Soluble Fc Receptor for 1gM in Sera From Subsets of Patients With Chronic Lymphocytic Leukemia as Determined by a New Mouse Monoclonal Antibody

The FcR for IgM (Fc mu R) is the newest member of the FcR family, selectively expressed by lymphocytes, and distinct from FcRs for switched Ig isotypes that are expressed by various immune cell types and non-hematopoietic cells. From studies of Fcmr-ablated mice, Fc mu R was shown to have a regulatory function in B-cell tolerance, as evidenced by high serum titers of autoantibodies of the IgM and IgG isotypes in mutant mice. In our previous studies, both cell-surface and serum Fc mu R levels were elevated in patients with chronic lymphocytic leukemia (CLL), where antigen-independent self-ligation of BCR is a hallmark of the neoplastic B cells. This was assessed by sandwich ELISA using two different ectodomain-specific mAbs. To determine whether the serum Fc mu R is derived from cleavage of its cell-surface receptor (shedding) or its alternative splicing to skip the transmembrane exon resulting in a 70-aa unique hydrophilic C-terminus (soluble), we developed a new mouse IgG1 kappa mAb specific for human soluble Fc mu R (solFc mu R) by taking advantages of the unique nature of transductant stably producing His-tagged solFc mu R and of an in vivo differential immunization. His-tagged solFc mu R attached to exosomes and plasma membranes, allowing immunization and initial hybridoma screening without purification of solFc mu R. Differential immunization with tolerogen (membrane Fc mu R) and immunogen (solFc mu R) also facilitated to generate solFc mu R-specific hybridomas. The resultant solFc mu R-specific mAb reacted with serum Fc mu R in subsets of CLL patients. This mAb, along with another ectodomain-specific mAb, will be used for verifying the hypothesis that the production of solFc mu R is the consequence of chronic stimulation of BCR.