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Infect Immun, June 1998, p. 2905-2913, Vol. 66, No. 6
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Signal Transduction during Legionella pneumophila Entry into Human Monocytes

Patricia Y. Coxon,1,* James T. Summersgill,1,2 Julio A. Ramirez,2 and Richard D. Miller1

Department of Microbiology and Immunology1 and Division of Infectious Diseases, Department of Medicine,2 University of Louisville, Louisville, Kentucky 40292

Received 21 January 1998/Returned for modification 6 March 1998/Accepted 26 March 1998

Legionella pneumophila causes Legionnaires' disease by replication in alveolar macrophages and monocytes. The bacteria are internalized most efficiently by opsonin-dependent, CR3-mediated phagocytosis. This investigation focused on determining the role of actin polymerization and phosphorylation signals in this uptake mechanism. Uptake inhibition assays and confocal microscopic analysis indicated that entry of L. pneumophila activated tyrosine kinase (TK) and protein kinase C (PKC) and induced actin polymerization at the site of bacterial entry. Upon L. pneumophila entry, six major cellular proteins (75, 71, 59, 56, 53, and 52 kDa) were TK phosphorylated in soluble fractions of monocytes, and three of these proteins (52, 53, and 56 kDa) were consistently found in insoluble (i.e., cytoskeletal) fractions of monocytes as well. Tyrosine phosphorylation was suppressed when cells were pretreated with the kinase inhibitor genistein, tyrphostin, or staurosporine. A similar tyrosine-phosphorylated protein pattern was observed with CR3-mediated entry of avirulent L. pneumophila, Escherichia coli, or zymosan into monocytes. This study has shown that PKC and TK signals which activate actin polymerization during the process of phagocytosis are induced upon L. pneumophila entry. In addition, CR3 receptor-mediated phagocytosis into monocytes may involve tyrosine phosphorylation of similar proteins, regardless of the particle being phagocytosed. Therefore, the tyrosine-induced phosphorylation observed during opsonized L. pneumophila entry is not a virulence-associated event.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, School of Medicine, University of Louisville, Louisville, KY 40292. Phone: (502) 852-4120. Fax: (502) 852-7531. E-mail: pycoxo01{at}ulkyvm.louisville.edu.

Infect Immun, June 1998, p. 2905-2913, Vol. 66, No. 6
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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