The early pathogenesis of foot-and-mouth disease in cattle after aerosol inoculation.

摘要

The goal of the efforts described in this dissertation was to characterize the early pathogenesis of foot-and-mouth disease (FMD) in cattle after simulated natural infection. More specifically, emphasis was placed upon two critical knowledge gaps: identification of the primary site(s) of infection of FMD virus (FMDV) and the mechanism of establishment of viremia. In order to investigate these processes, novel systems were developed for (1) consistent experimental aerosol infection of steers, (2) molecular and virological detection of FMDV in bovine tissues, and (3) microscopic localization of FMDV antigens in bovine tissues. These novel tools were then applied to a thorough, prospective, time-course analysis of bovine FMD.;Screening of antemortem samples indicated that establishment of primary infection in the respiratory tract was detectable between 4 - 6 hours post aerosol inoculation (hpa); establishment of viremia was detectable between 24 -- 48 hpa. Examination of tissue samples collected postmortem demonstrated that in previremic steers, FMDV was most consistently localized to nasopharyngeal tissues by all detection methods indicating this region as the most important site of primary viral replication. The earliest site of microscopic localization of FMDV antigens was the lymphoid follicle-associated epithelium of the pharyngeal mucosa -- associated lymphoid tissue (PALT) of the nasopharynx. At early time points after aerosol inoculation, viral antigens colocalized with cytokeratin-positive pharyngeal epithelial cells; intraepithelial, FMDV-negative, MHCII/CD11c-double positive dendritic cells were present in close proximity to FMDV-positive cells. Onset of viremia coincided with marked increase of viral loads in pulmonary tissues and substantial decrease of viral detection in nasopharyngeal tissues. These data indicate that subsequent to aerogenous exposure to FMDV, the temporally defined critical pathogenesis events are (1) primary replication in epithelial cells of the PALT crypts, (2) subsequent widespread replication in pneumocytes in the lungs which coincides with (3) the establishment of sustained viremia.;This body of work is unique for its breadth and depth of investigation of FMD in cattle; the importance of the conclusions described herein may be separated into three tiers. The detailed characterization of the early virus-host interactions provides a greater level of understanding of the pathogenesis of this important disease and thus directly contributes to basic science knowledge. Additionally, the novel techniques described herein may be applied to a wide range of subsequent pathogenesis studies which will further elucidate FMDV-host interactions in various species and stages of infection. However, the translational implications of the findings are likely to, ultimately, have greatest significance by contributing to the development of rationally designed FMDV vaccines and biotherapeutics. Specifically, the data described in the chapters which follow strongly suggest that improved mucosal immunity (particularly in the nasopharynx) should be a high-priority goal of "next generation" FMDV prophylaxis.