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Equine infectious anemia virus (EIAV) has been used as a model to investigate protective mechanisms against lentiviruses. Unlike other lentiviruses, EIAV replication can be eventually controlled in most infected horses leading to an inapparent carrier state free of overt clinical signs which can last for many years. Maintenance of this carrier state is absolutely dependent on active immune responses as evidenced by the fact that immunosuppressive drugs can induce the recurrence of disease. However, the immune mechanisms that are responsible for this control of infection are not yet identified. As the resolution of the initial infection is correlated with the appearance of the virus-specific cytotoxic T lymphocytes (CTL), it appears that cellular immune responses play an important role. However, most studies into this protective mechanism have been limited to the identification of specific epitopes, usually at a single time point in the infection. Few studies have examined the cellular immune responses to the viral antigens throughout the infection period. Since the virus undergoes rapid mutation following infection, the adaptive immune response must also evolve to meet this challenge. Previously, the EIAV envelope (gp90) protein was shown to be the primary determinant of vaccine efficacy. Here, we hypothesized that the maturation of cellular immune responses is a lengthy process and involves envelope-specific T cell recognition shifting from immunodominant variable determinants to conserved immunorecessive determinants during the initial stages of the EIAV infection. The first part of this dissertation was to develop a new in vivo method to identify envelope-specific T cell responses. The second part of this dissertation was to investigate whether envelope-specific T cell recognition evolved in EIAV-infected ponies. Finally, the mechanisms for this T cell immunodominant shifting were also investigated from the point of both virus sequence mutation and T cell clone expansion and contraction. Also, a new EIAV attenuated vaccine which contained a consensus gp90 sequence was tested to see if it facilitated T cell recognition of the more conserved regions early in the infection. Our results indicated that envelope-specific T cell recognition patterns changed over time. Early after infection, dominant immune responses to the peptides in the carboxyl-terminus variable region were identified. By six months post infection, the recognized peptides spanned the entire envelope sequence, with a shift to the amino-terminus conserved region. The mechanisms responsible for this change remain unclear, but analysis of T cell receptor repertoire indicated that T cell clonal expansion and contraction might be one of the reasons. Our demonstration that envelope-specific peptide recognition shifts from the variable to the more conserved regions provides evidence that the maturation of cell mediated immune response is parallelled with long-term control of this infection.

Das Gen für die Expression von Helicobacter pylori Urease wurde in das Genom von Salmonella typhi Ty21a (Typhoral() integriert. Der resultierende Stamm erhielt die Bezeichnung Salmonella typhi Ty21a(pDB1). Neun gesunden Probanden wurde Salmonella typhi Ty21a(pDB1) verabreicht. In der Kontrollgruppe erhielten drei gesunde Probanden Salmonella typhi Ty21a. Schwerwiegende Nebenwirkungen wurden bei keinem der Probanden beobachtet. Zehn von 12 Probanden zeigten eine humorale Immunantwort gegen Antigene von Salmonella typhi, nachgewiesen über die Detektion spezifischer Antikörper produzierender Zellen, aber bei nur zwei Probanden ließ sich eine Serokonversion nachweisen. Eine zelluläre Immunantwort gegen das H-Antigen von Salmonella typhi konnte bei insgesamt fünf Probanden nachgewiesen werden. Insgesamt sechs Probanden zeigten eine zelluläre Immunantwort gegen Urease von Helicobacter pylori in zumindest einem von drei der zur Anwendung gebrachten Testsysteme (Proliferation, T-cell-Elispot, IFN-gamma Elisa). Eine humorale Immunantwort gegen Urease von Helicobacter pylori konnte bei keinem der Probanden nachgewiesen werden. Ty21a(pDB1) ist somit ein geeigneter Prototyp zur Optimierung einer Salmonella basierte Impfung zur Induktion einer zellulären Immunantwort, die zu einer protektiven Immunität gegenüber Helicobacter pylori führen könnte. / Helicobacter pylori urease was expressed in the common live typhoid vaccine Salmonella typhi Ty21a (Typhoral() yielding Salmonella typhi Ty21a(pDB1). Nine volunteers received Salmonella typhi Ty21a(pDB1) and three control volunteers received Salmonella typhi Ty21a. No serious adverse effects were observed in any of the volunteers. Ten out of 12 volunteers developed humoral immune responses to the Salmonella carrier as detected by antigen-specific antibody-secreting cells but only two volunteers seroconverted. A total of five volunteers showed cellular responses to the carrier. Six out of nine volunteers that had received Salmonella typhi Ty21a(pDB1) showed a T-cell response to Helicobacter urease in at least one of the three assays for detection of cellular response (Proliferation, T-cell-Elispot, IFN-gamma Elisa). No volunteer had detectable humoral responses to urease. Salmonella typhi Ty21a(pDB1) is a suitable prototype to optimize Salmonella-based vaccination for efficient cellular responses that could mediate protective immunity against Helicobacter.