BACKGROUND: The fetal immune system is characterized by a Th2 bias but it is unclear how the Th2 predominance is established. Natural killer T (NKT) cells are a rare subset of T cells with immune regulatory functions and are already activated in utero. To test the hypothesis that NKT cells are part of the regulatory network that sets the fetal Th2 predominance, percentages of Vα24(+)Vβ11(+) NKT cells expressing Th1/Th2-related chemokine receptors (CKR) were assessed in cord blood. Furthermore, IL-4 and IFN-γ secreting NKT cells were quantified within the single CKR(+) subsets.RESULTS: Cord blood NKT cells expressed the Th2-related CCR4 and CCR8 at significantly higher frequencies compared to peripheral blood NKT cells from adults, while CXCR3(+) and CCR5(+) cord blood NKT cells (Th1-related) were present at lower percentages. Within CD4(neg)CD8(neg) (DN) NKT cells, the frequency of IL-4 producing NKT cells was significantly higher in cord blood, while frequencies of IFN-γ secreting DN NKT cells tended to be lower. A further subanalysis showed that the higher percentage of IL-4 secreting DN NKT cells was restricted to CCR3(+), CCR4(+), CCR5(+), CCR6(+), CCR7(+), CCR8(+) and CXCR4(+) DN subsets in cord blood. This resulted in significantly decreased IFN-γ /IL-4 ratios of CCR3(+), CCR6(+) and CCR8(+) cord blood DN NKT cells. Sequencing of VA24AJ18 T cell receptor (TCR) transcripts in sorted cord blood Vα24Vβ11 cells confirmed the invariant TCR alpha-chain ruling out the possibility that these cells represent an unusual subset of conventional T cells. CONCLUSIONS: Despite the heterogeneity of cord blood NKT cells, we observed a clear Th2-bias at the phenotypic and functional level which was mainly found in the DN subset. Therefore, we speculate that NKT cells are important for the initiation and control of the fetal Th2 environment which is needed to maintain tolerance towards self-antigens as well as non-inherited maternal antigens.