Chlamydia trachomatis is an obligate intracellular bacterium with a tropism for epithelial cells lining mucosal surfaces. Chlamydia-induced disease has been reported following infection of the genital, rectal, pulmonary, or conjunctival mucosa, affecting millions of individuals worldwide. Yet there is no vaccine available against this pathogen and no current methodology for delivery of a vaccine that will provide protection at multiple mucosal sites, parenteral immunization strategies having failed in this regard. It is now clear that the capacity of leukocytes to emigrate into distinct tissue sites is not random but is directed by a program of interactions between leukocyte adhesion receptors and endothelial ligands. Furthermore, the homing program for mucosal tissues appears to be distinct from that for systemic sites, providing a rational basis for the failure of parenteral vaccines to protect against mucosal infections. Information on mucosal homing programs is currently derived from analyses of receptor-ligand interactions required for trafficking to the intestinal mucosa. To determine whether the same program extended to the mucosa of the genital tract, we analyzed the receptor-ligand interactions involved in lymphocyte migration to the genital mucosa in mice infected vaginally with Chlamydia trachomatis. Vaginal infection with C. trachomatis stimulates a vigorous inflammatory response comprised initially of neutrophils and later of mononuclear cells, primarily lymphocytes. Using flow cytometry and immunohistochemistry, we analyzed the distribution of T cell subsets and adhesion receptors on infiltrating lymphocytes and the expression of vascular ligands by local venule endothelium. Mononuclear infiltrates were composed predominantly of CD4+ T lymphocytes expressing the alphaLbeta2 and alpha4beta1 adhesion molecules, receptors traditionally associated with lymphocyte homing to systemic sites of inflammation. Only 15% of cells expressed the alpha4beta7 adhesion receptor that is required for lymphocyte homing to the intestinal mucosa. The apparent existence of two homing programs for the genital mucosa, one shared with the systemic circulation and one with the intestinal mucosa, was corroborated by detection of both corresponding vascular ligands on genital tract endothelium. Interestingly, the distribution of these ligands showed regional specialization such that alpha4beta7+ T cells had access to vaginal and cervical tissues only while alpha4beta1+ T cells could infiltrate any part of the female genital tract. Since alpha4beta7+ T cells derived from the intestinal mucosa appear to be functionally distinct from their peripheral counterparts in that they are more easily tolerized to bacterial antigens, we postulate that regional specialization within the genital mucosa may reflect a physiologic requirement for functional segregation of lymphocyte populations. Results also suggest that parenteral immunization may be an appropriate route for induction of immunity within the genital tract since it should stimulate an alpha4beta1+ T cell subset capable of protecting the uterine mucosa.
