Cross-Dressing (Immunology)

Cross-dressing in immunology is an unconventional pathway of antigen presentation in which an antigen-presenting cell (APC)—most notably a conventional dendritic cell (cDC)—acquires intact, preformed peptide–major histocompatibility complex class I (pMHC-I) or class II (pMHC-II) complexes directly from the plasma membrane of a live donor cell, apoptotic body, or secreted extracellular vesicle (exosome) and displays them on its own surface to prime or stimulate T lymphocytes without internalizing, proteolytically cleaving, or reloading the antigen. Coined in 2006 by Brian P. Dolan, Kenneth D. Gibbs Jr., and Suzanne Ostrand-Rosenberg alongside Uwe D. Staerz and Kenneth L. Rock, cross-dressing represents a third major pathway of antigen presentation alongside endogenous direct presentation and canonical cross-presentation. Because the transferred pMHC complex remains structurally intact throughout intercellular transfer, cross-dressing bypasses intracellular proteasomal degradation and transporter associated with antigen processing (TAP) translocation, preserving the exact peptide repertoire generated inside the donor cell—including proteasome-dependent viral, tumor, and minor histocompatibility epitopes that are poorly generated by cross-presentation.

In classical cross-presentation (discovered by Michael J. Bevan in 1976), a recipient CD8α⁺/XCR1⁺ type 1 conventional dendritic cell (cDC1) phagocytoses exogenous proteins or dead donor cells, exports the intact polypeptides from phagosomes into the cytosol via Sec61 or HSP90/NOX2-mediated endosomal rupture, degrades them via the immunoproteasome, transports the resulting octamer–decamer peptides via TAP1/TAP2 into the endoplasmic reticulum (or phagosome), and loads them onto the dendritic cell's own endogenous MHC class I molecules.

By contrast, in cross-dressing, the recipient dendritic cell captures an already assembled donor pMHC complex (comprising the donor MHC heavy chain, β₂-microglobulin, and bound peptide) and integrates that intact trimolecular complex into its own lipid bilayer:

Contact-Dependent Trogocytosis (Membrane Gnawing)

The primary mechanism of cross-dressing in lymphoid organs and inflamed tissues is trogocytosis (from the Greek trogo, "to gnaw"), a rapid, contact-dependent process occurring within minutes of conjugate formation between a live or dying donor cell and a recipient dendritic cell. Driven by phosphatidylserine recognition receptors (TIM-4, MerTK, Axl), scavenger receptors (SCARB1), or C-type lectins (DNGR-1 / CLEC9A), actin cytoskeleton remodeling at the intercellular contact zone pinches off nanometer-scale plasma membrane patches from the donor cell and fuses them into the recipient dendritic cell membrane. Consequently, the cross-dressed dendritic cell simultaneously displays acquired donor pMHC complexes alongside its own high-density CD80, CD86, CD40, and ICAM-1 costimulatory molecules and paracrine IL-12/IL-15 signals across the immunological synapse.

Extracellular Vesicle and Exosome Capture

A second major route of cross-dressing operates at a distance without direct cell-to-cell contact. Parenchymal cells, tumor cells, and donor allograft endothelial cells release extracellular vesicles (EVs)—including 30–150 nm endosomal multivesicular-body-derived exosomes and plasma-membrane microvesicles—whose lipid bilayers are enriched up to 10-fold in pMHC-I and pMHC-II complexes alongside tetraspanins (CD9, CD63, CD81) and milk fat globule-EGF factor 8 (MFG-E8 / lactadherin). Subcapsular sinus macrophages and follicular or conventional dendritic cells in draining lymph nodes capture these exosomes via integrin α_vβ₃/α_vβ₅, TIM-4, or sialoadhesin (CD169). Rather than routing all captured exosomes to lysosomes for degradation, recipient dendritic cells retain intact exosomal patches on their plasma membrane or fuse exosomal bilayers with the cell surface, presenting the donor pMHC complexes directly to naive and memory T cells.

1. The "Semi-Direct" Pathway of Allorecognition and Acute Graft Rejection

For decades, transplantation immunology divided allorecognition into two mutually exclusive pathways:

• The Direct Pathway: Donor "passenger leukocytes" (donor dendritic cells) migrate out of the allograft into host lymph nodes and present intact donor MHC molecules directly to host T cells.
• The Indirect Pathway: Host dendritic cells phagocytose shed donor proteins, degrade them into peptides, and present those donor peptides bound to host MHC molecules to host CD4⁺ T cells.

A long-standing paradox was that donor passenger dendritic cells are rapidly eliminated by host natural killer (NK) cells and cytotoxic T cells within the first days after transplantation, yet direct-pathway CD8⁺ and CD4⁺ T-cell responses against intact donor MHC molecules dominate acute rejection for weeks or months. In 2004, Robert I. Lechler, Giovanna Lombardi, and O. B. Herrera resolved this paradox by discovering the semi-direct pathway of allorecognition (allogeneic cross-dressing). As validated in vivo by Fadi G. Lakkis (2016) and Gilles Benichou (2016) using two-photon intravital microscopy and MHC-deficient recipients, host dendritic cells infiltrate the vascularized graft or capture graft-derived exosomes, cross-dress with intact donor MHC class I and class II molecules, and present them on the host dendritic cell surface. Moreover, because a single cross-dressed host dendritic cell simultaneously presents intact donor MHC-I (via cross-dressing) and processed donor peptide on host MHC-II (via the indirect pathway), an indirect-pathway CD4⁺ helper T cell and a direct-pathway CD8⁺ cytotoxic T cell can dock on the exact same host dendritic cell, licensing lethal CD8⁺ CTL alloreactivity or, under costimulatory blockade, enabling tolerogenic dendritic cells to enforce linked suppression and split tolerance.

