What is a Mixed Lymphocyte Reaction?

In short, Mixed Lymphocyte Reactions (MLR’s) are cell proliferation assays. There are a few main types of MLR’s (one-way, two-way, and three-way), and many different ways to set them up so that they meet a scientist’s or organization’s guidelines, as well as the regulatory guidelines that they must follow. In an effort to keep this guide simple, we will only discuss the two main types of MLR’s.

One-way MLR- One-way MLR’s are a cell proliferation assay where one population of lymphocytes (responder cells) is stimulated to proliferate by another population of lymphocytes (stimulator cells) which have been rendered non-proliferative. These two populations are then incubated together, generally for around 5-6 days. After the initial incubation, Bromodeoxyuridine (BrdU) is added to each mixture, and is incorporated into the DNA of the proliferating cells while they are incubated for an additional 12-24 hours. Finally, the BrdU is washed out of the mixture and an Enzyme-linked Immunosorbant Assay (ELISA) is performed to quantify the amount of BrdU as a measure for cell proliferation. In the end, you are evaluating the proliferation response of the target cells that occurs when you mix the two populations of lymphocytes.

Two-way MLR- A two-way MLR differs from a one-way MLR in that both of the cell populations are able to proliferate. As both cell populations are able to proliferate, the specific experimental design must take this into account. The analysis for the amount of cell proliferation again utilizes BrdU incorporation into DNA with a subsequent BrdU-specific ELISA.

In a nutshell, MLR’s are pretty much what they sound like; mixing populations of lymphocytes together, and measuring the reaction that occurs. In a one-way MLR, only one lymphocyte population can respond or proliferate. In a two-way MLR, both populations can proliferate. Now that we’ve established what a Mixed Lymphocyte Reaction is, let’s see why anybody would perform one!