Pan-neuronal expression of human mutant huntingtin protein in Drosophila impairs immune response of hemocytes

Huntington's disease (HD) is a late-onset; progressive, dominantly inherited neurological disorder marked by an abnormal expansion of polyglutamine (poly Q) repeats in Huntingtin (HTT) protein. The pathological effects of mutant Huntingtin (mHTT) are not restricted to the nervous system but systemic abnormalities including immune dysregulation have been evidenced in clinical and experimental settings of HD. Indeed, mHTT is ubiquitously expressed and could induce cellular toxicity by directly acting on immune cells. However, it is still unclear if selective expression of mHTT exon1 in neurons could induce immune responses and hemocytes' function. In the present study, we intended to monitor perturbations in the hemocytes' population and their physiological functions in Drosophila, caused by pan-neuronal expression of mHTT protein. A measure of hemocyte count and their physiological activities caused by pan-neuronal expression of mHTT protein highlighted the extent of immune dysregulation occurring with disease progression. We found that pan-neuronal expression of mHTT significantly alters crystal cells and plasmatocyte count in larvae and adults with disease progression. Interestingly, plasmatocytes isolated from diseased conditions exhibit a gradual decline in phagocytic activity ex vivo at progressive stages of the disease as compared to age-matched control groups. In addition, diseased flies displayed elevated reactive oxygen species (ROS) in circulating plasmatocytes at the larval stage and in sessile plasmatocytes of hematopoietic pockets at terminal stages of disease. These findings strongly implicate that neuronal expression of mHTT alone is sufficient to induce non-cell-autonomous immune dysregulation in vivo.