Cell adhesion properties of hemolin, an insect immune protein in the Ig superfamily

The isolation of antibacterial peptides from the giant silkmoth Hyalophora cecropia has opened the area of animal antibiotics [Boman, H. G. (1991) Cell 65, 205-207] and the study of insect immune genes has revealed striking similarities to many immune response genes in mammals [Hultmark, D. (1994) Nature 267, 116-117]. However, the molecules and mechanisms behind primordial immune recognition are not understood. One candidate for one such recognition molecule is hemolin, a 48-kDa immunoglobulin-related protein first isolated from H. cecropia, where it is up-regulated upon infection and secreted into the hemolymph. Hemolin was shown to bind to bacteria and to hemocytes, giving rise to changes in hemocyte adhesiveness and intracellular phosphorylation patterns [Faye, I. & Kanost, M. (1997) in Molecular mechanisms of immune responses in insects (Brey, P. T. & Hultmark, D., eds) Chapman and Hall, London]. In the present publication, we give evidence for the presence of a 52-kDa membrane form of hemolin on hemocytes, based on flow-activated cell sorting and membrane protein extractions. In addition we reveal calcium-dependent homophilic binding properties of hemolin, using hemolin-coated microspheres. When biotinylated recombinant hemolin was allowed to bind to hemocyte membranes, higher molecular-mass complexes were formed. Furthermore, we used immunological methods and Northern-blot analysis to demonstrate the presence of hemolin in embryos and retinal discs, suggesting that hemolin is expressed in several tissues at different developmental stages. These results show novel cell adhesion features of hemolin, corroborating its multifunctional character with putative roles in cellular and humoral immunity and in development.