|Natural Killer (NK) Cell Assay|
The bulk of the cells detected that express CD56 on their surface have a particular function that accounts for their name, natural killer cell. This function is, of course, the ability to identify and then kill foreign or abnormal cells. Identification of the number and activity of these cells gives us an idea of the strength of this killing capacity. However, it has been known for some time, that the only way to assess the activity of these cells in the body is to measure their activity in the laboratory. The most precise way to accomplish this is to mix the patients natural killer cells with target cells and then measure the amount of killing that the target cells experience. This killing activity is measured very precisely using a flow cytometer.
As was described in the A Guide to Interpreting the Results of the Reproductive Immunophenotype document, natural killer cells have a number of diverse functions. Aside from directly killing targeted cells, they can produce cytokines that direct the killing activity of other cells. One of these cytokines is tumor necrosis factor (TNF).
The test for NK cell activity is performed routinely in the Centers laboratory. Natural Killer cells from the woman are separated from her blood and are cultured at different dilutions with target cells that they can kill. The target cells used are from a cancer cell line that, serendipitously, share many similarities to placentas and embryos. After two to four hours of culturing the patients NK cells with the target cells, a special DNA dye called propidium iodide is added to the culture. This dye is taken up by the DNA of the cells that have been killed. The living cells do not take up the dye. The cell suspensions are then put into the flow cytometer and the percentage of dead to live cells at the different dilutions determined.