Heat-induced expression of MHC-linked HSP70 genes in lymphocytes varies at the single-cell level

The expression of MHC-linked heat shock protein 70 (HSP70) genes HSP70-1 and HSP70-2 has been studied in human and rat lymphocytes and Concanavalin A (con A)-induced lymphoblasts by in situ hybridization and flow cytometry. In in vitro experiments transcripts of these genes were observed only after heat shock, and mitogen stimulation per se did not lead to induction. Heat shock-induced expression of HSP70-1 and HSP70-2 mRNA varied at the single-cell level. The fraction of HSP70-positive lymphocytes increased continuously with the severity of heat shock. However, 15-35% of the cells always remained HSP70 transcript negative. After fever induction in vivo similar variation of HSP70-1 and HSP70-2 mRNA expression could also be observed. Analysis of heat shocked lymphocytes by flow cytometry demonstrated that HSP70 induction varied also at the protein level, a fraction of cells always remaining unresponsive. Thus, HSP70 induction does not appear to occur in each cell and to a similar extent when a given cell population is exposed to the same stress.