Optimum culture conditions for specific and nonspecific activation of whole blood and PBMC for intracellular cytokine assessment by flow cytometry

Abstract

The assessment of cytokine production is an important component of studies of cell-mediated immune responses (CMI) to immunological challenges. In this study, we present a method to enhance the detection of cytokine-producing cells by allowing antigen-specific cells to expand in long-term culture. We investigated the influence of the degree of dilution of whole blood and the duration of the incubation period on whole blood as well as peripheral blood mononuclear cells (PBMCs), cultured in the absence or presence of mitogens, superantigens or specific antigens, for intracellular cytokine production (IFNγ, TNFα, IL-2, IL-4, IL-10 and IL-13) by CD4 + and CD8 + T lymphocytes using four-colour flow cytometry. Whole blood was diluted 1/1, 1/2, 1/5 and 1/10, and cultured for 6, 24, 48, 72 and 120 h in the presence of antibodies against the co-stimulatory molecules CD28 and CD49d, and, during the last 4 h of culture, in the presence of brefeldin A. Optimum conditions for detection of a high number of IFNγ-positive cells were observed after 72 h of culture in blood diluted 1/10. Median frequencies of IFNγ + cells obtained after activation by PMA-ionomycin, PHA or SEA-B were 29.3%, 20.0% and 6.8% for CD4 + cells, and 67.8%, 20.6% and 6.8% for CD8 + cells. In blood samples diluted 1/5 or 1/10, and cultured in the presence of cytomegalovirus (CMV) or varicella-zoster virus (VZV), mean peak levels of 2.8% and 1.4% IFNγ +CD4 + cells were recorded at 120 h. The levels of cells producing cytokines other than IFNγ were generally much lower and, in the case of IL-4 and IL-13, difficult to distinguish from background levels recorded in cultures with medium only. Kinetic studies of cytokine production by PBMCs showed a pattern similar to that of whole blood with peak levels of IFNγ-producing cells recorded at 72 h. The increased levels of IFNγ production after culture for 72 h were due to an expansion of the numbers of cytokine-producing cells responsive to a specific stimulus. Antigen-specific cells are usually present only at low levels in peripheral blood and may not be detected following simple activation for a few hours. To reach a level of detection in such cases, culture of diluted blood for several days is recommended.