Ex Vivo Kinetics of Early and Long-Term Multifunctional Human Leukocyte Antigen E-Specific CD8+ Cells in Volunteers Immunized with the Ty21a Typhoid Vaccine

Salmonella enterica serovar Typhi, the causative agent of typhoid fever, is a facultative intracellular bacterial pathogen (5, 7, 37). In industrialized countries, Salmonella infection is rare, with most infections occurring in military personnel and in individuals traveling to areas where typhoid fever is endemic. However, it remains an important public health priority in undeveloped parts of the world. Overall, it is estimated that 16 million new cases and 600,000 deaths occur annually (5, 12, 18). Our laboratory has demonstrated that oral immunization of volunteers with attenuated typhoid vaccine strain Ty21a triggers the generation of CD8+ T cells that kill S. Typhi-infected cells displaying human leukocyte antigen E (HLA-E) on the cell surface (27). T cells are likely to play an important role in the host defense against S. Typhi. They might contribute to Salmonella control by secreting cytokines such as gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin-2 (IL-2) and by directly killing bacterium-infected cells (27, 28, 30-32, 38-40). HLA-E is a protein that is part of a family of molecules known as class Ib or nonclassical HLA molecules (26). HLA-E is expressed ubiquitously on the peripheral blood mononuclear cell (PBMC; e.g., B cells, T cells, natural killer [NK] cells, and macrophages) surface at various levels, depending on the cell type (15, 34). Because HLA-E molecules are essentially monomorphic, they are likely to present a more conserved set of bacterial peptides that could be universally recognized by CD8+ T cells from most, if not all, individuals. Given the potential significance of HLA-E-restricted CD8+ responses in resistance to Salmonella infection, it is of great importance to study these responses in depth, including their kinetics of appearance and persistence, which currently remains largely unknown. These responses might provide key insights into the open question concerning the dual role of the HLA-E molecules in short-lived innate and long-lived adaptive immunity (35). In fact, HLA-E molecules were primarily described as ligands for CD94/NKG2A, -B, and -C receptors present on the innate NK cell surface (3). However, HLA-E-restricted T cells have recently been implicated in adaptive immunity not only to Salmonella but also to cytomegalovirus and Mycobacterium (10, 22). Oral immunization with the licensed Ty21a typhoid vaccine is a particularly relevant model of Salmonella infection, which is transmitted by the ingestion of virulent S. Typhi. In the present studies, volunteers were immunized with the standard regimen of four spaced doses of Ty21a given every other day (8, 28). This vaccination regimen results in levels of protection ranging from 60 to 80% (9). Using cells from Ty21a vaccinees, we investigated the kinetics of appearance and longevity of HLA-E-restricted CD8+ T-cell responses for up to 2 years after immunization with the Ty21a typhoid vaccine and defined in detail the various memory T-cell subsets and effector functions mediating these responses.