IO Summit #1: Development of A Novel Engineered B2M Mouse Strain in B-NDG Background Specifically Eliminates MHC I Expression While Retaining FcRn-mediated Antibody Metabolism
Severely immunodeficient mouse strains, such as NOD.Cg-Prkdcscid Il2rg-/-, have been widely used to evaluate human immune responses, including applications in immuno-oncology in recent years. However, when human peripheral blood mononuclear cells (PBMCs) are engrafted in these mice, severe acute xenogeneic graft versus host disease (GvHD) arises due to engagement of human T cell receptors (TCRs) and murine major histocompatibility complex (MHC) I and II. GvHD shortens animal life span and results in only a brief window of evaluation of human immune cell functions in such mouse models. Efforts have been undertaken to reduce human PBMC-induced GvHD and to subsequently extend the window of human immunity assessment by eliminating mouse MHC I and MHC II (ref 1-3). While these MHC deficient mice show few symptoms of GvHD, the use of beta-2-microglobulin(B2M) null allele in order to eliminate expression of all MHC I molecules resulted in an unintended consequence in that these mice had rapid antibody clearance. This is because B2M is also associated with neonatal Fc receptor (FcRn) and is important to FcRn-mediated in vivo antibody recycling and retention (ref 4). Low half-lives of antibodies in B2M null mice render them unsuitable for antibody efficacy assessment. To overcome this undesired pharmacokinetic (PK) limitation of B2M null allele, while retaining its desired effect on eliminating MHC I expression and alleviating GvHD, we engineered a B2M knock-out strain in the background of B-NDG mice (NOD.Cg-Prkdcscid Il2rg-/-/Bcgen) that was deficient in MHC I expression but not antibody retention. In present study, we describe genetic, phenotypic, pharmacokinetic, and functional characterizations of the resulting mouse strain, which we believe will be a valuable addition to the collection of severely immunodeficient mouse models that permit extended and appropriate interrogation of antibody therapeutics on human immune cells.