Mucormycosis is an emerging fungal infection with extremely high mortality rates in patients with defects in their innate immune response, specifically in functions mediated through phagocytes. However, we currently have a limited understanding of the molecular and cellular interactions between these innate immune effectors and mucormycete spores during the early immune response. Here, the early events of innate immune recruitment in response to infection by Mucor circinelloides spores are modeled by a combined in silico modeling approach and real-time in vivo microscopy. Phagocytes are rapidly recruited to the site of infection in a zebrafish larval model of mucormycosis. This robust early recruitment protects from disease onset in vivo. In silico analysis identified that protection is dependent on the number of phagocytes at the infection site, but not the speed of recruitment. The mathematical model highlights the role of proinflammatory signals for phagocyte recruitment and the importance of inhibition of spore germination for protection from active fungal disease. These in silico data are supported by an in vivo lack of fungal spore killing and lack of reactive oxygen burst, which together result in latent fungal infection. During this latent stage of infection, spores are controlled in innate granulomas in vivo. Disease can be reactivated by immunosuppression. Together, these data represent the first in vivo real-time analysis of innate granuloma formation during the early stages of a fungal infection. The results highlight a potential latent stage during mucormycosis that should urgently be considered for clinical management of patients.

In the thymus, distinct cortex and medulla areas emphasize the division of labor in selection events shaping the abT cell receptor repertoire. For example, MHC restriction via positive selection is a unique property of epithelial cells in the thymic cortex. Far less clear are the events controlling tolerance induction in the medulla. By acting in concert through multiple roles, including antigen production/presentation and chemokine-mediated control of migration, we propose that medullary epithelium and dendritic cells collectively enable the medulla to balance T cell production with negative selection and Foxp3 + regulatory T cell (Treg) development. We examine here the features of these medullary resident cells and their roles in T cell tolerance, and discuss how imbalance in the thymus can result in loss of T cell tolerance. Thymic Medulla and the Control of T Cell ToleranceThe thymus produces multiple T cell types that play key roles in both innate and adaptive immune responses. The importance of these responses has now been shown in sister lineages of vertebrates, long after the function of the thymus was proven for jawed vertebrates in the 1960s [1,2]. Indeed, a key feature of adaptive immunity lies in its ability to generate a wide diversity of antigen receptors that target non-self. For T cells, this is achieved by T cell receptor (TCR) gene rearrangements that take place during T cell development in the thymus. Because of the random nature of gene rearrangement, the thymus imposes stringent mechanisms that shape the abTCR repertoire. Such processes are crucial because they ensure that abT cells (see Glossary) leaving the thymus are not only biased towards the recognition of self-MHC proteins but also tolerant to self-antigens. To achieve this the thymus creates a division of labor: the cortex imposes MHC restriction via positive selection, while the medulla imposes T cell tolerance via a combination of negative selection and Foxp3 + Treg development.The generation of abT cells in the thymus involves immature thymocytes being subjected to sequential checkpoints as they undergo intrathymic migration. To ensure that the thymus produces abT cells capable of antigen recognition in peripheral tissues, the cortex supports positive selection of immature CD4 + CD8 + double-positive (DP) thymocytes that recognize selfpeptide/MHC complexes produced and expressed by cortical thymic epithelial cells (cTECs). This process rescues DP thymocytes from cell death and triggers further differentiation, including expression of the chemokine receptor CCR7 that guides newly selected thymocytes into the medulla [3,4].Because positive selection results in the survival of thymocytes that recognize self-peptide/ MHC, additional selection mechanisms ensure that T cell development produces functional thymocytes that are tolerant to self-antigens. The thymic medulla provides a specialized microenvironment to support these events, and medullary thymic epithelial cells (mTECs) and dendritic cells (DCs) combine to enforce two key proces...