Investigation of atypical western blot (immunoblot) reactivity involving core proteins of human immunodeficiency virus type.

Serum specimens which originally exhibited a narrow (indeterminate) 24-kilodalton core protein (p24) or p24/p55 pattern of reactivity with human immunodeficiency virus (HIV) in the Western blot (immunoblot) test were studied to gather information on antibody specificity. A total of 12 specimens were initially reevaluated with an indirect immunofluorescence assay (IFA), three enzyme-linked immunosorbent assays (ELISAs), and Western blot analyses. Five of the specimens were IFA positive and contained anti-gp160/gp120 antibodies which were observed only when an HIV Western blot antigen rich in gp160 and gp120 was used. The remaining seven serum specimens were nonreactive by IFA and showed variable reactivity in HIV antibody ELISAs. The specimens did not cross-react with core antigens for human T-cell leukemia virus types 1 and 2 or contain detectable levels of HIV p24 antigen. The p24/p55 reactivity of six of the seven indeterminate specimens could be reduced or eliminated by preincubating the specimens with disrupted, HIV-infected H9 cells but not with uninfected H9 cells. The six specimens also exhibited discernible reactivity with recombinant HIV p24 antigen. When an additional 23 indeterminate specimens were assayed, all of the serum specimens were nonreactive by IFA while 65% (15 of 23) showed various degrees of reactivity with the recombinant p24 protein. There was no indication that any of the HIV core antibody reactivity was caused by HIV infection. Indeterminate results for five patients with specific p24 reactivity, who were retested after a period of weeks or months, remained indeterminate for HIV antibody with no significant change in ELISA or Western blot reactivity.