Id3 and Id2 act as a dual safety mechanism in regulating the development and population size of innate-like gd t cells

The innate-like T cells expressing Vg1.1 and Vd6.3 represent a unique T cell lineage sharing features with both the gd T and the invariant NKT cells. The population size of Vg1.1+Vd6.3+ T cells is tightly controlled and usually contributes to a very small proportion of thymic output, but the underlying mechanism remains enigmatic. Deletion of Id3, an inhibitor of E protein transcription factors, can induce an expansion of the Vg1.1+Vd6.3+ T cell population. This phenotype is much stronger on the C57BL/6 background than on the 129/sv background. Using quantitative trait linkage analysis, we identified Id2, a homolog of Id3, to be the major modifier of Id3 in limiting Vg1.1+Vd6.3+ T cell expansion. The Vg1.1+Vd6.3+ phenotype is attributed to an intrinsic weakness of Id2 transcription from Id2 C57BL/6 allele, leading to an overall reduced dosage of Id proteins. However, complete removal of both Id2 and Id3 genes in developing T cells suppressed the expansion of Vg1.1+Vd6.3+ T cells because of decreased proliferation and increased cell death. We showed that conditional knockout of Id2 alone is sufficient to promote a moderate expansion of gd T cells. These regulatory effects of Id2 and Id3 on Vg1.1+Vd6.3+ T cells are mediated by titration of E protein activity, because removing one or more copies of E protein genes can restore Vg1.1+Vd6.3+ T cell expansion in Id2 and Id3 double conditional knockout mice. Our data indicated that Id2 and Id3 collaboratively control survival and expansion of the gd lineage through modulating a proper threshold of E proteins.