Dendritic Cells Phagocytose and Are Activated by <em>Treponema pallidum</em>

Departments of Microbiology^{and Dermatology,}^{The University of Texas Southwestern Medical Center, Dallas, Texas 75390}

^{Corresponding author. Mailing address: Department of Microbiology, U.T. Southwestern Medical Center, 6000 Harry Hines Blvd., Dallas, TX 75390. Phone: (214) 648-5900. Fax: (214) 648-5905. E-mail:}ude.demws.wstu@dragron.

Received 2000 Aug 10; Revisions requested 2000 Sep 28; Accepted 2000 Oct 6.

Abstract

Cell-mediated immune processes play a prominent role in the clinical manifestations of syphilis, a sexually transmitted disease of humans caused by spirochetal bacterium Treponema pallidum. The immune cell type that initiates the early immune response to T. pallidum thus far has not been identified. However, dendritic cells (DCs) are the first immune-competent cells to encounter antigens within skin or mucous membranes, the principal sites of early syphilitic infection. In the present study, immature DC line XS52, derived from murine skin, was utilized to examine T. pallidum-DC interactions and subsequent DC activation (maturation). Electron microscopy revealed that T. pallidum was engulfed by DCs via both coiling and conventional phagocytosis and was delivered to membrane-bound vacuoles. The XS52 DC line expressed surface CD14 and mRNA for Toll-like receptors 2 and 4, molecules comprising important signaling components for immune cell activation by bacterial modulins. Both T. pallidum and a synthetic lipopeptide (corresponding to the 47-kDa major membrane lipoprotein) activated the XS52 DC line, as indicated by the secretion of interleukin-12 (IL-12), IL-1β, tumor necrosis factor alpha, and IL-6 and elevated surface expression of CD54. The combined data support the contention that DCs stimulated by T. pallidum and/or its proinflammatory membrane lipoproteins are involved in driving the cellular immune processes that typify syphilis.

Abstract

Syphilis, a sexually transmitted disease of humans caused by spirochetal bacterium Treponema pallidum, remains a global public health problem, with an estimated 12 million new cases annually (15). A syphilis eradication program for the United States, recently announced, includes the development of a syphilis vaccine (36). However, basic immune mechanisms that might lead to protective immunity in humans (32) are largely unknown and, as such, a more thorough comprehension of syphilis immunology is essential for designing new approaches for the development of a syphilis vaccine.

The first clinical symptom of syphilis typically is the primary genital ulcer (chancre) at the local site of T. pallidum tissue invasion (30); dermal cellular infiltrates are composed chiefly of lymphocytes, macrophages, and plasma cells (30). A key aspect of understanding syphilis immunology has been the identification of the treponemal factor(s) that elicits this intense inflammatory response. Of note, T. pallidum lacks lipopolysaccharide (LPS) (14, 17), a potent proinflammatory agonist. However, treponemes contain abundant membrane lipoproteins (6, 14). There is now a large body of evidence, derived from both in vitro and in vivo studies, to support the notion that T. pallidum's membrane lipoproteins are the principal proinflammatory mediators during syphilitic infection (1, 10, 37, 38, 43, 45, 49, 59). Lipoproteins efficiently activate various immune effector cells, particularly those of the monocyte/macrophage lineage, and their immunomodulatory properties are engendered by the acyl configuration of their N termini. In this regard, in many immune cell activation studies that now have been performed synthetic analogs (lipopeptides) modeled after the N termini of bacterial lipoproteins have been used as surrogates for native lipoproteins (10, 37, 38, 40, 45, 59).

The rabbit experimental model of syphilis has provided for an analysis of the course of cellular events during syphilitic lesion development (29). Following intradermal or intratesticular inoculation of rabbits with T. pallidum, T lymphocytes appear 6 days after infection, reaching maximal numbers on days 10 to 13 (paralleling maximal T. pallidum replication) (29). Macrophages become apparent on day 10 and reach maximal numbers on day 13, coincident with a dramatic clearance of the spirochetes (29). Whereas these former studies provided seminal information about the various immune cell types present in syphilitic lesions, they focused primarily on characterizing immune cells involved at times relatively late (i.e., several to many days) in the cellular processes of syphilis, well beyond the time when key triggering events of the innate immune response occur. The actual cell type(s) that potentiates the early immune response to dermal invasion by T. pallidum thus far has not been identified.

There is now overwhelming evidence that dendritic cells (DCs) (e.g., Langerhans cells of the epidermis) are among the most potent antigen-presenting cells of the immune system and are crucial for the initiation of primary T-cell responses to foreign antigens, particularly bacterial antigens (4, 47). During foreign antigen insult, immature DCs in the peripheral tissues capture antigens (via phagocytosis and pinocytosis) and then, under microenvironmental signals (e.g., cytokines), migrate to the draining lymph nodes while undergoing maturation (4). The maturing DCs, now with a reduced capacity for phagocytosis but with an increased ability to present antigens, enter the lymph nodes where they home to T-cell-rich areas and induce an antigen-specific primary T-cell response (4). Primed T cells then migrate via the efferent pathway back to the site of antigen deposition. In this scenario, DCs act as an important sentinel between the external environment and the host's immune system, as well as serve as the cellular interface for the transition to the adaptive immune response (4, 47).

Given the contemporary understanding of DC biology (4), it is plausible that the DC is the pivotal immune effector cell that initiates the cellular inflammatory response during syphilis. Thus far, DCs in the cellular immune responses to T. pallidum have not been studied. Furthermore, at areas of inflammation in skin and mucous membranes, the principal sites of syphilitic infection, DCs are the first immune-competent cells to encounter antigens (4, 34). The present study thus was designed with two principal objectives. The first was to characterize the interaction of virulent T. pallidum with DCs, with emphasis on examining phagocytic processes, using a well-characterized immature DC line (72) as a model system. The second was to assess the impact of T. pallidum on DC maturation. Additionally, given the proinflammatory properties of T. pallidum membrane lipoproteins (1, 10, 37, 38, 43, 45, 49, 59), we also investigated whether DCs were activated by a synthetic lipopeptide corresponding to T. pallidum's major membrane lipoprotein.

ACKNOWLEDGMENTS

We thank Kayla Hagman and Martin Goldberg for assistance with treponemes, Dennis Bellotto for electron microscopy thin sectioning, Dale Edelbaum and Richard Kitchens for technical advice, Robert Modlin and Justin Radolf for supportive discussions, and Petra Cravens for critical review of the manuscript.

This work was supported by Public Health Service grant AI-16692 from the Sexually Transmitted Diseases Branch of the National Institute of Allergy and Infectious Diseases, NIH, by an NIH-supported (P30-AR-41940) Pilot and Feasibility Grant from the U.T. Southwestern Skin Disease Research Center, and by grant 010019-0096-1999 (Advanced Technology Program) from the Texas Higher Education Coordinating Board.

ACKNOWLEDGMENTS

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