SununarySuggestive evidence indicates that immunoglobulin E (IgE)-dependent activation of mononuclear phagocytes plays an important pathogenic role in allergic tissue inflammation. Prevailing opinion holds that low affinity IgE receptors are the relevant IgE-binding structures on monocytes/ macrophages and that functional events occurring after cross-linking of membrane-bound IgE on these cells are mediated by these receptors. Here we demonstrate that Ix-ripheral blood monocytes can bind monomeric IgE via the high affinity IgE receptor (FceRI) and that FceRI expression on these ceils is upregulated in atopic persons. Further, we demonstrate that, upon monocyte adherence to substrate, bridging of monocyte FcdLl is followed by cell activation. We propose that direct interaction of multivalent allergen with Fc~RI+-bound IgE on mononuclear phagocytes results in cell signaling via Fcd~I and that the biological consequences of this event may critically influence the outcome of allergic reactions.

The discovery of marker proteins of human blood (BECs) and lymphatic endothelial cells (LECs) has allowed researchers to isolate these cells. So far, efforts to unravel their transcriptional and functional programs made use of cultured cells only. Hence, it is unknown to which extent previously identified LEC-and BEC-specific programs are representative of the in vivo situation. Here, we define the human BEC-and LEC-specific in vivo transcriptomes by comparative genomewide expression profiling of freshly isolated cutaneous EC subsets and of non-EC skin cells (fibroblasts, mast cells, dendritic cells, epithelial cells). Interestingly, the expression of most of the newly identified EC subset-discriminating genes depends strictly on the in vivo tissue environment as revealed by comparative analyses of freshly isolated and cultured EC subsets. The identified environment-dependent, EC subsetrestricted gene expression regulates lineage fidelity, fluid exchange, and MHC class II-dependent antigen presentation.As an example for a BEC-restricted in vivo function, we show that non-activated BECs in situ, but not in vitro, assemble and display MHC class II protein complexes loaded with self-peptides. Thus, our data demonstrate the key importance of using precisely defined native IntroductionThe microvasculature is actively involved in metabolism and immune cell trafficking. Although the supply with oxygen and nutrients as well as the attraction of leukocytes is accomplished by blood vessels, the removal of extracellular fluid from the tissues and the guidance of immune cells to lymph nodes are carried out by lymphatic vessels. At the molecular level, these different functional capabilities are likely encoded by the transcriptional repertoires of blood vessel endothelial cells (BECs) and lymphatic ECs (LECs). Recently, the discovery of the LEC-specific marker proteins podoplanin (PDPN) and LYVE-1 allowed researchers to isolate and propagate BECs and LECs in vitro. 1,2 Accordingly, several techniques, including genomewide expression profiling coupled to bioinformatic approaches, have been used to identify the spectrum of genes that are expressed in cultured LECs and BECs. [3][4][5][6] However, probably as a result of technical limitations, genomewide analyses were not performed on freshly isolated ECs. Thus, it is possible that ECs, in their tissue-resident state, have a transcriptional and, thus, functional repertoire that differs from that of cultured ECs. In fact, the specialized ECs of high endothelial venules rapidly lose EC subset-defining antigen expression and morphology when these cells were placed in culture. 7 This supports that active signal exchange between ECs and the local tissue environment can also be of critical importance for the maintenance of differentiation and function of LECs and BECs.To address whether and, if so, to which extent tissue-resident LECs or BECs resemble their cultured counterparts, we established for the first time the complete transcriptomes of freshly isolated cutaneous LECs, BECs, a...

The skin contains a population of tissue-resident memory T cells (Trm) that is thought to contribute to local tissue homeostasis and protection against environmental injuries. Although information about the regulation, survival program, and pathophysiological roles of Trm has been obtained from murine studies, little is known about the biology of human cutaneous Trm. Here, we showed that host-derived CD69+ αβ memory T cell clones in the epidermis and dermis remain stable and functionally competent for at least 10 years in patients with allogeneic hematopoietic stem cell transplantation. Single-cell RNA sequencing revealed low expression of genes encoding tissue egress molecules by long-term persisting Trm in the skin, whereas tissue retention molecules and stem cell markers were displayed by Trm. The transcription factor RUNX3 and the surface molecule galectin-3 were preferentially expressed by host T cells at the RNA and protein levels, suggesting two new markers for human skin Trm. Furthermore, skin lesions from patients developing graft-versus-host disease (GVHD) showed a large number of cytokine-producing host-derived Trm, suggesting a contribution of these cells to the pathogenesis of GVHD. Together, our studies highlighted the relationship between the local human skin environment and long-term persisting Trm, which differs from murine skin. Our results also indicated that local tissue inflammation occurs through host-derived Trm after allogeneic hematopoietic stem cell transplantation.