Level of expression of ILT2 in different leukocyte subsets of PBMCs after HPV immunization. The MFI of ILT2 expression in different subsets of PBMCs was determined by flow cytometry in 23 healthy volunteers before (T0) and 15 days after the first (T1, day 15) and third (T2, day 195) quadrivalent HPV (type 6/11/16/18) vaccine doses. Panels show the MFIs of ILT2 expression in CD8+ lymphocytes (A), CD4+ cells (B), NK (CD3− CD56+) cells (C), CD3+ CD56+ cells (D), B cells (CD19+) (E), and monocytes (CD14+) (F). Cells were stimulated in vitro for 72 h with the quadrivalent HPV (type 6/11/16/18) vaccine (10 ng/ml of L1 protein of HPV-6 and -18 and 20 ng/ml of HPV-11 and -16). Data correspond to the arithmetic mean and SD. *, P < 0.05; **, P < 0.01; ***, P < 0.001 (two-way repeated measures analysis of variance).