Estimation of the infectious viral load required for transfusion‐transmitted human T‐lymphotropic virus type 1 infection ( TT ‐ HTLV ‐1) and of the effectiveness of leukocyte reduction in preventing TT ‐ HTLV ‐1

BACKGROUND AND OBJECTIVES: The risk of transfusion-transmitted human T-lymphotropic virus type 1 infection (TT-HTLV-1) after prestorage leucocyte reduction (LR) remains unknown, as the proviral load in the blood component that would cause TT-HTLV-1 is undetermined. On the basis of the distribution of HTLV-1 proviral load among HTLV-1-sero-positive blood donors, we attempted to estimate the proviral load for transfusion-related infectivity. We also discuss the effectiveness of LR in preventing TT-HTLV-1. MATERIALS AND METHODS: The HTLV-1 proviral load in 300 HTLV-1-sero-positive blood donors was determined by real-time polymerase chain reaction analysis. The proviral load required for transfusion-related infectivity was estimated using historical TT-HTLV-1 frequency data from a retrospective study on patients who had received blood from HTLV-1-sero-positive blood donors and the distribution pattern of HTLV-1 proviral load among blood donors. RESULTS: HTLV-1 proviral loads ranged between < 0.01 and 25.0 copies per 100 leucocytes. Historical data showed TT-HTLV-1 frequency to be 80%. Assuming that 80% of the 300 sero-positive samples are infectious, it is estimated that the transfer of ≥ 9 × 10(4) cells containing the HTLV-1 provirus is required to establish TT-HTLV-1. CONCLUSION: The residual number of HTLV-1-infected cells after LR is substantially lower than the viral load necessary for TT-HTLV-1. LR therefore appears to be effective in minimizing the incidence of TT-HTLV-1.