C-Terminal Defect of CD20 on T-Cell Leukemia/Lymphoma Cells: Implications for Resistance to Rituximab

Rituximab is commonly used for treatment of B-cell leukemia/lymphoma and its efficiency on CD20+ B-cell malignancies has been established. On the other hand, the effect of rituximab on CD20⁺ T cell leukemia/lymphoma is unclear. Two patients CD20⁺ T-cell malignancy (1 with T-cell prolymphocytic leukemia (T-PLL) and 1 with peripheral T-cell lymphoma, unspecified (PTCL-u)) were recently treated with rituximab without appreciable effects. The treatment failure prompted a study of the mechanisms underlying the resistance to rituximab therapy using leukemia/lymphoma cells of these patients and another CD20⁺ T-PLL patient. Patient 1: A 68-year old Japanese male was diagnosed as having T-PLL (small cell variant) characterized by CD2⁺CD3⁺CD4⁻ CD5⁺CD8⁺CD20⁺TCR-Vβ8⁺. Southern blotting of these T-PLL cells showed a TCR-β gene rearrangement. The systemic lymphadenopathy and high-fever of the patient were ameliorated by the administration of prednisolone (20 mg/d). Rituximab was administered weekly, but no clinical improvement was observed. In vitro treatment of the T-PLL cells with rituximab did not show any complement-dependent cytotoxicity (CDC) and antibody-dependent cell-mediated cytotoxicity (ADCC). Patient 2: A 75-year old Japanese male was diagnosed as having T-PLL (small cell variant) with a phenotype of CD2⁺CD3⁺CD4⁺CD5⁺CD8⁺CD20⁺. Southern blotting of these T-PLL cells showed a TCR-β gene rearrangement. The patient is being followed without treatment because his pancytopenia is non-progressive. Patient 3: A 67-year old Japanese male developed a left thigh tumor which proved to be PTCL-u. The phenotype of lymphoma cells was CD2⁺CD3⁻CD4⁻CD5⁻CD8⁺CD20⁺ and Southern blotting of these cells showed a TCR-β gene rearrangement. The patient’s PTCL-u was refractory to CHOP and rituximab plus EPOCH therapy. The reactivity of various monoclonal Abs specific to CD20 molecules against these patients’ leukemia/lymphoma cells is summarized in Table 1. CD20 molecules of these 3 patients’ leukemia/lymphoma cells were detectable by flow cytometry using anti-CD20 mAbs (L27 and rituximab) and also by Western blotting using anti-CD20 mAbs which recognize N-terminal region of CD20. However, the CD20 molecules could not be revealed either by immunohistochemistry using anti-CD20 mAbs (L26) or by Western blotting using anti-CD20 mAbs which recognize C-terminal region of CD20. The sequence of the full-length CD20 cDNA derived from Patient 1’s T-PLL cells proved to be intact. These results indicate that the CD20 molecules of the 3 patients’ leukemia/lymphoma cells have defects in the cytoplasmic C-terminal region which may impair the cytotoxic effect of rituximab such as CDC and ADCC and the defect may be due to post translational changes such as phosphorylation of serine and threonine residues.

Table 1. Reactivity to different Abs specific to CD20