Detection of T cell CD4 epitopes in HIV-infected individuals

HIV antigens can be detected in the circulation of HIV-infected patients and are associated with active virus production. Free virions and shedded gp120 bind CD4 with high affinity. We have studied the expression of Leu3a and OKT4 epitopes on a CD4+ T cell line (HPB-ALL), pretreated with HIV rgp120, and on CD4+ pheripheral blood T lymphocytes of HIV-infected patients. The associated determination of these epitopes (the Leu3a mapping at the gp120 binding site of CD4 and the OKT4 mapping at a site independent of gp120 binding) allowed to monitor binding of gp120 to surface CD4 and maintenance of CD4 expression. The comparison of MFI of gp120-treated versus untreated Leu3a+ HPB-ALL cells suggested that the Leu3a epitope was masked by treatment with 20 micrograms/ml rgp120, while with 1 microgram/ml rgp120 masking was undetectable, although gp120 was bound to cells. The determination of the Leu3a and OKT4 epitopes in 105 HIV-infected individuals and in 50 normal controls, showed that the Leu3a epitope is detected equally well in HIV-infected and in normal subjects, provided the anti-Leu3a is used at saturation. Therefore the binding to epitopes distinct from the gp120-binding site does not seem to be a requisite for the selection of anti-CD4 mAbs for immunophenotyping. To optimize the detection of CD4 masking, a limiting amount of conjugated anti-Leu3a has to be used. Measurements of CD4 binding by gp120 in terms of gp120-free CD4 molecules, as detected by reactivity with anti-Leu3a, may be used to monitor disease progression in HIV-infected subjects.