CD1a expression on developing CD34⁺CD1a^–thymic precursors is impaired by type I IFNs endogenously produced by pDCs after stimulation with CpG or HIV-1_LAI. Five-day cocultures of CD34⁺CD1a^– thymic progenitors on OP9-Jag1 cells were stimulated with (A) CpG (2216), CpG (2243) (both at 1 μg/mL), recombinant IFN-α (1000 U/mL), or left untreated or with (B) HIV-1_LAI (range, 2 to 150 ng p24 per milliliter) or control PM1 supernatant. Stimulations were performed in the presence or absence of neutralizing antibodies against type I IFNs. Three days after stimulation, cells were analyzed by flow cytometry for CD1a surface expression. CD1a expression levels of stimulated cells are shown relative to corresponding controls on day 8, which were set as 1. Average MFI values of at least 3 independent experiments are shown. Error bars represent the range in MFI values of at least 3 independent experiments. Statistical analysis was performed using a paired 2-tailed Student t test; *P < .05, **P < .001.