BAFF and APRIL in Chronic Lymphocytic Leukemia: Clinico-Biological Correlates and Prognostic Significance.

Abstract 1235

Poster Board I-257

BAFF (B-cell activating factor) and APRIL (a proliferation-inducing ligand) are TNF family proteins that upregulate anti-apoptotic genes through the NF-kB pathway. Studies in vitro suggest that BAFF and APRIL protect neoplastic B cells from apoptosis in chronic lymphoproliferative disorders (CLPD) including chronic lymphocytic leukemia (CLL). Serum BAFF levels have been previously shown to be lower in CLL than in other CLPD or normal subjects. To contribute to a better understanding of their role in CLL, we analyzed BAFF and APRIL at mRNA and protein serum levels and their receptors [transmembrane activator and CAML interactor (TACI), B-cell maturation antigen (BCMA) and BAFF receptor (BAFF-R)] by flow cytometry, in 82 patients with CLL, 36 with other CLPD and 35 age- and sex-matched controls. mRNA BAFF and APRIL levels were calculated as a the percentage of expression referred to an internal control and the receptor expression as the ratio between the mean fluorescence intensity (MFI) of the receptor antibody and the MFI of the isotype control. Patients with CLL showed significantly lower median BAFF and APRIL levels (0.63 μg/ml and 3.18 μg/ml) than those with other CLPD (1.27 μg/ml and 5.51 μg/ml) (p<0.05). Moreover, BAFF but not APRIL was lower in CLL than in healthy subjects (0.63 μg/ml vs. 0.77 μg/ml; p<0.0001). Serum BAFF levels and blood lymphocyte counts were inversely correlated. Likewise, in follicular lymphoma patients who had circulating neoplastic B cells, median BAFF levels was 0.84 μg/ml vs. 1.46 μg/ml in those without detectable neoplastic cells in blood (p<0.05). We also examined the expression of BAFF and APRIL in purified CD19+ cells from 19 CLL patients and 10 healthy controls. All CLL and normal B cells expressed BAFF and APRIL although heterogeneously. Nevertheless, BAFF and APRIL were lower in CLL than in normal B cells (median: 6.24% and 12.73% in CLL vs. 11.54% and 42.26% in controls). In CLL, mRNA BAFF expression inversely correlated with BAFF serum levels. As far as BAFF and APRIL receptors are concerned, BAFF-R was the one most highly expressed in CLL and normal B cells (MFI ratios of 167.3 and 157.2, respectively). TACI and BCMA were also expressed in all CLL cells and normal B cells (MFI ratios TACI: 1.70 and 2.41; BCMA: 9.51 and 4.72, respectively), but at a significantly lower level than BAFF-R (p<0.001). Furthermore, whereas BCMA MFI ratio was significantly higher in CLL than in normal B cells (p<0.05), no differences were observed in the expression of TACI and BAFF-R. TACI expression was heterogenous in CLL cells. BAFF-R inversely correlated with BAFF and APRIL serum levels. From a clinical standpoint, there is some indication that BAFF and APRIL serum levels as well as their expression in CLL cells may correlate with clinical and biological characteristics of the disease. No significant relationship was observed between BAFF and APRIL and IGVH mutational status, ZAP-70, CD38 or cytogenetics. However, an inverse correlation was observed between BAFF serum levels and blood lymphocyte counts as well as advanced clinical stage (p<0.05). In contrast, APRIL serum levels were only correlated with CD38 expression, the higher the expression of CD38 the higher the APRIL. Although blood lymphocyte counts and BAFF serum levels are correlated, a multivariate analysis showed that these two variables along with poor risk cytogenetics were independent predictors of progression (poor risk cytogenetics RR=11.699, p<0.05; high blood lymphocyte count RR=9.780, p<0.05 and low serum BAFF RR= 6.098, p<0.05). In summary this study confirms that BAFF and APRIL serum levels are lower in CLL than in other CLPD. In patients with CLL, BAFF serum and mRNA levels correlate with blood lymphocyte count and advanced clinical stage but not with other well known prognostic factors. Finally, although BAFF correlates with blood lymphocyte counts, our results suggest that BAFF serum levels have independent prognostic significance.