Thymic Dendritic Cell Subsets Display Distinct Efficiencies and Mechanisms of Intercellular MHC Transfer

Abstract

Thymic dendritic cells (DC) delete self-Ag-specific thymocytes, and drive development of FoxP3-expressing immunoregulatory T cells. Unlike medullary thymic epithelial cells (mTEC), which express and present peripheral self-Ag, DC must acquire self-Ag to mediate thymic negative selection. One such mechanism entails the transfer of surface MHC-self peptide complexes from mTEC to thymic DC. Despite the importance of thymic DC “cross-dressing” in negative selection, the factors that regulate the process, and the capacity of different thymic DC subsets to acquire MHC and stimulate thymocytes are poorly understood. Here intercellular MHC transfer by thymic DC subsets was studied using a MHC-mismatch-based in vitro system. Thymic conventional DC (cDC) subsets SIRPα+ and CD8α+ readily acquired MHC class I and II from TEC but plasmacytoid DC (pDC) were less efficient. Intercellular MHC transfer was donor cell-specific; thymic DC readily acquired MHC from TEC plus thymic or splenic DC, whereas thymic or splenic B cells were poor donors. Furthermore DC origin influenced cross-dressing; thymic versus splenic DC exhibited an increased capacity to capture TEC-derived MHC, which correlated with direct expression of EpCAM by DC. Despite similar capacities to acquire MHC-peptide complexes, thymic CD8α+ cDC elicited increased T cell stimulation relative to SIRPα+ cDC. DC cross-dressing was cell-contact dependent and unaffected by lipid raft disruption of donor TEC. Furthermore, blocking PI3K signaling reduced MHC acquisition by thymic CD8α+ cDC and pDC but not SIRPα+ cDC. These findings demonstrate that multiple parameters influence the efficiency of and distinct mechanisms drive intercellular MHC transfer by thymic DC subsets.

Toll-like receptor signaling in thymic epithelium controls monocyte-derived dendritic cell recruitment and Treg generation

“…It has been previously documented that distinct subtypes of thymic DCs vary in their capacity to acquire antigens from TECs 6,10,11,16 . Whereas CAT of TdTOM from TECs to cDC1 and cDC2 is very potent in the Foxn1 Cre ROSA26 TdTOMATO system, it is limited in the case of pDC (Fig.…”

“…While cDC1 arise primarily in the thymus, cDC2 and pDCs originate extrathymically and then migrate to the thymic medullary region 14,15 . mTEC-derived antigens are transferred both to thymic resident cDC1 6,10 and cDC2 16,17 . Although it has been shown that the migration of cDC1 and cDC2 to the vicinity of mTECs is affected by a gradient of Xcl1 18 and Ccr2/Ccr7 ligands, respectively 19,20 , the potential involvement of other chemokines in the regulation of CAT still awaits resolution.…”

Class II MHC antigen processing in immune tolerance and inflammation

“…Antigen transfer via trogocytosis, in which an immune synapse forms between cells to mediate intercellular transfer of proteins, has also been shown to allow for capture and presentation of MHCpeptide complexes by both APCs and non-APCs (Cone et al 1972;Joly and Hudrisier 2003;Nakayama 2014). In the thymus, peptides presented as a result of trogocytosis between DCs and mTECs is posited to aid in increasing presentation of rare thymic antigens by MHC-II (Klein et al 2009;Kroger et al 2017;Millet et al 2008). Intercellular MHC-II transfer has been shown to occur in a number of other peripheral immune cell subsets as well, including between DCs and NK cells (Nakayama 2014;Nakayama et al 2011).…”

The Importance of Dendritic Cells in Maintaining Immune Tolerance

“…While pDC and migratory DC specialize in the presentation of peripheral antigens, resident DCs provide immature T cells with a distinct self-antigenic repertoire. In addition, although thymic resident and migratory cDC can uptake MHC-I and MHC-II from thymic epithelial cells in a cell-contact dependent manner, this process is dependent on PI3K pathway only for CD8α + resident cDCs (40), further supporting the view that each DC subset provides a non-redundant role in antigen-presentation to T cells and in the maintenance of central tolerance.…”