A humanized NOG-EXL mouse model for producing severe fever with thrombocytopenia syndrome virus-reactive human antibodies.

BACKGROUND: Humanized mouse models are essential for studying the human immune response and antibody development. However, conventional models show limited B cell maturation and antigen-specific humoral responses. To overcome these limitations, we used the NOG-EXL mice expressing human interleukin 3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) to enhance myeloid and B-cell lineage differentiation. METHODS: Human CD34(+) hematopoietic stem cells (HSC) were transplanted into NOG-EXL mice to produce humanized immune systems. After immune cell reconstitution was confirmed across 12 weeks, the mice were immunized twice with inactivated severe fever with thrombocytopenia syndrome virus (SFTSV) antigens. Peripheral blood mononuclear cells and splenocytes were analyzed using multicolor flow cytometry to assess human immune cell subsets. Antigen-specific immunoglobulin G (IgG) production was quantified using enzyme-linked immunosorbent assay (ELISA), and virus-specific B cells were isolated using antigen-labeled recombinant protein probes. RESULTS: Twelve weeks after transplantation of HSCs into NOG-EXL mice, they exhibited robust engraftment of human leukocytes, including T, B, and dendritic cells, compared to NOG mice. Unlike NOG mice, humanized NOG-EXL mice exhibited an increase in human IgG levels, indicating the production of human antibody responses to antigens. Humanized NOG-EXL mice were immunized twice every 2 weeks with inactivated SFTSV, and antigen-specific human antibodies against the virus were detected in the mouse sera by ELISA. Sera from SFTSV-immunized humanized mice demonstrated neutralizing activity against SFTSV, confirming the induction of functional virus-specific neutralizing antibodies. Antigen-binding IgG-positive human B cells were isolated from mouse splenocytes using recombinant protein probes. CONCLUSION: This model provides a valuable platform for evaluating humoral immunity and isolating B cells using high-affinity human monoclonal antibodies without genetic engineering.