Isotype switch and idiotype variation in hairy cell leukemia

Hairy cell leukemia (HCL) cells are B lymphocytes expressing monoclonal surface bound immunoglobulins, B cell specific antigens, and activation markers. Based on immunophenotyping analysis, HCL cells have been reported to express simultaneously multiple Ig heavy chain isotypes. This is in contrast with the majority of other B cell malignancies. To analyse the tumor cell Ig in more detail, we have prepared somatic cell hybridomas from two patients with HCL. In both cases we found that the tumor cell derived hybridomas secreted IgM, IgG, or IgA indicating that a class-switch event does occur within the monoclonal tumor cell population. However these hybridomas expressed only single isotypes. The clonal relatedness of the tumor-derived Ig was shown by Ig gene analysis and by reactivity with a panel of idiotype specific antibodies. The tumor Ig of one patient analysed showed a striking variability in idiotope expression, indicative of somatic mutations of the expressed Ig genes. To our knowledge this is the first example of HCL and of a B cell malignancy other than follicular lymphoma for which somatic mutations have been demonstrated. This finding might be important for the therapeutic approach based on monoclonal anti-idiotype antibodies.