2. Priming of Memory CD8⁺ T Cells in Viral Infection (Wakim & Bevan, 2011)

In 2011, Linda M. Wakim and Michael J. Bevan demonstrated in Nature that during vesicular stomatitis virus (VSV) and lymphocytic choriomeningitis virus (LCMV) infections, CD8α⁻/CD11b⁺ conventional dendritic cells (cDC2s) and plasmacytoid dendritic cells acquire preformed viral pMHC-I complexes from infected parenchymal cells via cross-dressing. Using bone marrow chimeras in which dendritic cells lacked endogenous MHC class I (H-2K^bD^b−/− or Tap1^−/−) while non-hematopoietic parenchymal cells expressed wild-type MHC-I, Wakim and Bevan proved that cross-dressed dendritic cells drive the rapid proliferative recall of memory CD8⁺ T cells even when canonical TAP-dependent cross-presentation is completely ablated. Subsequent studies by Li-Rung Huang and Percy A. Knolle (2013) in the liver showed that hepatic sinusoidal endothelial cells and Kupffer cells cross-dressed with viral pMHC-I complexes drive local T-cell expansion during viral hepatitis.

3. Essential Role in Antitumor CD8⁺ T-Cell Priming (MacNabb et al., 2022; Duong et al., 2022)

In 2022, independent landmark studies by Brendan W. MacNabb and Justin P. Kline and by Ellen Duong and Stefani Spranger demonstrated that cross-dressing of intratumoral BATF3-dependent cDC1s and ISG⁺ (interferon-stimulated gene-positive) dendritic cells with intact tumor-derived pMHC-I complexes is essential for priming antitumor CD8⁺ T cells against immunogenic solid tumors. When tumor cells express neoantigens whose peptides are destroyed by endolysosomal proteases or require specialized immunoproteasomal splicing not replicated during cross-presentation, trogocytic transfer of preformed tumor pMHC-I complexes onto cDC1s and Type I interferon-activated ISG⁺ DCs enables robust CD8⁺ T-cell priming even in Tap1^−/− or MHC-I-mismatched dendritic cell compartments.

How do experimental immunologists prove that T-cell activation occurred via cross-dressing rather than canonical cross-presentation?

To isolate cross-dressing from cross-presentation in vivo, investigators construct bone-marrow chimeric mice in which the recipient dendritic cells either lack the specific MHC class I allele required to present the epitope (e.g., H-2K^b-negative DCs in a host whose parenchymal or tumor cells express H-2K^b) or lack the peptide transporter TAP1 (Tap1^−/− DCs). Because H-2K^b-deficient or Tap1^−/− dendritic cells cannot synthesize or load H-2K^b–peptide complexes internally through canonical cross-presentation, any H-2K^b–peptide complex detected on their surface by conformation-specific monoclonal antibodies (such as 25-D1.16, which recognizes SIINFEKL bound to H-2K^b) and any activation of SIINFEKL-specific OT-I CD8⁺ T cells must result from the transfer of intact preformed pMHC-I complexes from the donor cell (cross-dressing).

What is the "semi-direct pathway" of allorecognition in organ transplantation?

The semi-direct pathway is the transplantation counterpart of cross-dressing. Instead of donor dendritic cells migrating to host lymph nodes (direct pathway) or host dendritic cells degrading donor proteins into peptides loaded onto host MHC molecules (indirect pathway), host dendritic cells acquire intact, undigested donor MHC class I and class II complexes via trogocytosis or donor exosomes and display them on the host dendritic cell surface. This explains how direct anti-donor MHC T-cell responses persist long after donor passenger leukocytes have died.

Can T cells themselves become cross-dressed during immune responses?

Yes. When activated CD4⁺ or CD8⁺ T cells form an immunological synapse with an antigen-presenting dendritic cell, the T cell strips intact pMHC complexes and costimulatory ligands (CD80/CD86) from the dendritic cell via TCR-mediated trogocytosis. While this cross-dressing can allow T-cell-to-T-cell antigen presentation, displaying cognate viral or tumor pMHC-I complexes on a CD8⁺ T cell's own membrane also renders that T cell a target for lysis by neighboring cytotoxic T cells of the same specificity—a process known as immunological fratricide.

1. Dolan, B. P., Gibbs, K. D., Jr., & Ostrand-Rosenberg, S. (2006). "Dendritic cells cross-dressed with peptide MHC class I complexes prime CD8⁺ T cells." The Journal of Immunology, 177(9), 6018–6024. https://doi.org/10.4049/jimmunol.177.9.6018
2. Wakim, L. M., & Bevan, M. J. (2011). "Cross-dressed dendritic cells drive memory CD8⁺ T-cell activation after viral infection." Nature, 471(7340), 629–632. https://doi.org/10.1038/nature09863
3. Liu, Q., Rojas-Canales, D. M., Divito, S. J., Shufesky, W. J., Stolz, D. B., Erdos, G., Sullivan, M. L. G., Gibson, G. A., Watkins, S. C., Larregina, A. T., & Morelli, A. E. (2016). "Donor dendritic cell–derived exosomes promote allograft-targeting immune response." Journal of Clinical Investigation, 126(8), 2805–2820. https://doi.org/10.1172/JCI84577
4. MacNabb, B. W., Tumuluru, S., Chen, X., Godfrey, J., Kasal, D. N., Yu, J., Jongsma, M. L. M., Spaapen, R. M., Kline, D. E., & Kline, J. (2022). "Dendritic cells can prime anti-tumor CD8⁺ T cell responses through MHC class I cross-dressing." Immunity, 55(6), 982–997. https://doi.org/10.1016/j.immuni.2022.04.016