Three-cell interactions in T cell-mediated suppression? A mathematical analysis of its quantitative implications

Aiming to further our understanding of T cell-mediated suppression, we inve stigate the plausibility of the hypothesis that regulatory T cells suppress other T cells (target cells), while both cells are conjugated with one APC . We use a mathematical model to analyze the proliferation inhibition score d during in vitro suppression assays. This model is a radical simplificatio n of cell culture reality, assuming that thymidine incorporation is proport ional to the number of target cells that would instantaneously form conjuga tes with APCs that are free of regulatory cells. According to this model th e inhibition index should be mainly determined by the number of regulatory cells per APC and should be insensitive to the number of target cells. We r eanalyzed several published data sets, confirming this expectation. Further more, we demonstrate that the instantaneous inhibition index has an absolut e limit as a function of the number of regulatory cells per APC. By calcula ting this limit we find that the model can explain the data under two non-m utually exclusive conditions. First, only similar to 15 % of APCs used in t he suppression assays form conjugates with T cells. Second, the growth of t he regulatory cell population depends on the target cells, such that the nu mber of regulatory cells per APC increases when they are cocultured with ta rget cells and overcomes its limit. However, if neither of these testable c onditions is fulfilled, then one could conclude that suppression in vitro d oes not require the formation of multicellular conjugates.