viruses Review Insights into Thymus Development and Viral Thymic Infections Francesco Albano 1,2,* , Eleonora Vecchio 1, Maurizio Renna 3, Enrico Iaccino 1, Selena Mimmi 1, Carmen Caiazza 3, Alessandro Arcucci 4, Angelica Avagliano 4, Valentina Pagliara 5, Giuseppe Donato 6, Camillo Palmieri 1, Massimo Mallardo 3, Ileana Quinto 1 and Giuseppe Fiume 1,* 1 Department of Experimental and Clinical Medicine, University “Magna Graecia” of Catanzaro, 88100 Catanzaro, Italy 2 Institute for Endocrinology and Oncology, National Research Council, 80131 Napoli, Italy 3 Department of Molecular Medicine and Medical Biotechnologies, University of Napoli “Federico II”, 80131 Napoli, Italy 4 Department of Public Health, University of Napoli “Federico II”, 80131 Naples, Italy 5 Department of Medicine, Surgery and Dentistry, University of Salerno, 84081 Salerno, Italy 6 Department of Health Sciences, University “Magna Graecia” of Catanzaro, 88100 Catanzaro, Italy * Correspondence: [email protected] (F.A.); fi[email protected] (G.F.) Received: 14 August 2019; Accepted: 6 September 2019; Published: 9 September 2019 Abstract: T-cell development in the thymus is a complex and highly regulated process, involving a wide variety of cells and molecules which orchestrate thymocyte maturation into either CD4+ or CD8+ single-positive (SP) T cells. Here, we briefly review the process regulating T-cell differentiation, which includes the latest advances in this field. In particular, we highlight how, starting from a pool of hematopoietic stem cells in the bone marrow, the sequential action of transcriptional factors and cytokines dictates the proliferation, restriction of lineage potential, T-cell antigen receptors (TCR) gene rearrangements, and selection events on the T-cell progenitors, ultimately leading to the generation of mature T cells. Moreover, this review discusses paradigmatic examples of viral infections affecting the thymus that, by inducing functional changes within this lymphoid gland, consequently influence the behavior of peripheral mature T-lymphocytes. Keywords: thymus; viral infections; T-cell development 1. Introduction The thymus gland is a primary lymphoid organ, in which the essential development of T cells occurs. Within the thymus, hematopoietic precursors, deriving from bone marrow, enter the route of differentiation that will generate αβ- or γδ- mature T cells. Finally, these cells will exit the thymus and will colonize secondary lymphoid organs, where they will provide immunity against specific antigens. The murine thymus has two lobes that are divided into multiple lobules by connective tissue, made of fibroblasts, collagen, and elastic fiber, as does the human thymus. The basic structural unit of the thymus is the lobule, composed of two distinct areas, the cortex and the medulla [1]. Each lobe has an outer cortex and an inner medulla. The cortex is densely populated by immature T-lymphocytes, while the medulla contains only a few mature T-lymphocytes. The thymus medulla contains some Hassall’s corpuscles that are populated by eosinophilic type-VI epithelial reticular cells arranged concentrically, containing keratohyalin and bundles of cytoplasmic fibers (Figure1). These cells di fferentiate from medullary thymic epithelial cells, following the inactivation of the autoimmune regulatory (AIRE) gene. Viruses 2019, 11, 836; doi:10.3390/v11090836 www.mdpi.com/journal/viruses VirusesViruses 20192019,, 1111,, x 836 FOR PEER REVIEW 22 of of 20 19 A third compartment can be found in the thymus, defined as the perivascular space (PVS). This compartment,A third compartment despite surrounding can be found blood in vessels the thymus, within definedthe capsule, as the is perivascularphysically separated space (PVS). from This the thymiccompartment, epithelial despite space. surrounding The role of the blood PVS vessels has been within often the overlooked, capsule, is physicallydue to its minimal separated extension from the duringthymic epithelialinfancy and space. because The role it ofdoes the PVSnot hasappear been to often be overlooked,a functional due site to actively its minimal involved extension in thymopoiesis.during infancy However, and because as it the does thymus not appear ages, to bethe a functionalPVS enlarges site activelyand progressively involved in thymopoiesis.replaces the epithelialHowever, area as the [2]. thymus ages, the PVS enlarges and progressively replaces the epithelial area [2]. Figure 1. Impact of viral infections on intra-thymic T cell development. Two major populations of Figurethymic 1. epithelial Impact of cells viral (TECs) infections are presenton intra-thymic in the thymic T cell parenchyma.development. InTwo particular, major populations cortical TEC of thymic(cTEC) epithelial cells are identifiedcells (TECs) in are the present cortical in compartment the thymic parenchyma. of the parenchyma, In particular, whereas cortical medullary TEC (cTEC) TEC cells(mTEC) are identified cells are locatedin the cortical internally. compartment Dendritic of cells the parenchyma, (DCs, shown whereas in green), medullary fibroblasts TEC (blue) (mTEC) and cellsmacrophages are located (yellow), internally. contribute Dendritic to the cells tissue (DCs, organization. shown in green), In humans, fibroblasts during (blue) the maturation and macrophages process, (yellow),bone marrow-derived contribute to T-cellthe tissu progenitorse organization. arrive in In the humans, thymus during via the bloodstream,the maturation where process, cTEC bone cells marrow-derivedcontribute to positive T-cell selection progenitor maturation,s arrive andin the mTEC thymus cells contributevia the bloodstream, to the negative where selection cTEC process, cells contributerespectively. to Also,positive DCs selection are likely responsiblematuration, forand negative mTEC selection cells contribute in the cortex. to the Once negative the selection selection has process,been successfully respectively. completed Also, DCs and are self-reactive likely responsibl cells havee for beennegative removed selection by apoptosis, in the cortex. mature Once CD4 the+ selectionor CD8+ hasT cells been leave successfully the thymus completed to populate and the self-r peripheraleactive cells blood have and been secondary removed lymphoid by apoptosis, organs. matureViral infections CD4+ or are CD8 able+ T to cells affect leave thymus thefunctionality thymus to populate in multiple the ways, peripheral but the blood most severeand secondary effects are lymphoidobserved whenorgans. positive Viral infections or negative are selection able to isaffe impaired,ct thymus as functionality these processes in drivemultiple key ways, steps ofbut T-cell the mostdevelopment. severe effects In the are figure, observed lightning when bolts positive indicate or negative which viruses selection can is specifically impaired, target as these and processes influence drivethe activity key steps of cTEC of T-cell or mTEC development. populations. In the figure, lightning bolts indicate which viruses can specifically target and influence the activity of cTEC or mTEC populations. Thymic natural killer (NK) cells arise from early thymocyte precursors, and their development strictlyThymic relies natural on Interleukin-7 killer (NK) (IL7)cells arise signaling from andearly requires thymocyte the precursors, transcription and factor their GATA-3development [3–5]. strictlyThymic relies NK cells on areInterleukin-7 cytotoxic and(IL7) characterized signaling and by therequires expression the transcription of CD127 (IL-7 factor receptor GATA-3α). CD1-d[3–5]. Thymicrestricted NK invariant cells are Natural cytotoxic Killer and T characterize (iNKT) cellsd areby athe major expression producer of ofCD127 cytokines (IL-7 withinreceptor the α thymus,). CD1- dsuch restricted as IFN γinvariantand IL-4. Natural Specifically, Killer IL4, T (iNKT) produced cells in combinationare a major producer with IL-13, of triggering cytokines type-2 within IL-4R the thymus,signaling, such is a keyas IFN regulatorγ and IL-4. of the Specifically, migration during IL4, produced development in combination [6]. with IL-13, triggering type-2In IL-4R mice, signaling, three subsets is a key of iNKT regulator cells (NKT1-2-3)of the migration have duri beenng characterized. development NKT2[6]. are responsible for theIn IL-4mice, secretion, three subsets reside of in iNKT the thymic cells (NKT1-2-3) medulla, and have secrete been cytokinescharacterized. even NKT2 after infection are responsible in order forto regulatethe IL-4 secretion, and induce reside the developmentin the thymic medulla, of the surrounding and secrete immune cytokines cells even [7 ].after Thymic infection NK cellsin order are, toindeed, regulate essential and induce for the the modulation development of thymopoiesis of the surrounding and the maintenanceimmune cells of [7]. thymic Thymic architecture, NK cells are, and, indeed,also have essential a role in for dendritic the modulation cell (DC) of maturation thymopoies [8].is and the maintenance of thymic architecture, and, alsoInterestingly, have a role the in thymus dendritic contains cell (DC) macrophages maturation and [8]. dendritic cells that originate from the bone marrowInterestingly, and arrive the in thymus the gland contains through macrophages blood vessels. and dendritic More specifically, cells that originate at least threefrom the diff boneerent marrowpopulations and ofarrive macrophages in the gland have beenthrough identified blood withinvessels. the More thymus: specifically, dendritic, at round, least