Generation of human antigen-specific monoclonal IgM antibodies using vaccinated "human immune system" mice

Background: Passive transfer of antibodies not only provides immediate short-term protection against disease, but also can be exploited as a therapeutic tool. However, the 'humanization' of murine monoclonal antibodies (mAbs) is a timeconsuming and expensive process that has the inherent drawback of potentially altering antigenic specificity and/or affinity. The immortalization of human B cells represents an alternative for obtaining human mAbs, but relies on the availability of biological samples from vaccinated individuals or convalescent patients. In this work we describe a novel approach to generate fully human mAbs by combining a humanized mouse model with a new B cell immortalization technique. Methodology/Principal Findings: After transplantation with CD34⁺CD38^- human hematopoietic progenitor cells, BALB/c Rag2^-/-IL-2Rγc^-/- mice acquire a human immune system and harbor B cells with a diverse IgM repertoire. "Human Immune System" mice were then immunized with two commercial vaccine antigens, tetanus toxoid and hepatitis B surface antigen. Sorted human CD19⁺CD27⁺ B cells were retrovirally transduced with the human B cell lymphoma (BCL)-6 and BCLXL genes, and subsequently cultured in the presence of CD40-ligand and IL-21. This procedure allows generating stable B cell receptor-positive B cells that secrete immunoglobulins. We recovered stable B cell clones that produced IgM specific for tetanus toxoid and the hepatitis B surface antigen, respectively. Conclusion/Significance: This work provides the proof-of-concept for the usefulness of this novel method based on the immunization of humanized mice for the rapid generation of human mAbs against a wide range of antigens.