Anti-N-Terminal CD20 Monoclonal Antibody and L26 Dual Staining Identifies An Irreversible Resistant Mutant Genes

Recently, we reported that gene mutations of CD20 were involved in resistance to rituximab therapy, and we proposed that C-terminal deletion mutations of CD20 might be related to relapse/resistance after rituximab therapy. Many of these cases were diagnosed as CD20 negative by the immunohistochemical analysis using the L26 monoclonal antibody used routinely in most clinical laboratories. L26 recognizes the cytoplasmic region of CD20 molecules, but no more detailed information about its epitope had been reported. So, we could not distinguish whether protein expression of CD20 extremely decreased or whether the epitope of the antibody was lost by these mutations. To make this clear, we determined the binding site of L26 antibody on CD20 protein in the present study. In addition, we developed new antibodies that recognize amino acid sequence close to the amino terminal of CD20 molecule. Then we investigated clinical specimens with these antibodies together with L26 to elucidate characteristics of CD20 molecules having C-terminal mutations.

To determine the binding site of L26 antibody on CD20, we made a series of constructs of the CD20 molecules with deletion mutations in the C-terminal cytoplasmic domain and introduced them into retrovirus vectors. A CD20 negative multiple myeloma cell line, KMS12PE cells were then transformed, and we established six kinds of sub-lines with the various C-terminal deletion mutations of CD20 and used them for epitope-mapping. On the other hand, we screened the CD20 gene sequence of the clinical specimen of rituximab-resistant patients and identified several cases with the mutation in the C-terminal cytoplasm region. The immunochemistry using L26 and newly developed antibodies, as well as membrane expression of CD20 molecules using the rituximab were analyzed.

The epitope analysis of L26 antibody using a series of CD20 deletion mutations revealed that L26 recognizes near the C-terminus of CD20 cytoplasmic region. These results showed that most of CD20 molecules with the C-terminal deletion mutation and frame-shift mutation could not be recognized by L26. The immunohistochemical analysis performed for clinical specimens revealed that the cells that were stained by antibodies recognizing N-terminal region of CD20 but not by L26 were indeed included in some rituximab-resistant cases. DNA sequencing analysis revealed that all these cases had mutated CD20 genes in its C-terminal cytoplasmic region. In addition, a cell-surface expression analysis using flowcytometry demonstrated that the cells having these mutations has reduced cell surface expression of CD20 compared with those of normal CD20.

In this study, we determined the recognition site of L26 and demonstrated that L26 couldn't recognize CD20 with the resistant mutations. In contrast, newly developed antibodies against N-terminal region of CD20 could stain even these CD20 molecules. These results suggest that combination use of these antibodies and L26 enables to detect the onset of irreversible rituximab-resistant clones with the CD20 mutations.

Hatake:Chugai Pharmaceutical Co., Ltd: Honoraria, Research Funding.