Evaluation of the Human T‐Cell Leukemia Virus Type I Seropositivity of Blood Donors by the Particle Agglutination Inhibition Test

In the HTLV‐I seroscreening of blood donor sera by gelatin particle agglutination (PA), more than 50% (55.6%) of the PA‐positive sera were negative by immunofluorescence assay (IF). However, when donors were divided into age groups, there were increasing numbers of IF‐positive/PA‐positive donors with age. Among the PA‐positive donors in the 50–64 age group, 65.9% were IF‐positive compared to 16.0% in the 16–19 age group. The serological specificities of the IF‐negative/ PA‐positive specimens were tested by using a newly developed PA inhibition (PAD test. The HTLV‐I specificity of the PAI test was confirmed by the observation that agglutinations with anti‐HTLV‐I p19 and gp21 monoclonal antibodies as well as IF‐positive sera were specifically inhibited with HTLV‐I preparations or HTLV‐I‐positive cell extracts and not with HTLV‐I‐negative cell extracts. Sixty of the 104 specimens collected randomly from the IF‐negative/PA‐positive donors were PAI‐positive. The majority (80%) of such PAI‐positive sera showed more than two bands of HTLV‐I gag‐encoded polypeptide, p19, p24, p2S and p53 on Western blotting. Some of the PAI‐positive sera were also positive by enzyme immunoassay. These results indicate that at least some of the IF‐negative/ PA‐positive donors possess HTLV‐I‐specific antibody and may be potential HTLV‐I carriers who will become IF‐positive at a later age