Developmental effects on immunity: Hormonal and proteinase control in Manduca sexta, the tobacco hornworm

Insects are able to survive a plethora of pathogens despite their lack of adaptive immunity.  Insects have robust innate immune systems that are divided into cell-mediated and humoral branches.  Activation of cell-mediated responses results in phagocytosis, nodule formation, and encapsulation by hemocytes.  Activation of humoral responses results in production of anti-microbial peptides (AMPs) and phenoloxidase (PO).  These immune responses, like many physiological processes, are plastic with development within an instar. We seek answer two main research questions:  how do immune responses vary within an instar and what drives changes in immunity within an instar?  AMP and PO activity decline after a single day of development in the 5^th instar of Manduca sexta, the tobacco hornworm. Many changes occur within an instar that could affect immunity. Protein expression of matrix metalloproteinase (MMP), which is involved in immune responses in other insects, decreases throughout the 5^th instar in M. sexta in the critical immune tissues, hemolymph and fat body.  Further, MMP mRNA expression increases after Escherichia coli injection in hemocytes and fat body in early 5^th instar M. sexta. Juvenile hormone (JH) peaks early and then decreases within an instar until molting. Though JH is immunostimulatory in other insects, we found JH analog application to late 5^th instar M. sexta did not affect AMP or PO activity. JH analog may still affect other indexes of immunity, such as survival. Because of the importance of insects as pests and pollinators, understanding how immunity varies throughout development and the underlying mechanisms is critical.