Purified enzymes improve isolation and characterization of the adult thymic epithelium

The reproducible isolation and accurate characterization of thymic epithelial cell (TEC) subsets is of critical importance to the ongoing study of thymopoiesis and its functional decline with age. The study of adult TEC, however, is significantly hampered due to the severely low stromal to hematopoietic cell ratio. Non-biased digestion and enrichment protocols are thus essential to ensure optimal cell yield and accurate representation of stromal subsets, as close as possible to their in vivo representation. Current digestion protocols predominantly involve diverse, relatively impure enzymatic variants of crude collagenase and collagenase/dispase (col/disp) preparations, which have variable efficacy and are often suboptimal in their ability to mediate complete digestion of thymus tissue. To address these issues we compared traditional col/disp preparations with the latest panel of Liberase products that contain a blend of highly purified collagenase and neutral protease enzymes. Liberase enzymes revealed a more rapid, complete dissociation of thymus tissue; minimizing loss of viability and increasing recovery of thymic stromal cell (TSC) elements. In particular, the recovery and viability of TEC, notably the rare cortical subsets, were significantly enhanced with Liberase products containing medium to high levels of thermolysin. The improved stromal dissociation led to numerically increased TEC yield and total TEC RNA isolated from pooled digests of adult thymus. Furthermore, the increased recovery of TEC enhanced resolution and quantification of TEC subsets in both adult and aged mice, facilitating flow cytometric analysis on a per thymus basis. We further refined the adult TEC phenotype by correlating surface expression of known TEC markers, with expression of intracellular epithelial lineage markers, Keratin 5 and Keratin 8. The data reveal more extensive expression of K8 than previously recognized and indicates considerable heterogeneity still exists within currently defined adult TEC subsets.