High Frequency of Surface Immunoglobulin IgG in B-Cell Chronic Lymphocytic Leukaemia.

Background: A common diagnostic feature of chronic lymphocytic leukaemia (CLL) is the weak surface immunoglobulin (sIg) expression of the IgM/D isotypes. The occurrence of sIgG is believed to be an atypical finding in CLL.

Aims: To demonstrate that sIgG expression is a real and not artefactual by molecular and functional assays.

Methods: 194 unselected, consecutive cases of CLL were analysed during routine diagnostic testing for sIgG and sIgM. Flow cytometry was performed using fresh whole blood and red cell lysis (Pharmalyse solution, BD). For confirmation of these results and demonstration of functionality, we used 4-colour flow cytometry, PCR for mRNA detection, proliferation assay (³H incorporation), and calcium flux assay.

Results: 23% (44/194) of all CLL cases displayed sIgG, with 39 being sIgG+ alone. PCR results showed a distinct band indicating the presence of mRNA for IgG. Four-colour flow cytometry identified single cells that co-expressed CD19/CD160/sIgG and ruled out sIgM expression. In vitro proliferation assays demonstrated that sIgG is functional when triggered by F(Ab’)2 fragments specific for sIgG isotype. The strength of the signal in one case after stimulation via sIgG led to a 3.5-fold increase in cellular proliferation. Calcium flux assays also demonstrated that the IgG isotype was functional, one case showing a 50% increase in fluorescence intensity after stimulation via sIgG.

Discussion: 23% of CLL cases have sIgG expression. In addition to a high frequency of sIgG expression, this study has shown sIgG to be capable of signalling. If antigen-drive is important in CLL, an IgG B-cell receptor should have higher affinity for a putative antigen, which potentially would be a poor prognostic feature. However, such cases would also be expected to show mutated VH genes, indicating a better prognosis. Further study is required to elucidate the pathological significance of sIgG expression and its role in CLL.