Cryopreservation does not alter the frequency of regulatory T cells in peripheral blood mononuclear cells.

With great interest we read the recent article in the Journal of Immunological Methods by Elkord (2009), in which the author reports a reduction of human regulatory T (Treg) cell numbers in peripheral blood mononuclear cells (PBMCs) after cryopreservation. Treg cells were identified by flow cytometric measurement of CD25 or ForkheadBoxP3 (FoxP3) expression in CD4 T cells. CD25 and the transcription factor FoxP3 are known to be expressed by functionally suppressive Treg cells (Sakaguchi et al., 2006), but are not specific for this subset of CD4 T cells, as recently activated T cells are also able to transiently express CD25 and FoxP3 (Wang et al., 2007).Weare interested in the application of cryopreservation of PBMCs, as it allows the simultaneous analysis of samples obtained at multiple time points during a clinical trial. Given the pivotal