Studies on T cell activation, differentiation and plasticity in humans

The current proposal is the continuation of our previous studies of the human T cell response to microbes that have advanced our understanding of CD4+ T cell functional heterogeneity, in particular with the discovery of a distinct non-classic Th1* subset involved in the response to Mycobacteria (Acosta-Rodriguez et al, 2007; Lindestam Arlehamn et al, 2013) and the characterization of two types of Th17 cells specific for extracellular bacteria or fungi (Zielinski et al, 2012). The new approaches that we have developed combining antigenic stimulation and T cell receptor (TCR) deep sequencing have also revealed that the CD4+ T cell response induced by pathogens or vaccines comprise not only clones polarized toward a single fate, but also clones whose progeny have acquired multiple fates (Becattini et al, 2015), demonstrating an unexpected degree of intraclonal diversification and suggesting a progressive and selective model of CD4+ T cell differentiation (Sallusto, 2016). In Aim 1 of the current proposal, we will address the mechanisms whereby a single naïve T cell can give rise to daughter cells with identical TCR but that adopt different fates. In this “one cell, multiple fates model”, the challenge is to determine whether the process is deterministic or stochastic and the temporal events leading to the heterogeneous progeny (Reiner et al, 2007). These studies should lead to a better definition on the mechanisms of T cell differentiation and on the lineage relationship between different T cell subsets and between T helper cells with polarized or mixed cytokine profiles. In Aim 2 we will further dissect the heterogeneity of human CD4+ T cell subsets, with particular emphasis on the characterization of T cells that can be distinguished based on the ability to produce the anti-inflammatory cytokine IL-10. This part of the proposal stems from our previous work that identified in humans two distinct subsets of IL-10+ and IL-10- Th17 cells that possess different pathogen specificity and have distinct differentiation requirements (Zielinski et al, 2012). A better understanding on the different fates CD4 T cells can adopt will provide insights not only into mechanisms of protection or pathology but also facilitate the development of new therapies. In Aim 3 we will characterize the phenotypes, functional capacities and frequencies of malaria-specific memory CD4+ T cells in volunteers after whole attenuated sporozoite vaccination and controlled human malaria infection. The goal is to identify determinants of cellular malaria-specific immunity in this cohort and to describe correlates of protection or risk from malaria and provide guidance for vaccine design.