Modulation of the Dendritic Cell Function by Activating/Inhibitory Receptors and the Half-Life of the TCR/pMHC Interaction. Implications on Immunological Synapse Function and T Cell Activation
Pontificia Universidad Católica de Chile
Dendritic cells (DCs) play a fundamental role in the initiation and regulation of adaptive immune responses. Thcse cells can, in an antigcn-specific manner, either prevent or promote adaptive immunity. DCs are specialized antigen presenting cells with the unique ability to prime antigen-specific na'ive T cells and initiate adaptive immunity. T cell activation, a critical event to promote anti-pathogen and antí-tumoral immune responses, results from productivc T cell receptor (TCR) engagement by a cognate peptide-MHC (pMHC) complex on the DC surface. Thís process leads to the polarization of the T cell secretory machinery toward the DC interface, which is known as imnmnological synapse (IS). Since IS assembly is required to generate ef:ficient T cell responses, IS modulation is currently a matter of active research. As T cell responses depend on T cells as well as on DCs, manyfactors can influence their final outcome. Among these factors, the quality of the TCR/pMHC interaction and the phenotype of DC could have a pro:found influence on IS assembly and T cell responses. We have previously shown that the half-life of the TCR/pMHC interaction and the density of pMHC on DCs are two parameters determining T cell activation. However, ít remains unclear whether the half-life of the TCR/pMHC interaction can also modulate the efficiency of T cell secretory machinery polarization toward DC. In this thesis, by using altered peptide ligands conferring different half-lives to the TCR/pMHC interaction, we have tested how this parameter can control T cell polarization. We observed that only TCR/pMHC interactions with intennediate half-lives were able to promote the assembly of synapses that lead to T ce11 activation. Strikingly,intermediate half-life interactions could be competed out by short half-life interactions, which could cfl:iciently promote T cell polarization and antagonize T cell activation thatwas induced by activating intermediate half-life interactions. However, short TCR/pMHC interactions failed at promoting phosphorylation of signaling molecules at the T cell-DC contact interface, which are needed for T cell activation. These data suggest that although intermcdiate half-life pMHC ligands can promote assembly of activating synapses, thisprocess can be inhibited by short half-lifc antagonistic pMHC ligands, which promote the assembly of non-activating/inhibitory synapscs. In addition, we evaluated thc effect ofaltering the phenotype of DCs on T ceU responses. It has becn shown that enzymehemc-oxygenase -1 (H0-1) inhibits immune responses and inf1ammatory reactions via the catabolism of heme into carbon monoxide (CO), Fe2 + and biliverdin. As part of this thesis, we addressed the capacity of H0-1 to regulate antigen presentation in DCs and their abilityto activate T cells. Our results showed that H0-1 induction and CO treatment inhibited LPS-induced secretion of IL-12 and TNFa by DCs, but did not affect LPS-induced upregulation of costimulatory molecules. H0-1 induction and CO treatment inhibited DCs ability to induce T cell activation when DCs were pulsed with proteins or immune complexes, but not when DCs were pulsed with exogenously added antigenic peptides.Remarkably, the capacity of DCs to uptake antigens was not affected by H0-1 activity, but the degradative capacity of DCs was completely abolished. Taken together, our data suggest that H0-1 índuction and CO treatment can inhibit the ability of DC to cross-presentantigens to T cells through an antigen-processing defect in DCs. Finally, we evaluated whether phenotypic alterations on DCs correlate with autoimmune development. We studied a cohort of lupus-patients and hcalthy controls to determine the phenotype of theirDCs. We found that costimulatory molccules and the Fcy receptors (FcyRs) are altered on DCs from lupus-patients. Remarkably, the degree of FcyRs alterations correlated with the activity of lupus. In conclusion wc found in this thesis that TCR/pMHC kinetics and DC phenotype are key factors controlling T cell activation, and that DC phenotypic alterations could explain autoimmunity activity.