BALB/c-hCD3E Transgenic Mice: An Ideal Animal Model for In Vivo Efficacy Study of CD3-Bispecific Antibodies

T cell-mediated immunotherapy is an important strategy for treatment of a variety of tumors.T-cell bispecific antibody (TCB), which binds to a tumor associated antigen (TAA) and human CD3ε (hCD3ε) and directs specific killing of tumor cells carrying the TAA. However, there is still an unmet need for suitable animal models to evaluate the therapeutic efficacy of TCB candidates. Although immune-deficient mice transfused with human PBMCs can be used to evaluate the TCBs, these mice are not fully immune competent and have limitation in studying immune system activation by TCBs. Herewe developed a human CD3ε-transgenic mouse model with BALB/c background (BALB/c-hCD3E) to overcome the limitation.

CD3ε plays a critical role in formation and function of the TCR-CD3 complex. However, hCD3ε shares only 47% homology with mouse CD3ε (mCD3ε) in the extracellular domain, indicating that hCD3ε and mCD3ε are not functionally interchangeable. Thus, we established BALB/c-hCD3E mice that carry the human CD3E gene and its entire regulatory sequence and co-express hCD3ε and mCD3ε in over 90% of T cells. BALB/c-hCD3E mice were phenotypically normal and had normal T-, B- and NK-cell populations compared to wild-type BALB/c mice. In addition, splenic T cells from BALB/c-hCD3E mice could be activated by either anti-hCD3 or anti-mCD3 antibodies to produce IFNg, IL2 and TNFa in vitro. Moreover, we knocked out mCD3e in BALB/c-hCD3E mice to generate BALB/c-hCD3E/mCd3e-KO mice that displayed a marked reduction in the number of splenic T cells. BALB/c-hCD3E/mCd3e-KO mice also had reduced percentages and numbers of CD4⁺ and CD8⁺T cells. Importantly, anti-mCTLA4 antibodies strongly inhibited the growth of subcutaneously inoculated CT26 tumor cells in BALB/c-hCD3E but not BALB/c-hCD3E/mCd3e-KO mice.

Taken together, these findings demonstrate that mCD3ε is indispensable for T-cell development and function in BALB/c-hCD3E transgenic mice and these mice are a novel and valuable model to assessing the therapeutic efficacy of TCBs.