Single-round HIV type 1 neutralization measured by ELISPOT technique in primary human cells.

To accurately quantify HIV-1 neutralizing antibody titers in primary human cells, we developed a single round, focus-forming unit (FFU) reduction assay in human peripheral blood mononuclear cells (PBMC). Infected PBMC were enumerated by a reverse ELISPOT technique in which they were incubated under agarose in the presence of a protease inhibitor in anti-p24 antibody-coated microtiter plates. Viral p24, secreted in the immediate vicinity of infected cells and captured by immobilized antibodies, was subsequently stained using gold-labeled anti-p24-antibody and a precipitating silver substrate. The resulting spots were counted visually, without the aid of a microscope, and percent neutralization titers were determined using curve-fitting software. Results of this ELISPOT neutralization assay (ENA) for 15 HIV-positive human specimens were compared with results from a standard PBMC neutralization assay (standard assay) that measured neutralization as a function of p24 concentration by enzyme immunoassay (EIA). The ENA measures FFU reduction of both syncytium-inducing (SI) and non-syncytium-inducing (NSI) primary isolates. Completed assay plates may be retained as a physical record of results or saved as an image using a flat-bed computer scanner.