Expression of cholera toxin B–proinsulin fusion protein in lettuce and tobacco chloroplasts – oral administration protects against development of insulitis in non‐obese diabetic mice

Summary Lettuce and tobacco chloroplast transgenic lines expressing the cholera toxin B subunit–human proinsulin (CTB‐Pins) fusion protein were generated. CTB‐Pins accumulated up to ~16% of total soluble protein (TSP) in tobacco and up to ~2.5% of TSP in lettuce. Eight milligrams of powdered tobacco leaf material expressing CTB‐Pins or, as negative controls, CTB–green fluorescent protein (CTB‐GFP) or interferon–GFP (IFN‐GFP), or untransformed leaf, were administered orally, each week for 7 weeks, to 5‐week‐old female non‐obese diabetic (NOD) mice. The pancreas of CTB‐Pins‐treated mice showed decreased infiltration of cells characteristic of lymphocytes (insulitis); insulin‐producing β‐cells in the pancreatic islets of CTB‐Pins‐treated mice were significantly preserved, with lower blood or urine glucose levels, by contrast with the few β‐cells remaining in the pancreatic islets of the negative controls. Increased expression of immunosuppressive cytokines, such as interleukin‐4 and interleukin‐10 (IL‐4 and IL‐10), was observed in the pancreas of CTB‐Pins‐treated NOD mice. Serum levels of immunoglobulin G1 (IgG1), but not IgG2a, were elevated in CTB‐Pins‐treated mice. Taken together, T‐helper 2 (Th2) lymphocyte‐mediated oral tolerance is a likely mechanism for the prevention of pancreatic insulitis and the preservation of insulin‐producing β‐cells. This is the first report of expression of a therapeutic protein in transgenic chloroplasts of an edible crop. Transplastomic lettuce plants expressing CTB‐Pins grew normally and transgenes were maternally inherited in T 1 progeny. This opens up the possibility for the low‐cost production and delivery of human therapeutic proteins, and a strategy for the treatment of various other autoimmune diseases.