Date Published: January 4, 2019
Publisher: Public Library of Science
Author(s): Natalia S. Baez, Fabio Cerbán, Constanza Savid-Frontera, Deborah L. Hodge, Jimena Tosello, Eva Acosta-Rodriguez, Laura Almada, Adriana Gruppi, Maria Estefania Viano, Howard A. Young, Maria Cecilia Rodriguez-Galan, David Sacks.
Innate CD8+ T cells express a memory-like phenotype and demonstrate a strong cytotoxic capacity that is critical during the early phase of the host response to certain bacterial and viral infections. These cells arise in the thymus and depend on IL-4 and IL-15 for their development. Even though innate CD8+ T cells exist in the thymus of WT mice in low numbers, they are highly enriched in KO mice that lack certain kinases, leading to an increase in IL-4 production by thymic NKT cells. Our work describes that in C57BL/6 WT mice undergoing a Th1 biased infectious disease, the thymus experiences an enrichment of single positive CD8 (SP8) thymocytes that share all the established phenotypical and functional characteristics of innate CD8+ T cells. Moreover, through in vivo experiments, we demonstrate a significant increase in survival and a lower parasitemia in mice adoptively transferred with SP8 thymocytes from OT I—T. cruzi-infected mice, demonstrating that innate CD8+ thymocytes are able to protect against a lethal T. cruzi infection in an Ag-independent manner. Interestingly, we obtained similar results when using thymocytes from systemic IL-12 + IL-18-treated mice. This data indicates that cytokines triggered during the acute stage of a Th1 infectious process induce thymic production of IL-4 along with IL-15 expression resulting in an adequate niche for development of innate CD8+ T cells as early as the double positive (DP) stage. Our data demonstrate that the thymus can sense systemic inflammatory situations and alter its conventional CD8 developmental pathway when a rapid innate immune response is required to control different types of pathogens.
The thymus is the primary lymphoid organ where T cell development takes place in the host. In physiological conditions, several T cells lineages arise in the organ including conventional αβT cells, γδT cells, regulatory T cells and NKT cells. Most recently more lineages have been added to the list and these include several types of innate T cells[1–3].
We have previously demonstrated that migration of mature T cells from SLO to the thymus, that occurs under inflammatory/infectious Th1 processes, is not necessary Ag-driven. However, due to the capacity of T. cruzi to infect the thymus, we speculated that specific T cells might be recirculating to the organ as well. To confirm this hypothesis, we performed immunofluorescence staining, as it has been reported that intracellular amastigotes can be observed inside the infected cells. As hypothesized, Fig 1A shows that T. cruzi can infect adherent cells from the thymi that are either CD11b+ cells (thymic Mϕ, Fig 1B) or CD11b- cells with large and oval-shaped features that resemble thymic fibroblasts (Fig 1C). The presence of the parasite in the thymi of infected mice suggests that Ag-specific T cells might be migrating to the organ. Using a tetramer linked to TSKB20, the most important and immunogenic antigen of the Tulahuen strain of T. cruzi, we evaluated by flow cytometry the presence of specific T cells in the thymi of T. cruzi-infected WT mice. When we analyzed the SP8 compartment, we observed that the percentage of Ag-specific T cells is higher in the CD44hi cell subset and is very low in the CD44lo cell subset (Fig 1D). This observation is expected as effector/memory T cells express high levels of CD44 after TCR activation (Fig 1D).
Development of CD8+ T cells in the thymus generates a predominant population of conventional naïve cells, along with minor populations of “innate” T cells that resemble memory cells. When analyzing the innate populations that arise in the thymus in a variety of KO mice that have an impaired TCR signaling pathway, several studies have demonstrated the presence of an increased number of IL-4-dependent innate CD8+ T cells (as compiled by Lee et al.). These KO mouse models all converge on the fact that a population of thymic cells (PLZF+: NKT, γδ T cells or CD4 CD44hi cells) ultimately produces increased levels of IL-4 that drives innate CD8+ T cell development[9, 11, 12, 14]. However, the question remains as to whether a similar pathway regulates innate CD8+ T cell development in normal mice. Interestingly, inbred strains of mice were shown to vary in their frequency of IL-4-producing invariant NKT (iNKT) cells, with BALB/c mice on top of the spectrum and C57BL/6 mice on the low end. This data that correlates with higher percentages of SP8 CD44hi CD122hi Eomeshi innate cells in the thymus of BALB/c mice compared to C57BL/6 mice under physiological conditions. Moreover, the importance of IL-4-producing iNKT cells in innate CD8+ cells development in the thymus is supported by the fact that no innate CD8+ T cells are found in BALB/c IL-4R KO and Cd1d KO mice.