iNKT cells impaired in proliferation and perforin production are partially infected with HTLV-1 and CD4^– iNKT cells are depleted in HTLV-1–infected persons. (A) Proliferative response ratio of iNKT cells. PBMCs from HDs (n = 3), ACs (n = 3), and HAM/TSP (n = 3) and ATL patients (n = 3) were cultured without any treatment, with 100 ng/mL α-GalCer, or with α-GalCer (100 ng/mL) + IL-2 (100 U/mL). The fold change in the number of iNKT cells was calculated as the ratio of the absolute number of iNKT cells on day 14 divided by the absolute number of iNKT cells on day 0. Data are represented as mean ± SEM. (B) Representative FACS histogram of perforin expression by iNKT cells from HDs, ACs, and HAM/TSP patients. PBMCs were stained for perforin, Vα24Jα18, and CD3, and the expression levels of perforin in CD3⁺Vα24Jα18⁺ cells are plotted. (C) Levels of perforin expression (mean fluorescence intensity [MFI]) by CD3⁺Vα24Jα18⁺ iNKT cells of HDs, ACs, and HAM/TSP patients. PBMCs from 7 HDs, 4 ACs, and 6 HAM/TSP and 2 ATL patients were stained for perforin, Vα24Jα18, and CD3, and the expression level of perforin in each CD3⁺Vα24Jα18⁺ cell was plotted. In 2 of the 6 HAM/TSP patients and 2 ATL patients, perforin expression could not be analyzed because the frequency of iNKT cells was low. The perforin expression levels were lower in iNKT cells of ACs (P = .038), and HAM/TSP patients (P = .038) were lower than in those of HDs. The Mann-Whitney U test was used for statistical analysis. The horizontal bar represents the mean value of each group. (D) The HTLV-1 proviral load in CD3⁺Vα24Jα18⁺ (iNKT) cells and CD3⁺Vα24Jα18⁻ (CD3⁺ cells lacking iNKT cells) cells of 4 ACs and 1 HAM/TSP patient. Paired t test was used for statistical analysis; n.s. indicates not significant. (E) The percentage of CD4⁻ iNKT cells among total iNKT cells. PBMCs from 6 HDs and 7 HTLV-1–infected persons (1 AC, 5 HAM/TSP patients, and 1 ATL patient with detectable iNKT cells) were stained for CD3, CD4, and Vα24Jα18, the percentage of CD4⁺/CD4⁻ cells among CD3⁺Vα24Jα18⁺ iNKT cells was analyzed by FACS, and the percentage of CD4⁻ iNKT cells among total iNKT cells of each person was calculated. CD4⁻ iNKT cells were found to be dominantly depleted in HTLV-1–infected persons compared with HDs (P = .001 by Student t test).