Effector Vγ9Vδ2 T cells dominate the human fetal γδ T-cell repertoire

Significance Despite their enormous potential for diversity (in excess of 10 ¹⁵ theoretical receptor specificities), the human γδ T-cell repertoire is dominated by a specific subset expressing the T-cell receptor containing the γ-chain variable region 9 and the δ-chain variable region 2 (Vγ9Vδ2) known to react to a set of pathogen-derived small molecules (phosphoantigens). Overrepresentation of this restricted set of γδ T cells in adults has been thought to reflect an antigen-specific selection process resulting from postnatal exposure to pathogens. However, we demonstrate here that restricted Vγ9Vδ2 cells with preprogrammed effector function represent the predominant γδ T-cell subset circulating in human fetal blood. This observation suggests that, despite developing in a sterile environment, the human fetal γδ T cell repertoire is enriched for pathogen-reactive T cells well before pathogen exposure. γδ T cells are unconventional T cells recognizing antigens via their γδ T-cell receptor (TCR) in a way that is fundamentally different from conventional αβ T cells. γδ T cells usually are divided into subsets according the type of Vγ and/or Vδ chain they express in their TCR. T cells expressing the TCR containing the γ-chain variable region 9 and the δ-chain variable region 2 (Vγ9Vδ2 T cells) are the predominant γδ T-cell subset in human adult peripheral blood. The current thought is that this predominance is the result of the postnatal expansion of cells expressing particular complementary-determining region 3 (CDR3) in response to encounters with microbes, especially those generating phosphoantigens derived from the 2-C-methyl- d -erythritol 4-phosphate pathway of isoprenoid synthesis. However, here we show that, rather than requiring postnatal microbial exposure, Vγ9Vδ2 T cells are the predominant blood subset in the second-trimester fetus, whereas Vδ1 ⁺ and Vδ3 ⁺ γδ T cells are present only at low frequencies at this gestational time. Fetal blood Vγ9Vδ2 T cells are phosphoantigen responsive and display very limited diversity in the CDR3 of the Vγ9 chain gene, where a germline-encoded sequence accounts for >50% of all sequences, in association with a prototypic CDR3δ2. Furthermore, these fetal blood Vγ9Vδ2 T cells are functionally preprogrammed (e.g., IFN-γ and granzymes-A/K), with properties of rapidly activatable innatelike T cells. Thus, enrichment for phosphoantigen-responsive effector T cells has occurred within the fetus before postnatal microbial ex