Identification of a Loss of Function Allele That Spontaneously Arose in a Widely Used C57BL/6 Substrain Reveals a Novel Role for Dock2 in CD8 T Cell Homeostasis and Differentiation

Abstract

Commercially available C57BL/6 mice are widely in biomedical research laboratories. We used whole genome sequencing to identify a homozygous copy number variant that disrupts the function of DOCK2 in a commercially available C57BL/6 mouse substrain (C57BL/6NHsd). This strain of C57BL/6 mice, from Harlan-Sprague (acquired by Envigo Biosciences), exhibits several striking immune phenotypes that have also been described in the context of Dock2 deficiency including the loss of marginal zone B cells, decrease in plasmacytoid dendritic cells and invariant NKT cells. To our knowledge this allele has been introduced into four independently generated knockout mice (Siae-/-, Cmah-/-, Irf5- /- and Nod2-/-) during backcrossing into the C57BL/6 strain. Given the widespread use of the C57BL/6NHsd substrain across the world, it is likely that this allele has been introduced into other gene targeted mice. As a result, published studies where C57BL/6NHsd mice have been used as controls, as experimental animals, or for backcrossing into the C57BL/6 background will need to be reinterpreted as this mutation may be alter or be responsible for phenotypes ascribed to other genes. We built on this finding by identifying a 3-5 fold expansion of memory phenotype CD8+ T cells in DOCK2 deficient mice that correlates with increased resistance to intracellular infection. Bone marrow chimera and adoptive transfer studies indicate that these cells arise in a cell intrinsic manner following thymic egress. In addition, close inspection of their transcriptional profile, TCR repertoire and cell surface marker expression shows that Dock2-/- naive CD8+ T cells directly convert into “virtual memory” cells bypassing the effector T cell stage. This phenomenon also occurs in the context of a restricted TCR repertoire as DOCK2 deficient T cells expressing the OT1 transgene show a similarly dramatic expansion of their memory compartment. Direct conversion to memory is associated with TCR hypersensitivity to ex vivo weak agonist peptide and in vivo self-peptide triggering. Collectively, these findings suggest that DOCK2 sets the threshold for entry into the “virtual” memory compartment by negatively regulating tonic TCR triggering.