Enzyme-Linked Immunospot Assay for Detection of Human Respiratory Syncytial Virus F Protein-Specific Gamma Interferon-Producing T Cells

Respiratory syncytial virus (RSV) causes significant disease in elderly adults, and we have previously reported that individuals 65 years of age and older have reduced RSV F protein-specific gamma interferon (IFN-gamma)-producing T cells compared to healthy younger adults. To measure RSV F-specific memory T cell responses in the elderly following infection or vaccination, we optimized and qualified an IFN-gamma enzyme-linked immunospot (ELISPOT) assay. Since peripheral blood mononuclear cells (PBMC) from the elderly could be more fragile, we established optimal cryopreservation techniques and minimal viability acceptance criteria. The number of cells per well, types and concentrations of stimulation antigens, and incubation times were evaluated to maximize assay sensitivity and precision. The optimized assay uses 300,000 cells/well, 2 mu g/ml of an RSV F peptide pool (RSV Fpp), and incubation for 22 +/- 2 h in serum-free CTL-Test medium. The assay was qualified by 3 analysts using 3 RSV F-responding donor PBMC samples (high, medium, and low responders) tested on 5 different assay days. The assay sensitivity or limit of detection (LOD) was determined to be 21 spot-forming cells (SFC) per 10(6) PBMC, and the lower limit of quantitation (LLOQ) was estimated to be 63 SFC/10(6) PBMC. The intra-and interassay percent coefficients of variation (CV) were < 10.5% and < 31%, respectively. The results of the qualification study demonstrate that a robust, precise, and sensitive IFN-gamma ELISPOT assay has been developed that is fit for measuring RSV F-specific IFN-gamma T cell responses in subjects enrolled in a vaccine clinical trial or in epidemiology studies.