Discrepancy of CD20 Protein Expression In IHC and FCM Analyses In Primary B-Cell Lymphoma: Relationship Between FCM-Negative Phenotype and Rituximab Binding with Lymphoma Cells

Rituximab is an anti-CD20 chimeric-monoclonal antibody, and its effectiveness for treatment of CD20-positive B-cell lymphomas has been proven over the past 10 years. Although rituximab is now a key molecular targeting drug for CD20-positive lymphomas, some patients with rituximab resistance have emerged. We previously reported that the CD20-protein-negative phenotypic change after using rituximab is one of the critical mechanisms in rituximab resistance (Hiraga J, Tomita A, et al., Blood, 2009., Sugimoto T, Tomita A, et al., Biochem Biophys Res Commun, 2009.). Recently, we have recognized that some newly-diagnosed B-cell lymphomas show CD20-protein-positive in immunohistochemistry (IHC) but -negative in flow cytometry (FCM) analyses. For these patients, so far, neither the molecular mechanisms of CD20 IHC(+)/FCM(−) phenotype, nor the relationship between this phenotype and rituximab resistance are clear. Thus, the clinical significance of introducing rituximab therapy for these patients must be elucidated.

Analyses of the molecular backgrounds of CD20 IHC(+)/FCM(−) phenotype in primary B-lymphoma cells, and confirmation of the effectiveness of rituximab therapy for the patients who show CD20 IHC(+)/FCM(−) phenotype.

Primary B-cell lymphoma (diffuse large B-cell (DLBCL), follicular, MALT, mantle cell, and Burkitt) tissues and cells were analyzed by IHC and FCM. Four newly-diagnosed B-cell lymphoma patients showed IHC CD79(+)/CD20(+) and FCM CD19(+)/CD20(−) phenotype using anti-CD20 antibodies L26 for IHC and B1 for FCM, and all were diagnosed as DLBCL. Chromosomal analysis showed complex karyotypes in 3 out of 3 patients analyzed, and no shared abnormalities were confirmed. Primary lymphoma cells from 3 patients were available for further molecular analyses, and the genomic DNA, the total RNA, and the protein from whole cell lysate were obtained from these lymphoma cells. DNA sequencing analysis indicated no significant genetic mutations on the coding sequences (CDS) of MS4A1 (CD20) gene. Semi-quantitative and quantitative RT-PCR indicated that CD20 mRNA expression was almost normal in 2 patients and ≂~f10 times lower in 1 patient compared to the positive control B-lymphoma/leukemia cells. Almost the same expression tendency with RT-PCR was confirmed in immunoblot analysis using whole cell lysate and the two different anti-CD20 antibodies. The molecular weight of the CD20 protein in immunoblotting corresponded to the wild type in these patients. Rituximab binding assay in vitro was performed using primary lymphoma cells from a patient and the fluorescent-labeled rituximab (Alexa488-rituximab). Interestingly, rituximab binding on the surface of the CD19 positive lymphoma cells was confirmed in vitro. Rituximab containing combination chemotherapy was performed, resulting in complete response in all 4 cases after completing 4 to 8 courses.

CD20 IHC(+)/FCM(−) phenotype was confirmed in newly-diagnosed DLBCL patients. Significant abnormalities in CD20 protein and mRNA expression in immunoblotting and RT-PCR were not confirmed, and genetic mutations on CDS of MS4A1 gene, resulting in the conformation change of CD20 protein, were not detected. The possibility of abnormal post-translational modification or aberrant localization of CD20 protein, leading to interference with antibody binding, can not be excluded. Rituximab binding with CD19-positive primary lymphoma cells was confirmed in a patient, suggesting that CD20 IHC(+)/FCM(-) phenotype does not directly indicate the ineffectiveness of rituximab for these cells. Further investigations, performing in vitro CDC and ADCC assay using primary lymphoma cells, are still warranted to show rituximab effectiveness and sensitivity to those cells.

Kinoshita:Zenyaku Kogyo Co.: Research Funding; Chugai Pharmaceutical Co., Ltd.: Research Funding. Naoe:Zenyaku Kogyo Co.: Research Funding; Chugai Pharmaceutical Co., Ltd.: Research Funding.