mean fold induction=0.8) whereas 26/36 patients displayed a mean fold induction of anti-BAFF reactivity of 3.5. These observations prompted us to next quantitate CLL B cell BBR expression. All patient CLL B cells expressed BAFF-R but at significantly lower levels than observed in normal B cells (p=0.0009). When CLL patients were categorized into IGHV mutated (M; n=22) and unmutated (UM; n=24), UM patients were observed to express higher levels of BAFF-R (ΔMFI =8.9) than M patients (ΔMFI =5.24). Regarding TACI, we previously demonstrated that normal memory B cells uniformly express TACI (ΔMFI =12.7; n=10) and there is a small population of activated naïve B cells that express TACI at lower levels (ΔMFI =8.3; n=10). In our CLL cohort, 14/22 M patients were TACI+ (ΔMFI =7.0) and 19/24 UM patients were TACI+ (ΔMFI =4.7). Finally, whereas normal PB B cells completely lack BCMA expression, 7/22 M and 4/22 UM patients expressed BCMA. Thus, using the BBR profile and analysis of expression levels relative to normal PB B cells, the following subgroups of B-CLL can be defined:
BAFF-R+; BAFF-R/TACI+; BAFF-R/BCMA+; BAFF-R/TACI/BCMA+.
It remains to be determined if these BBR profiles correlate with aspects of clinical disease. In addition, given the putative importance of BAFF in this disease, it is interesting to note that in general, CLL B cells display overall lower levels of prebound BAFF. Current studies are focused on determining whether this reflects CLL B cell activation status, increased competition for BAFF, and/or reduced levels of BBR expression.