Invariant natural killer T (iNKT) cells constitute a subpopulation of T cells that recognize glycolipids presented by CD1d molecules. They are characterized by their prompt production of interleukin-4 (IL-4) and interferon-γ (IFN-γ), which enables them to modulate diverse immune responses. Recently, we enlarged this concept by identifying a distinct IL-17-producing iNKT cell subset, named iNKT17 cells. The mechanisms leading to the acquisition of this new iNKT cell activity are unknown. Herein we show that IL-17-producing iNKT cells are already present in the thymus, predominantly among a subset regarded so far as an immature stage of thymic iNKT cell development, the CD1d tetramer ^pos CD44 ^pos NK1.1 ^neg CD4 ^neg cells. Using EGFP reporter mice, we demonstrate that the transcription factor ROR-γt is critical for the thymic differentiation of this subset because only ROR-γt ^pos iNKT cells are capable of massively secreting IL-17. Moreover, IL-17-producing CD1d tetramer ^pos CD44 ^pos NK1.1 ^neg CD4 ^neg thymic iNKT cells have reached a mature differentiation stage because they fail to generate other cell subsets in fetal thymic organ culture. Conversely, thymic ROR-γt ^neg iNKT cell precursors give rise to progeny, but acquire neither ROR-γt expression nor the ability to secrete IL-17. In conclusion, our findings demonstrate an alternative thymic pathway leading to the development of iNKT17 cells that requires ROR-γt expression.

Treg are immune cells that play a critical role in the regulation of the immune response. Although the transcription factor Foxp3 is widely accepted as the standard marker of Treg, specific surface markers are needed to better characterize these cells and decipher their mechanisms of action. Neuropilin‐1 (Nrp‐1), a membrane protein primarily involved in the nervous system, was identified as a specific marker of murine Treg, but its expression has not been rigorously investigated in human Treg. Here we show that in contrast to murine Treg and regardless of their origins (blood, thymus, spleen, lymph node or tonsil), human Foxp3⁺ Treg do not specifically express Nrp‐1. However, a population of Foxp3⁻ Nrp‐1⁺ T cells can be detected in human secondary lymphoid organs, and Nrp‐1 expression is induced on peripheral blood T lymphocytes upon in vitro activation. We conclude that Nrp‐1 cannot be used as a specific marker of human Treg, but might represent a novel activation marker of human T cells both in vitro and in vivo.

Cell migration is crucial for thymocyte differentiation, and the cellular interactions involved now begin to be unraveled, with chemokines, extracellular matrix (ECM) proteins, and their corresponding receptors being relevant in such oriented movement of thymocytes. This notion derives from in vitro, ex vivo, and in vivo experimental data, including those obtained in genetically engineered and spontaneous mutant mice. Thymic microenvironmental cells produce both groups of molecules, whereas developing thymocytes express chemokine and ECM receptors. It is important that although chemokines and ECM proteins can drive thymocyte migration per se, a combined role of these molecules likely concurs for the resulting migration patterns of thymocytes in their various differentiation stages. In this respect, among ECM moieties, there are proteins with opposing functions, such as laminin or fibronectin versus galectin-3, which promote, respectively, adhesion and de-adhesion of thymocytes to the thymic microenvironment. How chemokines and ECM are produced and degraded remains to be more clearly defined. Nevertheless, matrix metalloproteinases (MMPs) likely play a role in the intrathymic ECM breakdown. It is interesting that these molecules also degrade chemokines. Thus, the physiological migration of thymocytes should be conceived as a resulting vector of multiple, simultaneous, or sequential stimuli, involving chemokines, adhesive, and de-adhesive ECM proteins. Moreover, these interactions may be physiologically regulated in situ by matrix MMPs and are influenced by hormones. Accordingly, one can predict that pathological changes in any of these loops may result in abnormal thymocyte migration. This actually occurs in the murine infection by the protozoan Trypanosoma cruzi, the causative agent of Chagas disease. In this model, the abnormal release of immature thymocytes to peripheral lymphoid organs is correlated with the higher migratory response to ECM and chemokines. Lastly, the fine dissection of the mechanisms governing thymocyte migration will provide new clues for designing therapeutic strategies targeting developing T cells. The most important function of the thymus is to generate T lymphocytes, which once leaving the organ, are able to colonize specific regions of peripheral lymphoid organs, the T cell zones, where they can mount and regulate cell-mediated, immune responses. This intrathymic T cell differentiation is a complex sequence of biological events, comprising cell proliferation, differential membrane protein expression, gene rearrangements, massive programmed cell death, and cell migration. In this review, we will focus on the mechanisms involved in controlling the migration of thymocytes, from the entrance of cell precursors into the organ to the exit of mature T cells toward peripheral lymphoid organs. Nevertheless, to better comprehend this issue, it appeared worthwhile to briefly comment on some key aspects of thymocyte differentiation and the tissue context in which it takes place, the thymic microenvironment.