The effect of bacterial challenge on the midgut physiology and development of sand fly Lutzomyia longipalpis

Phlebotomine sand flies are vectors for parasites of the genus Leishmania, the causative agents of leishmaniasis, a disease endemic to 98 countries. During development within the vector, Leishmania parasites face many barriers; including the vectors own innate immune responses. Studies in Diptera models indicate that upon bacterial infection, innate immunity and epithelial homeostasis pathways are activated leading to the restructuring of the midgut epithelium.  Surprisingly, our results suggest that infection during late larval stages leads to an inhibition of the canonical immune response in favor of autophagy. This physiological switch allows the larvae to conserve and recycle the necessary nutrients required to progress through larval stages. Briefly, 3rd instar larvae were fed LB agar plates containing GFP-labeled Pantoea agglomerans or Bacillus subtilis versus controls containing LB or paraquat. Midguts were dissected at 4, 12, 24, 36 hours post infection and processed for qPCR analysis of changes in immune, homeostatic, and autophagic transcript levels. Additionally, midguts were processed separately for localization of infection, size differences, and apoptosis. Our results suggest that a negative regulator of the immunodeficiency response, PIRK, is responsible for inhibiting immunity in favor of autophagy in late larval stages of sand flies. We also observed differential cellular apoptosis and restructuring of the midgut upon different treatments. Our results provide evidences of the mechanistic underpinnings required for understanding the complex environment of an immature sand flies' development into adult. It may also provide novel stage specific targets for vector control.