Different Expression of FcgammaRIIb in Chronic Lymphocytic Leukemia and Human Normal B Lymphocytes

The Fcgamma receptors (FcγRs) are a family of molecules that modulate immune responses. FcγRIIb is an inhibitory FcγR that bears immunoreceptor tyrosine-based inhibitory motifs which transduce inhibitory signals on coligation with the surface membrane Ig of the B-cell antigen receptor (BCR). The role of FcγRIIb in controlling B cell activation through inhibition of BCR signaling has been extensively studied in animal models. Nevertheless, data on FcγRIIb are scant in human normal and neoplastic B cells, this being due to the lack of a specific antibody for human FcγRIIb. Consequently, there is little information on this receptor in chronic lymphocytic leukemia (CLL). Considering the activated nature of CLL cells and the central role of the BCR in the biology of the disease, studies of FcγRs are warranted. We used a novel specific mAb directly conjugated with Alexa 488 fluorophore that solely reacts with the human FcγRIIb (MacroGenics, Inc.) to investigate the receptors expression on CLL and normal human B cells. The study population included 84 patients with CLL and 24 age- and sex-matched controls. FcγRIIb expression was assessed as the mean fluorescence intensity (MFI) of surface membrane staining. In CLL cells, FcγRIIb was measured on CD19⁺CD5⁺ cells in combination with CD38, CD49d or CD69. Normal B cells were immunostained for CD19, CD5, IgD and CD38 expression and B cell subsets: naïve (IgD⁺CD38⁻), activated (IgD⁺CD38⁺) and memory B cells (IgD⁻CD38⁻) were studied for their relative expression of FcγRIIb. FcγRIIb expression was found significantly higher in naïve B cells compared to activated and memory B cells [median MFI: 17420 (11960–21180) vs. 11.140 (7899–16970) and 11.830 (6984–17100); p<0.001]. Significant differences were also observed between CD5⁻ and CD5⁺ normal B cells. In contrast, FcγRIIb expression was lower in CLL cells than in CD5⁺ and CD5⁻ normal B lymphocytes [median MFI: 6901(1034–42600), 10180 (5856–14820) and 12120 (7776–16040); p<0.05)]. Interestingly, FcγRIIb expression was variable within individual CLL clones, this being higher in CD38⁺ and CD49d⁺ cells than in CD38⁻ and CD49d⁻ cells (p<0.05). Furthermore, the highest density of FcγRIIb was observed on those cells which coexpressed CD38 and CD49d. In contrast, no significant differences were observed between FcγRIIb and the expression of the activation antigen CD69.

Although CD69 and CD38 expression was significantly higher on unmutated IGHV cases, no correlation was found between FcγRIIb levels and IGHV mutational status. Similarly, there was no correlation between FcγRIIb and other poor prognostic variables such as ZAP-70 (≥20%), CD38 (≥ 30%) or high risk cytogenetics. Nevertheless, cases with ≥ 30% CD49d⁺ cells had higher FcγRIIb expression than those with <30% CD49d⁺ cells (p=0.006). The findings presented in this study suggest a hierarchy of FcγRIIb expression in normal B-cells, CLL cells and their subpopulations: circulating normal CD5⁻ B cells > circulating normal CD5⁺ B cells > circulating CD5⁺ CLL B cells. In addition, although FcγRIIb is present on all normal B cell subsets its expression is higher in naïve B cells. Furthermore, in CLL FcγRIIb density is greater in CD38⁺ and CD49d⁺ cells within the clone. Although CD49d and FcγRIIb on CLL clones is linked in a direct manner, there is no relationship with FcγRIIb density and IGHV mutations, ZAP-70, CD38 and unfavorable cytogenetic markers. Finally, the relationship between FcγRIIb expression on CLL cells and functional responses to BCR and other receptor-mediated signals deserve further investigation.