HIV-1-Infected CD4+ T Cells Present MHC Class II-Restricted Epitope via Endogenous Processing.

HIV-1-specific CD4+ T cells (TCD4+s) play a critical role in controlling HIV-1 infection. Canonically, TCD4+s are activated by peptides derived from extracellular ("exogenous") Ags displayed in complex with MHC class II (MHC II) molecules on the surfaces of "professional" APCs such as dendritic cells (DCs). In contrast, activated human TCD4+s, which express MHC II, are not typically considered for their APC potential because of their low endocytic capacity and the exogenous Ag systems historically used for assessment. Using primary TCD4+s and monocyte-derived DCs from healthy donors, we show that activated human TCD4+s are highly effective at MHC II-restricted presentation of an immunodominant HIV-1-derived epitope postinfection and subsequent noncanonical processing and presentation of endogenously produced Ag. Our results indicate that, in addition to marshalling HIV-1-specific immune responses during infection, TCD4+s also act as APCs, leading to the activation of HIV-1-specific TCD4+s.

Recombinant Poxviruses: Versatile Tools for Immunological Assays.

The study of antigen processing and presentation is critical to our understanding of the mechanisms that govern immune surveillance. A typical requirement of assays designed to examine antigen processing and presentation is the de novo biosynthesis of a model antigen. Historically, Vaccinia virus, a poxvirus closely related to Cowpox virus, has enjoyed widespread use for this purpose. Recombinant poxvirus-based expression has a number of advantages over other systems. Poxviruses accommodate the insertion of large pieces of recombinant DNA into their genome, and recombination and selection are relatively efficient. Poxviruses readily infect a variety of cell types, and they drive rapid and high levels of antigen expression. Additionally, they can be utilized in a variety of assays to study both MHC class I restricted and MHC class II restricted antigen processing and presentation. Ultimately, the numerous advantages of poxvirus recombinants have made the Vaccinia expression system a mainstay in the study of processing and presentation over the past two decades. In an attempt to address one shortcoming of Vaccinia virus while simultaneously retaining the benefits inherent to poxviruses, our laboratory has begun to engineer recombinant Ectromelia viruses. Ectromelia virus, or mousepox, is a natural pathogen of murine cells and performing experiments in the context of a natural host-pathogen relationship may elucidate unknown factors that influence epitope generation and host response. This chapter will describe several recombinant poxvirus system protocols used to study both MHC class I and class II antigen processing and presentation, as well as provide insight and troubleshooting techniques to improve the reproducibility and fidelity of these experiments.

Human MAIT cells exit peripheral tissues and recirculate via lymph in steady state conditions.

Mucosal-associated invariant T cells (MAIT cells) recognize bacterial metabolites as antigen and are found in blood and tissues, where they are poised to contribute to barrier immunity. Recent data demonstrate that MAIT cells located in mucosal barrier tissues are functionally distinct from their blood counterparts, but the relationship and circulation of MAIT cells between blood and different tissue compartments remains poorly understood. Previous studies raised the possibility that MAIT cells do not leave tissue and may either be retained or undergo apoptosis. To directly address if human MAIT cells exit tissues, we collected human donor-matched thoracic duct lymph and blood and analyzed MAIT cell phenotype, transcriptome, and T cell receptor (TCR) diversity by flow cytometry and RNA sequencing. We found that MAIT cells were present in the lymph, despite being largely CCR7- in the blood, thus indicating that MAIT cells in the lymph migrated from tissues and were capable of exiting tissues to recirculate. Importantly, MAIT cells in the lymph and blood had highly overlapping clonotype usage but distinct transcriptome signatures, indicative of differential activation states.