Low expression level but potent antigen presenting function of CD1d on monocyte lineage cells

CD1d is a key antigen‐presenting molecule involved in the selection and activation of a highly conserved T cell subset known as NK T cells. In this study, we analyzed the expression, regulation and function of human CD1d by various antigen‐presenting cells (APC) of myeloid origin, including circulating monocytes, monocyte‐derived dendritic cells and macrophages. CD1d was expressed as a mature glycoprotein by these cells, and unlike the other members of the human CD1 family its expression was constitutive and was not strongly up‐regulated by GM‐CSF and IL‐4 or a range of other cytokines. Despite their remarkably low surface expression of CD1d, all myeloid lineage cells tested were extremely potent APC for responses of NK T cell clones to the synthetic glycolipid antigen, α‐galactosyl ceramide. Prominent localization of CD1d to the endocytic system of monocyte lineage cells was observed, and functional studies suggested that this was important for achieving efficient antigen loading onto CD1d. Overall, these results support the view that monocyte lineage cells are important stimulators of CD1d‐restricted immune responses, while also underscoring the unique regulation of CD1d expression by these cells.