Expression of Fcγ receptors type II (FcγRII) in chronic lymphocytic leukemia B cells

We have read with interest the paper from Damle et al¹  analyzing the surface membrane phenotype of B lymphocytes in chronic lymphocytic leukemia (B-CLL). The authors stated that the leukemic cells from all B-CLL patients evaluated (irrespective of immunoglobulin heavy chain (IgV_H) gene mutational status) bear the phenotype of antigen-experienced B cells based, among other features, on the very low expression of Fcγ receptors type IIb (FcγRIIb, CD32), which is the main isoform of FcγRII in B lymphocytes.²  We would like to comment on this issue on the basis of our own results, which differ from those of Damle et al.

We analyzed membrane expression of FcγRII by flow cytometry in leukemic cells from 52 B-CLL patients who were classified by Rai stage system as indolent (0-I), intermediate (II), or aggressive (III-IV) disease. We have used 3 different monoclonal antibodies (mAbs): clones AT10 and 2E1, which recognize all isoforms of FcγRII; and clone IV.3, which recognizes FcγRIIa when used as Fab fragment but is capable of reacting with FcγRIIb when used as a whole molecule.³  IV.3 Fab was tested because, to our knowledge, it is the only mAb capable of discriminating between FcγRII isoforms by fluorescence-activated cell sorter analysis; in fact, mAb II8D2 used by Damle et al has been shown to react with both isoforms.⁴  By using IV.3 Fab, we found that FcγRIIa is expressed only marginally in B-CLL cells from some patients (data not shown). On the other hand, more than 95% of leukemic cells, in all samples analyzed, were stained with mAbs AT10, 2E1, or IV.3 (whole molecule), which can be attributed to the presence of FcγRIIb. Moreover, we found that B-CLL cells displayed comparable or even higher levels of FcγRII expression than B lymphocytes from healthy volunteers (Figure 1). No significant differences in FcγRII expression were observed between CD5⁺ and CD5^- B lymphocytes from control donors (not shown). Discrepancy between Damle et al's findings and ours could not be attributed to antigen loss due to cryopreservation, as we obtained comparable results with fresh and thawed B-CLL cells.

In conclusion, our findings show that B-CLL cells from patients in early or advanced stage disease express comparable or even higher levels of FcγRII than normal peripheral B lymphocytes, suggesting that FcγRII expression is not a useful parameter to define antigen experience of B-CLL cells. However, given that FcγRII is far from being just a cell marker, we believe that the receptors' role in B-CLL deserves further analysis. The main isoform expressed by B cells, FcγRIIb, functions as an inhibitory receptor.^2,5  Its coaggregation with B-cell receptors (BCRs) dampens B-cell activation by recruitment of a limited number of Src homology 2 domain (SH2)-containing phosphatases, predominantly SHIP (SH2-containing inositol phosphatase), which causes a dramatic and immediate hydrolysis of PIP3 (phosphatidylinositol 3,4,5-trisphosphate).⁵  FcγRIIb and SHIP are able to inhibit not only BCR-mediated signals but also signals induced by other cell surface receptors that require PIP3 generation.⁶  On the other hand, FcγRIIb can also signal independently of BCR colligation to directly mediate an apoptotic response.⁷  Whether or not this receptor is functional in B-CLL cells remains to be solved.