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Phagocytosis of Borrelia burgdorferi, the Lyme Disease Spirochete, Potentiates Innate Immune Activation and Induces Apoptosis in Human Monocytes

Adriana R. Cruz,^1,, Meagan W. Moore,^1, Carson J. La Vake,¹ Christian H. Eggers,^1, Juan C. Salazar,² and Justin D. Radolf^1,3*

Departments of Medicine,¹ Genetics and Developmental Biology, University of Connecticut Health Center, Farmington, Connecticut 06030-3715,³ Department of Pediatrics, Division of Infectious Diseases, Connecticut Children's Medical Center, Hartford, Connecticut 06106²

Received 27 July 2007/ Returned for modification 7 September 2007/ Accepted 8 October 2007

We have previously demonstrated that phagocytosed Borrelia burgdorferi induces activation programs in human peripheral blood mononuclear cells that differ qualitatively and quantitatively from those evoked by equivalent lipoprotein-rich lysates. Here we report that ingested B. burgdorferi induces significantly greater transcription of proinflammatory cytokine genes than do lysates and that live B. burgdorferi, but not B. burgdorferi lysate, is avidly internalized by monocytes, where the bacteria are completely degraded within phagolysosomes. In the course of these experiments, we discovered that live B. burgdorferi also induced a dose-dependent decrease in monocytes but not a decrease in dendritic cells or T cells and that the monocyte population displayed morphological and biochemical hallmarks of apoptosis. Particularly noteworthy was the finding that apoptotic changes occurred predominantly in monocytes that had internalized spirochetes. Abrogation of phagocytosis with cytochalasin D prevented the death response. Heat-killed B. burgdorferi, which was internalized as well as live organisms, induced a similar degree of apoptosis of monocytes but markedly less cytokine production. Surprisingly, opsonophagocytosis of Treponema pallidum did not elicit a discernible cell death response. Our combined results demonstrate that B. burgdorferi confined to phagolysosomes is a potent inducer of cytosolic signals that result in (i) production of NF-B-dependent cytokines, (ii) assembly of the inflammasome and activation of caspase-1, and (iii) induction of programmed cell death. We propose that inflammation and apoptosis represent mutually reinforcing components of the immunologic arsenal that the host mobilizes to defend itself against infection with Lyme disease spirochetes.

Published ahead of print on 15 October 2007.

Editor: F. C. Fang

A.R.C. and M.W.M. contributed equally to this research.

Present address: Centro Internacional de Entenamiento e Investigaciones, Cali, Colombia.

Present address: Department of Biomedical Sciences, Quinnipiac University, Hamden, CT 06518.