Constitutive overexpression of the major inducible 70 kDa heat shock protein mediates large plaque formation by measles virus

Induction of the cellular stress response elevates cytoplasmic levels of heat shock proteins (HSPs) belonging to multiple families. When infected with canine distemper virus or measles virus (MV), cells containing elevated HSPs support increased viral gene expression and cytopathic effect. The present work tests the hypothesis that increases in the major inducible 70 kDa HSP (hsp72) are sufficient to mediate the effect of stress response induction on infection phenotype, Human astrocytoma cells (U373) were stably transfected with the human hsp72 gene under control of the p-actin promoter. Constitutive overexpression of hsp72 was demonstrated in multiple clones by Western blot analysis of cytoplasmic total protein. Southern blot analysis of cell DNA confirmed the recovery of genetically distinct clones. Infection of these clonal populations with MV resulted in increased viral transcript production relative to infected control cell lines. Increased transcript production was associated with increased viral membrane glycoprotein expression and cytopathic effect (i.e., mean plaque area). Increases in cytopathic effect were due to the emergence of a large plaque phenotype from a small plaque-purified inoculum, mimicking the effect of cellular stress response induction upon viral infection phenotype. Large plaque phenotypic variants reported in the literature are associated with enhanced neurovirulence, a fact that highlights the potential significance of physiologic elevations in hsp72 (e.g., fever-induced) that accompany in vivo viral infection.