We describe the unique clinical characteristics of patients with
idiopathic thrombocytopenic purpura (
ITP) who are infected by human T-lymphotropic virus type I (HTLV-I). Thirty-seven patients with
ITP were examined in the present study: 10 patients had
HTLV-I infection, and the remaining 27 did not. The mean age of the group with
HTLV-I infection was significantly older than that of the group without
infection (57.8 +/- 14.0 and 42.4 +/- 20.1, P = 0.022). The difference in mean platelet counts at diagnosis between the two groups was not significant, 29 x 10(9)/L and 21 x 10(9)/L, respectively. The levels of platelet associated
IgG, red blood cell count, white blood cell count, bone marrow cell count, and megakaryocyte count did not show any significant difference. Nine patients in the group with
HTLV-I infection were treated with
prednisolone (1 mg/kg, daily oral). Only 3 of them responded to the
therapy (one complete response [CR] and two partial responses [PR]). However, 17 of 22 patients not infected with HTLV-I were treated with
prednisolone successfully: 14 patients achieved CR, and 3 patients achieved PR. There was a significant difference in response to
prednisolone between the two groups (P = 0.034). Two patients with the
infection and one patient without the
infection achieved CR with
splenectomy. These results suggest that
HTLV-I infection may cause
immune thrombocytopenia by a different mechanism than classical
ITP; HTLV-I may modify the clinical features of
ITP through an unknown immune pathway.