Dendritic-cell (DC) cross-dressing — the acquisition and surface display of intact donor MHC–peptide complexes by host antigen-presenting cells — was shown by Li et al. (2020) to underlie priming of allo-specific CD8+ T cells by Batf3-dependent type 1 conventional DCs (cDC1s: lymphoid-resident CD8α+ and migratory CD103+ subsets) in mouse skin transplantation. The mechanistic basis of this pathway in vivo, however, remains undefined: the anatomical sites and migratory routes through which CD103+ cDC1s acquire donor MHC and prime naive allo-reactive CD8+ T cells, whether effective priming requires normal migratory capacity of these cells, and the molecular mediators that execute acquisition, membrane incorporation, and stable display of intact donor MHC class I are all open. The literature documents extracellular-vesicle and trogocytosis routes of MHC transfer and one phosphatidylserine-dependent acquisition mechanism from dead cells in a non-transplant context, but no study resolves the transplant-specific route or machinery, and targeted therapeutic modulation of cross-dressing remains unachieved.

Background

Transplantation of genetically dissimilar (allogeneic) tissue triggers rejection when recipient T cells recognize donor antigens. Three allorecognition pathways are distinguished. In the direct pathway, T cells recognize intact allogeneic major histocompatibility complex (MHC) molecules on donor-derived antigen-presenting cells (APCs, the classical 'passenger leukocytes'). In the indirect pathway, recipient APCs process donor proteins and present donor peptides on self-MHC. In the semi-direct pathway, recipient APCs capture intact donor MHC–peptide complexes and display them on their own surface, so that T cells with direct-pathway specificity are stimulated by host rather than donor cells; the capture process is called cross-dressing (also MHC dressing or intercellular MHC transfer). Cross-dressing can occur through trogocytosis ('nibbling' of membrane fragments during cell–cell contact), trans-synaptic receptor–ligand transfer, or uptake of extracellular vesicles (membrane-bound particles including exosomes released by cells). Cross-dressing was first demonstrated for antiviral and antitumor CD8+ T-cell responses (Wakim and Bevan, 2011; Li L et al., 2012), and in transplantation it explains how direct-pathway CD8+ responses can persist long after donor passenger leukocytes are eliminated (Smyth et al., 2017): recipient dendritic cells keep acquiring donor MHC class I for weeks after grafting. Within the DC compartment, type 1 conventional DCs (cDC1s) depend on the transcription factor Batf3; they comprise the CD8α+ subset resident in lymphoid organs and the CD103+ subset in peripheral tissues and skin-draining lymph nodes, both specialized in activating CD8+ T cells. Batf3−/− mice lack or have strongly reduced numbers of cDC1s, with migratory CD103+ DCs depleted to residual levels in most tissues. Li et al. (2020) showed that Batf3−/− mice reject allo-skin grafts slowly and fail to prime allo-specific CD8+ T cells; adoptive transfer of either purified CD8α+ or CD103+ DCs from wild-type mice into Batf3−/− recipients restored priming, and, using F5 transgenic T-cell-receptor CD8+ T cells (a virus-specific, H-2Db-restricted receptor that they showed to cross-react with the donor MHC class I molecule H-2Kd), they demonstrated that cross-dressing of these DCs primes allo-specific CD8+ responses in vitro and in vivo. Parallel work showed that host DCs inside islet and kidney allografts are extensively cross-dressed and sustain effector CD8+ T-cell responses within the graft (Hughes et al., 2020), that donor extracellular vesicles rather than passenger leukocytes deliver allo-MHC to recipient APCs in lymphoid organs (Marino et al., 2016; Zeng et al., 2021), and that PD-L1hi cross-dressed DCs can regulate antidonor responses in liver transplant tolerance (Ono et al., 2018; PD-L1 is the inhibitory ligand programmed death-ligand 1). Reviews through 2024 treat the semi-direct pathway as a central mechanism of T-cell-mediated rejection, yet the cellular route and the molecular execution of cross-dressing in vivo remain unmapped: it is still unknown which molecular interactions let host DCs acquire intact donor MHC class I, incorporate it into their plasma membrane, and display it stably enough to prime T cells (Benichou and Lancia, 2024; Hori et al., 2024; Charmetant et al., 2024).

Problem Statement

After allogeneic skin transplantation in the mouse, determine the in vivo route and the molecular machinery by which host Batf3-dependent type 1 conventional dendritic cells (lymphoid-resident CD8α+ and migratory CD103+ subsets) cross-dress with intact donor MHC class I–peptide complexes and prime allo-specific CD8+ T cells. This comprises three jointly linked questions: (1) the anatomical sites and migratory pathways of priming — whether CD103+ DCs acquire donor MHC–peptide complexes within the graft and migrate to draining lymph nodes to prime T cells, whether they transfer acquired donor MHC to resident CD8α+ DCs which perform priming, or whether graft-resident Langerhans cells or other migratory APCs relay antigen to Batf3-dependent DCs; (2) whether the alloresponses restored by adoptively transferred CD8α+ or CD103+ cDC1s require the normal migratory capacity of those cells, formally excluding a migration defect — either in the adoptively transferred cDC1s or in the rare residual CD103+ cDC1s retained in Batf3−/− mice — as an alternative explanation; and (3) identification of the molecular mediators that execute acquisition, membrane incorporation, and stable surface display of intact donor MHC-I/peptide complexes by host DCs in the transplant setting. Answers to (1)–(3) jointly constitute the mechanistic objective. Targeted modulation of the cross-dressing pathway — for example, engineering cross-dressing DCs to bear inhibitory ligands such as PD-L1, or specifically inhibiting the acquisition/display machinery — is a longer-term implication of resolving these questions; it motivates the work but is not part of the completion target.

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