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The lipoprotein from the outer membrane of Escherichia coli and other Enterobacteriaceae is a potent polyclonal activator for B lymphocytes. To determine the molecular structure responsible for the biologic activity of lipoprotein, a well-defined series of analogs of its N-terminal part was synthesized: S-(2,3-bis(palmitoyloxy)-(2-RS)-propyl)-N-palmitoyl-(R)-cysteine, -cysteine methyl ester, -cysteinyl-serine, -cysteinyl-seryl-serine, -cysteinyl-seryl-seryl-asparagine, and -cysteinyl-seryl-seryl-asparaginyl-alanine. All compounds were tested for mitogenic activity toward spleen cells from BALB/c, LPS-non-responder C3H/HeJ, and congenitally athymic C3H/Tif/Bom/nu/nu mice, measuring the incorporation of [3H]thymidine into DNA. Lymphocyte activation was confirmed by determination of the incorporation of [3H]uridine into RNA and [3H]leucine into protein. The synthetic lipopeptides were also investigated for their ability to stimulate B lymphocytes into immunoglobulin secretion, as shown by a hemolytic plaque assay. Throughout our studies, the compounds carrying two to five amino acids exhibited strong stimulation activity toward B lymphocytes comparable to native lipoprotein. In contrast, products containing only one amino acid, cysteine or cysteine methyl ester, were only marginally active, indicating that to obtain full biologic activity the presence of the hydrophilic dipeptide structure is necessary. All compounds exhibited only a marginal effect on thymocytes. Thus, a series of defined synthetic fragments of a bacterial outer membrane component exhibits a pronounced mitogenic and polyclonally stimulating activity towards B lymphocytes. The substances will be valuable tools for more detailed investigations on the molecular mechanisms of B cell activation.

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1 Jeder Organismus wird taglich mit einer Vielzahl von pathogenen Bakterien, Viren und weiteren Mikroorganismen konfrontiert. Hohere Lebewesen setzen dieser Bedrohung ein effizientes Abwehrsystem, das Immunsystem, entgegen. Die wichtigsten Fahigkeiten dieses Systems sind die Unterscheidung von Selbst und Nicht-Selbst, die hohe Spezifitat der Erkennung und ein ausgepragtes immunologisches Gedachtnis. Ein in den Organimus eingedrungener Fremdkorper wird Antigen genannt. Handelt es sich dabei um zellulare Eindringlinge, z. B. Bakterien, so wirken Teile ihrer Oberflache als Antigen. Die Abwehr erfolgt zunachst durch Granulocyten oder Makrophagen, die in der Lage sind, Fremdstoffe aufzunehmen, zu phagocytieren und zu verdauen. Makrophagen haben zusatzlich die Aufgabe, das Antigen auf ihrer Oberflache zu fixieren und den Lymphocyten zu prasentieren. Biologie in unserw k i t / 16. Jahrg. 1986 / Nr. 2 0 V C H Verlagsgesellschafi mbH, 0-6940 Weinheim, 1986 004J-205X/86/0204-0033 $ 02.50/0