Developmentaland ComparativeImmunology, Vol. 17, pp. 195- 200, 1993 0145-305X/93 $6.00 + .00 Printed in the USA. All rights reserved Copyright © 1993 Pergamon Press Ltd. SPECIFIC IMMUNE RECOGNITION OF INSECT HEMOLIN Otto Schmidt, Ingrid Faye, Ingrid Lindstr6m-Dinnetz, and Shao-Cong Sun Institut for Biologie III, Sch~nzlestr, 1, D-7800 Freiburg, Germany and Department of Microbiology, Universityof Stockholm, S-106 91 Stockholm, Sweden (Submitted September 1991;Accepted August 1992) [3Abstract--Bacteria entering the body cavi- ties of insects are recognized by hemolymph components and subsequently inactivated by phagocytosis and nodule formation. A he- molymph component called hemolin is appar- ently involved in the recognition process. It binds to a bacterial surface molecule and forms a stable complex with other hemolymph pro- teins. Hemolin binding is independent of bac- terial lipopolysaccharide (LPS) structure, whereas the complex formation is dependent on the presence of the carbohydrate moiety of LPS molecules. The specificity of immune rec- ognition in insects is discussed. rqKeywords-- Hemolin; Hemolymph; Hyalophora cecropia; Manduca sexta; Insect immunity. Introduction The molecular recognition process of foreign objects and microorganisms that enter the body cavities of insects is not known. The small numbers of blood cells in most invertebrates preclude any ex- tensive anticipatory immune reaction (I) with clonal selection of antibody-pro- ducing blood cells that improve specific- ity of antigen recognition and provide memory. Nevertheless, given the capa- bility to resist most microorganisms and parasites, insects must have at their dis- posal a subtle and adaptable immune de- fense system to distinguish their own tis- sues from components of foreign struc- tures. Address correspondence to Otto Schmidt, (present address) Waite Institute, University of Adelaide, Glen Osmond, S.A. 5064, Australia. Here we describe some of the features of hemolin, a recently discovered mole- cule in the hemolymph of Hyalophora cecropia (2) and Manduca sexta (3), that allow specificity of recognition in the ab- sence of a mammalian-type anticipatory immune system. Hemolin Hemolin is present in a low but de- tectable amount in the hemolymph of the normal diapausing pupae of the giant silk moth, Hyalophora cecropia. After bac- terial infection, its concentration may in- crease 18-fold. However, hemolin does not exhibit direct bactericidal or bacte- riostatic effects (4,5). From the nucleotide sequence of the coding cDNA, the deduced hemolin pro- tein, 413 amino acids in length, consists of four internal repeats characteristic of the Ig-like domains (2). Two cysteines and a number of other amino acids are highly conserved among the repeated do- mains of hemolin and of other members of the Ig superfamily (6). Sequence comparison revealed that, within the Ig superfamily, hemolin has the closest similarity to the neural cell adhesion molecules from insects and vertebrates. It has particularly good se- quence homology (about 38%) to the first four Ig-like domains of Drosophila neu- roglian (2,7). However, hemolin lacks the fibronectin-like domains present in neuroglian and many other neural cell adhesion molecules (Fig. 1). The pre- dicted secondary structure of hemolin 195