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Leishmania mexicana Infection Induces Impaired Lymph Node Expansion and Th1 Cell Differentiation Despite Normal T Cell Proliferation¹

Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104

Leishmania mexicana infections in C57BL/6 mice are associated with minimal immune responses and persistent cutaneous lesions. In contrast, Leishmania major elicits a robust Th1 response that promotes lesion resolution. We investigated whether the nonhealing phenotype associated with L. mexicana was due to a failure of L. mexicana to activate T cells. In vivo T cell responses to infection were assessed by tracking the behavior of labeled naive T cells following the transfer of these cells into congenic mice. Although L. mexicana infection was associated with minimal expansion of the draining lymph nodes, we observed no difference in the percentage of T cells proliferating in response to L. mexicana and L. major. Instead, differences in the size and cellularity of lymph nodes were associated with decreased recruitment of cells trafficking to the lymph node. Furthermore, we found that T cells responding to L. mexicana infection were less able to differentiate into IFN- producing cells, and this deficit extended to previously activated T cells as well. Coadministration of CpG-containing oligodeoxynucleotides at the time of infection overcame this deficit and promoted disease resolution. Taken together, our results identify two distinct components that contribute to the minimal immune response associated with L. mexicana infection. First, despite ample levels of T cell proliferation, L. mexicana fails to induce substantial lymph node expansion, which limits the number of responding T cells. Second, L. mexicana infection fails to drive the differentiation of the majority of responding cells into IFN- producers.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by the National Institutes of Health Grant AI35914.

² Address correspondence and reprint requests to Dr. Phillip Scott, Department of Pathobiology, School of Veterinary Medicine, Room 310 Hill Pavilion, 380 South University Avenue, Philadelphia, PA 19104-4539. E-mail address: pscott{at}vet.upenn.edu

³ Abbreviations used in this paper: LN, lymph node; B6, C57BL/6J; DC, dendritic cell; ODN, oligodeoxynucleotide.