The humoral antibacterial response of Drosophila adults

Hemolymph from a normal adult Drosophila melanogaster lacks factors that block the growth of Escherichia coli, but hemolymph from a fly previously inoculated with Enterobacter cloacae inhibits bacterial growth. Antibacterial activity appears within two hours after inoculation, and is still detectable sixty days later. Activity is potent, and can be detected in as little as a quarter of the hemolymph from a single inoculated male fly. After inoculation, at least eight new polypeptides not of bacterial origin appear in hemolymph with a time course similar to the appearance of antibacterial activity; these are called Antibacterial Response Polypeptides, or ARs. The most prominent polypeptides are AR24, AR22, and AR19 with molecular weights of about 24, 22, and 19 kilodaltons (kd). Other bands with as much as 75 kd and as little as 5 kd were also found. Electrophoresis of active hemolymph under non-denaturing conditions, and isoelectric focusing separate several protein species that block bacterial growth (Antibacterial Proteins, or ABs); one AB is neutral (AB7.1) and three are basic (AB8.7, AB9.0 and AB9.2). Two dimensional gels show that AR24, AR22 and AR19 have pIs identical to the basic antibacterial proteins. Radiolabelling experiments proved that the ARs were synthesized de novo after bacterial inoculation. ARs in six species of Drosophila showed fundamentally similar electrophoretic patterns.